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Boeing CH-47 Chinook

The Boeing CH-47 Chinook is a tandem-rotor helicopter originally developed by American rotorcraft company Vertol and now manufactured by Boeing Defense, Space & Security. The Chinook is a heavy-lift helicopter that is among the heaviest lifting Western helicopters. Its name, Chinook, is from the Native American Chinook people of Oregon and Washington state.

The Chinook was originally designed by Vertol, which had begun work in 1957 on a new tandem-rotor helicopter, designated as the Vertol Model 107 or V-107. Around the same time, the United States Department of the Army announced its intention to replace the piston-engine–powered Sikorsky CH-37 Mojave with a new, gas turbine–powered helicopter. During June 1958, the U.S. Army ordered a small number of V-107s from Vertol under the YHC-1A designation; following testing, some Army officials considered it to be too heavy for the assault missions and too light for transport purposes. While the YHC-1A would be improved and adopted by the U.S. Marine Corps as the CH-46 Sea Knight, the Army sought a heavier transport helicopter, and ordered an enlarged derivative of the V-107 with the Vertol designation Model 114. Initially designated as the YCH-1B, on 21 September 1961, the preproduction rotorcraft performed its maiden flight. In 1962, the HC-1B was redesignated CH-47A under the 1962 United States Tri-Service aircraft designation system.

The Chinook possesses several means of loading various cargoes, including multiple doors across the fuselage, a wide loading ramp located at the rear of the fuselage and a total of three external ventral cargo hooks to carry underslung loads. Capable of a top speed of 170 kn, upon its introduction to service in 1962, the helicopter was considerably faster than contemporary 1960s utility helicopters and attack helicopters, and is still one of the fastest helicopters in the US inventory. Improved and more powerful versions of the Chinook have also been developed since its introduction; one of the most substantial variants to be produced was the CH-47D, which first entered service in 1982; improvements from the CH-47C standard included upgraded engines, composite rotor blades, a redesigned cockpit to reduce workload, improved and redundant electrical systems and avionics, and the adoption of an advanced flight control system. It remains one of the few aircraft to be developed during the early 1960s – along with the fixed-wing Lockheed C-130 Hercules cargo aircraft – that has remained in both production and frontline service for over 60 years.

The military version of the helicopter has been exported to nations across the world; the U.S. Army and the Royal Air Force (see Boeing Chinook (UK variants)) have been its two largest users. The civilian version of the Chinook is the Boeing Vertol 234. It has been used by civil operators not only for passenger and cargo transport, but also for aerial firefighting and to support logging, construction, and oil extraction industries.

Initial development
During late 1956, the United States Department of the Army announced its intention to replace the Sikorsky CH-37 Mojave, which was powered by piston engines, with a new, gas turbine-powered helicopter. Turbine engines were also a key design feature of the smaller UH-1 "Huey" utility helicopter. Following a design competition, in September 1958, a joint Army–Air Force source selection board recommended that the Army procure the Vertol-built medium transport helicopter. However, funding for full-scale development was not then available, and the Army vacillated on its design requirements. Some officials in Army Aviation thought that the new helicopter should be operated as a light tactical transport aimed at taking over the missions of the old piston-engined Piasecki H-21 and Sikorsky H-34 helicopters, and be consequently capable of carrying about 15 troops (one squad). Another faction in Army Aviation thought that the new helicopter should be much larger, enabling it to airlift large artillery pieces and possess enough internal space to carry the new MGM-31 "Pershing" missile system.

During 1957, Vertol commenced work upon a new tandem-rotor helicopter, designated as the Vertol Model 107 or V-107. During June 1958, the U.S. Army awarded a contract to Vertol for the acquisition of a small number of the rotorcraft, giving it the YHC-1A designation. As ordered, the YHC-1A possessed the capacity to carry a maximum of 20 troops. Three underwent testing by the Army for deriving engineering and operational data. However, the YHC-1A was considered by many figures within the Army users to be too heavy for the assault role, while too light for the more general transport role. Accordingly, a decision was made to procure a heavier transport helicopter, and at the same time, upgrade the UH-1 "Huey" to serve as the needed tactical troop transport. The YHC-1A would be improved and adopted by the Marines as the CH-46 Sea Knight in 1962. As a result, the Army issued a new order to Vertol for an enlarged derivative of the V-107, known by internal company designation as the Model 114, which it gave the designation of HC-1B. On 21 September 1961, the preproduction Boeing Vertol YCH-1B made its initial hovering flight. During 1962, the HC-1B was redesignated the CH-47A under the 1962 United States Tri-Service aircraft designation system; it was also named "Chinook" after the Chinook people of the Pacific Northwest.

The CH-47 is powered by two turboshaft engines, mounted on each side of the helicopter's rear pylon and connected to the rotors by drive shafts. Initial models were fitted with Lycoming T55 engines rated at 2200 hp each. The counter-rotating rotors eliminate the need for an antitorque vertical rotor, allowing all power to be used for lift and thrust. The ability to adjust lift in either rotor makes it less sensitive to changes in the center of gravity, important for the cargo lifting and dropping. While hovering over a specific location, a twin-rotor helicopter has increased stability over a single rotor when weight is added or removed, for example, when troops drop from or begin climbing up ropes to the aircraft, or when other cargo is dropped. If one engine fails, the other can drive both rotors. The "sizing" of the Chinook was directly related to the growth of the Huey and the Army's tacticians' insistence that initial air assaults be built around the squad. The Army pushed for both the Huey and the Chinook, and this focus was responsible for the acceleration of its air mobility effort.

Improved and later versions

Improved and more powerful versions of the CH-47 have been developed since the helicopter entered service. Often the same airframe was upgraded to later standards, or sometimes the airframes were built to the newer upgrade. The U.S. Army's first major design leap was the now-common CH-47D, which entered service in 1982. Improvements from the CH-47C included upgraded engines, composite rotor blades, a redesigned cockpit to reduce pilot workload, improved and redundant electrical systems, an advanced flight control system, and improved avionics. The latest mainstream generation is the CH-47F, which features several major upgrades to reduce maintenance, digitized flight controls, and is powered by two 4733 hp Honeywell engines.

An example of a British upgraded version is the HC.4; the first HC.4 first flew on 9 December 2010. A commercial model of the Chinook, the Boeing-Vertol Model 234, is used worldwide for logging, construction, fighting forest fires, and supporting petroleum extraction operations. In December 2006, Columbia Helicopters Inc purchased the type certificate of the Model 234 from Boeing. The Chinook has also been licensed to be built by companies outside the United States, such as Agusta (now AgustaWestland) in Italy and Kawasaki in Japan.

Vietnam War

The Army finally settled on the larger Chinook as its standard medium-transport helicopter, and by February 1966, 161 aircraft had been delivered to the Army. The 1st Cavalry Division had brought its organic Chinook battalion (three Chinook companies) when it arrived in 1965 and a separate aviation medium helicopter company, the 147th, had arrived in Vietnam on 29 November 1965. This latter company was initially placed in direct support of the 1st Infantry Division. CH-47 crews quickly learned to mount an M60 machine gun in each of the forward doors. Sometimes they also installed an M2 machine gun to fire from the rear cargo door.

The most spectacular mission in Vietnam for the Chinook was the placing of artillery batteries in perilous mountain positions inaccessible by any other means, and then keeping them resupplied with large quantities of ammunition. The 1st Cavalry Division found that its CH-47s were limited to a 7000 lb payload when operating in the mountains, but could carry an additional 1000 lb when operating near the coast.

As with any new piece of equipment, the Chinook presented a major problem of "customer education". Commanders and crew chiefs had to be constantly alert that eager soldiers did not overload the temptingly large cargo compartment. It would be some time before troops would be experts at using sling loads. The Chinook soon proved to be such an invaluable aircraft for artillery movement and heavy logistics that it was seldom used as an assault troop carrier. Some of the Chinook fleet was used for casualty evacuation, and due to the very heavy demand for the helicopters, they were usually overburdened with wounded. Perhaps the most cost effective use of the Chinook was the recovery of other downed aircraft.

At the war's peak the US Army had 21 Chinook companies in Vietnam. Pilots discovered the CH-47A's transmission system could not handle the two gas turbines running at full power, and high humidity and heat reduced the maximum lift by more than 20% in the low lands and 30% in mountain areas. More powerful, improved transmission and strengthened fuselages arrived in 1968 with the CH-47B, followed a few months later by the CH-47C. The CH-47s in Vietnam were generally armed with a single 0.308 in (7.62 mm) M60 machine gun on a pintle mount on either side of the aircraft for self-defense, with stops fitted to keep the gunners from firing into the rotor blades. Dust filters were also added to improve engine reliability. Of the nearly 750 Chinook helicopters in the U.S. and South Vietnam fleets, about 200 were lost in combat or wartime operational accidents. The U.S. Army CH-47s supported the 1st Australian Task Force as required.

Four CH-47s were converted into ACH-47As by adding armor and improved engines. Its armament included two fixed forward firing M24A1 20 mm cannons, one turret with 40 mm automatic grenade launcher on the nose, five .50 in machine guns and two weapon pods on the sides that could carry either XM159B/XM159C 70 mm rocket launchers or 7.62 mm miniguns. They arrived in Vietnam in 1966, and they engaged in six months of operational testing at An Khê Army Airfield. They performed well in combat, but its high maintenance costs and demand for use in troop and cargo transport was stronger. Three ACH-47s were lost. One collided with a CH-47 while taxiing. Another had a retention pin shake loose on a 20 mm cannon and was brought down when its own gun fired through the forward rotor blades. The third was grounded by enemy fire and destroyed by enemy mortar rounds after the crew escaped.

Iran
During the 1970s, the United States and Iran had a strong relationship, in which the Iranian armed forces began to use many American military aircraft, most notably the F-14 Tomcat, as part of a modernization program. After an agreement signed between Boeing and Agusta, the Imperial Iranian Air Force purchased 20 Agusta-built CH-47Cs in 1971. The Imperial Iranian Army Aviation purchased 70 CH-47Cs from Agusta between 1972 and 1976. In late 1978, Iran placed an order for an additional 50 helicopters with Elicotteri Meridionali, but that order was canceled immediately after the revolution; 11 of them were delivered after multiple requests by Iran.

In the 1978 Iranian Chinook shootdown, four Iranian CH-47Cs penetrated 15 – into Soviet airspace in the Turkmenistan Military District. They were intercepted by a MiG-23M which shot down one CH-47, killing eight crew members, and forced a second helicopter to land. Chinook helicopters were used in efforts by the Iranian against Kurdish rebels in 1979.

During the Iran–Iraq War, Iran made heavy use of its US-bought equipment, and lost at least eight CH-47s during the 1980–1988 period, most notably during a clash on 15 July 1983, when an Iraqi Mirage F1 destroyed three Iranian Chinooks transporting troops to the front line, and on 25–26 February 1984, when Iraqi MiG-21 fighters shot down two examples. On 22 March 1982, in Operation Fath ol-Mobin, a key operation of the war, Iranian Chinooks were landed behind Iraqi lines, deployed troops that silenced their artillery, and captured an Iraqi headquarters; the attack took the Iraqi forces by surprise.

Despite the arms embargo in place upon Iran, it has managed to keep its Chinook fleet operational. Some of the Chinooks have been rebuilt by Panha. As of 2015, 20 to 45 Chinooks were operational in Iran.

Libyan wars
In 1976, the Libyan Air Force purchased 24 Italian-built CH-47C helicopters, 14 of which were transferred to the Libyan Army during the 1990s. The Libyan Air Force recruited Western pilots and technicians to operate the CH-47 fleet. The Libyan Chinooks flew transport and support missions into Chad to supply Libyan ground forces operating there in the 1980s. Chinooks were occasionally used to transport Libyan special forces in assault missions in northern Chad.

In 2002, Libya sold 16 helicopters to the United Arab Emirates, as due to the Western embargo and lack of funds, maintaining them was difficult. The sale to UAE was a $939 million package that included equipment, parts, and training. How many CH-47s are still in existence or operational during the ongoing Libyan civil wars that started in 2011 is not known.

Falklands War
The Chinook was used both by Argentina and the United Kingdom during the Falklands War in 1982.

The Argentine Air Force and the Argentine Army each deployed two CH-47C helicopters, which were widely used in general transport duties. Of the Army's aircraft, one was destroyed on the ground by 30 mm cannon fire from an RAF GR3 Harrier, while the other was captured by the British and reused after the war. Both Argentine Air Force helicopters returned to Argentina and remained in service until 2002.

Three British Chinooks were destroyed on 25 May 1982 when Atlantic Conveyor was struck by an Exocet sea-skimming missile fired by an Argentine Super Étendard. The sole surviving British Chinook, Bravo November, did outstanding service in the Falklands, lifting 81 troops on one occasion. Since April 2022, it has been on display at Royal Air Force Museum Cosford.

Afghanistan and Iraq wars
About 163 CH-47Ds of various operators were deployed to Saudi Arabia, Kuwait, and Iraq during Operation Desert Shield and the subsequent Operation Desert Storm in 1990–91.

The CH-47D has seen wide use in Operation Enduring Freedom in Afghanistan and Operation Iraqi Freedom in Iraq. The Chinook is being used in air assault missions, inserting troops into fire bases, and later bringing food, water, and ammunition. It is also the casualty evacuation aircraft of choice in the British Armed Forces. In combat theaters, it is typically escorted by attack helicopters such as the AH-64 Apache for protection. Its lift capacity has been found of particular value in the mountainous terrain of Afghanistan, where high altitudes and temperatures limit the use of helicopters such as the UH-60 Black Hawk; reportedly, one Chinook can replace up to five UH-60s in the air assault transport role.

The Chinook helicopters of several nations have participated in the Afghanistan War, including aircraft from Britain, Italy, the Netherlands, Spain, Canada, and Australia. Despite the age of the Chinook, it is still in heavy demand, in part due its proven versatility and ability to operate in demanding environments such as Afghanistan.

In May 2011, an Australian Army CH-47D crashed during a resupply mission in Zabul Province, resulting in one fatality and five survivors. The helicopter was unable to be recovered and was destroyed in place. To compensate for the loss, the ADF added two ex-U.S. Army CH-47Ds to the fleet which are expected to be in service until the introduction of the CH-47Fs in 2016.

On 6 August 2011, a Chinook crashed near Kabul, killing all of the 38 aboard. The Chinook was reportedly shot down with a rocket-propelled grenade by the Taliban while attempting to assist a group of U.S. Navy SEALs. The 38 were members of NATO and allied forces, including 22 Naval Special Warfare operators, five U.S. Army Aviation soldiers, three U.S. Air Force special operations personnel, and seven Afghan National Army commandos. A civilian translator and a U.S. military working dog were also killed in the crash. The crash was the single deadliest during the entire Operation Enduring Freedom campaign. The previous biggest single-day loss for American forces in Afghanistan involved a Chinook that was shot down near Kabul in Kunar Province in June 2005 with all aboard killed, including a 16-member U.S. Special Operations team.

Chinook helicopters participated in the 2021 Kabul airlift at the close of military operations in Afghanistan.

Disaster relief
The Chinook's ability to carry large, underslung loads has been of significant value in relief operations in the aftermath of natural disasters. Numerous operators have chosen to deploy their Chinook fleets to support humanitarian efforts in stricken nations overseas. Following the 2004 Asian tsunami, the Republic of Singapore Air Force assisted in the relief operations in neighboring Indonesia using its Chinooks; similarly, after the 2005 Kashmir earthquake, the Royal Air Force dispatched several Chinooks to Northern Pakistan to assist in recovery efforts.

In August 1992, six CH-47Ds were deployed from Fort Bragg in North Carolina to provide relief in the wake of Hurricane Andrew in what was one of the first major helicopter disaster relief operations on US soil. Then President George H. W. Bush ordered the military to assist. The Chinooks arrived at Miami-Opa Locka Executive Airport, just outside of the disaster zone, one day after the President's order; early on, they performed a wide loop over Homestead and Florida City to publicly display their presence, helping to curtail lawlessness and looting. The Chinooks initially flew twelve sorties per day out of Opa Locka, which expanded over time, often supporting distribution operations at Homestead AFB and Opa Locka, as well as delivering relief payloads via internal storage, not using sling loads, supplementing the two dozen distribution centers and trucks, proving essential as trucks could not reach the worst hit areas due to downed trees and power lines. They flew everyday for approximately three weeks, moving supplies and personnel around the disaster zone as well as carrying media and government officials, including then Congressman Bill Nelson. Ultimately, the Chinooks supplied 64 distribution sites throughout the zone and transported 1.2 million pounds of supplies before the urgent relief phase ended.

Three of Japan's CH-47s were used to cool Reactors 3 and 4 of the Fukushima Nuclear power plant following the 9.0 earthquake in 2011; they were used to collect sea water from the nearby ocean and drop it over the affected areas. In order to protect the crew from the heightened radiation levels present, a number of lead plates were attached to the floor of each Chinook; even with such measures, pilots had to keep their distance from the reactors while also limiting flight times in the vicinity to a maximum of 45 minutes to avoid excessive radiation exposure.

Other operations and roles
In April 2023, multiple Chinooks conducted the evacuation of the US Embassy in Khartoum, Sudan with Special Forces. (see also Sudanese civil war (2023–present)) Chinooks have been deployed to Mali for MINUSMA.

Since the type's inception, the Chinook has carried out various secondary missions, including medical evacuation, disaster relief, search and rescue, aircraft recovery, fire fighting, and heavy construction assistance. In February 2020, the Indian Air Force started using Chinooks at theatres such as Ladakh and Siachen Glacier to assist Indian forces deployed at the Indian borders with China and Pakistan. The Indian Space Research Organisation has also enlisted Indian Air Force Chinooks during the development of its Reusable Launch Vehicle Technology Demonstration Program, with Chinook helicopters air-dropping two test vehicles during the RLV-LEX-01 and RLV-LEX-02 conducted on April 2, 2023, and March 22, 2024, respectively.

CH-47F orders
In February 2007, the Royal Netherlands Air Force became the first international customer of the CH-47F model, expanding their Chinook fleet to 17. On 10 August 2009, Canada signed a contract for 15 extensively modified and upgraded CH-47Fs for the Canadian Forces, later delivered in 2013–2014 with the Canadian designation CH-147F. On 15 December 2009, Britain announced its Future Helicopter Strategy, including the purchase of 24 new CH-47Fs to be delivered from 2012. Australia ordered seven CH-47Fs in March 2010 to replace its six CH-47Ds between 2014 and 2017. In September 2015, India approved purchase of 15 CH-47F Chinooks. On 7 November 2016, Singapore announced that the CH-47F would replace its older Chinooks, which had been in service since 1994, enabling the Republic of Singapore Air Force to meet its requirements for various operations, including Search and Rescue (SAR), Aeromedical Evacuation (AME), and Humanitarian Assistance and Disaster Relief (HADR) operations. The German government announced in June 2022 that the CH-47F Block 2 was selected as the winner of its heavy helicopter program to replace its Sikorsky CH-53G Sea Stallion fleet. Germany is planning to buy 60 airframes to boost heavy lift capability.

Spain had 17 CH-47 which it is planning to upgrade CH-47F standard. By 2023, 13 CH-47D were upgraded to CH-47F, and additionally, 4 new build CH-47F were acquired.

Variants
There have been many versions of the Chinook over the decades, there are variants that involve major upgrades such as engines and avionics, ones for certain tasks, such as Special Operation missions, and finally there are version for certain countries, such as the J model built by and for Japan.

HC-1B
The pre-1962 designation for Model 114 development aircraft that would be redesignated CH-47 Chinook.

CH-47A
The all-weather, medium-lift CH-47A Chinook was powered initially by Lycoming T55-L-5 engines rated at 2200 hp, which were replaced by the T55-L-7 rated at 2650 hp engines or T55-L-7C engines rated at 2850 hp. The CH-47A had a maximum gross weight of 33000 lb, with a maximum payload about 10000 lb Delivery of the CH-47A Chinook to the U.S. Army began in August 1962. A total of 354 were built.

ACH-47A

The ACH-47A was known as the Armed/Armored CH-47A (or A/ACH-47A) before being designated ACH-47A as a U.S. Army Attack Cargo Helicopter, and unofficially referred to as Guns A Go-Go. Four CH-47A helicopters were converted to gunships by Boeing Vertol in late 1965. Three were assigned to the 53rd Aviation Detachment in South Vietnam for testing, with the remaining one retained in the U.S. for weapons testing. By 1966, the 53rd was redesignated the 1st Aviation Detachment (Provisional) and attached to the 228th Assault Support Helicopter Battalion of the 1st Cavalry Division (Airmobile). By 1968, only one gunship remained, and transport demands prevented more conversions. It was returned to the United States, and the program stopped.

The ACH-47A carried five 7.62 × 51 mm M60D machine guns or .50-caliber (12.7 mm) M2HB heavy machine guns, provided by the XM32 and XM33 armament subsystems, two 20 mm M24A1 cannons, two 19-tube 2.75 in Folding Fin Aerial Rocket launchers (XM159B/XM159) or sometimes two M18/M18A1 7.62 × 51 mm gun pods, and a single 40 mm M75 grenade launcher in the XM5/M5 armament subsystem (more commonly seen on the UH-1 series of helicopters). Rare newsreel footage shows one of the aircraft in action supporting the 8th Cavalry Regiment during an ambush at Bông Son, South Vietnam. The surviving aircraft, Easy Money, has been restored and is on display at Redstone Arsenal, Alabama.

CH-47B
The CH-47B was an interim upgrade while Boeing worked on a more substantially improved CH-47C. The CH-47B was powered by two Lycoming T55-L-7C 2,850 hp engines. It had a blunted rear rotor pylon, redesigned asymmetrical rotor blades, and strakes along the rear ramp and fuselage to improve flying characteristics. It could be equipped with two door-mounted M60D 7.62 mm NATO machine guns on the M24 armament subsystem and a ramp-mounted M60D using the M41 armament subsystem. Some CH-47 "bombers" were equipped to drop tear gas or napalm from the rear cargo ramp onto Viet Cong bunkers. The CH-47B could be equipped with a hoist and cargo hook. The Chinook proved especially valuable in "Pipe Smoke" aircraft recovery missions. The "Hook" recovered about 12,000 aircraft valued at over $3.6 billion during the war; 108 were built.

CH-47C

The CH-47C featured more powerful engines and transmissions. Three sub-versions were built, the first with Lycoming T55-L-7C engines delivering 2850 shp. The "Super C" had Lycoming T55-L-11 engines delivering 3750 shp, an upgraded maximum gross weight of 46000 lb, and pitch stability augmentation. The T55-L-11 engines were less reliable, as they had been hurriedly introduced to increase payload; thus, they were temporarily replaced by the more reliable Lycoming T55-L-7C. The Super C was distinguishable from the standard "C" by the uprated maximum gross weight.

The type was not approved by the FAA for civil aviation due to the nonredundant hydraulic flight boost system drive. The hydraulic system was redesigned for the succeeding CH-47D, allowing it to achieve certification as the Boeing Model 234. A total of 233 CH-47Cs were built. Canada bought eight CH-47Cs; deliveries of the type began in 1974. Receiving the Canadian designation "CH-147", these were fitted with a power hoist above the crew door; other changes included a flight engineer station in the rear cabin: operators referred to the configuration as the "Super C". The CH-47C was used widely during the Vietnam War, eventually replacing the older Piasecki H-21 Shawnee in the combat assault support role.

CH-47D

The CH-47D shares the same airframe as earlier models, with more powerful engines. Early CH-47Ds were powered by two T55-L-712 engines. The most common engine is the later T55-GA-714A. With its triple-hook cargo system, the CH-47D can carry heavy payloads internally and up to 26000 lb (such as 40 ft containers) externally. It was introduced into service in 1979. In air assault operations, it often serves as the principal mover of the 155 mm M198 howitzer, plus 30 rounds of ammunition, and an 11-man crew. The CH-47D has advanced avionics, including Global Positioning System. Nearly all US Army CH-47Ds were converted from previous A, B, and C models, a total of 472 being converted. The last U.S. Army CH-47D built was delivered to the U.S. Army Reserve, located at Fort Hood, Texas, in 2002.

The Netherlands acquired all seven of the Canadian Forces' surviving CH-147Cs and upgraded them to CH-47D. Six more new CH-47Ds were delivered in 1995 for a total of 13. The Dutch CH-47Ds are improved over U.S. Army CH-47Ds, including a long nose for Bendix weather radar, a "glass cockpit", and improved T55-L-714 engines. As of 2011, the Netherlands planned upgrading 11 of them to the CH-47F standard at a later date. As of 2011, Singapore has 18 CH-47D/SDs, which includes twelve "Super D" Chinooks, in service. In 2008, Canada purchased 6 CH-47Ds from the U.S. for the Canadian Helicopter Force Afghanistan for $252 million. With 1 CH-47D lost to an accident, the remaining five were sold in 2011 after the end of Canada's Afghanistan mission and replaced with seven CH-147Fs. The U.S. Army is surplusing many of its CH-47Ds for use in FAA restricted category.

MH-47D
The MH-47D was developed for special forces operations and has inflight refueling capability, a fast rope-rappelling system, and other upgrades. The MH-47D was used by U.S. Army 160th Special Operations Aviation Regiment. Twelve MH-47D helicopters were produced. Six were converted from CH-47As and six were converted from CH-47C models.

MH-47E

The MH-47E has been used by U.S. Army Special Operations. Beginning with the E-model prototype manufactured in 1991, a total of 26 Special Operations Aircraft were produced. All aircraft were assigned to 2–160th SOAR(A) "Nightstalkers", home based at Fort Campbell, Kentucky. E models were converted from CH-47C airframes. The MH-47E has similar capabilities as the MH-47D, but with increased fuel capacity similar to the CH-47SD and terrain following/terrain avoidance radar.

In 1995, the Royal Air Force ordered eight Chinook HC3s, effectively a lower cost MH-47E for special operations. They were delivered in 2001, but never became operational due to technical issues with their avionics fit, unique to the HC3. In 2008, work started to revert the HC3s to HC2 standard, to enable them to enter service. they were upgraded to HC5 standard with a digital automated flight control system.

CH-47F

In 2001, the CH-47F, an upgraded CH-47D, made its maiden flight. The first production model rolled out at Boeing's facility in Ridley Park, Pennsylvania, and first flew on 23 October 2006. Upgrades included 4868 shp Honeywell engines and airframe with better integrated construction for lower maintenance requirements. The milled construction reduces vibration, as well as inspection and repair needs, and eliminates flexing points to increase service life. The CH-47F can fly at speeds of over 175 mi/h with a payload of more than 21000 lb. New avionics include a Rockwell Collins Common Avionics Architecture System (CAAS) cockpit, and BAE Systems' Digital Advanced Flight Control System (DAFCS). AgustaWestland assembles the CH-47F under license, known as the Chinook ICH-47F, for several customers. Boeing delivered 48 CH-47Fs to the U.S. Army through August 2008; at that time Boeing announced a $4.8 billion (~$ in ) contract with the Army for 191 Chinooks.

A CH-47F Block 2 is being implemented. The Block 2 aims for a payload of 22000 lb with 4000 ft and 95 °F high and hot hover performance, eventually increased up to 6000 ft, to carry the Joint Light Tactical Vehicle; maximum takeoff weight would be raised to 54000 lb. It has 20% more powerful Honeywell T55-715 engines along with an active parallel actuator system (APAS) to enhance the digital advanced flight-control system, providing an exact torque split between the rotors for greater efficiency. A new fuel system combines the three fuel cells in each sponson into one larger fuel cell and eliminates intracell fuel transfer hardware, reducing weight by 90 kg and increasing fuel capacity. Electrical capacity is increased by three 60 kVA generators. The Advanced Chinook Rotor Blades (ACRB), derived from the cancelled RAH-66 Comanche, were intended to improve lift performance in hot/high altitude conditions by 2,000 lb; however, the US Army ultimately decided against implementing the ACRB due to persistent vibration during testing, Boeing denied the assertion that the vibration was a safety risk and believed it could be solved with dampeners. In addition, the aft rotor blade was stalling when in a swept back position.

The U.S. Army plans for a Block 3 upgrade after 2025, which could include a new 6000 hp class engine with boosted power capacity of the transmission and drive train developed under the future affordable turbine engine (FATE) program and a lengthened fuselage. The Future Vertical Lift program plans to begin replacing the Army's rotorcraft fleet in the mid-2030s, initially focusing on medium-lift helicopters, thus the CH-47 is planned to be in service beyond 2060, over 100 years after the first entered service.

MH-47G
The MH-47G Special Operations Aviation (SOA) version is similar to the MH-47E, but features more sophisticated avionics including a digital Common Avionics Architecture System (CAAS). The CAAS is a common glass cockpit used by helicopters such as MH-60K/Ls, CH-53E/Ks, and ARH-70As. The MH-47G also incorporates all of the new sections of the CH-47F.

The modernization program improves MH-47D and MH-47E Special Operations Chinooks to the MH-47G design specs. A total of 25 MH-47E and 11 MH-47D aircraft were upgraded by the end of 2003. The final MH-47G was delivered to the U.S. Army Special Operations Command (USASOC) on 10 February 2011. Modernization of MH-47D/Es to the MH-47G standard is due by 2015. On 1 September 2020, Boeing announced the delivery of the first MH-47G Block II to USASOC of an initial order of 24, with a stated ultimate requirement for 69. The MH-47G Block II includes all of the improvements from the CH-47F Block II, as well as inflight refueling capability, a comprehensive defensive aids suite and low-level/adverse weather piloting aids, such as forward-looking infrared and multi-mode/terrain-following radar. It is armed with two 7.62 mm M134 Miniguns and two M240 7.62 mm machine guns.

The British MOD confirmed that while the U.S. does not export the model, the two countries were in discussion regarding the MH-47G. On 19 October 2018, the Defense Security Cooperation Agency notified Congress of a possible sale of 16 H-47 Chinooks (Extended Range) to the UK.

CH-47J

The CH-47J is a medium-transport helicopter for the Japan Ground Self-Defense Force (JGSDF), and the Japan Air Self-Defense Force (JASDF). The differences between the CH-47J and the CH-47D are the engine, rotor brake and avionics, for use for general transportation, SAR and disaster activity like U.S. forces. The CH-47JA, introduced in 1993, is a long-range version of the CH-47J, fitted with an enlarged fuel tank, an AAQ-16 FLIR in a turret under the nose, and a partial glass cockpit. Both versions are built under license in Japan by Kawasaki Heavy Industries, who produced 61 aircraft by April 2001.

The Japan Defense Agency ordered 54 aircraft of which 39 were for the JGSDF and 15 were for the JASDF. Boeing supplied flyable aircraft, to which Kawasaki added full avionics, interior, and final paint. The CH-47J model Chinook (N7425H) made its first flight in January 1986, and it was sent to Kawasaki in April. Boeing began delivering five CH-47J kits in September 1985 for assembly at Kawasaki.

Over 110 had been produced by the early 21st century, including the CH-47JA model with improved fuel tanks and other upgrades.

HH-47
On 9 November 2006, the HH-47, a new variant of the Chinook based on the MH-47G, was selected by the U.S. Air Force as the winner of the Combat Search and Rescue (CSAR-X) competition. Four development HH-47s were to be built, with the first of 141 production aircraft planned to enter service in 2012. However, in February 2007 the contract award was protested and the GAO ordered the CSAR-X project to be re-bid. The CSAR-X program was again terminated in 2009. In February 2010, the USAF announced plans to replace aging HH-60G helicopters, and deferred secondary combat search and rescue requirements calling for a larger helicopter. In a long process the Air Force did not choose the larger Chinook, and the HH-60W entered service by 2020 to replace the older helicopters.

Sea Chinook
For years the U.S. Navy has been operating different versions of the single-rotor CH-53 helicopter as its heavy-lift helicopter. CH-47s regularly conduct ship-based operations for U.S. Special Forces and other international operators. Due to budget issues, technical problems and delays with CH-53K, the director of the Pentagon's cost assessment office directed US Navy to consider maritime versions of CH-47. Naval versions must be protected against the corrosive seaborne environment and be able to operate from aircraft carriers and amphibious ships.

Other export models

 * The Royal Air Force version of the CH-47C is designated Chinook HC1; its versions of the CH-47D are designated Chinook HC2 and HC2A. Additional types in RAF service include HC3,4,5,6, and 6A. (for further reading see Boeing Chinook (UK variants))
 * The export version of the CH-47C Chinook for the Italian Army was designated "CH-47C Plus".
 * The HH-47D is a search and rescue version for the Republic of Korea Air Force.
 * The CH-47DG is an upgraded version of the CH-47C for Greece.
 * While the CH-47SD (also known as the "Super D") is a modified variant for Singapore of the CH-47D, with extended range fuel tanks and higher payload carrying capacity; the CH-47SD is in use by the Republic of Singapore Air Force, Hellenic Army and the Republic of China Army.

Civilian models

 * British Airways Helicopters Boeing 234 G-BISP.jpg 234LR at Aberdeen Airport in 1985]]Model 234LR (long range): Commercial transport helicopter. The Model 234LR can be fitted out as an all-passenger, all-cargo, or cargo/passenger transport helicopter.
 * Model 234ER (extended range): Commercial transport version.
 * Model MLR (multi-purpose long range): Commercial transport version.
 * Model 234UT (utility transport): Utility transport helicopter.
 * Model 414: The Model 414 is the international export version of the CH-47D. It is also known as the CH-47D International Chinook.
 * CU-47: Built by Unical from parts from ex-Canadian Forces CH-47D/CH-147D for Coulson Aviation to be used for aerial firefighting.

Derivatives
In 1969, work on the experimental BV-347 was begun. It was a CH-47A with a lengthened fuselage, four-blade rotors, detachable wings mounted on top of the fuselage and other changes. It first flew on 27 May 1970 and was evaluated for a few years.

In 1973, the Army contracted with Boeing to design a "Heavy Lift Helicopter" (HLH), designated XCH-62A. It appeared to be a scaled-up CH-47 without a conventional body, in a configuration similar to the S-64 Skycrane (CH-54 Tarhe), but the project was canceled in 1975. The program was restarted for test flights in the 1980s and was again not funded by Congress. The scaled-up model of the HLH was scrapped in late 2005 at Fort Rucker, Alabama.

Operators

 * Australian Army
 * Royal Canadian Air Force
 * Royal Canadian Air Force
 * Republic of China Army
 * National Fire Agency
 * Egyptian Air Force
 * German Air Force Pending an order contract
 * Hellenic Army
 * German Air Force Pending an order contract
 * Hellenic Army
 * Hellenic Army
 * Hellenic Army
 * Indian Air Force
 * Indonesian National Board for Disaster Management
 * Islamic Republic of Iran Army
 * Italian Army
 * Japan Air Self-Defense Force operates 15 CH-47Js as of March 2022.
 * Japan Ground Self-Defense Force operates 47 CH-47J/JAs as of March 2022.
 * Italian Army
 * Japan Air Self-Defense Force operates 15 CH-47Js as of March 2022.
 * Japan Ground Self-Defense Force operates 47 CH-47J/JAs as of March 2022.
 * Japan Ground Self-Defense Force operates 47 CH-47J/JAs as of March 2022.
 * Republic of Korea Army
 * Republic of Korea Army
 * Libyan Air Force
 * Royal Moroccan Air Force
 * Royal Netherlands Air Force
 * Royal Saudi Land Forces
 * Republic of Singapore Air Force
 * Royal Netherlands Air Force
 * Royal Saudi Land Forces
 * Republic of Singapore Air Force
 * Republic of Singapore Air Force
 * Republic of Singapore Air Force
 * Spanish Army Airmobile Force
 * Turkish Army
 * United Arab Emirates Air Force
 * Turkish Army
 * United Arab Emirates Air Force
 * United Arab Emirates Air Force

 * Royal Air Force
 * United States Army
 * Army National Guard
 * United States Army Reserve
 * United States Army Reserve

Former operators

 * Argentina
 * Argentine Air Force
 * Argentine Army

 * Australia
 * Royal Australian Air Force (later transferred to the Australian Army)

 * South Vietnam
 * Republic of Vietnam Air Force

 * Thailand
 * Royal Thai Army

 * United Kingdom
 * British Airways Helicopters

 * United States
 * NASA

 * Vietnam
 * Vietnam People's Air Force

Accidents

 * On 4 May 1966, a CH-47A crashed near Di Linh, Lâm Đồng Province killing all 20 on board.
 * On 26 December 1967, a CH-47A carrying 33 military passengers and crew crashed in a landing descent at Phu Cat AFB, Binh Dinh Province, South Vietnam killing 8 of those on board.
 * On 6 May 1969 a CH-47 carrying 83 people crashed 3 mi southwest of Phước Vĩnh Base Camp, South Vietnam, killing 40 of those on board.
 * On 18 August 1971, CH-47A, airframe 66-19023, was operated by the 4th Aviation Company, 15th Aviation Group. The helicopter was transporting 33 soldiers of the Heavy Mortar Platoon, 2nd Battalion, 4th Infantry Regiment, 56th Field Artillery Brigade from battalion headquarters in Ludwigsburg to Grafenwöhr for live fire training exercises. Fatigue failure of the rear rotor blade led to its separation causing structural damage resulting in the crash and explosion that killed all 37 on board, including four crew members. A memorial plaque that was placed near the crash site in the forest outside Pegnitz was stolen in 2009.
 * On 28 November 1971, a CH-47C carrying five crew and 28 soldiers from the 101st Airborne Division on a flight from Da Nang to Phu Bai Combat Base, South Vietnam crashed into high ground killing all on board.
 * On 18 October 1974, a CH-47C, US serial 74-22058 assigned 147001 but was never marked with Canadian Forces. The aircraft was lost on its delivery flight to Canada following gear failure in main combining gear box, caused by undetected metal infraction in gear blank before machining. This failure led to drive shaft failure and loss of synchronization and resulted in five fatalities. After a lengthy litigation, it was replaced by 147009.
 * On 11 September 1982 at an airshow in Mannheim, Germany, a U.S. Army Chinook (serial number 74-22292) carrying parachutists crashed, killing 46 people. The crash was later found to have been caused by an accumulation of ground walnut shell grit used for cleaning machinery, which blocked lubrication from reaching transmission bearings. The accident resulted in the eventual discontinuation of the use of walnut grit as a cleaning agent.
 * On 4 February 1985, a Royal Australian Air Force (RAAF) CH-47C (A15-001) crashed into Perseverance Dam, Toowoomba, Queensland, Australia. The Royal Air Force (RAF) exchange pilot was rescued from the submerged cockpit but later died in hospital.
 * On 6 November 1986, a British International Helicopters Chinook crashed on approach to Sumburgh Airport, Shetland Islands resulting in the loss of 45 lives and the withdrawal of the Chinook from crew-servicing flights in the North Sea.
 * On 1 March 1991, Major Marie Therese Rossi Cayton was killed when her U.S. Army Chinook helicopter crashed into an unlit microwave tower during a dust storm. She was the first American woman to fly in combat, during Desert Storm in 1991.
 * On 2 June 1994, an RAF CH-47 carrying 25 British MI5, police, and military intelligence experts and 4 crew, flew from Northern Ireland to Scotland for a conference crashed on the Mull of Kintyre
 * On 30 October 1997, a civilian BV234UT operated by Columbia Helicopters (registration C-FHFH) engaged in logging operations crashed on Vancouver Island, Canada, killing both of the pilots. The investigation determined that the helicopter lost yaw control due to failure of flight control computer.
 * On 29 May 2001, a Republic of Korea Army (ROK Army) CH-47D installing a sculpture onto Olympic Bridge in Seoul, South Korea failed to unlatch the sculpture. The helicopter's rotors struck the monument; then the fuselage hit and broke into two. One section crashed onto the bridge in flames and the other fell into the river. All three crew members on board died.
 * On 22 February 2002, a U.S. Army special forces MH-47E crashed at sea in the Philippines, killing all ten U.S. soldiers on board.
 * On 11 September 2004, a Hellenic Army Aviation CH-47SD crashed into the sea off Mount Athos. All 17 people on board were killed, including four senior figures in the Greek Orthodox Church of Alexandria.
 * On 6 April 2005, the U.S. Army CH-47D known as "Big Windy 25" crashed during a sandstorm near Ghazni, Afghanistan, killing all eighteen aboard (fifteen soldiers and three contractors). The pilots had been disoriented by the dust storm.
 * On 7 January 2013, a BV-234 N241CH owned by Columbia Helicopters, crashed shortly after taking off from the airport in Pucallpa, Coronel Portillo Province, Peru. All seven crew members were killed.
 * On 20 June 2023, a Royal Canadian Air Force CH-147F crashed in the Ottawa River near CFB Petawawa. The helicopter belonged to the 450 Tactical Helicopter Squadron. Two crew members died while the other two were hospitalized with non-life-threatening injuries.

Argentina

 * H-91 - CH-47C on display at the Museo Nacional de Aeronáutica de Argentina in Morón, Buenos Aires.

Australia

 * A15-104 - CH-47D on display at the Australian Army Flying Museum in Oakey, Queensland.

Canada

 * 147201 - CH-47D on display at the National Air Force Museum of Canada in Trenton, Ontario.
 * 147206 - CH-47D preserved as a gate guardian at CFB Petawawa.

Italy

 * MM80840 - CH-47C on display at Volandia in Somma Lombardo.

United Kingdom

 * 83-24104 - Former US Army CH-47D forward section on display at Royal Air Force Museum London, modified to represent "Bravo November".

United States

 * 59-4984 - YCH-47B on display at the U.S. Army Transportation Museum in Fort Eustis, Virginia.
 * 60-3451 - CH-47A on display at the United States Army Aviation Museum in Fort Novosel, Alabama.
 * 61-2408 - CH-47A on display in a park across the street from the Don F. Pratt Memorial Museum in Fort Campbell, Kentucky.
 * 64-13149 - ACH-47A on display at the Redstone Arsenal in Alabama.
 * 65-7992 - Model 347 on display at the United States Army Aviation Museum in Fort Novosel, Alabama.
 * 85-24346 - CH-47D on display at the Combat Air Museum on Topeka Regional Airport (Forbes Field) in Topeka, Kansas.
 * 89-00153 - CH-47D on display at Castle Air Museum in Atwater, California.
 * 90-00222 - CH-47D on display outside at Fort Knox, Kentucky.

Vietnam

 * 65-8025 - CH-47A on display at the Khe Sanh Combat Museum.
 * 66-0086 - CH-47A on display at the War Remnants Museum in Ho Chi Minh City (Saigon).
 * 66-19082 - CH-47A on display at the Vietnam Military History Museum in Hanoi.

title: Boeing CH-47 Chinook


text:
Julian day

The Julian day is the continuous count of days since the beginning of the Julian period, and is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g. food production date and sell by date).

The Julian period is a chronological interval of 7980 years; year 1 of the Julian Period was 4713 BC (−4712). The Julian calendar year 2024 is year of the current Julian Period. The next Julian Period begins in the year AD 3268. Historians used the period to identify Julian calendar years within which an event occurred when no such year was given in the historical record, or when the year given by previous historians was incorrect.

The Julian day number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Universal Time, with Julian day number 0 assigned to the day starting at noon on Monday, January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar),   a date at which three multi-year cycles started (which are: Indiction, Solar, and Lunar cycles) and which preceded any dates in recorded history. For example, the Julian day number for the day starting at 12:00 UT (noon) on January 1, 2000, was $2,451,545$.

The Julian date (JD) of any instant is the Julian day number plus the fraction of a day since the preceding noon in Universal Time. Julian dates are expressed as a Julian day number with a decimal fraction added. For example, the Julian Date for 00:30:00.0 UT January 1, 2013, is $2,456,293.521$. This article was loaded at (UTC) – expressed as a Julian date this is.

Terminology
The term Julian date may also refer, outside of astronomy, to the day-of-year number (more properly, the ordinal date) in the Gregorian calendar, especially in computer programming, the military and the food industry, or it may refer to dates in the Julian calendar. For example, if a given "Julian date" is "October 5, 1582", this means that date in the Julian calendar (which was October 15, 1582, in the Gregorian calendar—the date it was first established). Without an astronomical or historical context, a "Julian date" given as "36" most likely means the 36th day of a given Gregorian year, namely February 5. Other possible meanings of a "Julian date" of "36" include an astronomical Julian Day Number, or the year AD 36 in the Julian calendar, or a duration of 36 astronomical Julian years). This is why the terms "ordinal date" or "day-of-year" are preferred.  In contexts where a "Julian date" means simply an ordinal date, calendars of a Gregorian year with formatting for ordinal dates are often called "Julian calendars", but this could also mean that the calendars are of years in the Julian calendar system.

Historically, Julian dates were recorded relative to Greenwich Mean Time (GMT) (later, Ephemeris Time), but since 1997 the International Astronomical Union has recommended that Julian dates be specified in Terrestrial Time. Seidelmann indicates that Julian dates may be used with International Atomic Time (TAI), Terrestrial Time (TT), Barycentric Coordinate Time (TCB), or Coordinated Universal Time (UTC) and that the scale should be indicated when the difference is significant. The fraction of the day is found by converting the number of hours, minutes, and seconds after noon into the equivalent decimal fraction. Time intervals calculated from differences of Julian Dates specified in non-uniform time scales, such as UTC, may need to be corrected for changes in time scales (e.g. leap seconds).

Variants
Because the starting point or reference epoch is so long ago, numbers in the Julian day can be quite large and cumbersome. A more recent starting point is sometimes used, for instance by dropping the leading digits, in order to fit into limited computer memory with an adequate amount of precision. In the following table, times are given in 24-hour notation.

In the table below, Epoch refers to the point in time used to set the origin (usually zero, but (1) where explicitly indicated) of the alternative convention being discussed in that row. The date given is a Gregorian calendar date unless otherwise specified. JD stands for Julian Date. 0h is 00:00 midnight, 12h is 12:00 noon, UT unless otherwise specified. ''Current value is at and may be cached. [] ''

 * The Modified Julian Date (MJD) was introduced by the Smithsonian Astrophysical Observatory in 1957 to record the orbit of Sputnik via an IBM 704 (36-bit machine) and using only 18 bits until August 7, 2576. MJD is the epoch of VAX/VMS and its successor OpenVMS, using 63-bit date/time, which allows times to be stored up to July 31, 31086, 02:48:05.47. The MJD has a starting point of midnight on November 17, 1858, and is computed by MJD = JD − 2400000.5
 * The Truncated Julian Day (TJD) was introduced by NASA/Goddard in 1979 as part of a parallel grouped binary time code (PB-5) "designed specifically, although not exclusively, for spacecraft applications". TJD was a 4-digit day count from MJD 40000, which was May 24, 1968, represented as a 14-bit binary number. Since this code was limited to four digits, TJD recycled to zero on MJD 50000, or October 10, 1995, "which gives a long ambiguity period of 27.4 years". (NASA codes PB-1–PB-4 used a 3-digit day-of-year count.) Only whole days are represented. Time of day is expressed by a count of seconds of a day, plus optional milliseconds, microseconds and nanoseconds in separate fields. Later PB-5J was introduced which increased the TJD field to 16 bits, allowing values up to 65535, which will occur in the year 2147. There are five digits recorded after TJD 9999.
 * The Dublin Julian Date (DJD) is the number of days that has elapsed since the epoch of the solar and lunar ephemerides used from 1900 through 1983, Newcomb's Tables of the Sun and Ernest W. Brown's Tables of the Motion of the Moon (1919). This epoch was noon UT on January 0, 1900, which is the same as noon UT on December 31, 1899. The DJD was defined by the International Astronomical Union at their meeting in Dublin, Ireland, in 1955.
 * The Lilian day number is a count of days of the Gregorian calendar and not defined relative to the Julian Date. It is an integer applied to a whole day; day 1 was October 15, 1582, which was the day the Gregorian calendar went into effect. The original paper defining it makes no mention of the time zone, and no mention of time-of-day. It was named for Aloysius Lilius, the principal author of the Gregorian calendar.
 * Rata Die is a system used in Rexx, Go and Python. Some implementations or options use Universal Time, others use local time. Day 1 is January 1, 1, that is, the first day of the Christian or Common Era in the proleptic Gregorian calendar. In Rexx January 1 is Day 0.

 * The Heliocentric Julian Day (HJD) is the same as the Julian day, but adjusted to the frame of reference of the Sun, and thus can differ from the Julian day by as much as 8.3 minutes (498 seconds), that being the time it takes light to reach Earth from the Sun.

Julian Period
The Julian day number is based on the Julian Period proposed by Joseph Scaliger, a classical scholar, in 1583 (one year after the Gregorian calendar reform) as it is the product of three calendar cycles used with the Julian calendar:

28 (solar cycle) × 19 (lunar cycle) × 15 (indiction cycle) = 7980 years

Its epoch occurs when all three cycles (if they are continued backward far enough) were in their first year together. Years of the Julian Period are counted from this year, 4713 BC, as year 1, which was chosen to be before any historical record.

Scaliger corrected chronology by assigning each year a tricyclic "character", three numbers indicating that year's position in the 28-year solar cycle, the 19-year lunar cycle, and the 15-year indiction cycle. One or more of these numbers often appeared in the historical record alongside other pertinent facts without any mention of the Julian calendar year. The character of every year in the historical record was unique – it could only belong to one year in the 7980-year Julian Period. Scaliger determined that 1 BC or year 0 was Julian Period (JP) 4713. He knew that 1 BC or 0 had the character 9 of the solar cycle, 1 of the lunar cycle, and 3 of the indiction cycle. By inspecting a 532-year Paschal cycle with 19 solar cycles (each of 28 years, each year numbered 1–28) and 28 lunar cycles (each of 19 years, each year numbered 1–19), he determined that the first two numbers, 9 and 1, occurred at its year 457. He then calculated via remainder division that he needed to add eight 532-year Paschal cycles totaling 4256 years before the cycle containing 1 BC or 0 in order for its year 457 to be indiction 3. The sum 4256 + 457 was thus.

A formula for determining the year of the Julian Period given its character involving three four-digit numbers was published by Jacques de Billy in 1665 in the Philosophical Transactions of the Royal Society (its first year). John F. W. Herschel gave the same formula using slightly different wording in his 1849 Outlines of Astronomy.

"Multiply the Solar Cycle by 4845, and the Lunar, by 4200, and that of the Indiction, by 6916. Then divide the Sum of the products by 7980, which is the Julian Period: The Remainder of the Division, without regard to the Quotient, shall be the year enquired after."

Carl Friedrich Gauss introduced the modulo operation in 1801, restating de Billy's formula as:

Julian Period year = (6916a + 4200b + 4845c) MOD 15×19×28

where a is the year of the indiction cycle, b of the lunar cycle, and c of the solar cycle.

John Collins described the details of how these three numbers were calculated in 1666, using many trials. A summary of Collin's description is in a footnote. Reese, Everett and Craun reduced the dividends in the Try column from 285, 420, 532 to 5, 2, 7 and changed remainder to modulo, but apparently still required many trials.

The specific cycles used by Scaliger to form his tricyclic Julian Period were, first, the indiction cycle with a first year of 313. Then he chose the dominant 19-year Alexandrian lunar cycle with a first year of 285, the Era of Martyrs and the Diocletian Era epoch, or a first year of 532 according to Dionysius Exiguus. Finally, Scaliger chose the post-Bedan solar cycle with a first year of 776, when its first quadrennium of concurrents, 1 2 3 4, began in sequence. Although not their intended use, the equations of de Billy or Gauss can be used to determined the first year of any 15-, 19-, and 28-year tricyclic period given any first years of their cycles. For those of the Julian Period, the result is AD3268, because both remainder and modulo usually return the lowest positive result. Thus 7980years must be subtracted from it to yield the first year of the present Julian Period, −4712 or 4713BC, when all three of its sub-cycles are in their first years.

Scaliger got the idea of using a tricyclic period from "the Greeks of Constantinople" as Herschel stated in his quotation below in Julian day numbers. Specifically, the monk and priest Georgios wrote in 638/39 that the Byzantine year 6149 AM (640/41) had indiction 14, lunar cycle 12, and solar cycle 17, which places the first year of the Byzantine Era in 5509/08BC, the Byzantine Creation. Dionysius Exiguus called the Byzantine lunar cycle his "lunar cycle" in argumentum 6, in contrast with the Alexandrian lunar cycle which he called his "nineteen-year cycle" in argumentum 5.

Although many references say that the Julian in "Julian Period" refers to Scaliger's father, Julius Scaliger, at the beginning of Book V of his Opus de Emendatione Temporum ("Work on the Emendation of Time") he states, "Iulianam vocauimus: quia ad annum Iulianum accomodata", which Reese, Everett and Craun translate as "We have termed it Julian because it fits the Julian year." Thus Julian refers to the Julian calendar.

Julian day numbers
Julian days were first used by Ludwig Ideler for the first days of the Nabonassar and Christian eras in his 1825 Handbuch der mathematischen und technischen Chronologie. John F. W. Herschel then developed them for astronomical use in his 1849 Outlines of Astronomy, after acknowledging that Ideler was his guide.

"The period thus arising of 7980 Julian years, is called the Julian period, and it has been found so useful, that the most competent authorities have not hesitated to declare that, through its employment, light and order were first introduced into chronology. We owe its invention or revival to Joseph Scaliger, who is said to have received it from the Greeks of Constantinople. The first year of the current Julian period, or that of which the number in each of the three subordinate cycles is 1, was the year 4713 BC, and the noon of January 1 of that year, for the meridian of Alexandria, is the chronological epoch, to which all historical eras are most readily and intelligibly referred, by computing the number of integer days intervening between that epoch and the noon (for Alexandria) of the day, which is reckoned to be the first of the particular era in question. The meridian of Alexandria is chosen as that to which Ptolemy refers the commencement of the era of Nabonassar, the basis of all his calculations."

At least one mathematical astronomer adopted Herschel's "days of the Julian period" immediately. Benjamin Peirce of Harvard University used over 2,800 Julian days in his Tables of the Moon, begun in 1849 but not published until 1853, to calculate the lunar ephemerides in the new American Ephemeris and Nautical Almanac from 1855 to 1888. The days are specified for "Washington mean noon", with Greenwich defined as 18$7980⁄28$ 51$285×Try⁄28$ 48$7980⁄19$ west of Washington (282°57′W, or Washington 77°3′W of Greenwich). A table with 197 Julian days ("Date in Mean Solar Days", one per century mostly) was included for the years –4713 to 2000 with no year 0, thus "–" means BC, including decimal fractions for hours, minutes and seconds. The same table appears in Tables of Mercury by Joseph Winlock, without any other Julian days.

The national ephemerides started to include a multi-year table of Julian days, under various names, for either every year or every leap year beginning with the French Connaissance des Temps in 1870 for 2,620 years, increasing in 1899 to 3,000 years. The British Nautical Almanac began in 1879 with 2,000 years. The Berliner Astronomisches Jahrbuch began in 1899 with 2,000 years. The American Ephemeris was the last to add a multi-year table, in 1925 with 2,000 years. However, it was the first to include any mention of Julian days with one for the year of issue beginning in 1855, as well as later scattered sections with many days in the year of issue. It was also the first to use the name "Julian day number" in 1918. The Nautical Almanac began in 1866 to include a Julian day for every day in the year of issue. The Connaissance des Temps began in 1871 to include a Julian day for every day in the year of issue.

The French mathematician and astronomer Pierre-Simon Laplace first expressed the time of day as a decimal fraction added to calendar dates in his book, Traité de Mécanique Céleste, in 1823. Other astronomers added fractions of the day to the Julian day number to create Julian Dates, which are typically used by astronomers to date astronomical observations, thus eliminating the complications resulting from using standard calendar periods like eras, years, or months. They were first introduced into variable star work in 1860 by the English astronomer Norman Pogson, which he stated was at the suggestion of John Herschel. They were popularized for variable stars by Edward Charles Pickering, of the Harvard College Observatory, in 1890.

Julian days begin at noon because when Herschel recommended them, the astronomical day began at noon. The astronomical day had begun at noon ever since Ptolemy chose to begin the days for his astronomical observations at noon. He chose noon because the transit of the Sun across the observer's meridian occurs at the same apparent time every day of the year, unlike sunrise or sunset, which vary by several hours. Midnight was not even considered because it could not be accurately determined using water clocks. Nevertheless, he double-dated most nighttime observations with both Egyptian days beginning at sunrise and Babylonian days beginning at sunset. Medieval Muslim astronomers used days beginning at sunset, so astronomical days beginning at noon did produce a single date for an entire night. Later medieval European astronomers used Roman days beginning at midnight so astronomical days beginning at noon also allow observations during an entire night to use a single date. When all astronomers decided to start their astronomical days at midnight to conform to the beginning of the civil day, on January 1, 1925, it was decided to keep Julian days continuous with previous practice, beginning at noon.

During this period, usage of Julian day numbers as a neutral intermediary when converting a date in one calendar into a date in another calendar also occurred. An isolated use was by Ebenezer Burgess in his 1860 translation of the Surya Siddhanta wherein he stated that the beginning of the Kali Yuga era occurred at midnight at the meridian of Ujjain at the end of the 588,465th day and the beginning of the 588,466th day (civil reckoning) of the Julian Period, or between February 17 and 18 JP 1612 or 3102 BC. Robert Schram was notable beginning with his 1882 Hilfstafeln für Chronologie. Here he used about 5,370 "days of the Julian Period". He greatly expanded his usage of Julian days in his 1908 Kalendariographische und Chronologische Tafeln containing over 530,000 Julian days, one for the zeroth day of every month over thousands of years in many calendars. He included over 25,000 negative Julian days, given in a positive form by adding 10,000,000 to each. He called them "day of the Julian Period", "Julian day", or simply "day" in his discussion, but no name was used in the tables. Continuing this tradition, in his book "Mapping Time: The Calendar and Its History" British physics educator and programmer Edward Graham Richards uses Julian day numbers to convert dates from one calendar into another using algorithms rather than tables.

Julian day number calculation
The Julian day number can be calculated using the following formulas (integer division rounding towards zero is used exclusively, that is, positive values are rounded down and negative values are rounded up):

The months January to December are numbered 1 to 12. For the year, astronomical year numbering is used, thus 1 BC is 0, 2 BC is −1, and 4713 BC is −4712. JDN is the Julian Day Number. Use the previous day of the month if trying to find the JDN of an instant before midday UT.

Converting Gregorian calendar date to Julian Day Number
The algorithm is valid for all (possibly proleptic) Gregorian calendar dates after November 23, −4713. Divisions are integer divisions towards zero; fractional parts are ignored.

JDN = (1461 × (Y + 4800 + (M − 14)/12))/4 +(367 × (M − 2 − 12 × ((M − 14)/12)))/12 − (3 × ((Y + 4900 + (M - 14)/12)/100))/4 + D − 32075

Converting Julian calendar date to Julian Day Number
The algorithm is valid for all (possibly proleptic) Julian calendar years ≥ −4712, that is, for all JDN ≥ 0. Divisions are integer divisions, fractional parts are ignored.

JDN = 367 × Y − (7 × (Y + 5001 + (M − 9)/7))/4 + (275 × M)/9 + D + 1729777

Finding Julian date given Julian day number and time of day
For the full Julian Date of a moment after 12:00 UT one can use the following. Divisions are real numbers.

$\begin{matrix}J\!D & = & J\!D\!N + \frac{\text{hour} - 12}{24} + \frac{\text{minute}}{1440} + \frac{\text{second}}{86400}\end{matrix}$|undefined

So, for example, January 1, 2000, at 18:00:00 UT corresponds to JD = 2451545.25 and January 1, 2000, at 6:00:00 UT corresponds to JD = 2451544.75.

Finding day of week given Julian day number
Because a Julian day starts at noon while a civil day starts at midnight, the Julian day number needs to be adjusted to find the day of week: for a point in time in a given Julian day after midnight UT and before 12:00 UT, add 1 or use the JDN of the next afternoon.

The US day of the week W1 (for an afternoon or evening UT) can be determined from the Julian Day Number J with the expression:

W1 = mod(J + 1, 7)

If the moment in time is after midnight UT (and before 12:00 UT), then one is already in the next day of the week.

The ISO day of the week W0 can be determined from the Julian Day Number J with the expression:

W0 = mod (J, 7) + 1

Julian or Gregorian calendar from Julian day number
This is an algorithm by Edward Graham Richards to convert a Julian Day Number, J, to a date in the Gregorian calendar (proleptic, when applicable). Richards states the algorithm is valid for Julian day numbers greater than or equal to 0. All variables are integer values, and the notation "a div b" indicates integer division, and "mod(a,b)" denotes the modulus operator.

For Julian calendar: For Gregorian calendar: For Julian or Gregorian, continue:
 * 1) f =  J + j
 * 1) f =  J + j + (((4 × J + B) div 146097) × 3) div 4 + C

D, M, and Y are the numbers of the day, month, and year respectively for the afternoon at the beginning of the given Julian day.

Julian Period from indiction, Metonic and solar cycles
Let Y be the year BC or AD and i, m and s respectively its positions in the indiction, Metonic and solar cycles. Divide 6916i + 4200m + 4845s by 7980 and call the remainder r.

If r>4713, Y = (r − 4713) and is a year AD.

If r<4714, Y = (4714 − r) and is a year BC.

Example

i = 8, m = 2, s = 8. What is the year?

(6916 × 8) = 55328; (4200 × 2) = 8400: (4845 × 8) = 38760. 55328 + 8400 + 38760 = 102488.

102488/7980 = 12 remainder 6728.

Y = (6728 − 4713) = AD 2015.

Julian date calculation
As stated above, the Julian date (JD) of any instant is the Julian day number for the preceding noon in Universal Time plus the fraction of the day since that instant. Ordinarily calculating the fractional portion of the JD is straightforward; the number of seconds that have elapsed in the day divided by the number of seconds in a day, 86,400. But if the UTC timescale is being used, a day containing a positive leap second contains 86,401 seconds (or in the unlikely event of a negative leap second, 86,399 seconds). One authoritative source, the Standards of Fundamental Astronomy (SOFA), deals with this issue by treating days containing a leap second as having a different length (86,401 or 86,399 seconds, as required). SOFA refers to the result of such a calculation as "quasi-JD".

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title: Julian day