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Landing gear

Landing gear is the undercarriage of an aircraft or spacecraft that is used for taxiing, takeoff or landing. For aircraft it is generally needed for both. It was also formerly called alighting gear by some manufacturers, such as the Glenn L. Martin Company. For aircraft, Stinton makes the terminology distinction undercarriage (British) = landing gear (US).

For aircraft, the landing gear supports the craft when it is not flying, allowing it to take off, land, and taxi without damage. Wheeled landing gear is the most common, with skis or floats needed to operate from snow/ice/water and skids for vertical operation on land. Retractable undercarriages fold away during flight, which reduces drag, allowing for faster airspeeds. Landing gear must be strong enough to support the aircraft and its design affects the weight, balance and performance. It often comprises three wheels, or wheel-sets, giving a tripod effect.

Some unusual landing gear have been evaluated experimentally. These include: no landing gear (to save weight), made possible by operating from a catapult cradle and flexible landing deck: air cushion (to enable operation over a wide range of ground obstacles and water/snow/ice); tracked (to reduce runway loading).

For launch vehicles and spacecraft landers, the landing gear usually only supports the vehicle on landing and during subsequent surface movement, and is not used for takeoff.

Given their varied designs and applications, there exist dozens of specialized landing gear manufacturers. The three largest are Safran Landing Systems, Collins Aerospace (part of Raytheon Technologies) and Héroux-Devtek.

Aircraft
The landing gear represents 2.5 to 5% of the maximum takeoff weight (MTOW) and 1.5 to 1.75% of the aircraft cost, but 20% of the airframe direct maintenance cost. A suitably-designed wheel can support 30 t, tolerate a ground speed of 300 km/h and roll a distance of 500000 km ; it has a 20,000 hours time between overhaul and a 60,000 hours or 20 year life time.

Retractable gear

To decrease drag in flight, undercarriages retract into the wings and/or fuselage with wheels flush with the surrounding surface, or concealed behind flush-mounted doors; this is called retractable gear. If the wheels do not retract completely but protrude partially exposed to the airstream, it is called a semi-retractable gear.

Most retractable gear is hydraulically operated, though some is electrically operated or even manually operated on very light aircraft. The landing gear is stowed in a compartment called a wheel well.

Pilots confirming that their landing gear is down and locked refer to "three greens" or "three in the green.", a reference to the electrical indicator lights (or painted panels of mechanical indicator units) from the nosewheel/tailwheel and the two main gears. Blinking green lights or red lights indicate the gear is in transit and neither up and locked or down and locked. When the gear is fully stowed up with the up-locks secure, the lights often extinguish to follow the dark cockpit philosophy; some airplanes have gear up indicator lights.

Redundant systems are used to operate the landing gear and redundant main gear legs may also be provided so the aircraft can be landed in a satisfactory manner in a range of failure scenarios. The Boeing 747 was given four separate and independent hydraulic systems (when previous airliners had two) and four main landing gear posts (when previous airliners had two). Safe landing would be possible if two main gear legs were torn off provided they were on opposite sides of the fuselage. In the case of power failure in a light aircraft, an emergency extension system is always available. This may be a manually operated crank or pump, or a mechanical free-fall mechanism which disengages the uplocks and allows the landing gear to fall under gravity.

Shock absorbers
Aircraft landing gear includes wheels equipped with solid shock absorbers on light planes, and air/oil oleo struts on larger aircraft.

Large aircraft

As aircraft weights have increased more wheels have been added and runway thickness has increased to keep within the runway loading limit. The Zeppelin-Staaken R.VI, a large German World War I long-range bomber of 1916, used eighteen wheels for its undercarriage, split between two wheels on its nose gear struts, and sixteen wheels on its main gear units—split into four side-by-side quartets each, two quartets of wheels per side—under each tandem engine nacelle, to support its loaded weight of almost 12 t.

Multiple "tandem wheels" on an aircraft—particularly for cargo aircraft, mounted to the fuselage lower sides as retractable main gear units on modern designs—were first seen during World War II, on the experimental German Arado Ar 232 cargo aircraft, which used a row of eleven "twinned" fixed wheel sets directly under the fuselage centerline to handle heavier loads while on the ground. Many of today's large cargo aircraft use this arrangement for their retractable main gear setups, usually mounted on the lower corners of the central fuselage structure.

The prototype Convair XB-36 had most of its weight on two main wheels, which needed runways at least 22 in thick. Production aircraft used two four-wheel bogies, allowing the aircraft to use any airfield suitable for a B-29.

A relatively light Lockheed JetStar business jet, with four wheels supporting 44,000 lb, needed a 10 in thick flexible asphalt pavement. The 210,000 lb Boeing 727-200 with four tires on two legs main landing gears required a 20 in thick pavement. The thickness rose to 25 in for a McDonnell Douglas DC-10-10 with 443,000 lb supported on eight wheels on two legs. The heavier, 558,000 lb, DC-10-30/40 were able to operate from the same thickness pavements with a third main leg for ten wheels, like the first Boeing 747-100, weighing 700,000 lb on four legs and 16 wheels. The similar-weight Lockheed C-5, with 24 wheels, needs an 18 in pavement.

The twin-wheel unit on the fuselage centerline of the McDonnell Douglas DC-10-30/40 was retained on the MD-11 airliner and the same configuration was used on the initial 275 t Airbus A340-200/300, which evolved in a complete four-wheel undercarriage bogie for the heavier 380 t Airbus A340-500/-600. The up to 775000 lb Boeing 777 has twelve main wheels on two three-axles bogies, like the later Airbus A350.

The 575 t Airbus A380 has a four-wheel bogie under each wing with two sets of six-wheel bogies under the fuselage. The 640 t Antonov An-225, the largest cargo aircraft, had 4 wheels on the twin-strut nose gear units like the smaller Antonov An-124, and 28 main gear wheels.

The 97 t A321neo has a twin-wheel main gear inflated to 15.7 bar (228 psi), while the 280 t A350-900 has a four-wheel main gear inflated to 17.1 bar (248 psi).

STOL aircraft
STOL aircraft have a higher sink-rate requirement if a carrier-type, no-flare landing technique has to be adopted to reduce touchdown scatter. For example, the Saab 37 Viggen, with landing gear designed for a 5m/sec impact, could use a carrier-type landing and HUD to reduce its scatter from 300 m to 100m.

The de Havilland Canada DHC-4 Caribou used long-stroke legs to land from a steep approach with no float.

Operation from water
A flying boat has a lower fuselage with the shape of a boat hull giving it buoyancy. Wing-mounted floats or stubby wing-like sponsons are added for stability. Sponsons are attached to the lower sides of the fuselage.

A floatplane has two or three streamlined floats. Amphibious floats have retractable wheels for land operation.

An amphibious aircraft or amphibian usually has two distinct landing gears, namely a "boat" hull/floats and retractable wheels, which allow it to operate from land or water.

Beaching gear is detachable wheeled landing gear that allows a non-amphibious floatplane or flying boat to be maneuvered on land. It is used for aircraft maintenance and storage and is either carried in the aircraft or kept at a slipway. Beaching gear may consist of individual detachable wheels or a cradle that supports the entire aircraft. In the former case, the beaching gear is manually attached or detached with the aircraft in the water; in the latter case, the aircraft is maneuvered onto the cradle.

Helicopters are able to land on water using floats or a hull and floats.

Shipboard operation
The landing gear on fixed-wing aircraft that land on aircraft carriers have a higher sink-rate requirement because the aircraft are flown onto the deck with no landing flare. Other features are related to catapult take-off requirements for specific aircraft. For example, the Blackburn Buccaneer was pulled down onto its tail-skid to set the required nose-up attitude. The naval McDonnell Douglas F-4 Phantom II in UK service needed an extending nosewheel leg to set the wing attitude at launch.

The landing gear for an aircraft using a ski-jump on take-off is subjected to loads of 0.5g which also last for much longer than a landing impact.

Helicopters may have a deck-lock harpoon to anchor them to the deck.

In-flight use
Some aircraft have a requirement to use the landing-gear as a speed brake.

Flexible mounting of the stowed main landing-gear bogies on the Tupolev Tu-22R raised the aircraft flutter speed to 550 kn. The bogies oscillated within the nacelle under the control of dampers and springs as an anti-flutter device.

Gear common to different aircraft
Some experimental aircraft have used gear from existing aircraft to reduce program costs. The Martin-Marietta X-24 lifting body used the nose/main gear from the North American T-39 / Northrop T-38 and the Grumman X-29 from the Northrop F-5 / General Dynamics F-16.

When an airplane needs to land on surfaces covered by snow, the landing gear usually consists of skis or a combination of wheels and skis.

Detachable
Some aircraft use wheels for takeoff and jettison them when airborne for improved streamlining without the complexity, weight and space requirements of a retraction mechanism. The wheels are sometimes mounted onto axles that are part of a separate "dolly" (for main wheels only) or "trolley" (for a three-wheel set with a nosewheel) chassis. Landing is done on skids or similar simple devices (fixed or retractable). The SNCASE Baroudeur used this arrangement.

Historical examples include the "dolly"-using Messerschmitt Me 163 Komet rocket fighter, the Messerschmitt Me 321 Gigant troop glider, and the first eight "trolley"-using prototypes of the Arado Ar 234 jet reconnaissance bomber. The main disadvantage to using the takeoff dolly/trolley and landing skid(s) system on German World War II aircraft—intended for a sizable number of late-war German jet and rocket-powered military aircraft designs—was that aircraft would likely be scattered all over a military airfield after they had landed from a mission, and would be unable to taxi on their own to an appropriately hidden "dispersal" location, which could easily leave them vulnerable to being shot up by attacking Allied fighters. A related contemporary example are the wingtip support wheels ("pogos") on the Lockheed U-2 reconnaissance aircraft, which fall away after take-off and drop to earth; the aircraft then relies on titanium skids on the wingtips for landing.

Rearwards and sideways retraction
Some main landing gear struts on World War II aircraft, in order to allow a single-leg main gear to more efficiently store the wheel within either the wing or an engine nacelle, rotated the single gear strut through a 90° angle during the rearwards-retraction sequence to allow the main wheel to rest "flat" above the lower end of the main gear strut, or flush within the wing or engine nacelles, when fully retracted. Examples are the Curtiss P-40, Vought F4U Corsair, Grumman F6F Hellcat, Messerschmitt Me 210 and Junkers Ju 88. The Aero Commander family of twin-engined business aircraft also shares this feature on the main gears, which retract aft into the ends of the engine nacelles. The rearward-retracting nosewheel strut on the Heinkel He 219 and the forward-retracting nose gear strut on the later Cessna Skymaster similarly rotated 90 degrees as they retracted.

On most World War II single-engined fighter aircraft (and even one German heavy bomber design) with sideways retracting main gear, the main gear that retracted into the wings was raked forward in the "down" position for better ground handling, with a retracted position that placed the main wheels at some distance aft of their position when downairframe—this led to a complex angular geometry for setting up the "pintle" angles at the top ends of the struts for the retraction mechanism's axis of rotation. with some aircraft, like the P-47 Thunderbolt and Grumman Bearcat, even mandating that the main gear struts lengthened as they were extended to give sufficient ground clearance for their large four-bladed propellers. One exception to the need for this complexity in many WW II fighter aircraft was Japan's famous Zero fighter, whose main gear stayed at a perpendicular angle to the centerline of the aircraft when extended, as seen from the side.

Variable axial position of main wheels
The main wheels on the Vought F7U Cutlass could move 20 inches between a forward and aft position. The forward position was used for take-off to give a longer lever-arm for pitch control and greater nose-up attitude. The aft position was used to reduce landing bounce and reduce risk of tip-back during ground handling.

Tandem layout
The tandem or bicycle layout is used on the Hawker Siddeley Harrier, which has two main-wheels behind a single nose-wheel under the fuselage and a smaller wheel near the tip of each wing. On second generation Harriers, the wing is extended past the outrigger wheels to allow greater wing-mounted munition loads to be carried, or to permit wing-tip extensions to be bolted on for ferry flights.

A tandem layout was evaluated by Martin using a specially-modified Martin B-26 Marauder (the XB-26H) to evaluate its use on Martin's first jet bomber, the Martin XB-48. This configuration proved so manoeuvrable that it was also selected for the B-47 Stratojet. It was also used on the U-2, Myasishchev M-4, Yakovlev Yak-25, Yak-28 and Sud Aviation Vautour. A variation of the multi tandem layout is also used on the B-52 Stratofortress which has four main wheel bogies (two forward and two aft) underneath the fuselage and a small outrigger wheel supporting each wing-tip. The B-52's landing gear is also unique in that all four pairs of main wheels can be steered. This allows the landing gear to line up with the runway and thus makes crosswind landings easier (using a technique called crab landing). Since tandem aircraft cannot rotate for takeoff, the forward gear must be long enough to give the wings the correct angle of attack during takeoff. During landing, the forward gear must not touch the runway first, otherwise the rear gear will slam down and may cause the aircraft to bounce and become airborne again.

Crosswind landing accommodation

One very early undercarriage incorporating castoring for crosswind landings was pioneered on the Bleriot VIII design of 1908. It was later used in the much more famous Blériot XI Channel-crossing aircraft of 1909 and also copied in the earliest examples of the Etrich Taube. In this arrangement the main landing gear's shock absorption was taken up by a vertically sliding bungee cord-sprung upper member. The vertical post along which the upper member slid to take landing shocks also had its lower end as the rotation point for the forward end of the main wheel's suspension fork, allowing the main gear to pivot on moderate crosswind landings.

Manually-adjusted main-gear units on the B-52 can be set for crosswind take-offs. It rarely has to be used from SAC-designated airfields which have major runways in the predominant strongest wind direction. The Lockheed C-5 Galaxy has swivelling 6-wheel main units for crosswind landings and castoring rear units to prevent tire scrubbing on tight turns.

"Kneeling" gear
Both the nosegear and the wing-mounted main landing gear of the World War II German Arado Ar 232 cargo/transport aircraft were designed to kneel. This made it easier to load and unload cargo, and improved taxiing over ditches and on soft ground.

Some early U.S. Navy jet fighters were equipped with "kneeling" nose gear consisting of small steerable auxiliary wheels on short struts located forward of the primary nose gear, allowing the aircraft to be taxied tail-high with the primary nose gear retracted. This feature was intended to enhance safety aboard aircraft carriers by redirecting the hot exhaust blast upwards, and to reduce hangar space requirements by enabling the aircraft to park with its nose underneath the tail of a similarly equipped jet. Kneeling gear was used on the North American FJ-1 Fury and on early versions of the McDonnell F2H Banshee, but was found to be of little use operationally, and was omitted from later Navy fighters.

The nosewheel on the Lockheed C-5, partially retracts against a bumper to assist in loading and unloading of cargo using ramps through the forward, "tilt-up" hinged fuselage nose while stationary on the ground. The aircraft also tilts backwards. The Messier twin-wheel main units fitted to the Transall and other cargo aircraft can tilt forward or backward as necessary.

The Boeing AH-64 Apache helicopter is able to kneel to fit inside the cargo hold of a transport aircraft and for storage.

Tail support
Aircraft landing gear includes devices to prevent fuselage contact with the ground by tipping back when the aircraft is being loaded. Some commercial aircraft have used tail props when parked at the gate. The Douglas C-54 had a critical CG location which required a ground handling strut. The Lockheed C-130 and Boeing C-17 Globemaster III use ramp supports. The unladen CG of the rear-engined Ilyushin IL-62 is aft of the main gear due to design decisions stemming from efforts to reduce overall weight, systems complexity and drag; to prevent the fuselage from tilting back when unloaded, the aircraft has a unique fully retractable vertical tail strut with castering wheels to allow towing or pushback. The strut is not intended for taxiing or flight, when the weight of the crew, passengers, cargo and fuel provide the necessary fore-aft balance.

Monowheel

To minimize drag, modern gliders usually have a single wheel, retractable or fixed, centered under the fuselage, which is referred to as monowheel gear or monowheel landing gear. Monowheel gear is also used on some powered aircraft, where drag reduction is a priority, such as the Europa Classic. Much like the Me 163 rocket fighter, some gliders from prior to the Second World War used a take-off dolly that was jettisoned on take-off; these gliders then landed on a fixed skid. This configuration is necessarily accompanied with a taildragger.

Helicopters
Light helicopters use simple landing skids to save weight and cost. The skids may have attachment points for wheels so that they can be moved for short distances on the ground. Skids are impractical for helicopters weighing more than four tons. Some high-speed machines have retractable wheels, but most use fixed wheels for their robustness, and to avoid the need for a retraction mechanism.

Tailsitter
Experimental tailsitter aircraft use landing gear located in their tails for VTOL operation.

Light aircraft
For light aircraft a type of landing gear which is economical to produce is a simple wooden arch laminated from ash, as used on some homebuilt aircraft. A similar arched gear is often formed from spring steel. The Cessna Airmaster was among the first aircraft to use spring steel landing gear. The main advantage of such gear is that no other shock-absorbing device is needed; the deflecting leaf provides the shock absorption.

Folding gear
The limited space available to stow landing gear has led to many complex retraction mechanisms, each unique to a particular aircraft. An early example, the German Bomber B combat aircraft design competition winner, the Junkers Ju 288, had a complex "folding" main landing gear unlike any other aircraft designed by either Axis or Allied sides in the war: its single oleo strut was only attached to the lower end of its Y-form main retraction struts, handling the twinned main gear wheels, and folding by swiveling downwards and aftwards during retraction to "fold" the maingear's length to shorten it for stowage in the engine nacelle it was mounted in. However, the single pivot-point design also led to numerous incidents of collapsed maingear units for its prototype airframes.

Tracked
Increased contact area can be obtained with very large wheels, many smaller wheels or track-type gear. Tracked gear made by Dowty was fitted to a Westland Lysander in 1938 for taxi tests, then a Fairchild Cornell and a Douglas Boston. Bonmartini, in Italy, fitted tracked gear to a Piper Cub in 1951. Track-type gear was also tested using a C-47, C-82 and B-50. A much heavier aircraft, an XB-36, was made available for further tests, although there was no intention of using it on production aircraft. The stress on the runway was reduced to one third that of the B-36 four-wheel bogie.

Ground carriage
Ground carriage is a long-term (after 2030) concept of flying without landing gear. It is one of many aviation technologies being proposed to reduce greenhouse gas emissions. Leaving the landing gear on the ground reduces weight and drag. Leaving it behind after take-off was done for a different reason, i.e. with military objectives, during World War II using the "dolly" and "trolley" arrangements of the German Me 163B rocket fighter and Arado Ar 234A prototype jet recon-bomber.

Steering
There are several types of steering. Taildragger aircraft may be steered by rudder alone (depending upon the prop wash produced by the aircraft to turn it) with a freely pivoting tail wheel, or by a steering linkage with the tail wheel, or by differential braking (the use of independent brakes on opposite sides of the aircraft to turn the aircraft by slowing one side more sharply than the other). Aircraft with tricycle landing gear usually have a steering linkage with the nosewheel (especially in large aircraft), but some allow the nosewheel to pivot freely and use differential braking and/or the rudder to steer the aircraft, like the Cirrus SR22.

Some aircraft require that the pilot steer by using rudder pedals; others allow steering with the yoke or control stick. Some allow both. Still others have a separate control, called a tiller, used for steering on the ground exclusively.

Rudder
When an aircraft is steered on the ground exclusively using the rudder, it needs a substantial airflow past the rudder, which can be generated either by the forward motion of the aircraft or by propeller slipstream. Rudder steering requires considerable practice to use effectively. Although it needs airflow past the rudder, it has the advantage of not needing any friction with the ground, which makes it useful for aircraft on water, snow or ice.

Direct
Some aircraft link the yoke, control stick, or rudder directly to the wheel used for steering. Manipulating these controls turns the steering wheel (the nose wheel for tricycle landing gear, and the tail wheel for taildraggers). The connection may be a firm one in which any movement of the controls turns the steering wheel (and vice versa), or it may be a soft one in which a spring-like mechanism twists the steering wheel but does not force it to turn. The former provides positive steering but makes it easier to skid the steering wheel; the latter provides softer steering (making it easy to overcontrol) but reduces the probability of skidding. Aircraft with retractable gear may disable the steering mechanism wholly or partially when the gear is retracted.

Differential braking
Differential braking depends on asymmetric application of the brakes on the main gear wheels to turn the aircraft. For this, the aircraft must be equipped with separate controls for the right and left brakes (usually on the rudder pedals). The nose or tail wheel usually is not equipped with brakes. Differential braking requires considerable skill. In aircraft with several methods of steering that include differential braking, differential braking may be avoided because of the wear it puts on the braking mechanisms. Differential braking has the advantage of being largely independent of any movement or skidding of the nose or tailwheel.

Tiller
A tiller in an aircraft is a small wheel or lever, sometimes accessible to one pilot and sometimes duplicated for both pilots, that controls the steering of the aircraft while it is on the ground. The tiller may be designed to work in combination with other controls such as the rudder or yoke. In large airliners, for example, the tiller is often used as the sole means of steering during taxi, and then the rudder is used to steer during takeoff and landing, so that both aerodynamic control surfaces and the landing gear can be controlled simultaneously when the aircraft is moving at aerodynamic speeds.

Tires and wheels
The specified selection criterion, e.g., minimum size, weight, or pressure, are used to select suitable tires and wheels from manufacturer's catalog and industry standards found in the Aircraft Yearbook published by the Tire and Rim Association, Inc.

Gear loading
The choice of the main wheel tires is made on the basis of the static loading case. The total main gear load $$F_\text{m}$$ is calculated assuming that the aircraft is taxiing at low speed without braking:

 * $$ F_\text{m} =\frac{l_\text{n}}{l_\text{m} + l_\text{n}} W.$$

where $$W$$ is the weight of the aircraft and $$l_\text{m}$$ and $$l_\text{n}$$ are the distance measured from the aircraft's center of gravity(cg) to the main and nose gear, respectively.

The choice of the nose wheel tires is based on the nose wheel load $$F_\text{n}$$ during braking at maximum effort:

 * $$ F_\text{n} =\frac{l_\text{m}}{l_\text{m} + l_\text{n}} (W-L) + \frac{h_\text{cg}}{l_\text{m} + l_\text{n}} \left(\frac{a_\text{x}}{g} W - D + T\right).$$

where $$L$$ is the lift, $$D$$ is the drag, $$T$$ is the thrust, and $$h_\text{cg}$$ is the height of aircraft cg from the static groundline. Typical values for $$\frac{a_\text{x}}{g}$$ on dry concrete vary from 0.35 for a simple brake system to 0.45 for an automatic brake pressure control system. As both $$L$$ and $$D$$ are positive, the maximum nose gear load occurs at low speed. Reverse thrust decreases the nose gear load, and hence the condition $$T = 0$$ results in the maximum value:

 * $$ F_\text{n} =\frac{l_\text{m} + h_\text{cg}(\frac{a_\text{x}}{g})}{l_\text{m} + l_\text{n}} W.$$

To ensure that the rated loads will not be exceeded in the static and braking conditions, a seven percent safety factor is used in the calculation of the applied loads.

Inflation pressure
Provided that the wheel load and configuration of the landing gear remain unchanged, the weight and volume of the tire will decrease with an increase in inflation pressure. From the flotation standpoint, a decrease in the tire contact area will induce a higher bearing stress on the pavement which may reduce the number of airfields available to the aircraft. Braking will also become less effective due to a reduction in the frictional force between the tires and the ground. In addition, the decrease in the size of the tire, and hence the size of the wheel, could pose a problem if internal brakes are to be fitted inside the wheel rims. The arguments against higher pressure are of such a nature that commercial operators generally prefer the lower pressures in order to maximize tire life and minimize runway stress. To prevent punctures from stones Philippine Airlines had to operate their Hawker Siddeley 748 aircraft with pressures as low as the tire manufacturer would permit. However, too low a pressure can lead to an accident as in the Nigeria Airways Flight 2120.

A rough general rule for required tire pressure is given by the manufacturer in their catalog. Goodyear for example advises the pressure to be 4% higher than required for a given weight or as fraction of the rated static load and inflation.

Tires of many commercial aircraft are required to be filled with nitrogen, and not subsequently diluted with more than 5% oxygen, to prevent auto-ignition of the gas which may result from overheating brakes producing volatile vapors from the tire lining.

Naval aircraft use different pressures when operating from a carrier and ashore. For example, the Northrop Grumman E-2 Hawkeye tire pressures are 260 psi on ship and 210 psi ashore. En-route deflation is used in the Lockheed C-5 Galaxy to suit airfield conditions at the destination but adds excessive complication to the landing gear and wheels

Future developments
Airport community noise is an environmental issue which has brought into focus the contribution of aerodynamic noise from the landing gear. A NASA long-term goal is to confine aircraft objectional noise to within the airport boundary. During the approach to land the landing gear is lowered several miles from touchdown and the landing gear is the dominant airframe noise source, followed by deployed highlift devices. With engines at a reduced power setting on the approach it is necessary to reduce airframe noise to make a significant reduction to total aircraft noise. The addition of add-on fairings is one approach for reducing the noise from the landing gear with a longer term approach to address noise generation during initial design.

Airline specifications require an airliner to reach up to 90,000 take-offs and landings and roll 500,000 km on the ground in its lifetime. Conventional landing gear is designed to absorb the energy of a landing and does not perform well at reducing ground-induced vibrations in the airframe during landing ground roll, taxi and take-off. Airframe vibrations and fatigue damage can be reduced using semi-active oleos which vary damping over a wide range of ground speeds and runway quality.

Accidents
Malfunctions or human errors (or a combination of these) related to retractable landing gear have been the cause of numerous accidents and incidents throughout aviation history. Distraction and preoccupation during the landing sequence played a prominent role in the approximately 100 gear-up landing incidents that occurred each year in the United States between 1998 and 2003. A gear-up landing, also known as a belly landing, is an accident that results from the pilot forgetting to lower the landing gear, or being unable to do so because of a malfunction. Although rarely fatal, a gear-up landing can be very expensive if it causes extensive airframe/engine damage. For propeller-driven aircraft a prop strike may require an engine overhaul.

Some aircraft have a stiffened fuselage underside or added features to minimize structural damage in a wheels-up landing. When the Cessna Skymaster was converted for a military spotting role (the O-2 Skymaster), fiberglass railings were added to the length of the fuselage; they were adequate to support the aircraft without damage if it was landed on a grassy surface.

The Bombardier Dash 8 is notorious for its landing gear problems. There were three incidents involved, all of them involving Scandinavian Airlines, flights SK1209, SK2478, and SK2867. This led to Scandinavian retiring all of its Dash 8s. The cause of these incidents was a locking mechanism that failed to work properly. This also caused concern for the aircraft for many other airlines that found similar problems, Bombardier Aerospace ordered all Dash 8s with 10,000 or more hours to be grounded, it was soon found that 19 Horizon Airlines Dash 8s had locking mechanism problems, so did 8 Austrian Airlines planes, this did cause several hundred flights to be canceled.

On September 21, 2005, JetBlue Airways Flight 292 successfully landed with its nose gear turned 90 degrees sideways, resulting in a shower of sparks and flame after touchdown.

On November 1, 2011, LOT Polish Airlines Flight LO16 successfully belly landed at Warsaw Chopin Airport due to technical failures; all 231 people on board escaped without injury.

Emergency extension systems
In the event of a failure of the aircraft's landing gear extension mechanism a backup is provided. This may be an alternate hydraulic system, a hand-crank, compressed air (nitrogen), pyrotechnic or free-fall system.

A free-fall or gravity drop system uses gravity to deploy the landing gear into the down and locked position. To accomplish this the pilot activates a switch or mechanical handle in the cockpit, which releases the up-lock. Gravity then pulls the landing gear down and deploys it. Once in position the landing gear is mechanically locked and safe to use for landing.

Ground resonance in rotorcraft
Rotorcraft with fully articulated rotors may experience a dangerous and self-perpetuating phenomenon known as ground resonance, in which the unbalanced rotor system vibrates at a frequency coinciding with the natural frequency of the airframe, causing the entire aircraft to violently shake or wobble in contact with the ground. Ground resonance occurs when shock is continuously transmitted to the turning rotors through the landing gear, causing the angles between the rotor blades to become uneven; this is typically triggered if the aircraft touches the ground with forward or lateral motion, or touches down on one corner of the landing gear due to sloping ground or the craft's flight attitude. The resulting violent oscillations may cause the rotors or other parts to catastrophically fail, detach, and/or strike other parts of the airframe; this can destroy the aircraft in seconds and critically endanger persons unless the pilot immediately initiates a takeoff or closes the throttle and reduces rotor pitch. Ground resonance was cited in 34 National Transportation Safety Board incident and accident reports in the United States between 1990 and 2008.

Rotorcraft with fully articulated rotors typically have shock-absorbing landing gear designed to prevent ground resonance; however, poor landing gear maintenance and improperly inflated tires may contribute to the phenomenon. Helicopters with skid-type landing gear are less prone to ground resonance than those with wheels.

Stowaways
Unauthorized passengers have been known to stowaway on larger aircraft by climbing a landing gear strut and riding in the compartment meant for the wheels. There are extreme dangers to this practice, with numerous deaths reported. Dangers include a lack of oxygen at high altitude, temperatures well below freezing, crush injury or death from the gear retracting into its confined space, and falling out of the compartment during takeoff or landing.

Launch vehicles

Landing gear has traditionally not been used on the vast majority of launch vehicles, which take off vertically and are destroyed on falling back to earth. With some exceptions for suborbital vertical-landing vehicles (e.g., the Masten Xoie or Armadillo Aerospace's Lunar Lander Challenge vehicle), or for spaceplanes that use the vertical takeoff, horizontal landing (VTHL) approach (e.g., the Space Shuttle orbiter, or the USAF X-37), landing gear have been largely absent from orbital vehicles during the early decades since the advent of spaceflight technology, when orbital space transport has been the exclusive preserve of national-monopoly governmental space programs. Each spaceflight system through 2015 had relied on expendable boosters to begin each ascent to orbital velocity.

Advances during the 2010s in private space transport, where new competition to governmental space initiatives has emerged, have included the explicit design of landing gear into orbital booster rockets. SpaceX has initiated and funded a multimillion-dollar reusable launch system development program to pursue this objective. As part of this program, SpaceX built, and flew eight times in 2012–2013, a first-generation test vehicle called Grasshopper with a large fixed landing gear in order to test low-altitude vehicle dynamics and control for vertical landings of a near-empty orbital first stage. A second-generation test vehicle called F9R Dev1 was built with extensible landing gear. The prototype was flown four times—with all landing attempts successful—in 2014 for low-altitude tests before being self-destructed for safety reasons on a fifth test flight due to a blocked engine sensor port.

The orbital-flight version of the test vehicles–Falcon 9 and Falcon Heavy—includes a lightweight, deployable landing gear for the booster stage: a nested, telescoping piston on an A-frame. The total span of the four carbon fiber/aluminum extensible landing legs is approximately 60 ft, and weigh less than 2100 kg; the deployment system uses high-pressure helium as the working fluid. The first test of the extensible landing gear was successfully accomplished in April 2014 on a Falcon 9 returning from an orbital launch and was the first successful controlled ocean soft touchdown of a liquid-rocket-engine orbital booster. After a single successful booster recovery in 2015, and several in 2016, the recovery of SpaceX booster stages became routine by 2017. Landing legs had become an ordinary operational part of orbital spaceflight launch vehicles.

The newest launch vehicle under development at SpaceX—the Starship—is expected to have landing legs on its first stage called Super Heavy like Falcon 9 but also has landing legs on its reusable second stage, a first for launch vehicle second stages. The first prototype of Starship—Starhopper, built in early 2019—had three fixed landing legs with replaceable shock absorbers. In order to reduce mass of the flight vehicle and the payload penalty for a reusable design, the long-term plan is for Super Heavy to land directly back at the launch site on special ground equipment that is part of the launch mount, but initial testing of the large booster is expected to occur with landing legs.

Landers
Spacecraft designed to land safely on extraterrestrial bodies such as the Moon or Mars are known as either legged landers (for example the Apollo Lunar Module) or pod landers (for example Mars Pathfinder) depending on their landing gear. Pod landers are designed to land in any orientation after which they may bounce and roll before coming to rest at which time they have to be given the correct orientation to function. The whole vehicle is enclosed in crushable material or airbags for the impacts and may have opening petals to right it.

Features for landing and movement on the surface were combined in the landing gear for the Mars Science Laboratory.

For landing on low-gravity bodies landing gear may include hold-down thrusters, harpoon anchors and foot-pad screws, all of which were incorporated in the design of comet-lander Philae for redundancy.

In the case of Philae, however, both harpoons and the hold-down thruster failed, resulting in the craft bouncing before landing for good at a non-optimal orientation.

title: Landing gear


text:
London Museums of Health & Medicine

The London Museums of Health & Medicine is a group that brings together some of the activities of several museums in London, England, related to health and medicine. The group was founded in 1991.

The museums and medical organisations are:

 * Alexander Fleming Laboratory Museum
 * Anaesthesia Heritage Centre
 * Barts Pathology Museum
 * Bethlem Museum of the Mind
 * British Dental Association Museum
 * British Optical Association Museum (College of Optometrists)
 * British Red Cross Museum
 * Chelsea Physic Garden
 * Florence Nightingale Museum
 * Foundling Museum
 * Freud Museum
 * Hunterian Museum
 * Langdon Down Museum of Learning Disability
 * Museum of the Order of St John

 * Old Operating Theatre Museum & Herb Garret
 * Royal Botanic Gardens, Kew
 * Royal College of Nursing Library & Heritage Centre
 * Royal College of Obstetricians and Gynaecologists
 * Royal College of Physicians
 * Royal London Hospital Museum
 * Royal Pharmaceutical Society Museum
 * Royal Society of Medicine
 * Science Museum
 * St Bartholomew's Hospital Museum
 * St George's Museum and Archives

 * Worshipful Society of Apothecaries

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: London Museums of Health & Medicine


text:
Western meadowlark

The western meadowlark (Sturnella neglecta) is a medium-sized icterid bird, about 8.5 in in length. It nests on the ground in open grasslands across western and central North America. It feeds mostly on bugs, but will also feed on seeds and berries. The western meadowlark has distinctive calls described as watery or flute-like, which distinguish it from the closely related eastern meadowlark. The western meadowlark is the state bird of six states: Kansas, Montana, Nebraska, North Dakota, Oregon, and Wyoming.

Taxonomy
The western meadowlark was formally described in 1844 by the American ornithologist John James Audubon under its current binomial name Sturnella neglecta. The specific epithet is from the Latin neglectus meaning "ignored", "overlooked", "neglected" or "disregarded". Audubon explained that although the account of the Lewis and Clark Expedition of 1803–1806 mentioned yellow larks, these had never been formally described. The type locality is Old Fort Union, North Dakota.

Despite having "lark" in their name due to their melodic song, they are not true Larks, as they are instead in the New World Blackbird family.

Two subspecies are recognised:

 * S. n. neglecta Audubon, 1844 – southwest, south central Canada through the west USA (except coastal Pacific Northwest) to central Mexico
 * S. n. confluenta Rathbun, 1917 – coastal southwest Canada and northwest USA (southwest British Columbia to Oregon)

Western meadowlarks will occasionally interbreed with eastern meadowlarks where their ranges overlap; however, resulting young appear to have low fertility.

Description
Western meadowlark adults have yellow underparts with a black "V" on the breast and white flanks streaked with black. Their upper parts are mostly brown, but also have black streaks. These birds have long, pointed bills and their heads are striped with light brown and black bands.

Measurements:
 * Length: 6.3 - 10.2 in
 * Weight: 3.1 - 4.1 oz
 * Wingspan: 16.1 in

These birds have a flute-like warbled song. These calls contrast with the simple, whistled call of the eastern meadowlark.

Distribution and habitat
The breeding habitats of western meadowlarks are grasslands, prairies, pastures, and abandoned fields, all of which may be found across western and central North America, as far south as northern Mexico. In regions where their range overlaps with the eastern species, these birds prefer thinner, drier vegetation; the two type of birds generally do not interbreed but do defend territory against one another. Their nests are situated on the ground and are covered with a roof woven from grass. There may be more than one nesting female in a male's territory. Nests are sometimes destroyed by mowing operations with eggs and young in them.

Western meadowlarks are permanent residents throughout much of their range. Northern birds may migrate to the southern parts of their range; some birds also move east in the southern United States.

It has also been introduced to Hawaii to control insects, and is now found on the island of Kauai. Attempted introductions to other islands failed, likely due to the presence of the small Indian mongoose, which is absent from Kauai.

Breeding
The nest is built by the female soon after arrival on the breeding grounds. The nest is placed in a concealed location on the ground. It is variable in form and ranges from a simple cup to a partially roofed structure with a runway extending from the nest entrance. Eggs are laid at daily intervals. Incubation only begins after the last egg is laid. The eggs measure on average 28 x and have brownish spots and blotches on a white ground. The eggs hatch over a period of one or two days after being incubated by the female for 13–15 days. The young are altricial and nearly naked; their eyes are closed until the 4th day. Only the female broods the chicks. The nestlings are almost exclusively fed insects. The male brings food to the nest which is then usually fed to the chicks by the female. The young fledge at 10-12 day but are only capable of sustained flight at around 21 days. The young receive parental care for around 2 weeks after fledging. A second brood is attempted.

The nests are parasitized by brown-headed cowbirds (Molothrus ater).

Food and feeding
These birds forage on the ground or in low to semi-low vegetation. They sometimes search for food by probing with their bills. They mainly eat insects, although they will consume seeds and berries. In winter, these birds often feed in flocks.

Relationship to humans
The western meadowlark is the state bird of six states: Kansas, Montana, Nebraska, North Dakota, Oregon, and Wyoming. The northern cardinal, which represents seven states, is the only bird to hold the status of state bird in more states.

During the 2017 regular session of the Oregon Legislature, there was a short-lived controversy over the western meadowlark's status as state bird versus the osprey. The sometimes-spirited debate included state representative Rich Vial playing the meadowlark's song on his smartphone over the House microphone. A compromise was reached in SCR 18, which was passed on the last day of the session, designating the western meadowlark as the state songbird and the osprey as the state raptor.

Books

 * Lanyon, W. E. 1994. Western Meadowlark (Sturnella neglecta). In The Birds of North America, No. 104 (A. Poole and F. Gill, Eds.). Philadelphia: The Academy of Natural Sciences; Washington, D.C.: The American Ornithologists’ Union.

Thesis

 * Davis SK. Ph.D. (2003). Habitat selection and demography of mixed-grass prairie songbirds in a fragmented landscape. The University of Regina (Canada), Canada.
 * Horn AG. Ph.D. (1987). Repertoires and song switching in western meadowlarks (Sturnella neglecta). University of Toronto (Canada), Canada.
 * Pierce AM. Ph.D. (1974). Energetics and Water Economy in the Western Meadowlark, Sturnella Neglecta. University of California, Irvine, United States, California.

Articles
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title: Western meadowlark


text:
Otford

Otford is a village and civil parish in the Sevenoaks District of Kent, England. It lies on the River Darent, 3 miles north of Sevenoaks. Otford's four churches are the Anglican Church of St Bartholomew in the village centre, the Otford Methodist Church, the Most Holy Trinity Roman Catholic Church, and the Otford Evangelical Church. By the village pond, there are pubs, cafes and shops. The village has three schools, Otford Primary School, St Michael's Prep School, and Russell House.

History
Otford's earliest history and archaeology shows occupation for at least 3,000 years. Occupants have included Iron Age farmers, Romans, archbishops, and royalty, and major events include two battles. The etymology of the village name is disputed: an article in the Kent and Sussex Courier claims that Otford is a contraction of Ottansford, meaning the ford of Otta, a local man of importance.

Roman Otford
A Roman villa in what is now Otford was abandoned during the 4th century, approximately 1700 years ago. The remains were re-discovered and excavated during the 1930s, and again in 2015 by the West Kent Archeological Society. It is at least twice the size of nearby Lullingstone villa.

Saxon Otford
Otford was made part of the Hundred of Codsheath,  one of the nine Hundreds making up the Lathe of Sutton at Hone in about the 6th century AD. Lathes were Saxon administrative regions.

From circa 650 to 750, during the Early Medieval period, Polhill Anglo-Saxon cemetery was used as a place of burial. The archaeologist Brian Philp suggested that the community who buried their dead at Polhill likely lived at Otford, noting that from the centre of the village, the cemetery was visible.

The name Otford may be a contraction of Otterford, possibly derived from Offa, the King of Mercia who fought the Kentish Saxons in 776 at the Battle of Otford. The History and Topographical Survey of the County of Kent: Volume 3, published 1797, indicates that Otford's Saxon name was Ottanford. It further notes that: "In 791, Offa, king of Mercia, whose gifts to the British churches and monasteries in general were great and munificent, gave Otteford to the church of Canterbury." Priest Werhard, kinsman of archbishop Wulfred briefly took possession of Otteford, but was commanded by the archbishop to return it to Canterbury in 830. It remained so until 1070.

The chronicler monk John of Worcester told that Edmund Ironside brought his army to Kent, and fought the Danes at the Battle of Otford in 1016. The Danes apparently fled to Sheppey.

Norman Otford
Otford is mentioned in the Domesday Book of 1086, where it shows that the Archbishop of Canterbury possessed 8 mills in the village, which is referred to as Otefort.

The History and Topographical Survey of the County of Kent: Volume 3 notes that the Textus Roffensis or Tome of Rochester, published between 1122 and 1124, uses Otteford as the name.

The Gough Map of Great Britain, dated between 1355 and 1366, shows Otford labelled with its current name, but on King Henry VIII's map Angliae Figura, the village is known as Otforde.

Otford in the Middle Ages
In 1514, Archbishop William Warham replaced an existing ecclesiastical building with Otford Palace, and in 1519, King Henry VIII stayed in the palace. Apparently, he liked it so much, Archbishop Thomas Cranmer felt compelled to give it to the king in 1537.

Otford in the Late Middle Ages
The History and Topographical Survey of the County of Kent says that: "Otford Parish is about nine miles in circumference, and contains about two thousand four hundred acres of land, of which about seventy are woodland. It lies for the greatest part of it in a low damp situation, which makes it far from being pleasant, and gives it a lonely and gloomy appearance, and in all probability it would have been but little known had it not been for the residence of the archbishops at it for such a length of time."

Victorian Otford
During the 19th century, Otford was still part of the Hundred of Codsheath, albeit with a population of 798 in 1841. In 1887, the Scottish theologian and historian John Hunt was appointed Vicar of the St Bartholomews Anglican church.

In 1882, Otford Station (see "Railway", below) opened for passenger traffic towards London and Sevenoaks.

20th-century Otford
Otford included the settlement of Dunton Green until 1909, when it was split off into a separate parish. In 1924, the Otford Watermill, dating from 1541, burnt down. In the late 1940s, Otford Medical Practice was formed and is now located at Lennards Avenue. On 16 January 1975 the village pond was granted listed building status. In 1999, Otford Medical Practice extended its reach to include a new branch surgery in Kemsing.

Recent history
In 2002, Otford won the Kent Village of the Year competition; the results were broadcast live on BBC Radio Kent, and reported in the Kent Messenger newspaper.

The census of 2011 lists Otford and Shoreham as having 4,595 residents (cf 798 in 1841, see above), in 1,852 households. The area is given as 2,147 hectares (cf 2,400 acres in the 1797 survey). The population density is averaged at 2.1 per hectare (0.85 per acre); however, this is perhaps misleading, as most housing is clustered.

In 2012, Otford was formally twinned with Neufchâtel-Hardelot, under a Twinning Charter signed in Neufchâtel-Hardelot on 17 September 2011 and Otford on 31 March 2012.

The pond one-way system was declared Roundabout of the Year by the Roundabout Appreciation Society in 2013.

In 2013 and 2014, Otford suffered considerable flooding from the river Darent. An assessment in 2011 by KCC highlighted significant risk for Otford. The risks were not overstated, with some shops out of business for several months.

Skyline
Notable landmarks are the Archbishop's Palace, the duck-pond one-way system and the model of the Solar System, at a scale of 1:4,595,700,000, or just under 1 km from the Sun to Pluto when Pluto is at its perihelion. The Anglican parish church of Otford is dedicated to Saint Bartholomew. Otford Methodist Church, Otford Evangelical Free Church and the Roman Catholic Church of the Most Holy Trinity also serve the village; the buildings were registered for marriages in 1936, 1959 and 1981 respectively.

Railway
Otford station is located in the village and has services northbound to central London via Bromley South to Victoria, Blackfriars and St Pancras and southbound to Ashford International via Maidstone East and to Sevenoaks. Otford is served by two railway companies, Southeastern and Thameslink Railway. The railway station has a substantial, chargeable, car park. Some parts of north western Otford are closer to Dunton Green station, which has services northbound to Charing Cross and Cannon Street as well as to Sevenoaks. This railway station also has a chargeable car park.

Road
Otford lies on the A225, a main road between Dartford and Sevenoaks. The presence of the site of a Roman Villa, in addition to the remains of an ancient trackway, suggests that this road has a long history. The High Street, C296, forms a junction with the A225 at the one-way system which encloses the pond. The M26 Wrotham Spur uses the valley here as part of its route, closely following the boundary between Otford and Dunton Green. M25 access from Otford is effected most easily via the A25 into Sevenoaks and M25 Junction 15, Chevening Interchange.

Buses

 * 2 to Swanley via Shoreham, Eynsford & Farningham or to Sevenoaks. Two journeys a day in each direction, Mondays to Saturdays only. Operated by Go-Coach.

Footpaths
The village is a key stopping-off point on the North Downs Way which runs through the village as it crosses the Darent Valley, intersecting the Darent Valley Path.

Pilgrims' Way
Passing through Otford is the Pilgrims' Way, the historic route supposed to have been taken by pilgrims from Winchester in Hampshire to the shrine of Saint Thomas Becket at Canterbury Cathedral.

Schools
The village includes a state primary school, Otford Primary School, and two independents (properly called public schools in the UK), St Michael's Prep School and Russell House School.

Nursery schools
There are three nursery schools in Otford: Otford Nursery, Gatehouse Nursery and Longacre.

Sports
Otford has a public recreation field of approximately 12 acres, marked for football with floodlighting, and cricket. There are also tennis courts available through Otford Tennis Club.

Village hall
Otford Village Memorial Hall is available for hire, and also regularly hosts local clubs ranging from dance to theatre, and bridge to embroidery.

Otford Heritage Centre
Built in 1999, in the old school house, this centre includes a scale model of the Archbishop's Palace, and details of local geology, archaeology and history, plus artefacts from Roman to recent times, including historical photos, a Percy Pilcher model, and a working model of a Kentish oast house. Percy Pilcher is often quoted as being the first man to fly a glider in the British Isles, on 12 September 1895 (although there are good records showing that Eilmer of Malmsbury actually accomplished this in the 11th century). He is considered to have invented the bi-plane form of stacked wings. He was killed in a gliding accident before he could go on to be the first man to fly a powered aircraft.

Shopping, eating and drinking
Otford now has just two pubs, a restaurant and two cafes, as the conversion of retail premises into accommodation continues. It also has a couple of antique shops, a milliner, flower shop, a tennis specialist, local Post Office, a convenience store and a popular builder's merchant. There is parking for free for a limited period in the village car-park.

Leisure
The village has allotments for hire, on land very close to the centre, which are managed by the Parish Council. Otford is covered by OS Landranger Map 188 (Maidstone and Royal Tunbridge Wells) at 1:50,000 and by OS Explorer 147 (Sevenoaks and Tonbridge) at 1:25,000.

Otford has its own television relay, situated at 51.321,0.199 or 51°19'16"N 0°11'55"E in the Otford Hills, serving Otford, Shoreham, and parts of Dunton Green and Kemsing. The television service is BBC London region, originating from the Crystal Palace transmitter. In some parts of Otford, the Crystal Palace transmitter can be received directly, and offers a richer set of multiplexes than the local relay. The BBC South East service is also available from the Bluebell Hill transmitter in some parts of Otford, which offers a similarly rich set of multiplexes but includes ITV Meridian not ITV London, and BBC Radio Kent rather than BBC London. Specialist installations can receive both sets at the same premises, as the multiplex channels are largely not overlapping, although two antennae are required for this.

The library is situated on the High Street, and is run by KCC. It is open Tuesdays and Fridays all day, Mondays, Thursdays and Saturdays mornings only, and is closed on Wednesday and Sunday. It is integrated into the Kent library systems, with common library cards. Printing, scanning and copying services are available, as well as computers for web access, several social clubs and a book club. There is limited parking.

Conservation areas
Otford Conservation Area covers some 16 hectares and contains about forty listed buildings and part of the scheduled monument of the Archbishop's Palace. It was designated in 1969 and extended in 1976, 1990 and 2006.

Crime rate
Reported crimes over the 12-month period May 2017 to April 2018 have occurred at a frequency of approximately 30 per month in the Otford and Shoreham areas. This is broadly similar to the rates across similar locations in Kent.

title: Otford