# Landing gear

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Landing gear is the undercarriage of an aircraft or spacecraft and is often referred to as such.

For aircraft, the landing gear supports the craft when it is not flying, allowing it to take off, land and usually to taxi without damage. Wheels are typically used but skids, skis, floats or a combination of these and other elements can be deployed depending both on the surface and on whether the craft only operates vertically (VTOL) or is able to taxi along the surface. Faster aircraft usually have retractable undercarriage, which folds away during flight to reduce air resistance or drag.

For launch vehicles and spacecraft landers, the landing gear is typically designed to support the vehicle only post-flight, and are not used for takeoff or surface movement.

## Aircraft landing gear

Aircraft landing gear usually includes wheels equipped with shock absorbers for solid ground, but some aircraft are equipped with skis for snow or floats for water, and/or skids or pontoons (helicopters).

The undercarriage is a relatively heavy part of the vehicle; it can be as much as 7% of the takeoff weight, but more typically is 4–5%.[1]

### Gear arrangements

A SAN Jodel D.140 Mousquetaire with conventional "taildragger" undercarriage.
A Mooney M20J with tricycle undercarriage.

Wheeled undercarriages normally come in two types: conventional or "taildragger" undercarriage, where there are two main wheels towards the front of the aircraft and a single, much smaller, wheel or skid at the rear; or tricycle undercarriage where there are two main wheels (or wheel assemblies) under the wings and a third smaller wheel in the nose. The taildragger arrangement was common during the early propeller era, as it allows more room for propeller clearance. Most modern aircraft have tricycle undercarriages. Taildraggers are considered harder to land and take off (because the arrangement is usually unstable, that is, a small deviation from straight-line travel will tend to increase rather than correct itself), and usually require special pilot training. Sometimes a small tail wheel or skid is added to aircraft with tricycle undercarriage, in case of tail strikes during take-off. The Concorde, for instance, had a retractable tail "bumper" wheel, as delta winged aircraft need a high angle when taking off. The Boeing 727 also has a retractable tail bumper. Some aircraft with retractable conventional landing gear have a fixed tailwheel, which generates minimal drag (since most of the airflow past the tailwheel has been blanketed by the fuselage) and even improves yaw stability in some cases.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }} Another arrangement sometimes used is central main and nose gear with outriggers on the wings. This may be done where there is no convenient location on either side to attach the main undercarriage or to store it when retracted. Examples include the Lockheed U-2 spy plane and the Harrier Jump Jet. ### Retractable gear The landing gear of a Boeing 767 retracting into the fuselage Schematic showing hydraulically operated landing gear, with the wheel stowed in the wing root of the aircraft To decrease drag in flight some undercarriages retract into the wings and/or fuselage with wheels flush against the surface or concealed behind doors; this is called retractable gear. If the wheels rest protruding and partially exposed to the airstream after being retracted, the system is called semi-retractable. Most retraction systems are hydraulically operated, though some are electrically operated or even manually operated. This adds weight and complexity to the design. In retractable gear systems, the compartment where the wheels are stowed are called wheel wells, which may also diminish valuable cargo or fuel space.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}

A Boeing 737-700 with main undercarriage retracted in the wheel wells without landing gear doors
A Ju 87D with a wheel spat on the right wheel, absent on the left.

Pilots confirming that their landing gear is down and locked refer to "three green" or "three in the green.", a reference to the electrical indicator lights from the nosewheel and the two main gears. Red lights indicate the gear is in the up-locked position; amber lights indicate that the landing gear is in transit (neither down and locked nor fully retracted).[2]

### Nautical

{{#invoke:main|main}} Some aircraft have landing gear adapted to take off from and land on water.

A floatplane has landing gear comprising two or more streamlined floats.

A flying boat has a hull, the bottom of which is shaped like a boat and gives buoyancy. Additional landing gear is often present, typically comprising wing-mounted floats.

Helicopters able to land on water may have floats or a hull.

An amphibious aircraft has landing gear for both land and water-based operation.

### Other types of landing gear

An Me 163B Komet with its two-wheel takeoff "dolly" in place
Bell Model 207 Sioux Scout with tubular landing skids
Hawker Siddeley Harrier GR7 (ZG472). The two mainwheels are in line astern under the fuselage, with a smaller wheel on each wing
A captured Mitsubishi A6M shows the Zero's nearly perpendicular main gear strut angle to its wing when extended

#### Detachable landing gear

Some aircraft use wheels for takeoff and then jettison them soon afterwards for improved aerodynamic streamlining without the complexity, weight and space requirements of a retraction mechanism. In these cases, the wheels to be jettisoned 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 then accomplished on skids or similar other simple devices.

Wheel-skis

### Tires and wheels

Two mechanics replacing a main landing gear wheel on a Lockheed P-3 Orion

The number of tires required for a given aircraft design gross weight is largely determined by the flotation characteristics. 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.

The choice of the main wheel tires is made on the basis of the static loading case. The total main gear load ${\displaystyle F_{\text{m}}}$ is calculated assuming that the aircraft is taxiing at low speed without braking:[6]

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

where ${\displaystyle W}$ is the weight of the aircraft and ${\displaystyle l_{\text{m}}}$ and ${\displaystyle l_{\text{n}}}$ 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 ${\displaystyle F_{\text{n}}}$ during braking at maximum effort:[6]

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

where ${\displaystyle L}$ is the lift, ${\displaystyle D}$ is the drag, ${\displaystyle T}$ is the thrust, and ${\displaystyle h_{\text{cg}}}$ is the height of aircraft cg from the static groundline. Typical values for ${\displaystyle {\frac {a_{\text{x}}}{g}}}$ on dry concrete vary from 0.35 for a simple brake system to 0.45 for an automatic brake pressure control system. As both ${\displaystyle L}$ and ${\displaystyle D}$ are positive, the maximum nose gear load occurs at low speed. Reverse thrust decreases the nose gear load, and hence the condition ${\displaystyle T=0}$ results in the maximum value:[6]

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

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.[6] From the flotation standpoint, a decrease in the tire contact area will induce a higher bearing stress on the pavement, thus eliminates certain airports from the aircraft’s operational bases. 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. 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.[7]

Tires of many commercial aircraft are required to be filled with nitrogen or low-oxygen air to prevent the internal combustion of the tire which may result from overheating brakes producing volatile hydrocarbons from the tire lining.[8]

### Landing gear and accidents

JetBlue Airways Flight 292, an Airbus A320, making an emergency landing on runway 25L at LAX in 2005 after the front landing gear malfunctioned

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.[9] A gear-up landing incident, also known as a belly landing, is an accident that may result from the pilot simply forgetting, or failing, to lower the landing gear before landing or a mechanical malfunction that does not allow the landing gear to be lowered. Although rarely fatal, a gear-up landing is very expensive, as it causes massive airframe damage. If the landing results in a prop strike, a complete engine rebuild may also be required. Many aircraft between the wars – at the time when retractable gear was becoming commonplace – were deliberately designed to allow the bottom of the wheels to protrude below the fuselage even when retracted to reduce the damage caused if the pilot forgot to extend the landing gear or in case the plane was shot down and forced to crash-land. Examples include the Avro Anson, Boeing B-17 Flying Fortress and the Douglas DC-3. The modern-day Fairchild-Republic A-10 Thunderbolt II carries on this legacy: it is similarly designed in an effort to avoid (further) damage during a gear-up landing, a possible consequence of battle damage.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }} Some aircraft have a stiffened fuselage bottom or added firm structures, designed to minimize structural damage in a wheels-up landing. When the Cessna Skymaster was converted for a military spotting role (the O-2 Skymaster), fiberglass railings were added to the length of the fuselage; they were adequate to support the aircraft without damage if it was landed on a grassy surface.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}

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.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }} On September 21, 2005, JetBlue Airways Flight 292 successfully landed with its nose gear turned 90 degrees sideways, resulting in a shower of sparks and flame after touchdown. This type of incident is very uncommon as the nose oleo struts are designed with centering cams to hold the nosewheels straight until they are compressed by the weight of the aircraft.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}

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.[10]

#### 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.[11]

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.[12]

### Stowaways in aircraft landing gear

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Unauthorized passengers have been known to stowaway on larger aircraft by climbing a landing gear strut and riding in the compartment. There are extreme dangers to this practice and numerous deaths reported, due to the lack of heating and oxygen in the landing gear compartments as well as lack of room due to the retracting gear.{{ safesubst:#invoke:Unsubst||date=__DATE__ |\$B= {{#invoke:Category handler|main}}{{#invoke:Category handler|main}}[citation needed] }}

## Spacecraft

### Launch vehicles

Landing gear has traditionally not been used on the vast majority of space launch vehicles, which take off vertically and are destroyed on falling back to earth. With some exceptions for suborbital vertical-landing vehicles (e.g., Masten Xoie or the Armadillo Aerospace' Lunar Lander Challenge vehicle), or for spaceplanes that use the vertical takeoff, horizontal landing (VTHL) approach (e.g., the Space Shuttle, 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.[13] Each spaceflight system to date has relied on expendable boosters to begin each ascent to orbital velocity. This is beginning to change.

Recent advances 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 multi-million dollar program to pursue this objective, known as the reusable launch system development program. As part of this program, SpaceX built, and flew eight times in 2012–2013, a first-generation orbital booster-test-vehicle 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.[14][15] A second-generation larger booster test vehicle has been built with extensible landing gear. The first prototype was flown five times in 2014 for low-altitude tests, and the second is expected to begin high-altitude test flights in New Mexico in late 2014.[16][17]

The orbital-flight version of the SpaceX design 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[18][19] is approximately Template:Convert, and weigh less than Template:Convert; the deployment system uses high-pressure Helium as the working fluid.[20] The first test of the extensible landing gear was successfully accomplished in April 2014 on a Falcon 9 rocket and was the first successful controlled ocean soft touchdown of a liquid-rocket-engine orbital booster.[21][22]

### Landers

Comet lander Philae showing landing gear. Each pad at the end of each of the lander legs has an ice screw, necessary for attachment to a celestial body with a very low gravitational field.

Spacecraft designed to land safely on extraterrestrial bodies such as the Moon or Mars usually have landing gear. Such landers include the Apollo Lunar Module as well as a number of robotic space probe landers. Examples include Viking 1 lander, the first lander to successfully land on Mars (November 1976),[23] and Philae which is currently in orbit around comet 67P/Churyumov–Gerasimenko after a 10-year transit and landed on the comet on 12 November 2014.[24][25][26][27]

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7. [1] Goodyear Tire & Rubber Co., Retrieved: 26 January 2012.
8. [2] FAA Ruling: "Use of Nitrogen or Other Inert Gas for Tire inflation in Lieu of Air" Docket No. 26147 Amendment No. 25-78 RIN 2120-AD87
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