The components of an aircraft or a spacecraft that support the weight of the craft and its load and give it mobility on ground or water.
Source: EUROCONTROL ATM Lexicon
The landing gear is the principal support of the airplane when parked, taxiing, taking off, or landing. The most common type of landing gear consists of wheels, but airplanes can also be equipped with floats for water operations or skis for landing on snow.
The wheeled landing gear on small aircraft consists of three wheels: two main wheels (one located on each side of the fuselage) and a third wheel positioned either at the front or rear of the airplane.
Landing gear with a rear mounted wheel is called conventional landing gear. Airplanes with conventional landing gear are sometimes referred to as tailwheel airplanes. The two main wheels are attached to the airframe ahead of its centre of gravity (CG) and support most of the weight of the aircraft. The tailwheel is located at the very back of the fuselage and provides a third point of support. This arrangement allows adequate ground clearance for a larger nose-mounted propeller and is more desirable for operations on unimproved fields. It is therefore popular with small, general aviation aircraft such as the PIPER L-18C and the C170. With the CG located behind the main landing gear (MLG), directional control is more difficult while on the ground. For example, if the pilot allows the aircraft to swerve while rolling on the ground at a low speed, they may not have sufficient rudder control and the CG will attempt to get ahead of the main gear, which may cause the airplane to ground loop. Touching down with the tailwheel may, depending on the speed, produce enough lift (due to the increased Angle of Attack (AOA)) and cause the aircraft to become airborne again. Diminished forward visibility when the tailwheel is on or near the ground is another disadvantage of tailwheel landing gear airplanes. Specific training is required to operate tailwheel airplanes.
When the third wheel is located on the nose, it is called a nosewheel, and the design is referred to as a tricycle gear. It has the following advantages compared to the conventional type:
- Allows more forceful application of the brakes during landings at high speeds without causing the aircraft to nose over.
- Tends to prevent ground looping (swerving) by providing more directional stability during ground operation since the aircraft’s CG is forward of the main wheels. This keeps the airplane moving forward in a straight line rather than ground looping.
- Provides better forward visibility for the pilot during takeoff, landing, and taxiing.
A steerable nosewheel or tailwheel permits the airplane to be controlled throughout all operations while on the ground. Most aircraft are steered by moving the rudder pedals, whether nosewheel or tailwheel. Airplane brakes are located on the main wheels and are applied by either a hand control or by foot pedals (toe or heel). Foot pedals operate independently and allow for differential braking, i.e. applying different force to the left and right main landing gear assemblies. During ground operations, differential braking can supplement nosewheel/tailwheel steering.
Landing gear can also be classified as either fixed or retractable. Fixed landing gear always remains extended and has the advantage of simplicity combined with low maintenance. Retractable landing gear is designed to streamline the airplane (reduce the drag) by allowing the landing gear to be stowed inside the structure during cruising flight. Fixed landing gear is common with slow (e.g. general aviation) aircraft and most commercial aircraft use retractable landing gear.
Heavier aircraft require more complex landing gear. These consist of multiple wheels and sometimes the MLG is made of more than two assemblies. For example, the Airbus A340 Family is equipped with a MLG comprising three parts (one under each wing and the third under the fuselage) and the AIRBUS A-380-800 and the Boeing B747 Series have four (one under each wing and two under the fuselage). Some large cargo aircraft, e.g. the ANTONOV An-124 Ruslan and ANTONOV An-225 Mriya also have nose landing gears comprising two assemblies (in addition to the complex MLG design).
Retractable landing gear is normally powered by the hydraulic system. In the case of failure, an emergency extension system is available. This may be a manually operated crank or pump, or a mechanical free-fall mechanism. Airflow is sometimes used to get the gear into the locked position.
Landing with the gear in the "up" position or with an unlocked gear can lead to loss of directional control on the ground, a Runway Excursion, extensive structural damage or Fire, Smoke & Fumes.
Accidents and Incidents
This section contains A&I examples that have landing gear as a contributory factor.
- A30B, Bratislava Slovakia, 2012 (On 16 November 2012, an Air Contractors Airbus A300 departed the left the side of the landing runway at Bratislava after an abnormal response to directional control inputs. Investigation found that incorrect and undetected re-assembly of the nose gear torque links had led to the excursion and that absence of clear instructions in maintenance manuals, since rectified, had facilitated this. It was also considered that the absence of any regulation requiring equipment in the vicinity of the runway to be designed to minimise potential damage to aircraft departing the paved surface had contributed to the damage caused by the accident.)
- A310, Vienna Austria, 2000 (On 12 July 2000, a Hapag Lloyd Airbus A310 was unable to retract the landing gear normally after take off from Chania for Hannover. The flight was continued towards the intended destination but the selection of an en route diversion due to higher fuel burn was misjudged and useable fuel was completely exhausted just prior to an intended landing at Vienna. The aeroplane sustained significant damage as it touched down unpowered inside the aerodrome perimeter but there were no injuries to the occupants and only minor injuries to a small number of them during the subsequent emergency evacuation.)
- A320, Khartoum Sudan, 2005 (On 11 March 2005, an Airbus A321-200 operated by British Mediterranean Airways, executed two unstable approaches below applicable minima in a dust storm to land in Khartoum Airport, Sudan. The crew were attempting a third approach when they received information from ATC that visibility was below the minimum required for the approach and they decided to divert to Port Sudan where the A320 landed without further incident.)
- A320, Los Angeles USA, 2005 (On 21 September 2005, an Airbus A320 operated by Jet Blue Airways made a successful emergency landing at Los Angeles Airport, California, with the nose wheels cocked 90 degrees to the fore-aft position after an earlier fault on gear retraction.)
- A320, Perth Australia, 2018 (On 14 August 2018, an Airbus A320 departed Perth without full removal of its main landing gear ground locks and the unsecured components fell unseen from the aircraft during taxi and takeoff, only being recovered after runway FOD reports. The Investigation identified multiple contributory factors including an inadequately-overseen recent transfer of despatch responsibilities, the absence of adequate ground lock use procedures, the absence of required metal lanyards linking the locking components not attached directly to each gear leg flag (as also found on other company aircraft) and pilot failure to confirm that all components were in the flight deck stowage.)
- A320, Singapore, 2015 (On 16 October 2015, the unlatched fan cowl doors of the left engine on an A320 fell from the aircraft during and soon after takeoff. The one which remained on the runway was not recovered for nearly an hour afterwards despite ATC awareness of engine panel loss during takeoff and as the runway remained in use, by the time it was recovered it had been reduced to small pieces. The Investigation attributed the failure to latch the cowls shut to line maintenance and the failure to detect the condition to inadequate inspection by both maintenance personnel and flight crew.)