Landing Gear
Landing Gear
Definition
Landing gear. 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
Description
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.
On 23 October 2022, an Airbus 330-300 failed to complete three night approaches to runway 22 at Mactan-Cebu due respectively to adverse weather, a low height ‘SINK RATE’ warning and a system fault. Touchdown followed the fourth approach but the aircraft failed to stop on the 3,300 metre-long runway and sustained major damage before coming to a stop 235 metres beyond the runway end. All occupants successfully evacuated without serious injuries. The lack of braking was found to have been caused by damage during brief aircraft ground contact just before the beginning of the runway during the second approach.
On 23 April 2019, the captain of an Embraer E170 which had just begun its standing takeoff at Yamagata, Japan, was unable to correct a deviation from the centreline and initiated a rejected takeoff at around 50 knots. This action was not immediately followed by maximum braking as required by company procedure for a rejected takeoff. The aircraft departed the side of the runway and continued on grass for almost 400 metres before stopping. The nose wheel steering system was found to be defective.
On 29 September 2019, an Airbus A330-200 received simultaneous indications of low pressure in two hydraulic systems soon after takeoff. An emergency was declared, and a return to land was followed by a stop on the runway due to a burst main wheel tyre. A manual valve for one of the hydraulic systems located in the left main gear wheel well had completely detached and impact-damaged a pipe in a nearby but separate hydraulic system. Both systems lost their fluid with valve detachment attributed to fatigue failure of the attachment screws, a risk addressed by an un-adopted non-mandatory Service Bulletin.
On 18 June 2021, a Boeing 787-8 being operated by British Airways was being loaded for a cargo flight at Heathrow whilst line engineering carried out checks required to permit despatch with a deferred minor defect for later rectification. The check required cycling the landing gear with locking pins inserted so that only the bay doors cycled but when this was done, the nose gear retracted and the front of aircraft dropped to the ground causing significant damage to the airframe and minor injuries to two people. The nose gear downlock pin had inadvertently been inserted into the wrong hole.
On 3 December 2021, a Boeing 737MAX-8 released to service with antiskid and autobrake systems inoperative in accordance with Minimum Equipment List procedures then operated two sectors. On the return to Singapore, both left main landing gear tyres were sufficiently damaged during landing to cause the bursting of one and deflation of the other. The cause of this was failure to deploy the speedbrakes manually as required. A similar error on the previous sector did not have the same outcome because the relatively more positive touchdown enabled automatic speedbrake deployment and wheel spin was accompanied by simultaneous manual braking.
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