The Ups and Downs of Landing Gear

When birds come into land, they do so gracefully rather than crashing and tumbling into the ground. How? The answer is all in the legs. You may have noticed a bird’s legs extend out from beneath their bodies as they approach the ground, which enables the birds to channel their center of gravity and stay upright. Aircraft landing gear works more or less on the same principle. Landing gear can come in many different forms such as floats, skis and wheels. The most common type for commercial aircraft is the tricycle setup which, as the name suggests, consists of three sets of wheeled landing gear located under the nose of the plane and two at the rear of the plane, just past the aircraft’s center of gravity. The back two sets of landing gear have 4 wheels in a setup that is called the bogie landing gear.

To reduce drag, modern aircraft have retractable landing gear that is stowed in the aircraft’s fuselage during flight and is deployed during take-off and landing. To operate the landing gear, the pilot has a lever inside the cockpit. Lights inside the cockpit indicate when the landing gear is down and locked in place or stored and locked in place. Also known as the undercarriage of the aircraft, the landing gear operates on a system of hydraulics that are sometimes assisted by electronics which can be activated with the push of the lever.

The basic gear up sequence begins with a selector valve that allows pressure from the hydraulic system to enter the components of the landing gear. The locks are pressurized and unlock the doors before the gears are retracted upwards. In the same instance, an adapter cylinder on each gear receives pressurized fluid to the gear-upside of the piston through a check valve, which drives the landing gear back into the well into the fuselage well. Only after the gears have fitted back into the well can the hydraulic sequence of the door close begin. The gears mechanically trigger the sequence valves to open and allow fluid to flow into the close side of the door actuator cylinders, which closes the door.

The key to correct landing gear operation is the timing. The hydraulics need to be punctuated by various valves that stall the door opening and closing sequence to allow the gears to settle in and out of the fuselage well. Backup systems are in place in case the hydraulic system lags in any way. An auxiliary backup unit is common on larger commercial aircraft. In some cases, a manual operated ‘free drop’ of the landing gear can take place. During MROs the landing gear mechanism is tested in detail. The aircraft is lifted off the ground and a procedure called the gear swing can be carried out. Mechanics closely monitor the movement and timings of the brakes to ensure there are no hesitations, cracks, or friction in the system.

Malfunctioning landing gears can render an aircraft not airworthy therefore leading to an expensive aircraft on ground status. The FAA stipulates a specific list of landing gear inspections that must take place during a 100-hour inspection. The list includes an inspection of the hydraulic lines, wheels, tires, retracting and locking mechanism, and shock absorbing device amongst others. Only when all of these checks have taken place and no visible signs of wear or tear have been detected can the aircraft be deemed airworthy.



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February 16, 2021

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