2.landing-gear. Landing Gear
The landing gear is the principal support structure of the airplane when it is on the surface. It absorbs landing loads, supports the airplane's weight during taxi, takeoff, and landing, and provides for braking and directional control on the ground. The most common configuration on modern training and general aviation airplanes is the tricycle gear, consisting of two main wheels and a single nosewheel. Older designs and many backcountry airplanes use conventional gear (a tailwheel configuration) with two main wheels forward of the center of gravity and a small wheel or skid at the tail.
Tricycle gear offers three principal advantages:
- It allows more forceful application of the brakes without nosing over, because the main wheels are aft of the CG.
- It provides better forward visibility for the pilot during taxi, takeoff, and landing.
- It tends to prevent ground looping (a violent uncommanded yaw on the runway), because the airplane's CG is forward of the main wheels. Any divergence from a straight ground track produces a restoring moment that returns the airplane to straight-ahead motion.
Conventional (tailwheel) gear places the CG behind the main wheels, which means any deviation from straight track is amplified rather than damped. Tailwheel airplanes therefore demand more rudder discipline, particularly during takeoff and landing rollouts in a crosswind, and they are more prone to ground looping. Their advantages include better propeller clearance from obstacles and improved performance on rough or unimproved surfaces.
Landing gear is further classified by whether it can be retracted in flight:
- Fixed gear is permanently extended. It is simpler, lighter, less expensive to maintain, and is found on most primary trainers (e.g., Cessna 172, Piper Cherokee).
- Retractable gear folds into the fuselage or wings to reduce parasite drag, increasing cruise speed and efficiency. The added complexity, weight, and maintenance burden are accepted in exchange for the aerodynamic improvement. Operation of retractable gear is regulated and requires specific training under 14 CFR 61.31(e) for airplanes designated as complex.
Most light airplanes use oleo-pneumatic struts (air-oil shock absorbers) on the nose gear and either oleo struts, spring-steel legs, or bungee cords on the main gear to absorb landing impact. Cessna's familiar tubular spring-steel main gear converts vertical landing energy into horizontal flexing of the legs; Piper-style trailing-link gear uses an oleo strut. Pre-flight inspection should verify proper strut extension — a flat strut indicates a loss of nitrogen pressure or hydraulic fluid and must be addressed before flight.
Brakes are independent on each main wheel and are operated by toe pressure on the rudder pedals (the tops of the pedals on most trainers). Differential braking — applying brake to one main wheel only — is used to assist with tight turns on the ground. Most light airplanes use hydraulically actuated single-disc brakes. The parking brake is set by depressing both toe brakes and pulling the parking-brake handle; it is normally released before flight to prevent overheating or a frozen brake on landing.
Directional control on the ground is provided in one of three ways:
- Steerable nosewheel, mechanically linked to the rudder pedals (e.g., Cessna 172). Travel is typically limited to about 10° each side of center, with greater turn angles achieved through differential braking.
- Free-castering nosewheel (e.g., Cirrus SR series, Diamond DA40), which is not linked to the rudders. All steering at low taxi speeds is done with differential braking; aerodynamic rudder authority takes over as speed increases.
- Steerable tailwheel on conventional-gear airplanes, linked to the rudder pedals through springs and usually able to unlock for sharp turns.
Tire condition is part of every preflight. Inspect for proper inflation, even tread wear, cuts, bulges, sidewall cracking, and slippage marks between tire and wheel. Underinflated tires flex excessively and overheat; overinflated tires wear in the center and reduce shock absorption.
Retractable gear systems typically use an electrically driven hydraulic pump to power actuators that raise and lower the gear. Required indicators include three green lights (one per gear leg) showing down-and-locked, and either a red light or a gear-in-transit indicator. Federal regulations (14 CFR 23 certification standards) and most POHs require an emergency extension method — usually a hand pump, free-fall release, or backup CO₂ bottle — and a gear warning horn that sounds when the throttle is reduced below a set manifold pressure with the gear retracted.
Example preflight check: with the airplane on the ramp, confirm correct strut extension (typically 2–4 inches of polished strut showing on the nosewheel), tire inflation and tread, brake disc and pad condition, no hydraulic leaks, secure cotter pins and torque links, and free movement of the steering linkage. After engine start, verify brake effectiveness by gently testing the brakes immediately as the airplane begins to roll.