6.1 Landing Gear, Wheels, Tires, and Brakes

Landing Gear Architecture: Fixed vs. Retractable

Landing gear supports the aircraft during taxi, takeoff, landing, towing, and parking; it absorbs the vertical descent energy of touchdown, provides directional control, and carries the braking loads. The Aviation Mechanic Airframe ACS draws gear questions from FAA-H-8083-31, Aircraft Technician Airframe Handbook, so memorize the system vocabulary precisely.

Fixed gear is bolted in the extended position permanently. It is simple, light, and reliable, common on trainers and many light singles, and may be faired with wheel pants to reduce drag. Retractable gear is raised after takeoff into wheel wells to cut parasite drag and raise cruise speed, but it adds actuators, locks, doors, sequencing, position indication, and a warning system.

By layout, the two dominant configurations are:

The mechanic's job is to inspect for cracks, corrosion, alignment, wear, and security, and to service the energy-absorbing components correctly.

Shock Struts and the Oleo Strut

Most transport and many light aircraft use an oleo-pneumatic shock strut (an "oleo strut"). It is a sealed telescoping cylinder containing hydraulic fluid and a compressed gas charge (air or, preferably, dry nitrogen). On landing the piston (inner cylinder) telescopes into the outer cylinder, forcing oil through a metering pin/orifice; the controlled fluid flow dissipates the impact energy as heat, while the gas charge acts as the spring that supports static weight and rebounds the strut. This dual action is why an oleo absorbs shock far better than a simple spring-steel or rubber-bungee gear.

Proper strut service requires both the correct fluid level and the correct gas pressure, in that order: fully compress and deflate, fill with the specified fluid, then re-inflate to the manufacturer's extension. A strut that bottoms out or shows the wrong exposed-piston (chrome) dimension with the aircraft on its wheels is low on air, low on fluid, or has an internal leak. Never loosen the air valve or any fitting until the gas charge is fully released — a charged strut stores lethal energy.

Simpler designs include spring-steel gear (Cessna singles, energy stored and released by bending) and rubber shock-cord (bungee) gear (Piper Cubs).

When inspecting any strut, check the exposed-piston (chrome) measurement against the chart for aircraft weight, look for fluid leakage past the wiper/packing (a film is normal, running fluid is not), and check for nicks or corrosion on the chrome that would cut the seals. Strut servicing is a frequent practical-test item: the candidate must show that fluid and gas are added in the correct order and that the air valve is never loosened on a charged strut.

Alignment matters too. Mechanics check and adjust toe-in/toe-out and camber on main gear; misalignment scrubs tires and causes uneven wear. Wheel bearings are cleaned, inspected, and repacked with the specified grease, and adjusted to the correct preload — too tight overheats, too loose allows play and shimmy.

Wheels, Tires, and Brakes

Modern aircraft wheels are split (two-piece) wheels bolted together around a tubeless tire, with fusible plugs that melt and deflate the tire if brake heat becomes extreme, preventing a wheel explosion. Tires are inflated with dry nitrogen, not shop air: nitrogen is inert, removes moisture, and avoids supporting combustion at the high temperatures of heavy braking. Always inflate inside a tire safety cage and verify the valve core.

Tire markings to read on the sidewall include the size (e.g., 6.00-6 = section width × rim diameter, ply rating), the skid/red dot or yellow balance mark (lightest point, aligned with the valve), serial number, and TSO data. Worn tires are removed at the tread wear limit (worn to the bottom of a groove) or for cuts exposing cord, flat spots, or sidewall damage.

Aircraft brakes are nearly all disc brakes. Light aircraft use a single-disc, floating-caliper design (one rotor keyed to the wheel, pistons squeeze linings against it). Transports use multiple-disc (stacked rotor/stator) brakes for the enormous energy of high-speed stops. Brakes may be energizing (servo action increases applied force) or non-energizing.

• Spongy pedals → air in the lines → bleed the brakes (top-down or bottom-up per the manual).
• Dragging brake → warped disc, hung-up piston, no return spring, or contaminated lining.
• Fading/grabbing → glazed or oil-soaked linings.

Linings and discs have published wear limits; measure, do not guess. Use the correct fluid — MIL-PRF-5606 (red) in light-aircraft systems, Skydrol (purple) in transports — and never mix the two.

Brake heat is the dominant hazard on heavy aircraft: a rejected takeoff can drive brake temperatures into the hundreds of degrees, melting the fusible (thermal) plugs so the tire deflates instead of bursting. After a hot stop, approach the wheels fore and aft, never from the side, because an overheated wheel can fail and throw the split-wheel halves laterally. Allow the brakes to cool before jacking or handling.

Jacking itself is a graded safety item: use only approved jack points, install safety locks/collars on the jacks, account for the CG shift when a heavy component is removed, and never work under an aircraft supported only by hydraulic jack pressure. Tail stands or ballast prevent tip-over when the empty CG moves aft of the mains.

Finally, document gear and brake work correctly in the maintenance record (the part, the action, return-to-service signature), because gear and brake discrepancies are airworthiness items, not deferrable comfort squawks.