Gear, Wheels, Brakes

Understanding Landing Gear, Wheels, and Brakes in Transport Aircraft

For ATPL students, a strong grasp of landing gear, wheel, and brake systems is essential to safe operations and exam success. In most commercial transport aeroplanes, the landing gear is hydraulically driven for normal extension and retraction, with an alternate/gravity extension backup if hydraulic pressure is lost. The gear assembly includes shock absorbers, wheels/tyres, braking systems, and steering components. Torsion links (scissor links) prevent rotation between the moving piston and the fixed cylinder of the oleo strut—critical to maintaining correct wheel alignment. On multi-wheel bogies, torsion link loads peak during tight-radius ground turns, a factor covered in aircraft handling procedures.

The primary shock absorber on modern aircraft is the oleo‑pneumatic strut, in which oil provides damping and nitrogen provides the spring. This combination dissipates landing energy smoothly and protects the airframe. Nose-wheel stability is managed by design geometry and, when fitted, a shimmy damper; “nose wheel shimmy” refers to a damaging vibration that can occur during ground roll if damping is inadequate or components are worn. Braking is typically by multiple-disc brakes with anti-skid. In hydroplaning conditions, the tyre–runway friction coefficient can drop to near zero, so pilots must follow AFM/QRH procedures for speed control, autobrake selection, and anti-skid use to maintain directional control.

Wheel safety features include thermal (fusible) plugs installed in the wheel rims. These plugs intentionally deflate the tyre when brake heat raises the wheel temperature beyond a set limit, thereby protecting against tyre explosion after heavy braking or rejected takeoff. Tyre management is also examined: tyre creep is the circumferential movement of the tyre relative to the wheel flange, monitored with creep marks; under‑inflated tyres exhibit shoulder wear and can overheat. Brake systems commonly include an accumulator to supply limited brake energy if normal hydraulics are lost—important for taxiing clear or stopping in an abnormal situation. On landing, the autobrake system is typically disengaged by pilot action as part of standard procedures. All of these topics integrate with aviation regulations and manufacturer limitations (AFM/FCOM), including brake energy limits, cooling times, and emergency extension procedures.

Key Concepts and Procedures

• Normal vs. alternate gear operation: Hydraulic extension/retraction with gravity extension as a fail-safe.
• Shock absorption: Oleo-pneumatic strut where oil damps motion and nitrogen provides spring force.
• Alignment and stability: Torsion/scissor links prevent strut rotation; shimmy control for nose gear.
• Braking systems: Multiple-disc brakes with anti-skid; autobrake use and pilot disconnection on landing.
• Wheel/tyre safety: Thermal plugs in wheel rims, tyre creep monitoring, and inflation/inspection practices.
• Performance and safety: Hydroplaning risk management and adherence to AFM/QRH procedures and aviation regulations.

What This Question Bank Covers

• Landing gear architecture and operation (hydraulic systems, gravity extension, bogie gear loads).
• Oleo strut functions, damping media (oil), and nitrogen spring behavior.
• Torsion/scissor links and their role in preventing oleo strut rotation.
• Brake system components: multiple-disc units, anti-skid, accumulators, autobrake usage.
• Tyre management: creep, inflation effects on wear, hydroplaning implications.
• Wheel safety devices: thermal/fusible plugs located in wheel rims and their protective function.
• Procedural knowledge aligned with ATPL standards, AFM/QRH guidance, and aviation regulations.

This integrated understanding of aircraft systems, regulations, and procedures prepares you to answer ATPL exam questions and, more importantly, to make sound operational decisions during landing, rollout, and taxi in both normal and abnormal scenarios.