6.2 Retraction, Steering, Anti-Skid, and Warning Systems

Retraction Systems and Locks

Retractable gear is raised and lowered by a power source — hydraulic (most transports and high-performance singles), electric (a reversible motor driving jackscrews or a gearbox), or manual (hand crank). Hydraulic systems route fluid through a selector valve to gear actuators, with sequence valves that force events to happen in the correct order: doors open, gear extends, doors close (or stay open). Restrictor/orifice check valves slow the gear so it does not slam.

Two locks are critical:

• Uplock — holds the gear retracted so it cannot free-fall and so air loads cannot force it down.
• Downlock — holds the gear extended for landing loads. It is commonly an over-center linkage (a drag/side brace that locks past center so loads cannot collapse it) backed by a downlock spring.

Every retractable system has an emergency extension method — gravity free-fall with the locks released, a hand pump, a blow-down nitrogen bottle, or a manual crank. Troubleshooting a gear that will not retract or extend follows the chain: power source → selector → sequence valves → actuators → locks → doors → switches → indication → mechanical rigging. Always put the aircraft on jacks before a retraction (swing) check.

During a swing check, watch the sequence and timing: doors and gear must move in the right order and within the time band in the manual, locks must engage with the proper rigging dimensions, and the indication must agree with the actual position at every step. A gear that extends but shows no down-and-locked light may have a perfectly good actuator and a mis-rigged or failed downlock switch — proving why you verify the mechanical position, not just the light. Hydraulic creep, a sluggish actuator, or a weak downlock spring all show up on a careful swing check that a quick functional test would miss.

Gear Position and Warning Systems

Gear indication tells the crew the gear's true position, and the warning system alerts them to an unsafe configuration. Standard light logic:

A separate aural warning horn sounds if the throttle is retarded toward idle (low power, approach configuration) with the gear not down-and-locked. Because these are safety systems, an intermittent or false indication is a real discrepancy — chase the wiring, the gear-position (squat/proximity) switches, the squat switch (weight-on-wheels) sensor, bulbs, and the annunciator logic before signing it off. Never dismiss a flickering gear light.

The squat switch (weight-on-wheels, WOW, switch) on the gear strut tells the aircraft whether it is on the ground or in the air, and it gates many systems beyond the gear horn: it can disable gear retraction on the ground (so you cannot raise the gear while parked), arm ground spoilers, and enable/disable other ground-only functions. A misadjusted or failed squat switch therefore produces confusing, multi-system symptoms. When indication and warning faults appear, confirm the squat switch rigging and condition early.

Bulbs are checked by press-to-test, and a dual-filament or dimming circuit is verified so a burned-out lamp is not mistaken for an unsafe gear.

Steering, Shimmy, and Anti-Skid

Nose-wheel steering is by direct linkage and rudder pedals on light aircraft, or by a hydraulic steering actuator (often through a steering metering valve and follow-up linkage) on larger aircraft, supplemented by differential braking and a free-castering capability for tight turns.

Shimmy is a rapid lateral oscillation of the nose wheel. A shimmy damper (a small hydraulic snubber or piston/vane unit) suppresses it. When shimmy appears, do not blame one item — inspect the chain: worn or loose torque links (scissors), worn wheel bearings, low or empty shimmy-damper fluid, tire imbalance or wear, loose attach hardware, and incorrect alignment/rigging.

Anti-skid (anti-lock) systems shorten stopping distance on wet, icy, or contaminated runways. A wheel-speed sensor on each wheel feeds a control box; when the box senses a wheel decelerating toward a skid (wheel speed dropping far below aircraft speed), it commands the anti-skid control valve to release or modulate brake pressure to that wheel, then reapply as the wheel spins back up. Functions include touchdown protection (no braking until wheels spin up) and locked-wheel protection. The system must be verified using the aircraft's built-in self-test; do not assume an anti-skid system is serviceable.

Anti-skid components a mechanic inspects include the wheel-speed transducers (and their wiring), the anti-skid control box/computer, the control (servo) valves, and the caution/inop annunciation. A common pitfall is that anti-skid is often automatically armed only above a certain ground speed and inhibited below it, so a failed self-test may be normal logic rather than a fault — read the manual before condemning a unit.

Steering systems are likewise rigged to limits: a steering metering valve and follow-up linkage make actual nose-wheel angle track the pedal/tiller input, and incorrect rigging produces over- or under-steering. Whenever a steering, anti-skid, or warning fault appears, identify whether the cause is electrical, hydraulic, or mechanical before swapping parts, because all three can mimic each other.