The retraction cycle
The normal sequence of a gear cycle is fixed by design: on selection UP, the down-locks are released, the doors open, the actuators fold the gear into its well (each leg through its own geometry), the up-locks capture it, and the doors close over it; hydraulic pressure is then removed and the gear hangs on its mechanical up-locks, not on pressure. Selection DOWN mirrors the sequence: doors open, up-locks release, the gear extends (helped by its own weight), the down-locks drive over-centre, and the doors sequence closed. The sequencing is interlocked so nothing moves out of order: a door that fails to open stops the cycle rather than letting the gear be driven through it.
Position indication and alerting report the locks, not the wheels: sensors on each lock feed the classic three-greens (each gear down and locked), with red or amber for a gear in transit or in disagreement with the lever, and no light for stowed and locked up. The alerting side is a configuration warning: if the aircraft descends towards landing (low height, low power, or flaps at landing setting) with the gear not down and locked, a dedicated warning sounds. The lesson of the indication design is precise: a green light means a lock has closed, which is why the drills for an unsafe indication start with the locks.
On the ground the danger reverses: the gear must not retract with the aircraft's weight on it. Protection devices against inadvertent retraction come in two layers:
Ground lock pins — physical pins with warning flags, inserted into the locks during turnarounds and towing;
Protections in the retraction mechanism — a weight-on-wheels sensor that disables the up-selection, or a mechanical baulk on the lever.
Taking off with ground pins still installed defeats nothing on the ground and everything in the air: the gear cannot retract past its pinned locks, so the aircraft is committed to a gear-down flight, with the climb, fuel and speed penalties that brings, until it lands again. The pins live with the aircraft's documents between flights precisely so the walkaround counts them.
Two speed limits belong to this cycle, and they are a naming pair:
VLO (maximum landing gear operating speed) — the fastest the aircraft may fly while the gear is being retracted or extended: moving gear and opening doors face the full airstream, and the actuators must beat it;
VLE (maximum landing gear extended speed) — the fastest the aircraft may fly with the gear locked down: usually higher than VLO, because a locked gear is braced in a way a moving one is not.
The emergency way down
When the normal system cannot lower the gear, every alternative follows the same three-step logic, and the sequence is the exam answer: unlocking, operating, down-locking: release the up-locks, get the gear travelling to the extended position, and confirm the down-locks close. The methods differ only in what does the work:
Gravity (free fall) — release the up-locks and let the gear's own weight, helped by the airstream and sometimes springs, carry it down and over-centre; the doors stay open and the extra drag remains, which the performance figures accept.
Air or nitrogen pressure — a stored gas bottle blows the gear down through dedicated lines, independent of the hydraulic system that failed.
Manual or mechanical — a crank, cable or hand pump lowers the gear by muscle: slow, certain, and found where the gear is light enough for it.
Nose-wheel steering
The operating principle of nose-wheel steering is a control loop in miniature: the pilot's input (tiller or pedals) is transmitted, mechanically or by wire, to a steering actuator (hydraulic on anything large) that rotates the nose-wheel assembly about the strut's axis; feedback closes the loop so the wheel angle follows the command and returns to it after every disturbance. Steering authority is deliberately split by speed on transport aeroplanes: large angles from the tiller for taxiing, small angles from the pedals for the take-off and landing roll, so no full-authority swerve is possible at speed.
Three arrangements cover the fleet, and each is examinable by name:
Differential braking with a free-castoring nose wheel — the nose wheel merely swivels; the aircraft is steered by braking one main wheel harder than the other. The simplest arrangement, standard on many light aircraft.
Tiller (hand-wheel) steering — a small wheel or handle on the flight deck commanding the full steering range through the powered system: the taxiing control of transport aeroplanes, one at each pilot's station.
Rudder-pedal nose-wheel steering — the pedals command a few degrees of nose-wheel angle alongside the rudder, giving directional control on the runway when the rudder alone is still too slow to bite or already fading.
Two mechanisms keep the steered wheel honest. The centring mechanism (centring cams inside the oleo, driven by the strut's own extension) aligns the nose wheel straight ahead as the weight comes off it at lift-off, so the gear retracts into its well aligned rather than jammed across it, and so the next touchdown happens on a straight wheel whatever was commanded in the air. And shimmy is the enemy of everything castored: a rapid oscillation of the wheel assembly about the strut's axis, fed by tyre flexibility and geometry the way flutter is fed by the airstream. Left alone it can shake a strut to destruction and its possible consequences reach both the nose and main wheels: violent vibration, worn torsion links, damaged tyres and struts. The shimmy damper, a small hydraulic damper across the steering rotation, exists to absorb that oscillation's energy and reduce the severity of shimmy; worn dampers and slack torsion links are why shimmy reports ground aircraft.
One more steering system belongs to the largest aeroplanes: main-wheel (body) steering, which steers the aft axles of the body-gear bogies opposite to the nose wheel in tight turns. Its purpose is the geometry problem the fuselage lesson's bogie-scrub discussion introduced: on a long wheelbase, a tight turn drags the rear tyres sideways across the pavement; steering them into the turn cuts the scrubbing loads, the tyre wear and the turn radius together.