Landing Gear, Wheels, Brakes, Antiskid and Tyres
Extension and retraction
Gear is extended and retracted by hydraulic or electric actuators, with uplocks and downlocks, and a sequence valve or control unit. Emergency extension may be by gravity, nitrogen bottle or manual crank. Know the indications: three green (down and locked), red (in transit or unsafe), and the horn or warning associated with gear-up landing with power reduced and flaps selected.
Brakes, antiskid and tyres
Brakes are normally hydraulic, with independent left and right systems, a parking brake valve and an accumulator for emergency braking. Antiskid systems sense wheel speed and modulate brake pressure to prevent skidding; a failed antiskid may leave brakes operative but with reduced effectiveness. Tyres are rated by ply rating and speed; under-inflation causes sidewall flexing and heat.
Traps and worked example
Traps: assuming a green light means the gear is down (it means down and locked only if the switch is on the downlock); forgetting that antiskid does not shorten the stopping distance on a dry runway but prevents wheel lock; and believing the parking brake can be set with no hydraulic pressure.
Worked example: after take-off the gear unsafe warning sounds with gear selected up. Reasoning: the warning is triggered by an unsafe gear condition with power reduced or flaps down; check whether the gear is still in transit, whether a downlock is not made, or whether the warning is a false indication. The correct action is to follow the checklist, not to assume the gear is up.