Landing Gear, Module 13, Certify66

Notes

Main ideas

The gear supports the aircraft and absorbs the landing shock through an OLEO-PNEUMATIC strut: on touchdown oil is forced through an orifice (DAMPING the energy as heat) while a charge of compressed gas acts as the SPRING.
Retraction/extension is normally HYDRAULIC, with up-locks and down-locks, sequenced doors and an alternate (gravity free-fall) extension; a position and warning system shows gear state.
WHEELS carry nitrogen-filled tyres and multi-disc BRAKES (steel or carbon); ANTI-SKID modulates brake pressure to prevent wheel lock, and AUTOBRAKE gives selectable deceleration; brake temperature is monitored.
Nose-wheel STEERING and an AIR/GROUND sensing system, the weight-on-wheels (squat) switch, tell many aircraft systems whether the aircraft is on the ground or airborne.
For the B2 technician the anti-skid electronics, position/warning sensing, WOW logic and indication are the focus.
⚠ Exam trap: the oleo strut absorbs shock by forcing OIL through an orifice (damping) with compressed GAS as the spring; the squat/weight-on-wheels switch provides the air/ground signal to the aircraft systems.

Solved examples

Why do so many aircraft systems need the weight-on-wheels (squat) switch signal?
Many functions must behave differently on the ground and in the air, for example ground spoilers and autobrake should only deploy on the ground, the pressurisation should reset, cabin/PA and door logic changes, and thrust-reverser and gear-retraction interlocks depend on it. The squat switch on the main gear strut (compressed on the ground, extended in the air) provides that single, reliable air/ground reference to all those systems.

Landing Gear