Landing Gear
The oleo-pneumatic strut, retraction/extension, wheels, brakes and anti-skid, steering and air/ground sensing.
Landing gear — summary notes
Free- 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.
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 concept map
FreeLanding Gear
Landing gear quiz
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