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Landing Gear
The oleo-pneumatic strut, retraction/extension, wheels, brakes and anti-skid, steering and air/ground sensing.
Landing gear — summary notes
FreeMain ideas
- The gear supports the aircraft on the ground and absorbs the landing shock through an OLEO-PNEUMATIC strut: on touchdown the strut compresses, forcing oil through an orifice (which DAMPS 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 gear doors, and an alternate (free-fall/gravity) extension for hydraulic failure; a position and warning system shows gear state.
- WHEELS carry tubeless tyres (nitrogen-filled) and multi-disc BRAKES (steel or carbon); ANTI-SKID modulates brake pressure to prevent wheel lock and skidding, and AUTOBRAKE gives selectable deceleration; brake temperature is monitored.
- Nose-wheel STEERING (hydraulic or electric), and an AIR/GROUND sensing system (the weight-on-wheels or 'squat' switch on the strut) tells many aircraft systems whether the aircraft is on the ground or airborne.
- Tyre condition, strut servicing (oil and gas charge) and brake wear are routine maintenance items.
- ⚠ Exam trap: the oleo strut absorbs shock by forcing OIL through an orifice (damping) with compressed GAS as the spring — not by a steel coil spring; the squat/weight-on-wheels switch provides the air/ground signal to the aircraft systems.
Solved examples
How does an oleo-pneumatic shock strut absorb a landing, and what are the two media doing?
As the strut telescopes on touchdown, it forces hydraulic OIL through a small metering orifice — this converts the impact energy into heat and DAMPS the motion so the aircraft doesn't bounce. At the same time a charge of compressed GAS (nitrogen) is squeezed, acting as the SPRING that supports the static weight and then extends the strut again. Oil = damping, gas = spring.
Landing gear concept map
FreeLanding Gear
Landing gear quiz
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