Landing Gear
The oleo-pneumatic strut, retraction/extension, wheels, brakes and anti-skid, steering and air/ground sensing.
Landing gear, summary notes
- 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.
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
Landing Gear
Landing gear quiz
Landing Gear, quiz
1. On the landing gear shown, the switch marked S is mounted on the shock strut so that it operates as the strut compresses. It is the:
Squat switchDownlock switchProximity heater2. An oleo-pneumatic strut absorbs shock using:
Oil forced through an orifice (damping) plus compressed gas (spring)A steel coil spring alone, chosen because it returns the stored energy smoothly on the reboundRubber blocks in compression only, which absorb the landing loads and need no servicing in service3. In the oleo strut, the compressed gas acts as the:
SpringDamperLubricant4. The weight-on-wheels (squat) switch tells systems whether the aircraft is:
On the ground or airborneTurning left or rightClimbing or descending5. Anti-skid braking works by:
Modulating brake pressure to prevent wheel lockApplying maximum pressure alwaysReleasing all brakes6. Gear retraction/extension is normally powered by:
HydraulicsThe batteryBleed air only7. An alternate gear extension typically uses:
Gravity free-fallThe engine starterThe APU8. Aircraft tyres are inflated with:
NitrogenOxygenHelium