Система уборки и выпуска шасси

Обзор

The landing gear retraction system is the electro-hydraulic mechanism responsible for raising and lowering the main and nose gear assemblies between their extended (down-and-locked) and retracted (up-and-locked) positions. On virtually all commercial transport aircraft, gear retraction occurs within seconds of liftoff and is one of the most safety-critical sequences in the flight cycle. A failure to retract increases aerodynamic drag substantially — on a narrowbody aircraft, an inadvertently extended gear increases drag by approximately 40 percent and fuel burn by 15–20 percent at normal cruise altitude. Conversely, a failure to extend prior to landing is immediately catastrophic, making the retraction system one of the most carefully redundant mechanisms on the aircraft.

Most commercial jets use a hydraulic system to power gear retraction, with a dedicated gear selector valve routing high-pressure hydraulic fluid to actuators on each gear assembly. Emergency extension, provided as a mandatory backup on all certified transport aircraft, allows the gear to free-fall under gravity and aerodynamic forces and lock in the extended position without hydraulic power.

Принцип работы

When the flight crew selects the gear handle to UP, the landing gear control valve directs hydraulic pressure to the retraction actuators on each strut and simultaneously releases the downlock mechanisms that secured the gear in the extended position. As each gear assembly retracts into its wheel well, a sequence valve ensures the gear doors open before the strut moves, the strut retracts fully, and the doors close over the retracted gear. An uplock hook engages over a roller on the gear structure, mechanically securing the gear in the retracted position and allowing hydraulic pressure to be relieved without the gear creeping down.

For extension, the sequence is reversed: doors open, hydraulic pressure releases the uplock and pushes the gear toward the extended position, the strut extends, downlock links snap over-center and engage mechanical downlocks, and doors close. Proximity sensors on the uplocks, downlocks, and gear doors transmit position information to the cockpit landing gear indication system. Three green lights (or equivalent ECAM/EICAS indications) confirm all three gear assemblies are down-and-locked before landing.

Emergency free-fall extension involves mechanically releasing the uplocks — either via a dedicated free-fall handle in the cockpit or through a solenoid-operated uplock release — allowing gravity and aerodynamic loads to swing the gear to the extended position. Downlocks engage mechanically without hydraulic assistance. Some designs use nitrogen pneumatic backup actuators to assist extension when aerodynamic forces alone are insufficient.

Основ��ые компоненты

• Retraction Actuators: Dual-acting hydraulic cylinders attached between the airframe structure and the gear trunnion or side strut brace. Convert hydraulic pressure into linear force sufficient to overcome gear weight, aerodynamic loads, and friction.
• Uplocks: Mechanical hooks or latches that capture the gear structure in the retracted position. Must withstand the gear's inertial loads in turbulence and during manoeuvres without creeping open.
• Downlocks: Over-center linkage or mechanical locks that prevent the gear from folding under landing loads. Their engagement is confirmed by proximity sensors feeding cockpit indication.
• Gear Selector Valve: Pilot-operated hydraulic valve directing pressure to retract or extend circuits. Typically incorporates a ground-safety solenoid (squat switch logic) that prevents gear retraction while weight is on wheels.
• Sequencing Valves: Pressure-operated valves that coordinate the door-open/gear-move/door-close sequence, ensuring doors do not contact gear structure during transit.
• Proximity Sensors (PSEU): Proximity switch electronics units reading uplock, downlock, and door position via inductive sensors, driving landing gear indication.
• Free-Fall Release Mechanism: Cable or solenoid system allowing mechanical uplock release for emergency gravity extension without hydraulic system availability.

Применение на воздушных судах

• Boeing 737-800 — hydraulic retraction via System A; free-fall backup standard; retraction time approximately 8–10 seconds
• Airbus A320-200 — Green hydraulic system primary; free-fall extension available; gear doors open and close during cycle
• Boeing 777-300ER — six-wheel main bogies retract inward; complex door sequencing; hydraulic Systems L & R plus Centre
• Boeing 787-9 — hydraulic retraction retained despite bleed-free architecture; composite gear doors reduce weight

Преимущества и ограничения

Hydraulic retraction systems are well-proven, powerful, and capable of generating the high forces needed to retract large multi-wheel bogies against aerodynamic loads. Their primary limitation is dependence on hydraulic system integrity; a hydraulic leak can simultaneously deprive the gear of both retraction and extension power, making the free-fall backup essential. Hydraulic systems also require careful sealing against fluid contamination and temperature extremes.

The mechanical complexity of uplocks, downlocks, and sequencing valves introduces multiple potential failure modes that require rigorous inspection intervals. Ground safety pins inserted in uplocks during maintenance prevent inadvertent retraction on the ground but represent a significant hazard if left installed at departure — "remove before flight" streamers are mandatory. As aircraft designs mature, some research programs explore electro-hydrostatic actuators (EHAs) for gear retraction, offering local hydraulic power generation without aircraft-level hydraulic plumbing, though the technology has not yet entered mainline commercial service for primary landing gear actuation.