On any aircraft equipped with retractable landing gear, the mechanism responsible for moving those gear legs up and down must be powerful enough to overcome significant aerodynamic and mechanical loads, yet reliable enough that a failure does not strand a crew without a safe way to land. Hydraulic actuation meets both demands elegantly: it transmits large forces through small-diameter lines, responds almost instantly to cockpit commands, and holds position indefinitely without continuous power input. Understanding how these systems are designed, operated, and maintained is essential knowledge for every Aviation Maintenance Technician (AMT) working on airframe systems, and it is thoroughly tested on the FAA Airframe Knowledge Test.
This article walks through the complete hydraulic landing gear actuation system — from the fluid source, through the control valve, down the actuating cylinders, and into the safety and indicating systems that protect both crew and aircraft.
How the System Works
A hydraulic landing gear actuation system is essentially a closed-loop power transmission circuit. Pressurized hydraulic fluid — typically Mil-Spec MIL-PRF-5606 (mineral-based, red), MIL-PRF-83282 (synthetic, red), or Skydrol (phosphate-ester, purple/green) on transport-category aircraft — is generated by an engine-driven pump, an electric motor-driven pump, or both. The pump maintains system pressure within a designed range, commonly between 1,000 and 3,000 psi on light aircraft and up to 5,000 psi or more on transport jets. An accumulator stores a reserve charge of pressurized fluid so that peak demands (like simultaneous gear retraction) can be met without overloading the pump.
From the pump, fluid flows through filters and a pressure relief valve to a selector valve (also called a control valve or directional control valve). The cockpit gear handle commands this valve — mechanically, electrically, or through a solenoid — to direct fluid either to the "up" port or the "down" port of each actuating cylinder. When the handle moves to the UP position, high-pressure fluid enters the retract side of the actuator pistons, pushing the gear into the wheel wells while return fluid from the extend side flows back to the reservoir. Moving the handle DOWN reverses flow, and gravity may also assist extension on many designs.
Actuating Cylinders
Each gear leg is typically driven by a linear actuating cylinder (sometimes called a hydraulic jack). These are double-acting cylinders, meaning fluid pressure can push the piston in either direction rather than relying on a spring for return. The cylinder is attached at one end to the airframe and at the other to the gear trunnion or drag brace. As the piston extends or retracts, it moves the gear through its arc of travel. Proper rigging — the precise adjustment of linkages, up-locks, and down-locks — is critical; if the actuator reaches the end of its stroke before the gear is fully seated in the lock, the system will neither protect the gear from inflight loads nor give a reliable position indication.
Up-Locks and Down-Locks
A landing gear that is simply held in position by hydraulic pressure is not safe: a hydraulic line failure would allow the gear to drift or collapse. That is why certificated retractable-gear aircraft use mechanical locks at both the up and down positions. The down-lock (also called an over-center lock or jury strut) snaps into place when the gear is fully extended, mechanically preventing retraction under any load. The up-lock holds the gear in the retracted position, unloading the hydraulic system so the pump does not have to run continuously. The actuating cylinder must first release these locks — often through a separate small hydraulic or mechanical release — before the main cylinder can move the gear. Maintenance technicians must verify lock engagement force, latch condition, and correct rigging on every inspection.
Sequence Valves and Priority Valves
On aircraft with wheel well doors, the gear cannot simply move in any order — doors must open before the gear extends, and close after the gear retracts. Sequence valves enforce this choreography hydraulically. A sequence valve is pressure-operated: it remains closed until upstream pressure reaches a set threshold (indicating the previous actuator has completed its stroke), then opens to allow flow to the next actuator in the sequence. This ensures, for example, that the main gear doors are fully open before the main gear struts begin to extend. Priority valves serve a related function on aircraft where multiple hydraulic systems compete for fluid: they guarantee that safety-critical systems (like gear extension) receive pressure before lower-priority systems.
Safety and Indicating Systems
No hydraulic gear system is complete without robust cockpit feedback. Three categories of safety devices are standard:
- Position indicators: Most aircraft use gear position lights — typically green for down-and-locked, red or amber for gear in transit or unsafe, and no light (or a dark lens) for gear up-and-locked. Some aircraft use a mechanical indicator (a "gear leg" button that protrudes through the wing or floor). The position switch itself is usually a microswitch actuated by the gear or lock linkage.
- Gear warning horn: A horn or aural alert sounds when throttle power is reduced below a threshold (simulating approach) and the gear is not in the down-and-locked position. On aircraft certificated under 14 CFR 23.729 or 25.729, this warning is a certification requirement tied to airspeed, power, and flap configuration, and it is one of the most important safety backups in all of aviation.
- Ground safety (squat) switch: A weight-on-wheels switch on the main gear oleo strut prevents the gear from being retracted while the aircraft is on the ground. Without this protection, a pilot who inadvertently moved the gear handle on the ground could retract the gear under a loaded aircraft. The squat switch is a primary focus of functional testing after any landing gear maintenance.
Emergency Extension Systems
Most retractable-gear aircraft certificated under 14 CFR Part 23 or Part 25 have a means of extending the landing gear independent of the normal hydraulic system, consistent with the certification requirements of 14 CFR 23.729 and 25.729. Common emergency extension methods include:
- Manual hydraulic handpump: The pilot pumps a cockpit-mounted handle to generate hydraulic pressure from the same reservoir, bypassing the engine-driven pump. This is common on light twins and some singles.
- Nitrogen (pneumatic) blow-down system: A high-pressure nitrogen bottle is plumbed into the gear circuit. Pulling an emergency handle releases nitrogen to push the gear down. This system is one-shot and non-resettable in flight; the cylinder must be recharged on the ground.
- Free-fall (gravity) extension: The up-lock is released (mechanically or hydraulically), and the gear falls and locks down under its own weight and aerodynamic loads. This is the simplest and most common method on light singles.
The design and certification of emergency extension systems is governed by the applicable Part 23 or Part 25 airworthiness standards, and ongoing operability is verified through the manufacturer's maintenance and inspection program under 14 CFR Part 43.
Key Numbers and Rules
- System pressure typically ranges from 1,000–3,000 psi on light aircraft; transport aircraft often operate at 3,000–5,000 psi.
- MIL-PRF-5606 and MIL-PRF-83282 are petroleum-based (mineral oil) and not compatible with Skydrol seals or reservoirs — mixing fluid types is a critical maintenance error.
- Skydrol is a phosphate-ester fluid requiring synthetic seals; it is corrosive to paint and irritating to skin and eyes — PPE is mandatory during servicing.
- The squat switch must be tested for correct operation after any landing gear removal, installation, or rigging per the aircraft maintenance manual.
- Hydraulic fluid level should be checked with the gear in the normal retracted position on aircraft where actuator displacement affects reservoir level, unless the manufacturer specifies otherwise.
- Sequence valve timing is verified during gear swing (retraction and extension test with aircraft on jacks) — a full gear swing is required after any landing gear rigging change.
- Under 14 CFR Part 43 and the applicable maintenance manual, any repair, adjustment, or replacement of landing gear components requires a logbook entry and, where specified, a return-to-service sign-off by an appropriately certificated AMT or IA.
Common Test Traps
- Confusing fluid types and compatibility: Test questions frequently ask which fluid type is compatible with which seal material. Petroleum-based fluids use nitrile (Buna-N) seals; phosphate-ester fluids (Skydrol) require ethylene-propylene (EPR/EPDM) seals. Mixing fluid types destroys seals and contaminates the system.
- Gear warning horn vs. squat switch: The gear warning horn activates in flight when gear is up and power is reduced. The squat switch prevents retraction on the ground. These are separate systems with separate functions — do not conflate them.
- Sequence valve vs. priority valve: A sequence valve controls the order in which actuators operate (doors before gear). A priority valve controls which system gets pressure first when demand exceeds supply. Questions often swap these definitions.
- Emergency extension is a one-shot system: Nitrogen blow-down systems cannot be reset in flight. If a test question asks about resetting after an emergency extension, the answer involves ground servicing — not an in-flight procedure.
- Checking fluid level with gear up or down: Because actuator pistons displace fluid volume, the reservoir level changes depending on gear position. Always follow the manufacturer's specific guidance; assuming gear-down is always correct can lead to overfilling and system damage.