Learn what an aircraft hydraulic failure affects, how redundancy preserves control, what pilots do, and why landing speed and steering may change.
During an aircraft hydraulic failure, one hydraulic system loses pressure or fluid, reducing or removing power from the actuators it supplies. Depending on the aircraft, this can affect flight controls, landing gear, flaps, spoilers, wheel brakes or nose-wheel steering. Redundant systems and mechanical, electrical or pneumatic backups often preserve essential control.
In our Aviation & Real-World Flying coverage, the crucial distinction is between losing one hydraulic source and losing all hydraulic capability. Many light aircraft have cable-operated primary controls, so a hydraulic problem may affect only brakes or retractable landing gear. Transport aircraft normally divide essential equipment between independent systems. Our guide to hydraulic pressure, actuators and system redundancy explains that underlying architecture.
Which systems are affected by a hydraulic failure?
Only equipment powered by the failed circuit is directly affected, although the resulting configuration can change the aircraft's handling and performance.
- Flight controls: Some ailerons, elevators, rudders or spoiler panels may become inactive. Other actuators usually retain control after a single-system failure, but control authority, response or protection may be reduced. The consequences differ sharply between the architectures covered in our comparison of cable, hydraulic and fly-by-wire controls.
- Flaps, slats and spoilers: Extension may be slower, limited to certain positions or unavailable. Losing ground spoilers also increases landing distance because less weight is transferred onto the wheels.
- Landing gear: Normal extension or retraction may fail. Gravity, free-fall or another alternate extension method is often provided, though gear doors may remain open and the gear may not retract again.
- Brakes and steering: Normal brakes, anti-skid or nose-wheel steering may be lost. Alternate brakes, an accumulator or a separate emergency-brake source may remain available.
- Other equipment: Depending on type, hydraulics can power thrust reversers, cargo doors, propeller controls or other secondary systems.
A single hydraulic failure can therefore feel almost normal in cruise yet become much more significant when the crew selects flaps, lowers the gear or applies the brakes.
How do pilots recognise hydraulic failure?
Pilots usually receive a low-pressure, low-quantity, pump-fault or overheat indication before they notice a major handling change.
A failed engine-driven pump often produces low pressure while reservoir quantity remains broadly normal. A ruptured pipe, seal or actuator usually causes both pressure loss and falling fluid quantity. An accumulator may temporarily maintain pressure, so the indication can worsen only when another hydraulic service is operated.
Typical causes include pump or drive failure, fluid leakage, reservoir pressurisation problems, overheating and contamination. Sensor faults are also possible, which is why crews cross-check pressure, quantity, temperature and the status of individual pumps rather than diagnosing the failure from one warning light.
Can hydraulic pressure be restored?
Pressure can often be restored after a pump or power-source failure, but usually not after most of the system's fluid has escaped.
| Observed pattern | Likely problem | Effect of a backup source |
|---|---|---|
| Low pressure with usable fluid quantity | Pump, drive or power-supply failure | An electric, standby or power-transfer source may restore pressure. |
| Low pressure with quantity falling | Leak in the reservoir, line or component | Running another pump may accelerate fluid loss; the leaking circuit normally has to be isolated. |
| High temperature or intermittent pressure | Overworked pump, low fluid, aeration or a developing fault | The checklist may require a pump to be switched off rather than adding more pressure. |
Check valves, shut-off valves and hydraulic fuses can contain some leaks, but they cannot repair the failed component. Reservoir quantity and system schematics determine what remains usable.
Can an aircraft still fly after hydraulic failure?
Most multi-system transport aircraft remain controllable after one hydraulic system fails, but that does not apply to every aircraft or to a multiple-system loss.
Independent pumps, reservoirs, pipes and actuators are arranged so that a single fault should not remove every primary flight control. The autopilot may disconnect, some control surfaces may be lost, and the crew may face higher workload or altered handling. Fly-by-wire aircraft can also revert to a reduced control law if the remaining actuator or sensor availability no longer supports normal protections.
A multiple-system or total hydraulic failure is far more serious. It can leave only limited control surfaces, manual reversion or another emergency mode, and in the worst case the aircraft may not remain controllable. Using differential engine thrust is not a normal substitute for working flight controls and does not guarantee a safe landing.
What do pilots do after a hydraulic failure?
The crew stabilises the aircraft, identifies what was lost and follows the aircraft-specific emergency or abnormal checklist.
- Maintain control: Confirm the effect on manual handling and autopilot operation before concentrating on system indications.
- Identify the failure: Compare pressure, quantity, temperature and pump status to distinguish a failed pump from a fluid leak.
- Apply the checklist: This may isolate a leaking component, switch pumps, activate a standby source or prohibit further operation of the failed system. Repeatedly resetting pumps without checklist authority can worsen a leak or overheat a pump.
- Plan the configuration: Determine how the gear and flaps will be extended, whether a go-around remains practical, and which flight controls, spoilers and brakes will be available.
- Recalculate the landing: A higher approach speed, reduced braking, unavailable thrust reversers or loss of steering can require a longer runway and more favourable wind conditions. Crews may also request emergency services and arrange towing after the aircraft stops.
What changes during a hydraulic-failure landing?
Landing may require alternate gear extension, a reduced-flap approach and more runway than normal.
Limited flap extension generally raises approach speed. Missing spoilers, anti-skid, normal brakes or thrust reversers can increase stopping distance, while loss of nose-wheel steering may make it difficult to leave the runway. Our explanation of alternate gear extension, braking and steering arrangements covers these backup systems in more detail.
Brake accumulators contain only a finite reserve. Pumping the brakes unnecessarily can consume that reserve, so crews use the braking method specified for the aircraft. A gravity-extended landing gear may also be impossible to retract, making a late go-around more demanding because of the extra drag.
Why does a backup pump sometimes fail to restore pressure in a simulator?
A backup pump can replace a failed pressure source, but it cannot pressurise a circuit that has lost its hydraulic fluid.
This distinction is modelled in detailed flight simulators and is a common source of confusion. On aircraft such as the A320, the power transfer unit transfers hydraulic power between systems without mixing their fluid; it cannot refill a leaking reservoir. Our A320 Yellow-system and PTU example shows how a single-system failure, electric pump and backup pressure interact.
A mistake we see constantly is switching on every available pump after a quantity-loss warning. That may be appropriate for a failed pump but wrong for a leak. Simulator implementations vary in depth, so the aircraft's own warning display and checklist should determine the response rather than a procedure copied from another model.