Hydraulic Failure and Emergency Gear Extension
On a night approach in a light twin, the pilot selects gear down at 130 KIAS. The left main green light illuminates; the right main and nose gear remain unsafe. The hydraulic pressure gauge reads 350 psi against a normal 1,500 psi, and the fluid quantity indicator sits at the bottom of the dipstick range. The electric standby pump runs when selected but pressure does not rise. A thin film of red fluid is visible on the left engine nacelle and the inboard flap underside. Selecting 10 degrees of flap produces no movement; the flaps are hydraulically operated. The aircraft is at 2,500 ft, 8 NM final, 140 KIAS, night, VMC, over an airport with a 1,800 m asphalt runway. The aircraft carries a nitrogen-charged accumulator and a separate emergency extension handle. The pilot must decide how to configure and land the aircraft.
1. Diagnose the failure and explain why the standby pump cannot restore pressure or flap movement.
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The pattern is a hydraulic leak in a shared system. Pressure falls to 350 psi, quantity is at the bottom of the range, fluid is visible on the nacelle and flap underside, and the standby pump runs without building pressure. A pump can only pressurise fluid it has; with the reservoir near empty the pump cavitates and circulates air, so it runs but produces no useful pressure. Because gear and flaps share the system, the flaps failing to move is consistent rather than a second fault.
The single green light tells you the left main reached down-and-locked before pressure was lost. The other two legs did not complete their travel. The accumulator is a stored-energy source of limited volume - enough for one emergency extension, not for repeated selection. Treat the fluid film as a fire risk near the exhaust and avoid prolonged ground running after landing.
2. Describe the emergency extension procedure and the airspeed considerations behind it.
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Slow to at or below the manual's emergency extension speed before operating the handle - typically well below normal gear speed. High airspeed creates aerodynamic loads on the doors and legs that can prevent the gear locking down or damage the mechanism, and it also increases the chance that a partially extended leg is forced back up.
Use the emergency extension handle once, deliberately, and allow the system time to work; each recycle consumes accumulator volume you will not get back. If the manual specifies a yaw or pitch input to assist gravity extension, apply it smoothly. Then confirm the indications.
If only two greens are shown, accept that a partial-gear landing may be required and stop trying to fix it in the air. Climb or hold clear of terrain while the checklist is completed, advise ATC of the emergency, and ask for the longest runway available with the best emergency-services access.
3. What are the key considerations for the landing itself and the subsequent actions?
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For a partial-gear landing, plan to touch down on the secure main gear and hold the nose off as long as elevator authority allows, so the aircraft settles gently rather than dropping onto the unsafe leg. A firm, positive touchdown close to the stall reduces the sideload and the risk of the leg collapsing. Keep the aircraft straight with rudder; differential braking may be needed once the unsafe leg touches.
Brief the cabin, secure loose items, and consider fuel balance - with a suspected hydraulic fluid leak near the left engine, shut that engine down after landing and switch off fuel and electrics as soon as directional control is assured. Expect a fire risk from fluid on hot exhaust and request fire cover if available. After stopping, evacuate and account for everyone before inspecting. Report the failure and preserve the evidence for maintenance: fluid samples, reservoir contents and the pump condition matter for the investigation.