Landing Gear Retraction Failure on a Light Twin
A light twin with hydraulically retracted landing gear is climbing after takeoff when the gear-up selection produces no green light extinguishing and no hydraulic pressure rise on the gear system gauge. The system uses an engine-driven pump on the left engine and an electric standby pump. The left engine oil pressure is normal, but the left engine-driven hydraulic pump output is zero. The electric standby pump is selected on and produces 2,100 psi, within the normal 2,000–2,300 psi range, but the gear still does not retract. The gear selector valve is electrically controlled and hydraulically actuated. The aircraft has a maximum gear-extended speed of 140 KIAS and is currently at 120 KIAS. The crew has 25 minutes of fuel to destination and 45 minutes to the alternate.
1. Explain the sequence of components that must function for gear retraction and identify the most likely failed element given the evidence.
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Retraction requires the selector valve to move to the up position, hydraulic pressure to reach the actuating cylinders, and the mechanical uplocks to release. The standby pump producing 2,100 psi shows the hydraulic supply is adequate and the reservoir is not empty. Because the gear still does not move with normal pressure, the fault is downstream of the pump: either the selector valve is not being commanded electrically, the valve is stuck, or the uplock release mechanism is jammed. The absence of a pressure rise on the gear gauge when the gear was first selected suggests the selector valve did not shift, because a shifted valve would direct pressure to the actuators and produce a transient pressure change. The crew should check the gear selector circuit breaker and the valve control circuit, since an electrical command failure is the most probable cause when hydraulic pressure is otherwise normal.
2. Recommend a course of action and justify it against the fuel and speed constraints.
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With gear down, the aircraft is limited to 140 KIAS and will burn more fuel. The crew has 25 minutes to destination and 45 to the alternate, so fuel is the binding constraint. The crew should not attempt repeated cycling of the gear, which wastes time and may worsen a jammed valve. Instead, they should reduce speed to the gear-extended limit, verify the gear is down and locked by the green lights and visual confirmation if possible, and plan to land at the nearest suitable airport rather than the original destination. The decision should account for the increased fuel burn with gear down, which may make the alternate unreachable. The conservative judgement is to divert to the nearest airport with adequate runway and emergency services, because a gear that will not retract may also have a compromised downlock, and landing sooner reduces exposure.