Reverser Unlocked Warning during Initial Climb
During initial climb at 3,000 ft in rain and 2 °C, the left engine REV UNLOCK amber message appears with a master caution. The left engine N1 is stable, there is no yaw or vibration, and the right engine is normal. The crew retards the left throttle to idle; the message stays on. The aircraft is 15 minutes from a maintenance base and 40 minutes from destination. The QRH directs reducing thrust to idle and landing at the nearest suitable airport. The first officer notes that the message appeared shortly after thrust was increased for climb, and that the reverser was stowed after the previous landing. Maintenance records show a recent reverser lock sensor replacement on the left engine. The crew must decide whether to divert to the maintenance base, continue, or return to departure, and what checks can be done in flight.
1. Explain the reverser locking architecture and why an unlocked indication is treated as a safety-of-flight issue rather than a simple status message.
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Thrust reversers normally have a primary mechanical lock and a secondary lock, plus an isolation valve that prevents hydraulic or pneumatic pressure from reaching the actuation system unless deployment is commanded. The locks are designed so that the reverser cannot deploy in flight even if a control valve leaks or a signal is spurious. An unlocked indication means the lock circuit is not confirming that at least one lock is engaged.
If the reverser were to deploy in flight, the affected engine would produce reverse thrust, causing severe yaw, possible loss of control, and very high structural loads on the nacelle and wing. The isolation valve is supposed to prevent that, but the crew cannot verify its position in flight. The QRH therefore treats the message as a land-as-soon-as-practical condition and requires reducing thrust to idle, which limits the consequences if deployment occurs.
The fact that the message appeared after a thrust increase could point to a false indication from the recently replaced lock sensor, but the conservative response is required until the system is proven safe on the ground.
2. How would you discriminate a false indication from an actual unlocked reverser in flight and on the ground?
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In flight, compare left and right engine parameters, look for yaw, vibration, or a change in reverser position indication, and note whether the message changes with thrust or bleed configuration. A false indication may be steady or intermittent and may correlate with a sensor or wiring fault. An actual unlock would more likely produce a position disagreement or abnormal vibration, but absence of those signs does not prove the reverser is locked.
On the ground, inspect the reverser sleeves and locks visually, check the lock sensor rigging and connector, and verify the isolation valve. Cycle the reverser with the aircraft parked and the reverser inhibited, following the maintenance manual. Check the recently replaced lock sensor for part number, rigging, and wiring chafe. A false indication may clear after rigging or a connector repair; an actual unlock may show a gap, a damaged lock, or a sleeve that moves unexpectedly. Do not dispatch until the system is tested and the message is resolved.
3. Decision: divert to maintenance base, continue to destination, or return?
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The maintenance base is the nearest suitable airport at 15 minutes, and it has the maintenance capability for the recently replaced lock sensor. The QRH requires landing at the nearest suitable airport, so the decision is to divert to the maintenance base with the left engine at idle. Continuing to destination for 40 minutes would extend exposure to a condition that could, if real, lead to a catastrophic asymmetric thrust event. Returning to departure is not necessary because the maintenance base is closer and better equipped.
During the diversion, minimize thrust changes, monitor the left engine for yaw or vibration, and be prepared to shut it down if the reverser actually deploys. After landing, do not reset and continue without a full ground test and inspection. If the message is proven false and the system tests satisfactory, the aircraft can be dispatched under the applicable MEL, but the event should be recorded and the sensor reliability reviewed.