An A320 aircraft equipped with IAE V2500 engines was prepared for dispatch under MEL with its ENG#2 thrust reverser deactivated following a thrust reverser fault logged during the previous flight.

At the gate, prior to departure, the ENG 2 REVERSE UNLOCKED ECAM caution appeared. The flight crew started the engines despite this alert and proceeded with the take-off.

The ENG 2 REVERSE UNLOCKED ECAM caution reappeared early in the climb. Moments later, due to the aerodynamic load, the translating sleeve started to move from its stowed position. When the position of the sleeve exceeded the threshold of the FADEC automatic IDLE function, the ENG#2 thrust automatically reduced to IDLE. The flight crew felt the aircraft vibrating, shut down the ENG#2 and then performed an in-flight turn back, landing the aircraft safely. 

Subsequent investigations revealed that the ENG#2 thrust reverser sleeve had moved, during flight, from its stowed position. The automatic IDLE function combined with the fly by wire technology, prevented the aircraft from any sudden lateral upset. It was also identified that the lockout pin was not installed correctly during the deactivation procedure.

Similar incidents have been reported to Airbus. During a review of these events, there are common factors identified that provide useful lessons learnt. These include a reminder of ECAM caution management (or local warning on A300) by the flight crew and correct application of the MEL maintenance deactivation task.

If the flight crew observes the ENG X REVERSE UNLOCKED ECAM caution before the start of the taxi-out phase (A300-600, A310, A320, A330, A340, A350, and A380) or REV UNLK local warning light (A300, A300-600, A310), they must refer to the MEL and ensure that the required maintenance actions are carried out.

If the alert triggers after the start of the taxi-out phase, the ECAM must be followed: it requires the flight crew to set the affected engine at idle and to shut the engine down. Consequently, the aircraft must return to the gate to perform maintenance actions.

(fig.1) Example of A320 ECAM action on ground

Several safety measures are described in the AMM/MP Thrust Reverser Deactivation task. These include:

1. Removing the hydraulic pressure or disconnecting electrical power from the actuator (depending on engine model)
2. Mechanically locking the thrust reverser by lockout pin installation 
3. Clearly indicating that a thrust reverser is deactivated by the ENG REV INHIBITION ECAM caution or a Warning placard

It is crucial that the maintenance personnel follow all the steps of AMM thrust reverser deactivation procedure and check that the ENG X REVERSE UNLOCKED caution does not appear on the ECAM.

This will prevent from any uncommanded deployment in flight.

The events reported to Airbus occurred on A320 aircraft equipped with V2500 engines. An incorrect installation of the thrust reverser lockout pin is a common contributor for all the events where the thrust reverser deployed in flight, as it was the case for the event described in this article.

In normal operation, when the thrust reverser is serviceable, the lockout pin is stowed in the pin stowage bracket assembly located in the middle of each translating sleeve (fig.2).

In order to lock out the translating sleeve and to prevent the thrust reverser from moving, it is important to:

1. Fully retract the translating sleeve
2. Install the lockout pin in the lockout assembly located above the lockout pin stowage (fig.2). With the lockout pin correctly installed, the translating sleeve is locked to the fixed structure of the nacelle.

(fig.2) Location of the reverser locking devices

The outer end of the lockout pin will be visible from the surface of the sleeve whereas the collar of the pin will be below the surface of the sleeve (fig.3). 

There are 3 typical cases of incorrect installation:

1. Lockout pin partially engaged due to insufficient or incorrect threading
2. Misalignment of the translating sleeve with the C-duct
3. Lockout pin not engaged in the lockout assembly

In this case, which is illustrated in (fig.4), the lockout pin has engaged with the outer cascade ring hole, but only partially engaged with 1 of the 2 anchor bracket holes.

In this case, which is illustrated in (fig.5, 6a and 6b), the translating sleeve is misaligned with the C-Duct due to an incorrect rigging of the translating sleeve. To correct this condition, check all the internal bumper shims and compression struts for clearance (Ref. IAE AMM TASK 78-30-00-820-010 Para. 4.B).

If the translating sleeve and the C-duct are still misaligned, then it is necessary to check the rigging of the translating sleeve actuator (Ref. IAE AMM TASK 78-30-00-820-010 Para. 4.C).

In this case, which is illustrated in (fig.7), the translating sleeve was not fully retracted and the lockout pin has engaged with the anchor bracket but NOT with in the aft cascade ring of the C-Duct.

This incorrect locking can happen if the sleeve has translated more than 20 mm from its fully stowed position. Any partial translation of the thrust reverser sleeve creates a visible gap between the translating sleeve and the fan cowl on both the right and left hand sides. 

This gap will be clearly visible during preflight walkaround inspection (fig.8).