What happened
A Boeing 717 experienced an in-flight thrust reverser deployment shortly after takeoff. The aircraft returned to the airport without further incident.
Prior to this flight, the thrust reverser had been left unlocked, though it was not deployed. During preflight inspections, the flightcrew observed a thrust reverser unlock light. Because the crew was unable to clear the indication, contract maintenance was contacted to perform a lockout of the reverser. The maintenance contractor, who was unfamiliar with the specific task, requested that the procedure be sent via fax. However, the contractor utilized an incomplete set of procedures to complete the lockout process.
The investigation
Post-incident inspections determined that the thrust reverser doors had over-deployed. Investigators found that four locking pins and their fork-shaped locking triggers exhibited nicks and gouges on their mating surfaces.
Boeing's Service Bulletin 717-78-004 noted that operators had reported five instances of thrust reversers failing to deploy and ten instances of side beam gouging. Inspections revealed that this gouging occurred between the overcenter link bolts and the side beam assembly. In one documented case, the gouging was deep enough to require a doubler to restore airworthiness.
Findings
Following the event, Boeing revised its dispatch guide. Maintenance is now required to verify that no more than one thrust reverser proximity sensor indicates an open position, and that the thrust reverser unlock indication is absent and red lockout pins are present on any inoperative reverser.
Additionally, the overcenter links were redesigned. Boeing and the thrust reverser manufacturer issued service bulletins to retrofit existing engines with a new link design, associated hardware, and rub plates. Aircraft currently in production incorporate these changes as a standard production modification. The modifications, including Rohr Incorporated Service Bulletin R715.78-008, aim to minimize side beam gouging and the possibility of inadvertent in-flight thrust reverser deployment.