History of Flight
On May 21, 2008, at 1745 hours eastern standard time, an Alitalia Boeing MD-11F, registration EI-UPO, experienced a partial thrust reverser translating cowling (transcowl) separation during landing rollout at Miami International Airport, Miami, Florida. According to the flight crew and digital flight data recorder information, the engine and alert display (EAD) position 3 'engine reverser unlock' message light remained on following thrust reverser activation. No loss of thrust control or airplane control was reported. There were no injuries.
Damage to Airplane
A post-event airplane inspection found that two of the three transcowl attachments had failed and that approximately 20 square feet of transcowl outer bondment material had liberated from the inboard transcowl of the No. 3 engine thrust reverser. The thrust reverser fixed structure was properly latched. Liberated thrust reverser material was found on the runway. There was minor damage to the airplane right wing's flap track cover.
Aircraft Information
The airplane was a Boeing MD-11 cargo airplane, powered by three General Electric CF6-80C2 turbofan engines. The CF6-80C2 thrust reverser was manufactured by Middle River Aircraft Systems (MRAS), Baltimore, Maryland, and the thrust reverser actuating system vendor was Honeywell, Phoenix, Arizona. The incident thrust reverser was manufactured on April 22, 1991, and was installed on the event airplane in December 2004. At the time of the incident, the thrust reverser had accumulated 51,078 hours since new, 23,104 hours since last inspection, and had never been overhauled.
The CF6-80C2 thrust reverser consists of 2 transcowl halves controlled by a thrust reverser actuating system (TRAS) that drives the transcowls to the deploy or stow position. Each transcowl is attached to the engine with 3 (upper, center, and lower) ballscrew actuators. The center ballscrew actuator for each transcowl is integral to a center drive unit (CDU) that incorporates a pneumatic motor driven by engine bleed air. Rod ends connect the ballscrew actuators to the transcowl at clevis assemblies that are riveted to longeron structure, which is bolted to the transcowl bondment.
Hardware Evaluation
Inspection of the incident thrust reverser found that the inboard transcowl's lower adjustable-length ballscrew actuator's 3/8-inch rod end, part number KBE6-59, was separated in the threaded area and that the rivets that attach the CDU ballscrew actuator clevis to the transcowl were sheared. The upper ballscrew actuator rod end remained intact. Metallurgical examination at the NTSB materials lab found that the rod end had fractured in fatigue and that the rivets had failed in overload.
Review of CF6-80C2 transcowl liberation incidents found that, in every case, a latent one actuator out (OAO) condition of a single transcowl attachment failure led to the failure of a second transcowl attachment during thrust reverser deployment, resulting in transcowl over-deployment, guide disengagement, and transcowl liberation.
MRAS and Honeywell performed a design validation of the TRAS and found that the fatigue capability of the CDU clevis assembly rivet fastener for the OAO condition under normal deploy loads was inadequate at minimum material properties. Analysis of an alternate Hi-Lok fastener determined that it provided adequate margin for all conditions. Fatigue analysis with torsional loading at the end of the deploy and stow strokes found that 4 of the 3/8-inch rod end part numbers, including P/N KBE6-59, lacked sufficient predicted fatigue life. One 3/8-inch rod end, P/N M81935/1-6, was shown to have much greater fatigue capability due to the thread forming method. The life capability of the 7/16-inch rod end exceeded the manufacturer-recommended component shop visit interval.
Additional Information
Maintenance of CF6-80C2 thrust reversers is on condition. Rod end time in service is in most cases unknown, although tracking rod end life can be accomplished by association with the ballscrew actuator.
As a result of the investigation findings, GE/MRAS published alert service bulletin A781162 on December 20, 2008, providing instructions for inspecting adjustable-length actuator 3/8-inch rod ends and torsion arms, clevis fasteners, clevis pins, and clevis pin retaining clips, and for replacing broken rod ends, damaged torsion arms, loose clevis fasteners, clevis pins that do not meet wear limits, and loose clip retainer nuts and bolts. It also provides instructions for replacing all 3/8-inch rod ends with P/N M81935/1-6 rod ends, and for replacing adjustable-length end actuators with 3/8-inch rod ends with adjustable-length actuators with 7/16-inch rod ends, or with 7/16-inch fixed-length actuators.
On October 27, 2009, the Federal Aviation Administration published Airworthiness Directive 2009-21-07, requiring across-the-fleet visual inspection of the hardware attaching the transcowls to the engine and hardware replacement as required, and the replacement of all 3/8-inch diameter rod ends installed on the thrust reverser adjustable-length end actuators, and repetitive replacements of the 3/8-inch rod ends with the optional terminating action of replacement with a 7/16-inch adjustable-length rod end or with a fixed-end actuator.