NTSB Factual Report

HISTORY OF FLIGHT

On May 21, 2008, at 1745 hours eastern standard time (EST), an Alitalia Boeing MD-11F, 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. 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. The National Transportation Safety Board (NTSB) investigated this incident after learning that a similar CF6-80C2 transcowl liberation event had occurred for which there was inadequate data to determine cause. Two additional transcowl separation events occurred while the investigation was ongoing, and the hardware from these events were examined as part of the NTSB investigation.

DAMAGE TO AIRPLANE

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 (GE) 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. Review of manufacturing and airplane service records for the incident thrust reverser found that it 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 that is driven by engine bleed air. The motor drives the fixed-length CDU ballscrew actuator, which drives (adjustable or fixed-length) upper and lower ballscrew actuators through flexible drive cables. 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.

When the thrust reverser design was introduced on the GE CF6-6 engine, the upper and lower ballscrew actuators were of the adjustable-length type, with 3/8-inch diameter (3/8-inch) rod end part number (P/N) KBE6-59. Since that time, additional 3/8-inch rod end P/Ns have been approved for use, and the thrust reverser design was certified for use on the GE CF6-50 and GE CF6-80C2 engine models, where it is subject to greater thrust loads. A fixed-length ballscrew actuator with a 7/16-inch diameter (7/16-inch) rod end was introduced into production in 1994 and an adjustable-length ballscrew actuator with a 7/16-inch rod end was introduced to the field in 1996. Field upgrade to the 7/16-inch adjustable-length ballscrew requires a shop visit for modification of the actuator body, and the update is not widely incorporated.

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, P/N 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 of the hardware fracture surfaces at the NTSB materials lab found that the rod end had fractured in fatigue and that the rivets had failed in overload.

Review of the C6-80C2 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 was performed to determine the capability of the various rod-ends qualified for use on CF6-80C2 series thrust reverser adjustable-length ballscrew actuators. The analysis found that 4 of the 3/8-inch rod end P/Ns, 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 a much greater fatigue capability than other 3/8-inch rod ends, due to the method in which the threads are formed. 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 (SB) A781162 on December 20, 2008, which provides 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 instructions 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 (AD) 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.

NTSB Probable Cause

The insufficient predicted fatigue life of the P/N KBE6-59 adjustable-length 3/8-inch rod end, and the inadequate fatigue margin at the transcowl CDU clevis assembly bracket attachment rivets for the one actuator out condition, which resulted in the failure of a second transcowl attachment during thrust reverser activation, leading to the partial liberation of the thrust reverser’s inboard transcowl.