UPS Crash Hearing Scrutinizes MD-11 Design Component

Day two of the National Transportation Safety Board’s hearing on the fatal crash of UPS Flight 2976 on November 4 in Louisville, Kentucky, took a deeper dive into design and structural testing of the MD-11’s wing pylon and components.

According to Boeing, during the design of the aircraft the emphasis was on the pylon maintaining structural integrity in the event of a gear-up or off-airport landing and to prevent fuel spillage. Boeing officials noted there was no structural requirement to ensure pylon separation did not occur, as the scenario of the pylon completely separating from the wing was not considered.

Much of Wednesday’s testimony in Washington, D.C., focused on the lugs designed to keep the pylon attached to the airframe.

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Boeing described the lugs as “fail-safe” with the definition being that, if one of them failed, the other would be able to support the load. However, in this case both the forward and aft lug failed simultaneously.

That definition struck a chord of interest with NTSB member John DeLeeuw, a former airline pilot and safety executive with more than 30 years experience.

“It took an accident for us to start to talk and ask these questions,” said DeLeeuw, referring to the definition of fail-safe and adding that there is a need for more data and detail to quantify it.

Several times during the two days of testimony it was noted that the spherical bearing—a critical part on the engine pylon attachment—was not considered a principal structural element (PSE) and as such it was not required to undergo frequent inspection.

According to Boeing, the bearing acts as an interface with the lugs that attach to the pylon box, allowing it to move slightly during flight in reaction to the loads placed on it—for example, during takeoff.

The post-crash inspection of the UPS MD-11 fleet found cracks in the lugs of three aircraft. The NTSB testimony noted the failure of the bearings affects the integrity of the lugs. Therefore, it was argued the spherical bearings should be considered a PSE and subject to more inspections.

Subsequent testimony revealed that neither the FAA nor Boeing was aware of the potential bearing issue when the MD-11 was certified in 1990. At the time, the inspection interval for the engine pylon lugs was 2,900 flight cycles, or takeoffs and landings. Since the accident, the inspection interval has been changed to 450 flight cycles.

However, in 2008 and 2011 service letters were issued to the operators of the MD-11, warning of potential issues with spherical bearings developing cracks or migrating. 

The operation of the engine fire warning system and fire handle shut-off also drew attention, as the cockpit voice recorder (CVR) did not pick up a fire warning during the takeoff roll. This system is supposed to activate when a fire is detected, sending a warning message to the cockpit.

Video of the accident after takeoff from Louisville Muhammad Ali International Airport (KSDF) clearly shows fire on the left engine before it departs the airframe. 

A Boeing representative noted that the system does not activate during the first 400 feet or so of flight—basically during the takeoff—because at that time the pilots are already task saturated and it could prove distracting.

NTSB Chairman Jennifer Homendy ended Wednesday’s hearing by thanking the members of the agency who responded to the crash.

“The accident happened during one of the longest government shutdowns on record,” Homendy said. “[NTSB employees went to the scene] documenting and collecting perishable evidence, working without pay, and working nights and weekends.”

Homendy also addressed the victims and their families, saying the NTSB is still in the fact-finding phase and noting the hearing has spurred more questions and the need for more information.

“It takes time to get to the right answer,” she said “…to get to the right solution and get the right recommendations.”

The agency now enters the analysis phase, which will lead to the probable cause and safety recommendations. The release of the final report is likely several months to a year away.