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US investigators probing UPS plane's maintenance history

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UPS crash similar to 1979 American Airlines DC-10 crash

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FAA ordered MD-11 and DC-10 jets temporarily grounded

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By Allison Lampert and David Shepardson

Nov 20 (Reuters) - The National Transportation Safety

Board said on Thursday it has found evidence of fatigue cracks

in a key part of a UPS MD-11 cargo jet that crashed this

month in Kentucky, an important clue into the cause of the

incident that killed 14.

Air safety experts said the discovery of fatigue cracks in a

support structure on the left pylon, which connects to the wing

and the plane's engine, is a significant finding in the

investigation into the November 4 crash in Louisville.

Besides evidence of fatigue cracks, which preceded the

accident in the left pylon aft mount lug, the NTSB also said in

its preliminary report that it found areas of overstress

failure.

As a result, one side of the pylon's supports failed due to

fatigue, and when it broke, the other side could not handle the

extra load.

"That's a major clue," said U.S. air safety expert Anthony

Brickhouse of the cracks, which he added developed over multiple

flights.

CRACKS WEAKEN SUPPORT STRUCTURE: EXPERT

U.S. safety expert John Cox said such fatigue cracks weaken

the support structure.

"It finally gets to a point where the force overcomes what

the structure can withstand and that's a point of failure," Cox

said. "There is a significant likelihood that that's what

occurred here."

Brickhouse said investigators need to figure out why the

crack was not discovered as part of the plane's maintenance, a

search that could go back for years.

U.S. safety investigators have already said they are probing the

maintenance history of the 34-year-old UPS cargo plane that was

in Texas for repairs weeks before crashing in flames after

takeoff. That maintenance was done at a site belonging to

Singapore's ST Engineering.

According to the report, the plane had not flown enough

cycles to warrant special detailed inspections of the part.

The preliminary report referenced the crash of an American

Airlines DC-10 jet in May 1979, which it listed under "similar

events." During the takeoff rotation of Flight 191 from Chicago

O'Hare Airport, the left engine and pylon assembly, and about

three feet of the leading edge of the left wing, separated from

the airplane and fell to the runway.

The NTSB found evidence of fatigue cracking in that

incident.

UPS and FedEx ( FDX ) grounded their MD-11 fleets this month

as a precaution and at the recommendation of U.S. planemaker

Boeing ( BA ). The Federal Aviation Administration also issued

directives that temporarily grounded the MD-11 and the DC-10 due

to their similar design.

NTSB Chair Jennifer Homendy told Reuters this week that

Boeing ( BA ) is doing some additional modeling and testing as part of

the investigation. Boeing ( BA ) has owned the broader MD-11 program

since its 1997 merger with McDonnell Douglas.

Preliminary reports are normally published around 30 days

after an incident, with a more comprehensive final report

unveiled about a year after the crash.