WASHINGTON - As part of the Federal
Aviation Administration's (FAA) efforts to reduce
engine-related incidents and improve reliability, the
agency today proposed to order tests for PW4000 engines
to prevent high-pressure compressor surges.
The proposed Airworthiness Directive (AD) is based on
reports of surge events during the critical takeoff and
climb phases of flight. A surge is a loss of engine power
that results from an imbalance in the flow of air through
the engine. Excessive clearances -- too much space --
between the engine blade tip and the outer casing of the
engine can cause unstable air flow and a subsequent
high-pressure compressor surge and stall. Aircraft
engines can restart and recover from a stall.
Work by the industry and FAA over the past seven years
has significantly reduced the surge frequency of PW4000
engines. The surge rate has decreased from a 1993 fourth
quarter high of about one surge per 8,000 flights to
about one surge per 32,000 flights during the first
quarter of 1998. The surge rate has since remained
"These tests should significantly reduce the
possibility of engine surges on the commercial
fleet," said FAA Administrator Jane F. Garvey.
"Knowing that pilots are well-trained and that the
PW4000 engine will automatically recover is not enough.
We want to eliminate the possibility of these engine
There are 546 engines in the U.S. fleet affected by this
AD out of a total of 2,200 engines worldwide. The total
estimated cost to U.S. industry is $8.7 million. The FAA
estimates that 192 engines will require annual on-wing
tests with an average cost of $2,000 per test, $384,000
per year. An estimated 60 engines will require annual
off-wing tests with an average cost of $12,000 per test,
$720,000 per year. The agency also estimates that 19
high-pressure compressor overhauls will be performed at a
cost of $400,000 each, $7.6 million. About 11 unscheduled
engine removals will cost $5,000 each, $55,000 total.
PW4000 engines are installed on certain models of Boeing
747, Boeing 767, McDonnell Douglas MD-11, and Airbus
Industrie A310 aircraft. Operators include Delta Air
Lines, Northwest Airlines, Trans World Airlines, United
Airlines and World Airways.