Investigators probing the recent string of air crashes in this country and abroad have stressed that no two are alike. The point remains technically valid. But the two most recent incidents in what has become the worst year in history for commercial-aviation fatalities appear to be engine-related.
Following the crash of a Midwest Express Airlines DC-9 shortly after takeoff from Milwaukee's Gen. Billy Mitchell Field, National Transportation Safety Board (NTSB) officials said they found parts of a compressor blade and other engine pieces along the plane's short flight path. All 31 people aboard Atlanta-bound Flight 105 were killed.
While cautioning that the engine parts could prove to be from another aircraft, NTSB chairman James Burnett has said that apparently the jet's right engine stopped running just before impact. Both of the Pratt & Whitney Group's JT8D-7 engines aboard the craft's tail have been recovered and will be analyzed.
The JT8D engine is the most widely used in commercial service, powering not only the McDonnell Douglas DC-9 but the Boeing 727 and 737.
It was a newer, class 15 version of that engine with a particular variation of the combustion chamber that exploded and caught fire Aug. 22 in Manchester, England. A crack in the chamber is suspected as the cause.
The Civil Aviation Authority in Britain ordered an urgent check of such engines; using x-ray devices, inspectors found a number of combustion-chamber cracks, which led to the withdrawal of 22 engines from service.
But Federal Aviation Administration (FAA) chief Donald Engen says no such cracks have appeared in aircraft used in this country. This is presumably because US engines are checked at more frequent intervals and are subject to less constant stress in the course of generally longer flights.
Indeed, the FAA's late-August demand that all US airlines inspect the combustion chamber of class 15 engines of the kind involved in the Manchester crash is proving to be far less sweeping than it originally sounded.
Pratt & Whitney spokesman David Long says there are only about 900 such engines in use.
A large proportion of them are aboard planes owned by major US airlines that are exempt from the FAA order. The exemptions stem from the fact that these airlines already monitor such engines on a continuous in-flight basis under an FAA-approved program.
``It's a system that uses computer analysis to predict engine difficulties and it's been very successful,'' says FAA spokesman Dennis Feldman.
To know how many US planes will actually be inspected under the special order, Mr. Feldman says he would have to poll all nine of the FAA's regional offices. ``We don't have a central repository for that kind of information,'' he says.
Still, it is expected to be some time before the FAA gets the final word on results of its special engine check.
The FAA asked that the combustion chamber of the engine model in question be checked within 7,500 flying hours of the last inspection. If the engine is new, it can go for 9,000 hours before a check, says the FAA. For some planes that time span could amount to four to five years of flying, says Pratt & Whitney's Long.
FAA maintenance requirements vary according to airline and aircraft. Often, the FAA approves a manufacturer's suggestions for care. If airlines want to vary these guidelines, they need FAA approval.
``Often as they gain experience, they come back to us to stretch out maintenance procedures -- sometimes we agree and sometimes we don't,'' says Feldman.
Is the recent FAA airworthiness directive really all that different from normal maintenance requirements?
``It won't do that much more than what the airlines are required to do right now -- they generally check their engines between 4,000-7,000 hours of flying time,'' says Christopher Witkowski, who heads Ralph Nader's Aviation Consumer Action Project. ``It's a little less rigorous than the average industry standard.''