IT appears the crash of a United Airlines DC-10 last week was caused by a rare, perhaps unique, set of circumstances. It involves a rear-engine failure powerful enough to send debris spewing through the engine casing. At least some of that debris, safety experts suspect, went spinning into the airplane's tail section, severing at least one hydraulic system. The loss of hydraulics rendered the DC-10 practically unmaneuverable as the pilot tried to land last Wednesday at the Sioux City, Iowa, airport. The resulting crash killed at least 110 of the 296 people aboard.
This scenario was pieced together by independent safety experts from the evidence released so far. The official investigation by the National Transportation Safety Board (NTSB) is not yet complete.
Here is what is known about Wednesday's crash:
About an hour after United Flight 232 lifted off from Denver on its way to Chicago, at least part of its tail engine blew apart with tremendous force. It knocked flight attendants to the floor and, apparently, blew off the engine's entire fan-rotor system. The pilot radioed that he had lost hydraulic control. An off-duty flight training instructor, who came up to the cockpit to help out, dropped to his knees to try to control the plane's throttle. After some 45 minutes of circling and desperate attempts to maneuver the airplane using the thrust of two remaining engines, one on each wing, the plane crashed just short of a runway in Sioux City.
Engine failure in itself is not rare, says John Gallipault, director of the Aviation Safety Institute, a private safety group. But a failure of this magnitude, ripping off the fan-rotor system, may be unprecedented.
``It's the first time for this type of engine that we've ever seen,'' according to NTSB spokesman Jim Burnett.
The General Electric CF6-6 engines, which powered the DC-10, have an industry record of one engine-caused inflight shutdown for every 30,000 flight hours. ``Those are pretty admirable figures,'' says company spokesman Dave Lane.
A catastrophic engine failure is ``very, very rare - to this destructive extent, certainly,'' says John Enders, president of Flight Safety Foundation, an independent safety organization. If only the fan blades had come off, for example, the engine casing probably would have contained them. But the force of the spinning disk and rotor is so great that it is virtually impossible to contain them within an airplane engine, he adds.
THE design philosophy has been to allow those parts to fly out of the engine but to armor the airplane sufficiently to protect sensitive areas. This time the armoring apparently was not enough. The NTSB has confirmed that at least one of the plane's three hydraulic systems was severed, leading to a loss of hydraulic pressure.
Some independent safety experts complain the DC-10 crash is another example of putting too much emphasis on fuel efficiency and not enough on safety.
When a few years ago the wing engine of a British airliner exploded, puncturing the fuel tank and setting the passenger cabin ablaze, the Federal Aviation Administration did start requiring much heavier metal to armor the wing tank, says Wayne Williams, founder of the independent National Transportation Safety Association. ``But it shouldn't have taken that event ... to prove we need heavier metal to protect the wing tank.''