When McDonell Douglas was seeking federal approval to build the DC10 jumbo jet, the manufacturer anitcipated almost precisely the sequence that led to the nation's worst air disaster but said the probablity of it occuring was less than one in a billion.
That is contained in a document released today with the opening of the National Transportation Safety Board's hearing into the May 25 DC10 crash at O'Hare Internatonal Airport here that claimed 273 lives.
Documents and testimony from first day witnesses also show that:
Safety board investigators have found that shims -- small metal strips used to fill gaps -- "appeared to be assembled in the wrong position" on the key part of the DC10 engine support pylon that broke and led to the crash. The break orginated with a 10-inch crack in the pylon bulkhead. That crack was induced by a maintenance technique, engineers believe. The crack was probably lengthened by the presence of the shims, a safety board metallurgist testified.
The DC10 has a history of difficulties that fill more than 200 pages of the safety board document. Many concern the hydraulic systems and include such problems as failed pumps and leaky lines. The hydraulic systems power the DC10s major controls.
An American Airlines mechanic who works on DC10 engine support pylons testified that on one occasion -- not necessarily on the crashed airplane -- he had to move the forklift that was supporting the nine-ton engine and pylon assemble. Such movements are suspected by engineers of having cause the 10-inch crack.
American Airlines Flight 191 from Chicago to Los Angeles lifted off the O'Hare runway May 25 just as its left engine and support pylon fell off the left wing.
In the process, hydraulic lines were served and electrical systems were shorted out. The plane climbed above 500 feet off the ground. The slats, metal plates extended on the front of the wings give added lift during takeoff, retracted on the left wing because of the loss of hydraulic fluid but stayed out on the right wing.
A June 1970 document released today shows that McDonnell Douglas anticipated the possibility of unplanned slat retracton on one wing. "The effect of such a failure is aircraft roll and yaw after takeoff," according to the document.
If a second problem such as an engine failure were combined with the slat retraction, according to the McDonnell Douglas safety analysts, there would be an increase "in the amount of yaw but it would be critical only under the most adverse flight or takeoff conditions. The probablity of both failures occuring is less than" one in a billion. One in a billion is the generally accepted mathematical equivalent of "extremely improbable," the words used in federal regulations setting aircraft safety standards.
In a letter to the Federal Aviation Administration last June 29, McDonnell Douglas wrote that the history of the DC10 now in service for eight years, showed that the probablity of the Chicago accident was one in 4.4 trillion based on the hours of flight the DC10 have now made, or one in three trillion based on the number of takeoffs DC10 have made.
The hearings will continue Tuesday.
American Airlines mechanic William A. Robinson testified he worked on DC10s at Tulsa, including the crashed airplane. He said he never saw written instructions on how to use the forklift to remove the engine and pylon together and that all of his training on that subject was on the job.
The FAA has outlawed the forklift procedure since the crash.
Robinson said he had to move the forklift once while working on a DC10 and that on that occasion a nick was noticed in the top of the pylon. He said he could not say what caused the nick or if that was the DC10 that crashed.