A top McDonnell Douglas engineer testified today that, in effect, critical flight control systems in the DC10 were not designed to survive the damage that occurred in the Chicago crash May 25, but no changes are needed in the systems.
"At this time we are not convinced that charges to the slats, electrical and hydraulic systems are justified," engineer Charles Dundore said.
All of those systems suffered partial failures just before the crash, which killed 273 people. Hydraulic fluid lines and electrical cables were shreded when an engine and its support pylon fell off the left wing of the American Airlines DC10 just as it was lifting off the runway.
"We did not design for the loss of a pylon," Dundore said. "That is an unacceptable failure mode."
With all the hydraulic fluid drained from the left wing controls, the slats on that wing retracted unexpectedly.
Slats are hugh metal plates that are extended from the front of a wing -- during takeoff to give a plane extra lift.
Other jumbo jets, including the Boeing 747 and Lockheed £1011, have locks that hold the slats out in the event of a similar failure. Those locks prevent an unbalanced aerodynamic condition in which one wing has more lift than the other.
The loss of electrical power turned off cockpit instrument that might have given the crew some warning as to the true condition of the airplane, including knowledge of the slat situation.
If McDonnell Douglas were to build new backup systems for the slats and the hydraulics, Dundore said, new complications would result. "We don't really feel these complications are justified as a preventable measure for what we consider an unacceptable failure mode."
Investigators have determined that a crack in the pylon of the crashed airplane was caused during maintenance by American Airlines. American Airlines charged that a shim installed by the Manufacturer in the pylon on the crashed plane made it possible for the crack to grow to a critical length.
McDonnell Douglas responded today with a highly technical presentation claiming that a shim would have made the pylon stronger, not weaker, and that the American Airlines maintenance procedure could have generated forces sufficient to produce the crack.
Under federal air regultions, when McDonnell Douglas designed the DC 10 it had either to include a method that would prevent the unbalanced slat condition from occuring or to demonstrate that the airplane would be controllable if the condition did occure. Boeing and Lockhead both chose prevention.
Since the accident, test pilots "flying" the McDonnell Douglas simulator at Long Beach have proven to the Federal Aviation Administration's statisfaction that the slat problem as presented to the pilot in Chicago could have been controllable, if the pilot had known he had the problem.
Two possible warnings would have been available to the crew if the electrical system had remained intact. One is a light on the panel between the pilot and co-pilot that, when illuminated, reads "slat disagree." Power to the light was disrupted when the pylon left the plane.
The flight engineer, who sits behind the pilot and co-pilot, could have restored power to the "slat disagree" light by moving his chair and reaching a switch above his head. According to pilots and American Airlines officials, it is unlikely the flight engineer had time to do that in the 31 seconds flight 191 was in the air.
The second warning is a "stick shaker," a device that literally shakes the pilot's control column to tell him the plane is in danger of stalling -- the technical problem the crew faced when the slats became unabalanced.
The motor that powered the stick shaker lost its electrically when the pylon fell and power could have been restored only if the crew restored to an unusual procedure that required reference to a manual, according to testimony. There was no time for that.
The FAA will require in a future regulation that both pilot and co-pilot have stick shakers and that both stick shakers will sense slat conditions on both wings. Currently, only the pilot has a stick shaker on a U.S.-operated DC10, although the slats on both wings are tested independently.
Dundore also said that McDonnell Douglas had chosen to use three hydraulic systems on the DC10 instead of four because a fourth system "would be additional piping and pumping and would be less simple. . ." Both the 747 and the £1011 have four systems.
Dunmore also said that the DC 10 has had "a series of problems in service" with its hydraulic systems and that detailed changes have been made.