Cause(s) of Accident
The National Transportation Safety Board (NTSB) determined that the probable cause of this accident was the asymmetrical stall and the ensuing roll of the aircraft because of the uncommanded retraction of the left wing outboard leading edge slats and the loss of stall warning and slat disagreement indication systems resulting from maintenance-induced damage leading to the separation of the number 1 engine and pylon assembly at a critical point during takeoff. The separation resulted from damage by improper maintenance procedures which let to failure of the pylon structure.
Structural and Mechanical Factors
After a thorough examination of the pylon attachment points, fractures and deformations at the separation points in the forward bulkhead and thrust link were all characteristic of overload. Testimony indicated the forklift was not powered for a period of time because it ran out of fuel. Post accident forklift tests showed that under these conditions leakage would allow a drift down of 1 inch in 30 minutes. Movement of 0.4 inch or less would produce a 7 inch fracture at the flange.
The design and interrelationship of the essential systems as they were affected by the structural loss of the pylon contributed to this accident. Flight control, hydraulic, and electrical systems in the aircraft were all affected by the pylon separation. When the engine separated from the pylon hydraulic pressure and fluid were lost and not recoverable. The separation also severed the electrical wire bundles inside the pylon which included the main feeder circuits between the generator and the No 1 a.c. generator bus. The flight crew was unable to restore power to the aircraft.
The failure of engineering to ascertain the damage-inducing potential of a procedure which deviated from the manufacturer’s recommended procedure was another contributed factor. The procedure in question was the removal of the pylon attaching hardware and the positioning of the forklift. As a result, maintenance personnel altered the sequence of hardware removal.
Investigation Board Findings
The engine and pylon assembly separated either at or immediately after liftoff. The flight crew was committed to continue the takeoff.
The aft end of the pylon assembly started to separate in the forward flange of the aircraft bulkhead. The structural separation of the pylon was caused by a complete failure of the forward flange of the aft bulkhead after its residual strength had been critically reduced by the fracture and subsequent service life. The length of the overload fracture and fatigue cracking was about 13 inches.
All electrical power to the number 1 a.c. generator bus and number 1 d.c. bus was lost after the pylon separated. The captains flight director instrument, stall warning system, and slat disagreement systems were rendered inoperative. Power was never restored.
The number 1 hydraulic system was lost at pylon separation. Hydraulic lines and follow up cables of the drive actuator for the left wing’s outboard leading edge slat were severed by the separation of the pylon and the left wing’s outboard slats retracted during climb out. The retraction of the slats caused an asymmetric stall and subsequent loss of control of the aircraft.
The pylon was damaged during maintenance performed on March 29 and 30, 1979 at the American Airlines Maintenance Facility in Tulsa, Oklahoma. Engineering personnel developed procedures for removing the pylon and engine that deviated from manufacturers procedures, and did so without performing proper tests.
The NTSB recommended that the Federal Aviation Administration (FAA) issue immediately an emergency Airworthiness Directive to inspect all pylon attach points by approved inspection methods.
Issue an Airworthiness Directive to require and immediate inspection of all DC-10 aircraft in which an engine pylon assembly had been removed and reinstalled for damage to the wing-mounted pylon aft bulkhead, including its forward flange and the attaching spar web and fasteners.
Issue a Maintenance Alert Bulletin directing FAA maintenance inspectors to contact their assigned carriers and advise them to immediately discontinue the practice of lowering and raising the pylon with the engine still attached and adhere to recommended manufacturer procedures.
After a series of post accident inspections disclosed damaged aft bulkheads in the wing to the engine pylons, the Administrator of the FAA issued an Emergency Order of Suspension on June 6, 1979, which suspended the DC-10 series aircraft type certificate until such time as it can be ascertained that the DC-10 aircraft meets the certification criteria of Part 25 of the FAR and is eligible for a Type Certificate.
Twenty days later the FAA issued Special Federal Aviation Regulation 40 which prohibited the operation of any model DC-10 aircraft within the airspace of the United States.
On July 13, 1979, after a series of formal investigations, the Administrator found that the DC-10 met the requirements for issuance of a type certificate. And the Emergency Order of Suspension was terminated.
In November 1979 the FAA fined American Airlines $500,000 for using faulty maintenance procedures on its DC-10 aircraft by using forklifts to mate the complete engine/pylon assembly with the wing attachment points. Continental Airlines was fined $100,000 on a similar charge.
Aviation Safety Network. Retrieved October 20, 2010, from http://aviation-safety.net/database/record.php?id=19790525-2
NTSB. (1979). Aircraft Accident Report, American Airlines, Inc. Flight 191. Retrieved October 20, 2010, from http://www.airdisaster.com/reports/ntsb/AAR79-17.pdf