National Airlines Flight 27 was a scheduled passenger flight between Miami and San Francisco with intermediate stops at New Orleans, Houston, and Las Vegas, operated by a DC-10-10 aircraft. On November 3, 1973, at about 4:40 p.m., while the aircraft was cruising at 65 miles southwest of Albuquerque, the No. 3 engine fan assembly disintegrated in an uncontained failure. Its fragments penetrated the fuselage, the Nos. 1 and 2 engine nacelles, and the right wing area. The resultant damage caused decompression of the aircraft cabin and the loss of certain electrical and hydraulic systems. One passenger was partially forced into the opening made by a failed cabin window, after it too was struck by engine fragments. He was temporarily retained in that position by his seatbelt. "Efforts to pull the passenger back into the airplane by another passenger were unsuccessful, and the occupant of seat 17H was forced entirely through the cabin window." The flight crew initiated an emergency descent, and the aircraft was landed safely at Albuquerque International Sunport 19 minutes after the engine failed. 115 passengers and 12 crew members exited the aircraft by using the evacuation slides. Of those, 24 people were treated for smoke inhalation, ear problems, and minor abrasions. The plane was repaired and was later flown by Pan Am. The New Mexico State Police and local organizations searched extensively for the missing passenger who was sucked out of the window. A computer analysis was made of the possible falling trajectories, which narrowed the search pattern. However, the search effort was unsuccessful, and the body of the passenger was not recovered until two years later, when a construction crew working on the tracks for the Very Large Arrayradio telescope came upon his skeletal remains, which took another year for the medical investigator in Albuquerque to identify.
Investigation
The National Transportation Safety Board determined the probable cause of this accident was the disintegration of the No. 3 engine fan assembly as a result of an interaction between the fan blade tips and the fan case. According to the NTSB, "the precise reason or reasons for the acceleration and the onset of the destructive vibration could not be determined conclusively," but enough was learned to prevent the occurrence of similar events. The speed of the engine at the time of the accident caused a resonance wave to occur in the fan assembly when the tips of the fan blades began to make contact with the surrounding shroud. The engine was designed to have a rearward blade retaining force of 18,000 pounds to prevent the blades from moving forward in their mountings slots and subsequently departing from the fan disk. The rearward force was not enough. As a result of this accident, GE re-designed the engine so that the blade retaining capability was increased to 60,000 pounds, and that change was incorporated into all engines already in service. In addition to this, it was found that between the 8th of August and the 12th of September 1973, there had been 15 problems reported about the third engine. The engine had been taken off the aircraft for repairs, and between the time it was replaced and the accident, a further 26 faults had been reported by the pilots. It was found that the bolts that had held the front covering in place, which had failed in the accident, were outside the tolerances laid down. An engineering dispatch was sent out to inspect these engines, and six more discrepancies were found in National Airlines fleet alone. Therefore, this dispatch was made compulsory for all early DC-10s in order to prevent the issue occurring again. The NTSB expressed concern about the cockpit crew conducting an unauthorized experiment on the auto-throttle system. They had been wondering where the system took its engine power readings from and to see if it was the N1 tachometer readout "the flight engineer pulled the three N1 tachometer " and then adjusted the autothrottle setting. The cockpit voice recorder proved that the engines altered their power setting when requested, proving to the crew that the system was powered from another source. The crew then manually reset the throttles to the normal cruising power before the flight engineer had closed the tachometer circuit breakers. It was considered whether the crew had accidentally over-speeded the engine when setting power without the tachometers, but there was insufficient evidence to deliver a certain verdict. Nonetheless; "regardless of the cause of the high fan speed at the time of the fan failure, the Safety Board is concerned that the flightcrew was, in effect, performing an untested failure analysis on this system. This type of experimentation, without the benefit of training or specific guidelines, should never be performed during passenger flight operations."
