National Airlines Flight 27

Investigation

The National Transportation Safety Board (NTSB) 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 lb 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, General Electric re-designed the engine so that the blade retaining capability was increased to 60,000 lb, and that change was incorporated into all engines already in service.

In addition to this, it was found that between August 8 and September 12, 1973, there had been fifteen 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 twenty-six faults had been reported by the flight crew. 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 the National Airlines fleet alone. Therefore, this dispatch was made compulsory for all early DC-10s in order to prevent the issue from occurring again.

The NTSB expressed concern about the flight 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 tachometers [circuit breakers]" and then adjusted the autothrottle setting. The cockpit voice recorder showed 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-sped 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 flight crew 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."