National Airlines Flight 27
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This article describes an accident to National Airlines Flight 27 in 1973. At 1640 local time 3 November 1973, a DC-10-10 (S/N N60NA) aircraft cruising at 39,000 ft experienced an uncontained failure of the #3 engine fan assembly. The cabin was penetrated by shrapnel from the engine and lost pressure. One seat belted passenger was violently ejected out the cabin through a failed window. The remains of the body were never found. The shrapnel penetrated the #1 engine oil tank which began to leak oil and had about one minute of usable oil left upon landing. Two of the three aircraft hydraulic systems were also inoperative. The #2 engine sustained minor damage from ingested shrapnel. The aircraft made an uneventful landing in Albuquerque, New Mexico.
[edit] Investigation
The subsequent investigation revealed some interesting facts.
During the flight, while on autopilot, the flight deck crew decided to experiment with the relationship between the fan speed indicator (N1) and the automatic flight control systems. The engineer disconnected the electrical circuit breakers for all three N1 tachometers while the pilot reduced the airspeed by 5 knots. Once the throttles automatically retarded, the pilot disconnected the autothrottle. The crew then heard the explosion.
The CF6-6D engines used on the aircraft were designed to be "red-lined" at 111% N1. The #3 engine failed at 100% N1 while the other two engines reached a maximum 107% N1. While the failure mechanism that triggered this event was never reached conclusively, 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 fan blades began to make contact with the fan shroud. This, compounded with the fact that no significant axial locking mechanism existed for the blades at the time allowed the blades to "walk" towards the front of the aircraft and part with the fan disk. The engine fan assembly was subsequently redesigned by GE.
It is important to note that although the pilots initiated the chain of events precipitating this accident, had it not happened, it would have taken place sometime in the future due to the design limitations of the CF6 engine.
