- National Airlines Flight 27
-
National Airlines Flight 27 Accident summary Date November 3, 1973 Type Uncontained failure Site New Mexico, United States Passengers 116 Crew 12 Injuries 24 Fatalities 1 Survivors 127 Aircraft type Douglas DC-10-10 Operator National Airlines Tail number N60NA On November 3, 1973, a National Airlines (NA) DC-10-10 aircraft (N60NA) was operating as a scheduled passenger flight between Miami and San Francisco with intermediate stops at New Orleans, Houston, and Las Vegas (NA Flight 27).
At about 4:40 p.m., while the aircraft was cruising at 39,000 feet (12,000 m) 65 miles southwest of Albuquerque, the No. 3 engine fan assembly disintegrated (uncontained failure) and its fragments penetrated the fuselage, the Nos. 1 and 2 engine nacelles (which contain those engines), and the right wing area. The resultant damage caused decompression of the aircraft cabin and the loss of certain electrical and hydraulic systems.[1]
The flight crew initiated an emergency descent, and the aircraft was landed safely at Albuquerque International Airport 19 minutes after the engine failed. The 115 passengers and 12 crewmembers exited the aircraft by using the evacuation slides. As a result of the accident, one passenger died and 24 persons were treated for smoke inhalation, ear problems, and minor abrasions. The plane was repaired and was later flown by Pan Am (as Clipper Meteor)
One passenger, G.F. Gardner of Beaumont, Texas,[2] was partially sucked into an opening left when a cabin window failed, 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."[3] The New Mexico State Police and local organizations searched extensively for the missing passenger. 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 crew constructing tracks for the Very Large Array radio telescope came upon his skeletal remains.[4]
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. The fan-tip rub condition was caused by the acceleration of the engine to an abnormally high fan speed which initiated a multiwave, vibratory resonance within the fan section of the engine. The precise reason or reasons for the acceleration and the onset of the destructive vibration could not be determined conclusively.
However, it is clear that the captain and flight engineer's irresponsible actions were to blame. They were experimenting with the autothrottle system, which supplied the instruments that measure the rotational speed of each engine's low pressure compressor. The cockpit voice recording contains the following conversation just prior to the number 3 engine exploding:
- Flight Engineer: "Wonder, wonder if you pull the N1 tach will that, -- autothrottle respond to N1?"
- Captain: "Gee, I don't know."
- Flight Engineer: "You want to try it and see?"
Captain William Brookes, who had been a National Airlines pilot since 1946 and who should have known better responds, "Yeah, let's see here."
- Flight Engineer: "You're on speed right now though."
- Captain: "Yeah."
- Flight Engineer: "You know what I mean if your annunciated speed - if you got, ---"
- Captain: "Still got 'em."
- Flight Engineer: "Well -- haven't got it --"
- Captain: "There it is."
- Flight Engineer: "I guess it does."
- Captain: "Yeah, I guess it does -- right on the nose."
[At the instant he says the word "nose" there is the sound of the number 3 engine exploding followed by ratcheting sounds.]
- Captain: "[expletive deleted] what was that?"
By playing with the autothrottle controls - in what amounted to an in-flight failure analysis test of the autothrottle system - the crew managed to produce a condition where the engines were pushed to higher rotation speeds than they were designed for. According to audio analysis of the CVR tape, all three engines surged (#1 to 105% of nominal maximum power, #2 to 107% and number 3, which failed, to 110%).
What the flight deck crew was trying to do was to determine the relationship between the fan speed indicator (N1) and the automatic flight control systems. The flight engineer disconnected the electrical circuit breakers for all three low speed fan tachometers while the pilot reduced the airspeed by 5 knots (9.3 km/h). Once the throttles automatically restarted, the pilot disconnected the autothrottle. The crew then heard the explosion.
The CF6-6D engines used on the aircraft were designed to never exceed the maximum N1 speed limit speed of 111% (of nominal maximum speed). The #3 engine was at 99% N1 at the time that the initial explosion sound was first heard. It continued to accelerate too, while the other two engines reached a maximum speed of 107% N1. "The speed of engine #3 was no longer discernible after achieving 110% N1 during acceleration, hence the maximum speed attained could not be determined."[1]
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 fan blades began to make contact with the fan shroud. The existing engines had a rearward blade retaining capability of 18,000 pounds to prevent the blades from "walking" towards the front of the aircraft and part with the fan disk. That 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.[1]
References
- ^ a b c "NTSB Report AAR75-02". NTSB, pgs. 20, 35, 36. http://www.airdisaster.com/reports/ntsb/AAR75-02.pdf.
- ^ "Featured Maps: Decompression Defenestration (3 November 2010)". http://www.gcmap.com/featured/20101103. Retrieved 2010-11-21.
- ^ Mondout, Patrick. "Curious Crew Nearly Crashes DC-10". http://www.super70s.com/Super70s/Tech/Aviation/Disasters/73-11-03%28National%29.asp. Retrieved 2010-11-21.
- ^ Harden, Paul (2010-06-05). "Aircraft Down". El Defensor Chieftain. http://www.dchieftain.com/dc/index.php/news/1488-aircraft-down.html. Retrieved 2010-11-21.
External links
Categories:- Accidents and incidents involving airliners
- Accidents and incidents involving the McDonnell Douglas DC-10
- 1973 in the United States
- In-flight airliner depressurization
- National Airlines accidents and incidents
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