Investigation Summary
At the time of the engine failure, the flight crew was conducting an unauthorized experiment to determine the source of the autothrottle's engine speed signal. The flight engineer had pulled the circuit breakers for the N1 tachometers on all three engines. Shortly after, the number 3 engine fan assembly disintegrated. The NTSB investigation revealed that the engine had reached a state of vibratory resonance, known as an 'interactive mode,' where the fan blade tips made contact with the fan case. This interaction caused the fan assembly to fail catastrophically. While the NTSB could not conclusively link the crew's experimentation to the engine overspeed, they strongly criticized the crew for performing 'untested failure analysis' during a passenger flight. The investigation also noted a troubled maintenance history for the No. 3 engine: between August 8 and September 12, 1973, fifteen problems were reported. After being removed for repairs and returned to service, an additional 26 faults were reported by crews in the weeks leading up to the accident.
Final Conclusions
The NTSB determined the probable cause was the disintegration of the No. 3 engine fan assembly resulting from an interaction between the fan blade tips and the fan case. This 'fan-tip rub' was triggered by the engine accelerating to an abnormally high fan speed, which initiated a multiwave vibratory resonance. The precise reason for the acceleration could not be determined. The NTSB report (NTSB-AAR-75-2) was issued in 1975.
Photographic Evidence (1)
Video Analysis
The Unsettling History of the DC-10 (National Airlines Flight 27) — A detailed look at the 1973 engine failure over New Mexico that resulted in a passenger being ejected from the aircraft during cruise.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Crew Experience
Captain
First Officer
Flight Engineer
Systems & Failure Modes
General Electric CF6-6D Engine
The high-bypass turbofan engine suffered a catastrophic fan assembly failure due to vibratory resonance and fan-tip rub.
Autothrottle / N1 Tachometers
The crew disabled the N1 tachometer signals to test the autothrottle system's response, potentially contributing to an undetected engine overspeed.
Cabin Pressurization
The loss of a cabin window caused an immediate explosive decompression at 39,000 feet.
Interesting Facts
- 01The aircraft, N60NA, was the first DC-10 delivered to National Airlines and was named 'Barbara'.
- 02One passenger was killed after being ejected from the aircraft; while his body was never recovered, a ranch hand later found his sunglasses and pipe near Alamo, New Mexico.
- 03The flight crew had pulled N1 tachometer circuit breakers just before the failure as part of an experiment.
- 04The No. 3 engine had 26 reported faults in the period between its last major repair and the accident.
- 05Debris from the No. 3 engine also damaged the No. 1 and No. 2 engines and two hydraulic systems.
Safety Actions & Advisories
Engine Redesign
General Electric redesigned the CF6 engine to increase fan blade retaining force from 18,000 pounds to 60,000 pounds.
Operational Discipline
The NTSB issued a warning against flight crews performing unauthorized experiments or 'untested failure analysis' during passenger operations.
Known Controversies & Unanswered Questions
Unauthorized In-Flight Experimentation
Reference ↗The NTSB investigation highlighted a significant controversy regarding the flight crew's decision to pull circuit breakers for the N1 tachometers to experiment with the autothrottle system during a passenger flight. While the NTSB could not conclusively prove this caused the overspeed, they issued a stern warning against 'untested failure analysis' by crews in flight.