Douglas DC-10-10 accident at Albuquerque, New Mexico, 3 Nov 1973

Accident narrative

On November 3, 1973, at about 1640 m.s.t., National Airlines Flight 27, a Douglas DC-10-10, experienced a catastrophic failure of its No. 3 engine while cruising at 39,000 feet about 65 nautical miles southwest of Albuquerque, New Mexico. Fragments from the disintegrating engine fan assembly penetrated the fuselage, the other two engine nacelles, and the right wing. The cabin depressurized, and a passenger was ejected through a shattered window. The flightcrew initiated an emergency descent and landed safely at Albuquerque International Airport. Of the 116 passengers and 12 crewmembers aboard, one passenger died, and 24 persons were treated for smoke inhalation, barotrauma, and minor abrasions.

### The flight

Flight 27 was a scheduled passenger flight from Miami, Florida, to San Francisco, California, with intermediate stops in New Orleans, Houston, and Las Vegas. The aircraft departed Houston at 1440 on an instrument flight rules clearance.

The flightcrew consisted of Captain William R. Broocke, who had 21,853 flight-hours including 801 in the DC-10; First Officer Eddie H. Saunders, with 7,086 flight-hours and 445 in the DC-10; and Flight Engineer Golden W. Hanks, who had 17,814 flight-hours with 1,252 in the DC-10. The weather in the accident area was reported as 10,000 feet scattered clouds with a broken ceiling at 25,000 feet.

### Sequence of events

The climb to 39,000 feet was conducted using the autopilot and autothrottle systems. Upon leveling off at a cruising speed of .82 Mach (257 knots indicated airspeed), the autothrottle was disengaged and power was set manually.

According to the captain, at about 1640, he engaged the autothrottle system in the airspeed mode with a target of 257 knots. He and the flight engineer then discussed the electronic interrelationship between the autothrottle system and the N1 (first stage fan) tachometers. To check the system's functions, the flight engineer pulled the N1 circuit breakers for all three engines. The captain stated he then retarded the speed bug on the airspeed indicator by about 5 knots to see if the throttles would follow. Noting that the throttles retarded slightly, he disengaged the autothrottles and remarked to the flight engineer that he was satisfied with the function. The flight engineer stated he then reset the N1 circuit breakers, though he could not remember if the throttles had been advanced when an explosion occurred.

The crew heard and felt an explosion, and the aircraft began to buffet severely. The flight engineer saw a fire warning light for the No. 3 engine. Unable to move the fuel shutoff handle, he activated the firewall shutoff and discharged two fire extinguisher bottles. Warning lights indicated failures of the No. 3 AC generator, No. 3 AC bus, No. 3 DC bus, No. 1 generator, and the left emergency AC bus. The captain switched on the emergency power, restoring his flight instruments and electrical power to the No. 1 AC and DC buses.

In the passenger cabin, the explosion was followed by blue-grey smoke that became progressively denser toward the rear. The aircraft depressurized. Fragments from the No. 3 engine had struck a cabin window at seat 17H, dislodging it from its frame. The passenger in that seat was partially forced through the 16 1/8 by 10 5/8-inch window opening. Despite efforts by another passenger to pull him back, he was forced entirely through the window. His body was not recovered.

Passenger oxygen masks presented automatically in the midsection, but took almost three minutes to drop in other sections, and failed to deploy in the left rear cabin. In the lower galley, two flight attendants lost consciousness when they stood up to obtain portable oxygen equipment.

The pilots initiated an emergency descent and alerted the Albuquerque Air Route Traffic Control Center via transponder. At 1645, radio contact was established with Albuquerque Approach Control, and the flight was cleared to descend to 8,000 feet. The aircraft landed safely at 1659. The occupants evacuated via emergency slides in about 60 seconds, though the left forward slide was accidentally jettisoned, the right forward slide required manual inflation, and the right overwing slide deployed uselessly on top of the wing.

### What the investigation found

Examination of the aircraft revealed that the No. 3 powerplant had disintegrated substantially at the first stage fan assembly. The nose cowl, the fan blade containment ring, and 32 of the 38 first stage fan blades separated in flight. Damage to the blade roots and fan disk showed that the blades moved forward past their restraining devices and out of their slots. Fragments punctured the lower fuselage skin in six areas, severed hydraulic lines and control cables, and struck the No. 1 and No. 2 engines.

The digital flight data recorder yielded no meaningful data. However, the cockpit voice recorder captured the crew's discussion about pulling the N1 tachometer circuit breakers, followed by the sound of the explosion. Postaccident examination of the aircraft's instruments, which froze when electrical power was lost, indicated engine pressure ratios and fuel flows compatible with an abnormally high power setting at the time of failure, though still within certificated maximum allowable limits.

A study of the autothrottle system revealed that pulling the N1 circuit breakers removes the limiting authority imposed by the thrust rating computer. In the speed mode, if an airspeed error is sensed requiring thrust, the throttles can advance without limit. The Board noted that the flightcrew was, in effect, performing an untested failure analysis on the autothrottle system.

Sound spectral analysis of the cockpit voice recorder tape indicated that the No. 3 engine accelerated rapidly just before the explosion, reaching an acceleration rate that requires a severe restriction of fan air flow. The Board compared this to two previous test-cell failures of CF6 engines, where rapid acceleration and a fan-tip rub condition initiated a multiwave, vibratory resonance between the fan rotor and the fan case. This interaction provided the vibratory forces necessary to move the fan blades forward and out of their slots.

The Board evaluated possible triggering mechanisms for the destructive vibration. One theory was that a piece of the inlet duct's inner acoustic panel separated and restricted airflow, causing a rapid fan acceleration. However, fragment damage to the honeycomb material indicated the liner was sheared by fragments of the disintegrating fan, not torn loose beforehand. The Board concluded the catastrophic failure was precipitated by a blade-tip rub condition caused by an acceleration of the engine to an abnormally high N1 speed, but the precise reason or reasons for the acceleration could not be determined conclusively.

### Probable cause

The National Transportation Safety Board determined that "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."