A Circuit Breaker Story

Alright then. This reminds me of an incident that occurred back on November 3, 1973 involving a National Airlines DC-10-10 on a regular passenger flight cruising at FL390.

About 2 hours into the cruise phase, the flight crew were discussing the system details of the autothrottle system. The aircraft had 2 independent autothrottles which worked in either airspeed mode or N1 rpm mode. The flight engineer wanted to know if the circuit breakers were pulled for the N1 tachs, would the autothrottle continue to function normally in airspeed mode.

So the crew decided to test this theory by engaging the autothrottle in airspeed mode and then pulling the circuit breakers to the N1 tachs for all 3 engines. They did this and no airspeed change was observed. So the "theory" was further tested by reducing the airspeed setting by 5 knots, and the autothrottle responded as expected. The test period was brief (only a minute or 2) and shortly thereafter the N1 tach circuit breakers were reset on all 3 engines.

Almost immediately the fan disk of the number 3 engine exploded (a CF6-6D). The explosion was uncontained and the hull was punctured leading to an immediate loss of cabin pressure. A cabin window was also destroyed leading to a passenger (Mr Gardner in seat 17H) being sucked bodily out of the window to his death during the depressurization.

The engine cowling was torn completely off of the number 3 engine which caused severe buffeting and vibration of the airframe. Wiring and control cables were severed in the lower fuselage, and electrical power was momentarily lost on all systems. The number 1 engine generator wiring was severed when fan disk debris passed under the fuselage and struck the port engine. It's oil and hydraulic pressures also went to low readings.

It took 5 minutes for the crew to restore electrical power to the aircraft systems while cabin crew helped the passengers with their oxygen masks. Two of the cabin crew in the lower galley passed out before they could get to their masks. The flight crew began an emergency descent and after electrical power was restored, they began an emergency descent and approach into Albuquerque. The leading and trailing edge lift devices extended normally in preparation for landing, but the landing gear had to be extended manually. With the oil and hydraulic pressures still deteriorating on the number 1 engine, the plane landed safely about 20 minutes after the incident began.

Post flight examination of the aircraft, CVR and FDR showed that the N1 speed of the starboard engine had reached 110 percent just prior to the fan disk explosion. The port engine suffered a punctured oil tank and complete severing of the generator wiring. The number 2 engine also suffered some fan blade leading edge damage and some compressor blade damage from ingesting small bits of the fan blades from the destroyed right engine.

So the moral of this story is that pilots should never perform the duties of a test pilot, unless you are a test pilot operating under test flight conditions.

Date: 03 NOV 1973
Type: McDonnell Douglas DC-10-10
Operator: National Airlines
Registration: N60NA
Msn / C/n: 46700/14
Year built: 1971
Total airframe hrs: 5954 hours
Crew: 0 fatalities / 12 on board
Passengers: 1 fatalities / 116 on board
Total: 1 fatalities / 128 on board
Location: Socorro, NM; nr (USA)
Phase: Cruise
Nature: Domestic Scheduled Passenger
Departure airport: Houston-George Bush Intercontinental Airport, TX (IAH)
Destination airport: Las Vegas-McCarran International Airport, NV (LAS)
Flightnumber: 27

National Airlines Flight 27 departed Houston for Las Vegas and climbed to FL390. Suddenly the No. 3 engine fan assembly disintegrated and fragments penetrated the fuselage the Nos. 1 and 2 engine nacelles, and the right wing area. As a result, the cabin depressurized and one cabin window, which was struck by a fragment of the fan assembly, separated from the fuselage. The passenger who was sitting next to that window was forced through the opening and ejected from the aircraft. The flightcrew initiated an emergency descent, and the aircraft landed safely at Albuquerque International Airport 19 minutes after engine failed. PROBABLE CAUSE: "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."


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