CHICAG0, IL, USA
N631US
BOEING 747-251B
THE FLT CREW DISCONNECTED THE NUMBER ONE ENG CONSTANT SPEED DRIVE (CSD) DURING A FLT ON THE DAY PRIOR TO THIS INCIDENT WHEN ITS OIL PRESSURE LIGHT ILLUMINATED. DUE TO OTHER, UNRELATED MAINTENANCE, THE AIRPLANE'S NEXT REVENUE FLT WAS CANCELLED. SECOND SHIFT MECHANICS WERE ASSIGNED MAINTENANCE ON THE AIRPLANE. THE MECHANICS DECIDED TO TROUBLESHOOT THE UNIT. THE CSD WAS MECHANICALLY RECOUPLED AND THE ENG WAS STARTED. AFTER THE ENG START, THE START VALVE LIGHT FAILED TO GO OUT AND THE ENGINE WAS SHUT DOWN. A MECHANIC REMOVED THE MAIN CSD WIRING HARNESS CANNON PLUG TO HELP WITH THE TROUBLE SHOOTING PROCESS. THE START VALVE WAS REPLACED AND THE ENG WAS RUNUP. THE CSD CANNON PLUG WAS NOT CONNECTED AT THE CONCLUSION OF THE ENG RUNUP. AFTER SHUTDOWN THE START VALVE CHANGE WAS ENTERED INTO THE AIRPLANE MAINTENANCE RECORDS. THE MECHANIC WHO RECOUPLED THE CSD INFORMED THE LEAD MECHANIC OF HIS ACTION BEFORE HE LEFT FOR THE DAY. ACCORDING TO THE COMPANY, THE THIRD SHIFT MECHANICS DID NOT HAVE ENOUGH TIME TO CHANGE THE CSD. THE NUMBER ONE ENG WAS '...CLOSED UP [AND] THE AIRCRAFT WAS DISPATCHED.' DURING TAXI FROM LANDING THE CSD CAUGHT FIRE.
On November 2, 1994, at 0920 central standard time, a Boeing 747- 251B, N631US, operated by Northwest Airlines, Incorporated, of Minneapolis, Minnesota, and piloted by an airline transport rated flight crew, received minor damage during taxi in from a landing at the Chicago O'Hare International Airport, Chicago, Illinois. The number one engine generator's constant speed drive was on fire and was extinguished by ramp personnel. The 14 CFR Part 91 positioning flight had been operating on an IFR flight plan. Visual meteorological conditions prevailed at the time of the incident. No injuries were reported by the three flight crew members on the airplane. The flight departed Detroit, Michigan, at 0902 eastern standard time (est). According to company records, the number one engine's constant speed drive (CSD) was disconnected from service during a flight on November 1, 1994, when its oil pressure light illuminated. A subsequent revenue flight was canceled due to another unrelated maintenance discrepancy. A lead mechanic assigned second shift mechanics to work on the N631US. A replacement CSD was scheduled to arrive after 2400 est on November 1, 1994. The mechanics decided to troubleshoot the CSD. One of the mechanics said the CSD check is done during an engine runup. He said the CSD was mechanically recoupled before starting the number one engine. After the engine started, its cockpit start valve light failed to go out and the engine was shut down. After replacement of the start valve the engine runup was performed. The engine was shut down and the mechanics signed off the start valve change. According to the company, a mechanic researched the fault isolation manual and decided that removal of the main CSD wiring harness cannon plug would help in the troubleshooting process. The cannon plug was removed from the CSD. The CSD had not arrived at the end of the second shift. The company's report on the incident states, "Based on MEL relief, the first day of deferral and down time required to replace the CSD, the engine was closed up, the aircraft was dispatched. The number one engine CSD had been mechanically recoupled, and the main harness disconnected, neither of which was documented." The maintenance shift change carry-over records showed the CSD as being on deferred maintenance. The records do not address the CSD recoupling by the mechanic. These records are reviewed by the lead mechanic. The second shift mechanic related to the CSD reengagement said, he told the lead mechanic what he had done. He said that the lead mechanic told him that he would have the next shift take care of the CSD change. An on-scene investigation revealed that the CSD's exterior was fire damaged. Wiring insulation attached to the CSD, and within 12 inches of its exterior was melted. Some smaller wires had completely melted through. The CSD's mechanical recoupling handle was found in the "IN" position, confirming earlier recoupling action. The CSD's engine nacelle access door was warped outward and flame damaged. The interior of the nacelle next to, and around, the area of the CSD was partially melted and warped.
inattentive maintenance service of the airplane by the mechanic who failed to disengage the constant speed drive (CSD) after the engine runup had been completed. A factor in this incident was the lead mechanic not performing maintenance record keeping after being informed that the CSD had been recoupled.
Source: NTSB Aviation Accident Database
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