NTSB Identification: CHI97IA072.
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Scheduled 14 CFR NORTHWEST AIRLINES
Incident occurred Thursday, February 20, 1997 in DES MOINES, IA
Probable Cause Approval Date: 02/29/2000
Aircraft: McDonnell Douglas DC-9-15, registration: N93S
Injuries: 36 Uninjured.

NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.

There was an electrical fire during cruise flight at FL310. The flight crew extinguished the flames by turning off both generators and the battery switch. The flight diverted to Des Moines, Iowa, where it landed without further incident. Inspection of the airplane revealed the cross tie relay along with four other relays and their associated wiring in the left main equipment rack near the left forward cabin door were charred. Investigation revealed the fire was a result of a phase-to-phase short in the cross tie relay arc box. The short resulted from relay contact wear products (metal dust) migrating through the arc box. The relay manufacturer recommended a 7,000 hour time between overhaul (TBO) for this relay when it is used as a cross tie relay. The aircraft manufacturer recommended a 7,000 hour TBO regardless of how the relay is used. The operator used a 14,000 hour TBO, based on an FAA approved TBO reliability program for the relay installed in a vertical or horizontal orientation. The failed relay had a total of 7,775.26 hours since overhaul.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

the cross tie relay shorted resulting in a fire in the left main electrical equipment rack.

Full narrative available

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