CHI97IA195

The captain and first officer noticed smoke coming from underneath the airplane's instrument panel glareshield while at cruise altitude. After putting on smoke goggles and oxygen masks the flight crew performed the emergency checklist. Upon shutting down the generators, the smoke cleared from the cockpit in about 4-minutes. The flight landed and its passengers deplaned without incident. Examination of the instrument panel area revealed burnt wiring and a charred position light switch. Further examination showed that the left wingtip's oscillating position light autotransformer had shorted out, and the position light circuit breaker was stuck in the closed position.

On July 5, 1997, at 1515 central daylight time, a McDonnell-Douglas DC-9-30, operated by Northwest Airlines, Incorporated, as Flight Number 1446, and piloted by an airline transport pilot certified crew, declared an emergency when the cockpit filled with smoke. Instrument meteorological conditions prevailed at the time of the incident. The 14 CFR Part 121 flight was operating on an instrument flight plan. The captain, first officer, two flight attendants, and 48 passengers reported no injuries. The flight departed Milwaukee, Wisconsin, at 1350 central daylight time.

Flight 1446 landed at the Bishop International Airport, Flint, Michigan, without incident. The passengers were deplaned through the concourse jetway. According to the captain, he and the first officer noted smoke coming from under the center instrument panel glareshield. The captain said the smoke was billowing out from under the center panel. He said the first officer and he put on their smoke goggles and oxygen masks and performed the emergency checklist.

The smoke immediately stopped when the generators were taken off line, according to the captain. He said emergency power was selected to "ON." He said the cockpit smoke had cleared completely within about 4-minutes. The smoke was confined to the cockpit and did not enter the passenger compartment.

The on-scene investigation revealed that about 8-inches of the insulation on a 20-gauge wire attached to the position light's switch was charred. The position light switch was stuck in a closed position. The 3-amp circuit breaker marked "Position Light" was found in the closed position.

The position light and oscillating position light circuit breakers, the position light switch, the sections of burnt and charred wire, and the position light assembly were sent to McDonnell Douglas for examination. According to the company's report, the position switch examination revealed that one of the internal contact arms was distended and not inline with the other." The report continues, "...the slider for the...contacts was melted... . The contact button corresponding to terminal "B3" was almost completely worn away. The contact arm for the "B" contacts had been partially annealed and was deformed to be higher in the switch body than the other contact arm. This condition would reduce pressure on the contact buttons and increase impedance."

According to the examination, the 3-ampere circuit breaker "...passed the acceptance test procedure." The second circuit breaker examination showed it "...was not able to unlock the mechanism latch to permit the breaker to open."

The position light assembly examination revealed its autotransformer had failed. Internal examination of the transformer revealed the "...individual winding wires...[were] without [any] insulation."

The examination report and wiring diagram are appended to this report.