INDIANAPOLIS, IN, USA
N148SK
Beech 1900D
Shortly after taking off, the crew of the airplane noted smoke in the cabin, and they returned and landed at the departure airport. Investigation revealed that a bearing in the forward cabin ventilation blower motor had failed. There was evidence that after the bearing had failed, the wires and resistors to the ventilation motor had overheated. The wires were charred and sooted on the blower motor housing. The resistors (for low speed fan operation) showed signs of heat damage. Current limiters were provided to protect the electrical circuit. However, research revealed that the wiring and resistors were not adequately protected in the low speed fan mode.
HISTORY OF FLIGHT On May 23, 1996, at 2000 central daylight time (cdt), a Beech 1900D, N148SK, experienced smoke in the cabin of the airplane, shortly after departing the Indianapolis International Airport, Indianapolis, Indiana. The 14 CFR Part 121 passenger flight returned to the Indianapolis International Airport, and landed on runway 05L. After stopping on the runway, the 18 passengers and two flight crew members performed an emergency evacuation of the airplane. No injuries were reported by the passengers or the flight crew. VFR conditions existed at the time of the incident. N148SK was operating on an IFR flight plan, with an intended destination of Milwaukee, Wisconsin. FLIGHT RECORDER The cockpit voice recorder was removed from the airplane. No useful information was retrieved from the cockpit voice recorder. FIRE The wires which control the forward cabin ventilation blower motor were charred and sooted on the blower motor housing. Both resistors which control the blower motor when it is in the low speed mode showed signs of heat damage. The floor immediately under the forward blower motor had a trace of soot on its bottom. TESTS AND RESEARCH The investigator in charge (IIC) obtained the forward blower motor, and the current limiter from the incident aircraft. The forward blower motor could not be rotated by hand, and the current limiter for the forward blower motor was tripped. One internal bearing of the forward cabin ventilation blower motor had failed, and was in numerous pieces when the motor was opened. A review of the aircraft's electrical system, and the blower motor was completed with the manufacturers of the aircraft and the blower motor. It was determined that if either the forward ventilation blower motor or the aft ventilation blower motor for the airplane stalled while in the low speed fan mode, the wires and resistors on the vent blower assembly were insufficiently protected by the current limiter. The failed blower motor had 2,321 hours total time in service, 2,838 total cycles, and 405 total days of service. There are two different ventilation blower manufacturers. One vent blower manufacturer lists the recommended overhaul time as 1,000 hours. The other vent blower manufacturer lists the overhaul time as 2,000 hours. The Beechcraft 1900D maintenance manual dated December 22, 1995 recommends to repair or replace the blower motor if it becomes inoperative. The blower motor manufacturers, and the aircraft manufacturer require bearing replacement at the time of blower motor overhaul. ADDITIONAL INFORMATION On July 26, 1996, at 1620 eastern standard time a Beech 1900D, N165YV, being operated as Liberty Express Flight 5991, reported smoke in the cockpit after departing from the Jamestown Airport, Jamestown, New York. The crew downed their oxygen masks and deployed the oxygen masks to the passengers. Flight 5991 returned to the Jamestown Airport, and landed. The 17 passengers, one lap child, and 2 flight crew members were uninjured. The aft blower motor from N165YV was disassembled in Wichita, Kansas, on July 30, 1996. This blower motor showed fire and heat damage similar to the blower motor on N148SK. One internal bearing had failed, the other bearing was worn. The blower motor from N165YV had been in service 1,597.2 hours, 3,306 cycles, and 318 days Parties to the investigation were Raytheon Aircraft, and Electromech Technologies.
failure of a forward cabin ventilation blower motor bearing, which resulted in binding of the blower motor; and inadequate design of the electrical system, which allowed overheating of the electrical circuit and smoke in the cabin of the aircraft.
Source: NTSB Aviation Accident Database
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