AL 1998-03
17 August 1998
Failure of Defog Blower Motor
Premature failure of defog blower motors is an ongoing problem with Cessna 550 aircraft. The Service Difficulty Report (SDR) database contains reports of nine failures, with most failures occurring during flight.
When the motor seizes, smoke enters the cockpit through the windshield defog ducts. In at least five reported incidents, the flightcrew declared an emergency and diverted to the nearest suitable airfield.
Although the same blower motor is used in the Cessna 560 aircraft, no failures have been reported for this model.
The blower motor that is failing (P/N 5703-3) does not incorporate a thermal cutout switch and the circuit breaker may not pop when the motor overheats.
A modified blower motor (P/N 5703-4), which incorporates a thermal cutout switch, is available from Cessna and is listed in the Cessna Illustrated Parts Catalogues (IPC) for the Cessna 550 and 560 aircraft. Blower motors returned for overhaul are updated with the thermal switch. However, since the incorporation of the thermal switch must be carried out by the manufacturer, no service bulletin (SB) has been issued and most operators are not aware of the additional safety feature of the modified blower motor.
Failure of these blower motors is a significant safety issue. Transport Canada recommends that operators of Cessna 550 aircraft make arrangements to install modified blower motors (P/N 5703-4) at the next convenient maintenance opportunity.
For further information contact a Transport Canada Centre, or Mr. Ian McLellan, Continuing Airworthiness, Ottawa, telephone (613) 952-4362, facsimile (613) 996-9178 or e-mail mclelli@tc.gc.ca.
For Director, National Aircraft Certification
M.J. Martin
Acting Chief, Continuing Airworthiness