What happened
On 06 October 2000, a Cessna 550, registration C-FJCZ, departed Rouyn-Noranda Airport, Quebec, for an instrument flight to Montréal/Dorval International Airport. The aircraft was carrying two pilots and one passenger. After climbing to 5000 feet, the captain detected smoke entering the cabin behind the first officer's seat.
The crew declared an emergency and requested an immediate return to Rouyln-Noranda under visual flight rules. During the descent, the crew encountered significant difficulties; both pilots struggled with oxygen masks, which hindered their ability to communicate and see clearly. The captain eventually had to raise his mask to maintain visibility for landing. Despite the smoke, the crew managed to land the aircraft safely on Runway 08. Upon arrival, the occupants evacuated the aircraft. There were no injuries.
The investigation
Investigators examined the aircraft's ventilation system and electrical components. They discovered that the smoke originated from a fan located beneath the cabin floor. A laboratory analysis of the motor revealed that a screw used to attach the fan armature had suffered a fatigue crack and eventually failed. This failure caused the screw to move and strike the fan armature, leading to overheating and the production of smoke.
Testing at the TSB Engineering Branch Laboratory demonstrated that the 20-amp circuit breaker failed to trip even when the motor was subjected to high current loads that generated significant smoke and heat. Furthermore, investigators found that the aircraft lacked the smoke-detection orifices in the cockpit air outlets that were specified in maintenance manuals for earlier models. The investigation also noted that the electrical load analysis for the aircraft incorrectly listed the cabin fan draw at 4 amps, whereas the installed fan actually drew 9 amps.
Findings
- The primary cause of the smoke was the failure of a cabin fan armature attachment screw due to fatigue.
- The 20-amp circuit breaker provided inadequate protection, failing to trip even as the fan overheated and produced smoke.
- The crew's ability to manage the emergency was compromised by a lack of familiarity with the oxygen masks and the difficulties in communicating while wearing them.
- The electrical system was under-protected because a single circuit breaker was used to protect both the non-essential cabin fan and the essential defog fan.