What happened
On 21 April 2003, a Cessna 550 Citation 11, registration G-VUEA, was performing a positioning flight from Edinburgh to Manchester. While descending through FL80 and being vectored for an approach to Runway 24 Right, the two-person crew detected a strong smell of burning electrical insulation. Shortly thereafter, the passenger cabin became filled with smoke, which began to encroach into the cockpit.
As the smoke entered the flight deck, causing breathing difficulties for the commander, the crew donned oxygen masks. The commander issued a MAYDAY call to Air Traffic Control. To manage the visibility issues, the commander performed a smoke removal drill from memory, which involved using the manual dump valve to depressurise the aircraft. This action partially cleared the smoke, allowing the crew to focus on the imminent landing.
During the visual final approach, the smoke concentration increased again, likely due to reduced airflow through the cabin as engine power was reduced for landing. The aircraft landed safely at Manchester Airport, and the crew successfully evacuated the aircraft. No injuries were reported, though the commander sustained a sore throat and chest from smoke inhalation.
The investigation
The AAIB examined the aircraft's air conditioning system and the specific component responsible for the smoke. The investigation focused on the cabin defog fan motor, which is part of the system providing air to the cockpit windows and foot warmers.
Technicians discovered that the circuit breaker protecting the fan motor had tripped due to a fault within the motor itself. Detailed examination of the motor revealed that the internal insulation had melted and degraded due to extreme overheating. The investigation found that one of the motor bearings had become very stiff, indicating a previous or ongoing seizure. This mechanical resistance likely caused the motor to draw excessive current and operate at a reduced speed, which in turn reduced the cooling airflow through the motor's vent holes.
Findings
- The primary cause of the smoke was the overheating of the cabin defog fan motor.
- A stiff motor bearing led to increased mechanical resistance and reduced rotational speed.
- The reduction in fan speed decreased the effectiveness of the cooling airflow through the motor's internal vents.
- The resulting high temperature caused the electrical insulation within the motor to degrade and emit smoke.
- The smoke was distributed into the cabin via the existing airflow paths provided by the air conditioning system.
- The commander's decision to prioritise the landing over completing the full emergency checklist was influenced by the immediate need to manage the approach and the difficulty of communicating via oxygen masks.