History of the Flight
The aircraft, a Cessna 550 Citation II registered G-VUEA, was conducting a positioning flight from Edinburgh to Manchester with only the two flight deck crew on board. While being vectored for an approach to Runway 24 Right at Manchester Airport, descending through approximately FL80, the crew detected a strong smell of burning electrical insulation. The passenger cabin became completely filled with smoke, obscuring its rearmost area. No smoke was initially present in the cockpit, despite only an open curtain separating the two sections.
The commander instructed the co-pilot, who was flying via the autopilot, to don an oxygen mask. As the co-pilot did so, the commander made a MAYDAY call to air traffic control, though thick smoke entering the cockpit caused breathing difficulties. The commander then donned his own oxygen mask, by which time smoke began to obscure the instrument panel and forward visibility.
With no malfunctions evident in the cockpit, the commander initiated the SMOKE REMOVAL emergency drill from memory, opening the dump valve to depressurize the aircraft. This partially cleared the smoke. The commander decided not to perform further emergency drills, prioritizing preparation for the approach and landing. Manchester ATC provided an expeditious routing to the localizer for Runway 24 Right, though communications were hampered by high cockpit noise from the oxygen masks. Weather was reported as CAVOK with light surface winds.
During the visual final approach, smoke concentration increased again, possibly due to reduced airflow from throttled-back engines, but forward vision remained adequate. The aircraft landed normally, and the crew stopped on the runway, shutting down both engines and removing all electrical power. Both pilots evacuated through the main door without difficulty as the Airport Fire Service arrived. No fire was found, though smoke continued to emit from the open door for an additional 20 minutes. The commander experienced a sore throat and chest from inhaling smoke during the initial distress call.
Aircraft Examination
The operator determined that the circuit breaker protecting the cabin defog fan blower motor had tripped, likely due to a motor fault. The defective motor was sent to the AAIB for further examination.
Component Examination
Inspection of the cabin defog fan revealed no visible damage to the fan unit, but considerable resistance to shaft rotation. The gauze filters on the motor's external vent holes were contaminated with a black sticky residue. Strip examination confirmed that the motor's interior had grossly overheated, melting and degrading much of its insulation. No point of concentrated burning or local overheating was found. The commutator was severely worn, but brushes remained in good condition. Black staining and sticky residue were present in two airflow paths from the motor to the fan case. One motor bearing was confirmed to be very stiff, indicating that seizure had occurred at some point.
The cabin defog fan had a service life of 3,500 airframe hours between overhaul or replacement. The failed unit had been installed for 2,090 airframe hours since new. The part number had been superseded, and the type was no longer supplied as a replacement, though there was no indication that the unit was not permitted to operate up to its quoted overhaul life. A continuing component reliability analysis program by the aircraft manufacturer did not produce conclusive statistical evidence of the unit's reliability.
Arrangement of the Air Conditioning System
The cabin defog fan is part of the air conditioning system, providing conditioned air to cockpit foot warmers and windshield and side-window defog outlets, and influencing airflow to the passenger cabin. A 28V DC electric motor drives a centrifugal fan that receives conditioned air from engine bleeds via the refrigeration unit and delivers it into the cabin duct system. The unit is located below the floor toward the rear of the cabin and can be selected with high and low speed settings.
A small volume of air for motor cooling is drawn into the fan housing from the motor interior via four vent holes at the fan end of the motor casing. This cooling air enters the motor casing from the under-floor area through four additional vent holes with gauze filters. Conditioned bleed air continues to flow through the defog fan to the cabin even when the fan is not operating. An Overhead Blower supplies conditioned, re-circulated, or fresh air to the cabin depending on temperature conditions. A manual emergency dump valve, part of the pressurization control system, can dump all cabin pressure by commanding both outflow valves to open fully.
Operational Procedures
Three emergency procedures were relevant to this incident: ELECTRICAL FIRE OR SMOKE, ENVIRONMENTAL SMOKE OR ODOUR, and SMOKE REMOVAL. The ENVIRONMENTAL SMOKE OR ODOUR procedure directs crews to don oxygen masks and smoke goggles, establish inter-crew communications, and turn off both the cabin fan and defog fan, then attempt to isolate the contaminated air source by selecting each engine bleed in turn. The ELECTRICAL FIRE OR SMOKE procedure includes memory items for oxygen masks, mic switches, and smoke goggles, and if the source is identified, directs isolation by pulling the associated circuit breaker. If unidentified, it involves removing power to much of the electrical system, leaving only essential battery power. The SMOKE REMOVAL procedure directs crews to don oxygen masks and smoke goggles, deploy passenger oxygen, and operate the pressurization dump valve.
The commander identified the smoke as electrical in origin and, recognizing the threat, decided to prioritize landing as soon as practicable, taking into account limited time available.
Analysis
The internal condition of the defog fan motor and contamination of cooling air passages indicated that smoke was drawn from the motor into the cabin airflow. The motor likely continued to operate at decreasing RPM with reduced cooling flow as bearing seizure progressed, causing overheating. The circuit breaker probably tripped after smoke production began, but the motor's residual heat and continued airflow through the fan allowed smoke to persist until the unit cooled. This was confirmed by smoke continuing to issue after landing and system shutdown. Either the ELECTRICAL FIRE OR SMOKE or ENVIRONMENTAL SMOKE OR ODOUR procedure would have isolated power to the fan, but the crew could not identify the source at the time.