What happened
On 7 October 2008, an ATR 42-300, registration EI-BYO, was performing a scheduled passenger service from Dublin to Ronaldsway, Isle of Man. As the aircraft began its descent from cruise altitude, the flight crew received an 'AFT SMK' warning on the flight deck.
Upon receiving the alert, the commander notified air traffic control of a potential cabin fire and requested priority landing. The cabin crew member investigated the rear of the aircraft and identified smoke within the toilet compartment. Following the aircraft's Quick Reference Handbook (QRH) procedures, the crew member used a BCF extinguisher to treat the area. After the smoke dissipated, the crew reported that the warning had extinguished.
The aircraft landed successfully at Ronaldsway with no injuries to the 17 passengers or 3 crew members. Emergency services met the aircraft on the taxiway, but subsequent inspections found no evidence of fire.
The investigation
The investigation focused on the source of the smoke and the functionality of the detection systems. It was noted that the 'AFT SMK' warning is linked to both the aft cargo area and the toilet compartment. While the crew followed the specific 'AFT SMK' procedure, the flight attendant panel indicated the smoke originated in the toilet area.
Maintenance inspections following the incident revealed that the cover for the illuminated 'RETURN TO SEAT' sign in the toilet had overheated, resulting in the failure of the bulb filaments. Although the correct bulbs had been installed, the specific cause of the overheating could not be identified during the initial inspection. The unit and relevant documentation were sent to the manufacturer for further analysis.
Findings
- The smoke was caused by an overheated light fitting within the toilet compartment.
- The overheating led to the failure of the bulb filaments in the 'RETURN TO SEAT' sign.
- The aircraft's smoke detection system functioned as intended, alerting the crew to the presence of smoke in the aft section of the cabin.