What happened
On 16 November 1999, a Boeing 737, registration OY-KKP, was performing a scheduled public transport flight from Stansted to Copenhagen. During the initial climb phase, approximately 5,000 feet above the ground and travelling at 320 knots, the crew received a fire warning originating from the wheel well. In response to the alert, the pilot declared a MAYDAY and commenced an immediate return to Stansted.
Upon landing, the fire warning ceased approximately one minute after the landing gear had been extended. However, cabin crew and several passengers reported the presence of a burning smell within the cabin. Following the landing, the crew deployed the engine and APU fire extinguishers. To ensure safety, the commander ordered an evacuation using only the cabin doors and slides, intentionally avoiding the over-wing exits to mitigate the risk of tyre bursts or injury to attendants. While the evacuation was successful and all 63 passengers were moved to a safe distance and subsequently bussed to a lounge, the emergency use of the equipment resulted in damaged emergency evacuation slides.
The investigation
Investigators examined the aircraft's overheat detection system, which utilizes a temperature-sensitive loop connected to an Overheat Detection Control Module. This system is designed to trigger an alarm only when temperatures reach 205°C or higher. Physical inspections of the wheel well and braking components showed no visible evidence of fire or overheating.
While the Overheat Detection Control Module was replaced and tested without finding any faults, the investigation focused on the sensing element. Testing of the removed sensing element revealed that it was prone to adopting a permanently low resistance, which is a condition that triggers an overheat warning. The investigation also noted that the aircraft had been subjected to maximum braking during its previous landing at Stansted and had been parked with the parking brake engaged for nearly 50 minutes prior to the incident flight.
Findings
- The primary cause of the incident was a faulty overheat sensing element that generated a false warning at temperatures significantly lower than the required 205°C.
- The warning was triggered specifically following the retraction of the landing gear, as the temperature in the wheel well rose sufficiently to activate the malfunctioning sensor.
- The aircraft was relatively new, having operated for only 29 flight hours, meaning the wheel well had likely not reached the high temperatures necessary to trigger the fault in previous flights.