Incident Overview
On 16 November 1999 at 1615 UTC, a Boeing 737 (registration OY-KKP) operating a public transport flight from Stansted to Copenhagen experienced a wheel well fire warning during initial climb. At approximately 5,000 feet and 320 knots, the warning activated. The crew executed the Non-Normal Checklist, declared a MAYDAY, and initiated a return to Stansted. The fire warning ceased one minute after landing gear extension. Cabin attendants and some passengers reported a burning smell.
Evacuation and Post-Landing Actions
After landing, the crew discharged the engine and APU fire extinguishers. The commander decided to evacuate passengers via the cabin doors and slides only, avoiding the over-wing exits due to the risk of tyre burst and potential injury. Passengers assembled approximately 200 metres from the aircraft and were transported to the terminal by buses.
System Examination
The wheel well fire detection system consists of a temperature-sensitive loop connected to an Overheat Detection Control Module, designed to trigger a warning at 205°C or more. Inspection of the wheel well and brakes revealed no evidence of heat or fire. However, the module indicator confirmed that a warning had been activated. The escape slides sustained damage during the evacuation.
Subsequent Testing
The Overheat Detection Control Module was replaced and sent for examination, which returned no fault found. The wheel well overheat sensing element was also replaced and the system reactivated. The aircraft was returned to service without further incidents. The removed sensing element underwent testing; initial tests showed anomalies, and further testing caused the element to adopt a permanently low resistance, consistent with an overheat warning.
Contextual Factors
On the previous landing at Stansted, maximum braking was used, though the landing weight was not high. The aircraft was then parked for 45 to 50 minutes with the parking brake engaged after a short taxi. On the incident flight, the warning activated after landing gear retraction when the wheel well temperature apparently rose sufficiently to trigger the sensing element at a temperature significantly below the design threshold of 205°C. The aircraft had only 29 flight hours since new, so the wheel well may not have previously reached such elevated temperatures. This was the operator's first experience of a wheel well fire warning on the Boeing 737.
Operator Recommendations
The warning and Non-Normal Checklist refer only to a wheel well fire, not an overheat. The operator planned further investigations to: 1) determine if additional information regarding the cause of the wheel well fire warning could be added to the Non-Normal Checklist, reflecting that it can be triggered by either overheat or fire; 2) assess whether the checklist should consider the significance of the warning persisting or ceasing after landing gear extension; and 3) evaluate if a burning smell can be sensed in the cabin area over the wing after landing gear retraction, and if so, inform crews accordingly.