What happened
On 17 July 1998, a Boeing 727 F (registration N6815) arrived at Edinburgh Airport on a freight flight from East Midlands Airport. After the aircraft had been parked and the brakes applied, the crew began shutdown procedures. During this process, smoke began to fill the flight deck. Unable to pinpoint the source of the smoke, the crew deactivated all electrical power, including ground power, opened the flight deck windows, and evacuated the aircraft.
Airport Fire Service personnel responded to the scene and discovered that the main nickel-cadmium battery, situated beneath the E3 electronic rack in the forward fuselage, was overheating and emitting smoke. Emergency responders removed the battery and moved it to a safe location. The incident resulted in no injuries to the three crew members. Damage was confined to the battery and its charger, with no structural damage sustained by the aircraft. Following the replacement of the faulty components, the aircraft departed for Aberdeen.
The investigation
The investigation examined the mechanics of nickel-cadmium battery overheating. While the operator did not provide specific details regarding the cause of this particular event, the investigation noted that such incidents are often linked to the charging process. Nickel-cadmium batteries are susceptible to rising charge currents as temperatures increase, which can lead to fire if overcharging occurs.
Investigators reviewed historical data from the CAA Occurrence Database, noting that while only 12 such incidents were recorded over a 20-year period, many similar events involving only battery and charger damage might go unreported. The investigation looked into potential failure points, such as the charger's inability to correctly pulse the current, the failure of the charger to terminate the charging cycle, or the failure of individual battery cells which can force the charger to overcharge the remaining cells.
Findings
- The primary cause of the smoke in the flight deck was the overheating of the nickel-cadmium battery.
- Potential contributing factors include malfunctions in the battery charger's protective circuitry or the failure of a battery cell, which can lead to a thermal runaway scenario if the overheat protection fails to interrupt the current.