What happened
On 13 August 1998, a Dart Herald 401, registration G-BEYF, was conducting a positioning flight when the crew received an engine fire warning while descending into Guernsey Airport. The left engine fire handle warning light illuminated, prompting the crew to execute the Engine Fire in Flight drill as specified in the Emergency Check List. Because the fire warning remained active after the initial discharge, the crew discharged the contents of both fire bottles into the left engine.
Following a 'PAN' call to Air Traffic Control, the aircraft landed safely. The Airport Fire Service inspected the left engine nacelle upon the aircraft's arrival, but no evidence of fire was found. The aircraft was subsequently taxied to its parking area and shut down.
The investigation
Post-flight inspections of the engine and the interior of the cowling confirmed that no actual fire had occurred. However, during subsequent testing by airline engineers, the left engine fire warning light and bell activated continuously while the aircraft was in a simulated flight condition.
Electrical testing identified a short circuit within the left engine fire warning element assembly, which was caused by a chafed section of the element. While replacing this section and performing necessary insulation and continuity tests initially appeared to resolve the issue, the aircraft experienced intermittent false fire warnings again over the following six days after returning to service.
Findings
- The initial fire warning was a false indication caused by a short circuit in the fire warning element assembly.
- The short circuit was traced to a chafed section of the element.
- Replacing the chafed section alone was insufficient to permanently resolve the issue, as intermittent warnings continued for six days.
- The fault was ultimately rectified by replacing the entire firewire element assembly, along with the associated relay and relay base.