What happened
On 3 June 2000, an AS3HS5F2 helicopter, registration G-EPOL, was conducting a police transport flight when the pilot observed a brief fire warning for the number one engine. Upon returning toward Boreham Airfield, the warning became persistent. Following established procedures, the pilot shut down the left engine and deployed both the primary and secondary fire extinguishers into the engine compartment.
Despite the use of both extinguishing bottles, the fire warning remained active, and a strong burning smell permeated the cabin. Shortly thereafter, smoke was observed emanating from an electrical panel located near the pilot's left knee. The pilot performed a successful forced landing on the airfield's grass area. There were no fatalities and no injuries to the crew or passengers during the incident.
The investigation
Investigators examined the aircraft's engine fire extinguishing system and the central console. They discovered that the circuit board and four current-limiting resistors for the number one engine switches had suffered severe overheating, resulting in charred board material.
While no actual fire was found in the engine compartment, investigators identified a broken electrical connection within the fire detection system. This failure, likely caused by vibration, had triggered the spurious engine fire warning. Furthermore, it was determined that one of the explosive squibs used to discharge the fire extinguisher had failed to melt its filament upon activation. This left the circuit closed, allowing continuous DC current to flow through the resistors, which caused them to overheat and produce the smoke seen in the cockpit.
Findings
- A broken wire in the fire detection system caused a false engine fire alert.
- The activation of the fire extinguisher system led to an electrical short-circuit because a squib filament remained intact.
- This short-circuit caused the current-limiting resistors to overheat and burn the circuit board.
- The AS355F2 Flight Manual at the time had not yet been updated with instructions to release the fire extinguisher switches after use, a measure previously implemented for the AS355N model to prevent such overheating.