What happened
On 22 July 2019, a Guimlam Cabri G2, registration G-CILR, landed at Wycombe Air Park following a private flight from Dunkeswell, Devon. During the standard engine shutdown procedure, the pilot observed unusual radio static and noted significant fluctuations on the ammeter. Shortly after the engine was shut down, the pilot noticed smoke emerging from the left side of the rotor mast, followed by visible flames within the engine cowling.
The pilot attempted to use the onboard fire extinguisher to suppress the flames near the rotor mast and engine underside, but the effort was unsuccessful. The pilot evacuated the aircraft and retrieved a personal bag before moving to a safe distance. Although emergency services arrived within ten minutes to extinguish the blaze, the helicopter was destroyed.
The investigation
Investigators examined the wreckage and found that the engine remained on its support frame, but the fire had consumed most of the airframe except for the tail boom. Laboratory analysis of the electrical wiring revealed that the cable connecting the alternator to the starter relay had suffered a break characterized by re-solidified molten material, indicating extreme heat from electrical arcing.
The investigation focused on the area where the cable passed through an aluminium engine baffle. It was determined that the cable's insulation had been breached, allowing it to contact the metal edge of the baffle. The movement of the engine during the disengagement of the clutch—which causes the engine to pivot and the baffle to move upward—likely provided the necessary tension to facilitate this contact.
Findings
- The fire was initiated by a short circuit between the alternator-to-starter relay cable and the aluminium engine baffle.
- The electrical arcing generated enough heat to ignite the cotton-impregnated phenolic clips and the carbon fibre cooling fan housing.
- The cable clips were likely incorrectly installed or missing, which prevented the cable from being held centrally within the baffle's pre-drilled holes.
- The design of the clips allowed them to be fitted in the same orientation rather than in opposition, failing to properly retain the cable.
- The mechanical movement of the engine during clutch disengagement caused the unsecured cable to rub against the sharp edge of the baffle, wearing through the insulation.