History of the Flight
The helicopter, a Guimbal Cabri G2 registered G-CILR, departed Dunkeswell Airfield in Devon at 1600 UTC on 22 July 2019 for a flight to Wycombe Air Park in Buckinghamshire. The pilot, who held a Private Pilot’s Licence (Helicopters) and had 174 hours total flying experience (110 on type), conducted pre-flight checks and found the helicopter in satisfactory condition. Prior to the flight, the helicopter had last flown on 17 June 2019 with no reported issues.
During engine start at Dunkeswell, the starter motor would not turn. The pilot suspected a low battery and charged it for approximately 2.5 hours using a 12V DC charger. After successful start, the engine idled for about 15 minutes before the pilot hover-taxied to the fuel pumps. The helicopter was refuelled, and the pilot completed inter-flight checks before departing at 1600 without issue. During the flight, the pilot observed the ammeter to confirm the alternator was charging and noticed nothing unusual.
Landing and Fire
After landing on helicopter pad 10 at Wycombe Air Park, the pilot initiated normal shutdown. During the shutdown, he noticed a static noise on the radio and observed the ammeter deflect fully negative for about 2 seconds before stabilizing at roughly one-third negative deflection. The exhaust gas temperature (EGT) read approximately 350°C (normally displaying '---' shortly after shutdown), and the carburettor temperature rose from 30°C to above 50°C. He applied the rotor brake and shortly before the rotor stopped, saw smoke rising over the left side of the helicopter.
The pilot evacuated and saw flames from within the cowling around the left side of the main rotor mast. He returned to pull the emergency fuel shut-off and switch off all electrical switches, then retrieved the on-board fire extinguisher. He discharged its contents half into the mast cowling and half onto the underside of the engine, but this had no effect. He retrieved a personal bag and retreated to a safe distance.
A witness called emergency services, and a local fire and rescue appliance arrived approximately ten minutes later. The fire was extinguished, but the helicopter had been destroyed. There were no injuries.
Examination and Cause
Examination of the wreckage identified that the electrical cable connecting the alternator to the starter relay had short circuited against the aluminium baffle surrounding the engine, probably as a result of the cable clips being incorrectly fitted. The cable was found in two pieces with fused, re-solidified molten material at the break. Laboratory analysis confirmed the fusing was due to localised heating from electrical arcing. The location of the break corresponded to where the cable passed through the baffle. The clips that supported the cables were not recovered, likely destroyed in the fire. The starter cable showed no evidence of arcing.
As a result of the investigation, the helicopter manufacturer issued a service bulletin instructing operators to inspect for correct installation of the cable clips and completed a redesign of the clips to prevent incorrect fitting.