What happened
On November 8, 2015, a HB 23/2400 Scanliner motor glider was involved in a ground fire at Hofkirchen Airfield (LOLH), Austria. The pilot had completed three successful sightseeing flights and was preparing for a fourth flight with a passenger. After restarting the warm engine and taxiing to the runway hold point, the pilot performed standard pre-takeoff checks.
During the application of full power, the engine failed to reach the required target RPM and produced an unusually rough running noise. The pilot subsequently noticed smoke developing behind the cockpit. He immediately aborted the taxiing process, returned to the parking area, and instructed the passenger to evacuate the aircraft. Upon exiting, the pilot observed flames emerging from the left engine cowling. Although the airfield's emergency services were alerted, the fire spread rapidly through the engine compartment. The fire eventually breached the engine cover and spread to the wooden airframe and the fuel tanks located in the wing center section. The aircraft was destroyed by the fire.
All occupants, including the pilot and one passenger, were able to exit the aircraft safely and sustained no injuries.
The investigation
The Austrian Federal Safety Investigation Board (SUB) examined the wreckage and engine components to determine the origin of the fire. The investigation focused on the engine's internal condition, the fuel system, and the sequence of events leading to the ignition. Investigators analyzed the engine's maintenance history, including the recent replacement of the cylinder head thermometer sensor and the fuel cock.
Findings
- The investigation identified a defective valve seat ring on the intake valve of cylinder 3.
- This defect caused the ring to strike the piston in cylinder 3, which prevented the intake valve from closing completely.
- The failure of the valve to close allowed hot combustion gases to escape into the intake system, leading to engine overheating.
- The fire originated at the right-hand carburetor.
- The fire spread from the engine compartment to the wooden structure and the wing fuel tanks, which contained between 28 and 49 liters of fuel at the time of the accident.
- The investigation also noted that the engine's running was rough, which is a primary indicator of engine damage.
Safety action
- The SUB issued a safety recommendation to the design state and the type certificate holder for the HB 23/2400 series. The recommendation states that the aircraft's flight manual should be updated to include a specific warning: rough engine running, particularly after warm-up, indicates engine damage that requires an immediate engine shutdown and a thorough inspection.