What happened
On 10 July 2003, a self-built Velocity 173 RG, registration HB-YHB, departed Grenchen aerodrome for a private VFR flight to Straubing, Germany. The aircraft was carrying the pilot and one passenger. Roughly 30 minutes into the flight, the pilot notified air traffic control of electrical system issues and expressed an intention to return to Grenchen. Shortly thereafter, the aircraft's transponder failed, and radio communication with controllers was lost.
Witnesses observed the aircraft trailing smoke and engulfed in flames before it struck a maize field near Würenlingen, Switzerland, at a steep angle. The impact and subsequent fire resulted in two fatalities and the total destruction of the aircraft.
The investigation
The investigation examined the aircraft's history, noting it was a kit-built aircraft constructed by the pilot. Investigators scrutinized the engine compartment, finding that the exhaust system had been modified with silencers to meet strict Swiss noise regulations. This modification involved routing exhaust components through the engine cowling, which contributed to significant thermal issues.
Evidence suggested that excessive heat in the engine compartment likely caused a fuel leak or the vaporization of fuel, leading to the fire. The investigation also noted that the pilot's decision to attempt a return to Grenchen, rather than terminating the flight immediately as per the flight manual, may have impacted the outcome. Furthermore, the pilot lacked sufficient visual aids, such as rear-view mirrors, to monitor the developing fire behind the cockpit.
Findings
- The primary cause of the accident was the pilot losing control of the aircraft during an attempt at an emergency landing.
Contributing factors included:
- An unsuitable construction of the exhaust system, which created severe thermal problems.
- A loss of engine power or an engine failure during the final phase of the emergency landing.
- Insufficient information available to the pilot in time to perform a comprehensive situational assessment.
Safety action
Following the accident, the Federal Office of Civil Aviation (FOCA) recommended that amateur-built pusher-configuration aircraft in Switzerland be retrofitted with a fire detection system. This recommendation aims to ensure that any fire in an engine compartment not visible from the cockpit can be promptly detected by the crew via a cockpit warning light.