What happened
On May 26, 2000, a McDonnell Douglas DC-3-S1C3G, registration HB-ISB, operated by Classic Air AG, was conducting a scheduled sightseeing flight from Zurich Airport. The aircraft was carrying 24 passengers when, during the landing phase on runway 34, the air traffic control tower alerted the crew to flames emanating from the right-hand engine.
Upon receiving the notification, the pilot checked the cockpit for signs of fire. Finding no visible flames within the cabin, the crew did not deploy fire extinguishers. Instead, they immediately shut down both engines and taxied the aircraft to the nearest taxiway. All passengers were able to evacuate the aircraft safely without injury. The aircraft was subsequently towed to its parking stand.
The investigation
Investigators examined the mechanical systems of the right engine, noting that the engine had recently undergone a carburetor replacement following an engine shutdown event on May 19, 2000. The investigation focused on the fuel delivery system, which utilizes a mechanical pump supplemented by an electric booster pump.
During ground testing at idle speed, investigators observed that fuel pressure exceeded the upper limit when the booster pump was active. Upon dismantling the fuel pump, the team discovered that the vulcanization on the membrane had detached, causing the bypass valve piston to become stuck. This defect allowed high-pressure fuel to bypass the regulation mechanism.
Findings
- The primary cause of the incident was a malfunction in the mechanical fuel pump.
- When the electric booster pump was engaged, the stuck bypass valve allowed excessive fuel pressure to reach the carburetor.
- This resulted in an over-rich fuel mixture, leading to the accumulation of unburned fuel within the exhaust system.
- At low power settings, this accumulated fuel ignited, producing the observed flames.
- Prior to the incident, an increased fuel consumption of approximately 60 liters per hour on the right engine compared to the left had been noted, but this was incorrectly attributed solely to the new carburetor rather than the underlying pump defect.