What happened
On June 29, 1973, a flight instructor and a student pilot were conducting emergency landing practice maneuvers near Grenchen Airport in a Beagle Pup HB-NAE. The flight began as a scheduled training session involving simulated engine failures. After successfully completing a simulated engine failure and a subsequent simulated landing on runway 2/25, the crew attempted a go-around.
During the climb, the instructor engaged the auxiliary fuel pump and carburetor heat, then gradually reduced power to allow the student to practice a landing approach at a different field. As the aircraft descended to an altitude of approximately 4 meters, the instructor attempted to increase power. However, the engine failed to respond to the throttle advancement and instead began to sputter. The crew was forced to perform an immediate landing in a rye field located about 1,100 meters southwest of the airport. Upon impact, the aircraft overturned, coming to rest on its back.
The investigation
The investigation focused on the mechanical state of the engine and the impact of fuel composition on engine performance. While the aircraft was found to be airworthy and the fuel system was intact, investigators noted that the engine performed normally during subsequent bench tests.
Analysis of the Avgas 100 L fuel revealed that the volatile components were below standard requirements, which could cause poor engine warm-up characteristics. Furthermore, the investigation examined the history of the carburetor, which had been swapped from another aircraft, HB-NAG, for reasons unrelated to the accident. Notably, HB-NAG experienced a nearly identical engine sputtering issue just three days after receiving the same carburetor, which was subsequently overhauled, resolving the problem.
Findings
- The primary cause of the engine malfunction was likely spark plug short-circuiting caused by lead deposits.
- The transition from Avgas 80/87 to Avgas 100 L increased lead content fourfold, which is known to cause lead-based accumulations on spark plugs, particularly during low-power operations.
- It is highly probable that the carburetor was set to a mixture that was too rich at idle, further contributing to the buildup of deposits.
- Both occupants of the aircraft escaped the incident without injury, though the aircraft sustained heavy damage.
Safety action
- The investigation highlighted the risks of operating engines originally designed for Avgas 80/87 with the higher-lead Avgas 100 L.
- The findings supported existing technical advisories regarding the management of engine deposits and the potential for ignition failures due to lead accumulation.