What happened
On April 22, 2017, at approximately 11:00 local time, an Alisport Silent 2 Targa, registration HB-5515, was performing a private flight takeoff from the grass runway 26L at Locarno Aerodrome (LSZL). Immediately after lifting off at an altitude of about one meter, the pilot noticed the aircraft was accelerating slower than usual and that the throttle lever was not providing maximum power.
After pushing the throttle forward to its limit, power briefly returned to normal before dropping abruptly again during the initial climb at an altitude of roughly 10 meters. To avoid a row of bushes located near the end of the runway, the pilot executed a right-hand turn. During this maneuver, the right wing struck the ground, causing the aircraft to spin around its vertical axis and strike the ground violently. The impact resulted in severe damage to the aircraft, including a broken rear fuselage, detached wingtip supports, and damage to the propeller and engine cowlings. The pilot escaped the incident uninjured.
The investigation
Investigators examined the engine and fuel systems to determine the cause of the power loss. The fuel tanks contained a 50/50 mixture of MOGAS and AVGAS, and testing showed no presence of water in the fuel or the refueling container.
Technical examination focused on the throttle control mechanism, which connects the cockpit lever to the carburetor butterfly valve via a flexible cable. The investigation revealed significant excessive play in this flexible control cable. Specifically, the butterfly valve only began to move from the idle position once the throttle lever was pushed past the halfway point. Even when the lever was pushed fully forward, the valve failed to open completely. Furthermore, when returning the lever to idle, the valve did not close immediately because the return spring force was insufficient to overcome the lag.
Findings
- The primary cause of the accident was a loss of engine power during the takeoff and initial climb phase.
- Excessive play in the flexible throttle control cable prevented the butterfly valve from reaching the full power position.
- The throttle valve failed to respond to movement until the lever passed the midpoint of its travel.
- The insufficient force of the return spring caused a delay in the valve returning to the idle position.
- The design of the throttle system, which relies on a spring to return the valve to idle in the event of a cable failure, presents an inherent risk during low-altitude climbs if power is unexpectedly reduced.