What happened
On April 14, 2018, at 13:35 LMT, a Socata Rallye 235ED departed from Warsaw Babice (EPBC) for the purpose of towing a glider into thermals. The weather conditions were VMC with an ambient temperature of 30°C. During the initial climb phase, the pilot noticed intermittent drops in engine RPM and power, which caused the towing speed to decrease to approximately 100 km/h.
At an altitude of roughly 100 meters, the pilot instructed the glider pilot to release the towline. The glider pilot successfully released the cable and landed safely without incident. The towing aircraft stabilized its speed at approximately 120 km/h and performed a 210-degree windward turn to return to the airfield. During the return flight and subsequent engine tests, the engine appeared to function normally, and the aircraft was not immediately removed from service.
The investigation
The investigation revealed that the incident was not reported to the PKBWL within the statutory 72-hour window; the operator notified the commission more than a single month after the event. While an inspection of the fuel and ignition systems was performed on April 21, 2018, it failed to identify any issues. However, following recurring performance problems, the aircraft was grounded for a more extensive inspection between May and July 2018.
This subsequent evaluation identified significant contamination of the air filter, which contained pieces of grass and substantial amounts of sand. Additionally, the inspection found an excessively rich fuel-air mixture and exhaust leakage from the muffler housing.