What happened
On May 22, 2020, at approximately 13:55 LMT, a PZL-104 Wilga 35A, registration SP-AGD, was performing a glider towing operation at Kielce-Masłów airport [EPKA]. While the pilot was searching for thermal lift in a circling pattern, the aircraft suddenly lost aileron control capability.
The pilot immediately radioed the towed SZD50-3 Puchacz glider, commanding, "Puchacz wyczep się" (Puchacz, release), and followed up seconds later with, "Puchacz wyczep się, nie mam lotek" (Puchacz, release, I have no ailerons). The glider pilot released the tow at an altitude of approximately 250 m AGL and transitioned to free flight within a thermal column.
Following the release, the pilot of the PZL-104 Wilga 35A notified the flight controller of the control failure while the aircraft was in a left turn over the northeastern part of the airfield. Following instructions from the flight controller to utilize the rudder for directional control, the pilot stabilized the flight and proceeded to land. The landing was completed without further incident, and the pilot sustained one minor injury.
The investigation
The PKBWL investigation focused on a technical analysis of the aileron control system. Investigators examined the lower end of the left aileron control pushrod (P/N:CE15509). The inspection revealed a fracture at the threaded end of the lower clevis end (P/N:X1148).
Technical analysis of the fracture surface showed characteristics of a fatigue break, including distinct striations and evidence of a long-term propagation of a crack covering approximately 70% of the circumference. The investigation also noted that the aircraft was an older model, having been in service since 1983, and that the physical design of the airframe made it difficult to inspect the pushrod ends inside the fuselage.
Findings
- The primary cause of the incident was a fatigue fracture of the lower clevis end of the left aileron control pushrod.
- The failure was likely caused by excessive forces acting on the control linkage, resulting from the failure to secure the ailerons during aircraft parking. This allowed wind gusts to cause uncontrolled aileron deflections, leading to fatigue loading.
- The long-term operation of the aircraft contributed to the fatigue failure.
- Difficult access to the pushrod ends within the fuselage hindered the detection of the developing crack during routine inspections.
Safety action
- The Civil Aviation Authority (ULC) was recommended to mandate inspections of the pushrod ends on all PZL-104 Wilga aircraft to check for cracks or signs of fatigue, to be performed during the next 100-hour maintenance interval.