What happened
On September 15, 2004, a Cessna 172 N, registration OK-JKV, was conducting a flight training mission for a student pilot in the airspace northeast of Prague Letňany. The flight, part of a PPL training syllabus, included maneuvers for practicing emergency landings.
Following the completion of the maneuvers, the crew attempted to retract the flaps. During this process, the left flap became stuck in a 20-degree extended position, while the right flap retracted normally. The flight instructor took control of the aircraft and declared an emergency, requesting landing instructions for runway 24 at Prague Kbely. Due to the asymmetric flap configuration, the aircraft's handling was restricted during the descent and approach. The instructor requested fire services as a precaution. The aircraft landed safely at Prague Kbely, and no injuries were reported to the crew.
The investigation
The ÚZPLN investigation focused on the mechanical failure of the flap actuation system. Technical examination of the aircraft revealed that while the cockpit flap selector was set to 0 degrees, the left flap remained at 15 degrees of extension.
Investigators found that during the retraction attempt, the flap tracks became jammed. The force applied through the flap control mechanism caused the middle section of the left flap to deform and bulge 15 mm to the left. This lateral displacement restricted the movement of the left aileron, thereby limiting the aircraft's lateral control capabilities. The investigation also noted that for aircraft of this specific serial number range, the manufacturer had implemented a simplified flap hinge assembly design starting in 1981.
Findings
- The primary cause of the incident was a technical failure of the bearing in the left flap hinge assembly.
- The failure of the needle bearing caused the flap to jam in the tracks.
- The resulting mechanical deformation of the left flap led to asymmetric lift and restricted aileron deflection.