What happened
On 29 October 2017, a Bellanca Decathlon, registration EI-BIV, was conducting a pilot training flight near Cork Airport. The flight involved demonstrating various aerobatic maneuvers and stall recovery techniques. During a specific maneuver intended to demonstrate recovery from a near-inverted attitude, the pilot encountered a sudden and severe flight control issue.
As the pilot attempted to roll the aircraft back to a level position, the ailerons became jammed. The controls could not be neutralized, with the ailerons stuck at a deflection of approximately 10 to 15 degrees (starboard up and port down). This imbalance caused the aircraft to maintain a continuous right-hand roll. The pilot declared a Mayday and managed the aircraft by using a combination of rudder input and airspeed adjustments to maintain a controlled flight path. To avoid potential engine fuel feed issues caused by prolonged flight in a slip, the pilot opted to land as soon as possible. After an initial unsuccessful approach on runway 35, the pilot successfully landed the aircraft on runway 25 using a crosswind from the right to assist the necessary control inputs.
The investigation
The AAIU examined the aircraft following the incident. Upon initial inspection, the control restriction was no longer present. However, a detailed technical examination of the wing structure revealed bright areas of primer damage on the starboard aileron bell-crank mechanism, indicating recent contact with a foreign object.
By tapping on the fabric wing surface, investigators located a large, loose bolt. Testing confirmed that this specific bolt, when positioned within the bell-crank mechanism, produced the exact control restriction reported by the pilot during the flight.
Findings
- The investigation identified the cause of the aileron jam as a loose bolt (an AN7-17A) that had become lodged in the aileron bell-crank mechanism.
- The bolt was likely left behind in a fuselage sill during a previous wing removal operation, possibly as far back as 2015.
- Over time, the bolt migrated through the wing root area into the outboard aileron mechanism.
- The specific aircraft attitudes during the upset recovery training allowed the bolt to move into a position that physically obstructed the control movement.
- The presence of the bolt was likely undetectable during normal flight due to the masking effect of engine and airflow noise.