What happened
During a training flight on 11 December 2009, an instructor and a student pilot were performing aerobatic maneuvers near Debe Dam, returning to Bisho Aerodrome. While executing a "Cuban 8" at approximately 7,300 feet, the student pilot discovered that the control stick could not be pulled back. The instructor took control and confirmed that the elevator flight control surface was jammed, preventing any upward elevator movement.
In an attempt to manage the descent, the crew utilized backward trim and maintained a level flight speed between 250 and 300 km/h. Because the surrounding terrain was unsuitable for a precautionary landing, the instructor opted for a forced landing at Bisho Aerodrome. During the approach, the crew was forced to use both hands to pull back on the control column, preventing them from performing other cockpit procedures.
Upon touchdown at a speed of approximately 250 km/h, the aircraft's inability to control the nose pitch caused it to bounce repeatedly. On the third impact, the nose wheel collapsed, causing the aircraft to slide into the mud alongside the runway. An immediate fire broke out in the engine compartment, though both occupants were able to evacuate safely before the fire was extinguished by responding personnel.
The investigation
SACAA AIID investigators examined the wreckage and the aircraft's maintenance history. The investigation focused on the sudden loss of elevator control during flight maneuvers. While the aircraft had no recorded mechanical defects and was within its inspection intervals, investigators discovered that an engine cowling lever knob had been missing for approximately three months.
During the recovery of the wreckage, the missing knob was located lodged within the elevator bell crank assembly. The investigation noted that the Yak-52's internal structure is largely open, leaving control cables and pushrods exposed to foreign object debris. It was determined that the knob had migrated into the bell crank, physically restricting the movement of the elevator control surface.
Findings
- The primary cause of the loss of control was an engine cowling lever knob that had jammed the elevator bell crank.
- The aircraft's design allows for the migration of loose objects into the aft fuselage area where flight controls are located.
- The owner was aware the knob was missing but had not located it or replaced it.
- The aircraft sustained substantial damage to the engine mount, propeller, and nose gear, and the engine compartment caught fire.
- Weather conditions were clear and not a contributing factor to the accident.