What happened
On 6 May 2022, a Tecnam P2008 JC, registration SP-SCS, was performing an aerodrome traffic circuit training flight near Chojna. While at an altitude of approximately 1000 ft AGL, the crew noticed increased resistance when moving the stabilator controls. The flight instructor immediately took control of the aircraft and initiated a descent at low speed to minimize the forces acting on the control system.
During the final approach to runway 27, approximately 10 seconds before touchdown, the resistance increased further until the stabilator became completely jammed. The instructor was forced to rely on the electric trim to complete the landing. The aircraft landed without further incident, and there were no injuries to the crew.
The investigation
The PKBWL investigation focused on the mechanical integrity of the stabilator suspension system. Physical inspections of the aircraft revealed that one bolt in the hinge assembly had broken, while a second bolt showed significant damage to its threaded portion.
Measurements of the original bolts (P/N UNI 5 .737-6-35) revealed that the unthreaded portion of the bolt was only 12 mm long. Investigators determined that the thickness of the connected elements—including hinges and washers—exceeded this 12 mm limit. Consequently, under operational loads, the components shifted onto the threaded part of the bolt, causing thread damage and eventual failure.
Findings
- The primary cause of the incident was a design flaw in the stabilator control system involving insufficient bolt length.
- The original 6 mm diameter bolts lacked a sufficient unthreaded section to accommodate the thickness of the mounting hardware.
- While the manufacturer had previously issued Service Bulletin SB 298 - CS to replace these bolts with a different specification (P/N AN4 - 11A) that provides a longer unthreaded section, this bulletin was not mandatory, allowing the risk to persist on aircraft where the update had not been implemented.
- Evidence from other aircraft operators suggested that this specific bolt failure had been observed in at least eight other instances during maintenance or inspections.