What happened
On September 21, 2019, a Duo Discus glider, registration HB-3411, conducted a local familiarization flight at the Bern-Belp Regional Aerodrome. The aircraft was occupied by a pilot and a flight instructor. During the flare phase of the first landing, the pilot experienced an unusual nose-heavy sensation. Although the pilot pulled the elevator control to the backstop, the aircraft landed more abruptly than expected on the grass runway. The pilot noted that the control feel seemed normal, but the aircraft's behavior was atypical for this specific model, which is equipped with trailing edge flaps.
Following the first landing, the aircraft was used for a second flight by another pilot. During this subsequent flight, the pilot also observed a nose-heavy tendency, noting that the aircraft nose could not be maintained in a horizontal position during turns despite applying full back elevator pressure. The pilot subsequently terminated the flight and landed safely.
The investigation
Investigators examined the aircraft's center of gravity and found it to be within the normal operating range. Upon inspecting the elevator deflections, the investigation established that the elevator had significantly reduced upward travel. After removing the front seat pan, investigators discovered a clamp containing a ferrite core lodged in the fuselage. This object was physically obstructing the elevator control linkage, preventing the control column from reaching its full mechanical stop.
Findings
- The investigation could not determine the origin of the clamp or how it became positioned under the seat pan; it may have fallen during maintenance or shifted during flight due to vibrations.
- A loose object obstructed the elevator control linkage, limiting the maximum possible control surface deflection.
- Standard pre-flight inspections and checklists typically verify the freedom of control movement and the integrity of rudder connections, but they do not involve measuring the specific degree of control surface deflection.
- Visual or tactile inspections of the controls may fail to reveal subtle limitations in the maximum range of motion.