Circumstances
On March 15, 2012, at 13:40 local time, a Robin DR 400-120, registration F-GXGB, was conducting a takeoff from runway 05R at Sarreguemines Neunkirch aerodrome. The runway was unpaved with a TODA of 714 m. During the takeoff roll, the student pilot reported pulling on the elevator control for rotation at a speed between 100 and 110 km/h, but the aircraft did not respond. The instructor then took over and applied full elevator deflection without any reaction from the aircraft. The instructor aborted the takeoff and applied brakes. The aircraft overran the runway longitudinally, crossed a ditch, nosed over, and came to a stop 160 m beyond the runway end. After stopping, the instructor observed flames at the front right of the engine. Both occupants evacuated the aircraft using the emergency handle. Approximately one minute after evacuation, an explosion occurred, and the aircraft was fully consumed by fire.
The instructor reported that the aircraft was configured for takeoff on runway 05 (preferential calm wind), takeoff power was set, and acceleration appeared consistent with indicated speed. No alarms or suspicious noises from the engine, cockpit, or rear were perceived.
Examination
Examination of the site and wreckage revealed the following:
- Wheel tracks of all three wheels were visible on the grass from 350 m from the threshold of runway 05 to the wreckage.
- The nose gear collapsed when crossing a ditch perpendicular to the runway, outside the aerodrome's easements.
- The continuity of the elevator control chain was found broken at the connection of the push and pull cables with the elevator control horn. The horn had melted during the fire and could not be examined.
- A deposit of molten metal trapped in the clevis of the push cable indicated that the connection between this clevis and the lower arm of the elevator horn was intact before the fire.
- The absence of molten material in the upper clevis of the pull cable indicated that the horn probably separated from its attachment before the fire.
Steel cables expand or contract with temperature variations. At low temperatures, they contract, resulting in increased tension. Repeated stress on the aluminum alloy elevator horn can cause damage at the connection hole, potentially leading to rupture.
During February 2012, the region experienced intense cold. Meteo France data from Doncourt les Conflans station (23 km from Chambley) showed a minimum temperature under shelter of -16°C and temperatures never above 0°C from February 1 to 12. From February 4 to 15, operations at Chambley were restricted by NOTAM due to ice on the runway. The aircraft flew for 9 days during this cold period.
Another instructor from the club at Chambley, where the aircraft was based, had noted significant tension and resistance when maneuvering the flight controls during the intense cold in February. This prompted a request to adjust the flight control cable tension, but the request was not acted upon. The perception was felt during preflight inspections in the early morning and faded during the day, disappearing completely when temperatures became positive a few days later.
The maintenance chief of another club based at an altiport, where aircraft operate part of the year in very low temperatures, stated that cable tension adjustments are systematically made every spring and autumn. By experience, cable tension is reduced in autumn within the limits recommended by the manufacturer (between 45 and 55 pounds).
Conclusion
The accident was probably due to the rupture of the upper end of the elevator control horn during the takeoff phase. The recent period of intense cold contributed to increased tension in the elevator control cables. This tension is likely the cause of premature wear on the end of the elevator horn. This hypothesis remains the most plausible given the state of the wreckage after the fire.