Too heavy, too wet, too short

Course of events
On Wednesday, 19 June 2019, the pilot and the passenger, both experienced pilots, met at Bellechasse airfield to conduct a local flight in the self-launching two-seater glider Arcus M, registered as D-KEJB. The fuel tank in the fuselage was completely filled and there were about 10 litres of water ballast in the fin. Shortly after 13:10 hrs, the D-KEJB left the stand next to the runway, initially in the direction of the threshold of runway 08, and then taxied along the entire runway to the take-off position of runway 26. There, the usual checks were carried out, including an engine check. According to the two pilots, the procedure in the event of an engine failure was discussed and it was decided at which point the take-off should be aborted at the latest (abort point).

Shortly before 13:15 hrs, the take-off run was initiated on runway 26. According to the pilot, the engine developed its usual power, but in his opinion, the glider did not accelerate normally, possibly because of the grass along the runway, which was somewhat taller. At the abort point, the required speed had almost been reached, so he continued the take-off run, but there was no further acceleration. The flaps were in the +2 position. Consequently, the glider overran the end of the runway and came to rest in an adjacent cornfield, where the corn was about 80 cm high, approximately 100 m from the end of runway 26. An eyewitness watching the take-off run from the edge of the runway saw how the aircraft seemed to lift off very slightly towards the end of the runway and then touched the crops in the adjacent field with the underside of the fuselage. Up until this point, he had not noticed any reduction in the engine regime. The glider then flew a short distance above an area of terrain depression, before descending in stable glide onto the corn field and came to a standstill after a turn of about 180° around the vertical axis. The two pilots were able to leave the glider uninjured. There was minor damage to one winglet. The pilots stated that the glider was in perfect technical condition. They did not claim any technical causes for the serious incident.

According to the two pilots, the glider did not reach the necessary speed during the take-off run. As a result, it rolled over the end of the runway and came to a halt in the cornfield. However, the eyewitness’s observation and the traces in the cornfield show that the glider was flying over the crops for some distance, probably aided by the depression in the terrain after the end of runway 26. In any case, the energy available at the end of the runway was so high that the glider, partly rolling, partly flying, still covered a distance of about 100 m before it was slowed down asymmetrically by the crops, which resulted in a turn of approximately 180° around the vertical axis. It is, therefore, more likely to have been an involuntary landing after a failed take-off attempt, than a simple runway excursion. In particular, the high energy present at the end of the runway and the observation of the eyewitness show that, at least up until this point, no attempt was made to abort take-off.

According to the flight preparation, the glider was close to the maximum take-off mass of 800 kg approved by the manufacturer, with a total mass of approximately 785 kg. According to the information in the flight manual (cf. figure 2), the take-off run on a hard surface runway is 283 m and the take-off distance over a 15 m obstacle is 548 m if the conservative values of a pressure altitude of 500 m and a temperature of 30 °C are assumed. If a 20 % distance increment for a firm, level grass runway is added, which in the present case was probably too optimistic in view of the grass height along the runway and the moisture that may still have been present, this results in values around 340 m and around 660 m respectively. These figures underline the fact that, in view of the existing runway length of 520 m, the distance margins were small.

The higher mass of the glider, the slightly taller grass, the approximately 80 cm high crops at the end of the runway, the temperature which was about 13 °C above the standard temperature for Bellechasse airfield according to the International Standard Atmosphere (ISA) as well as the lack of wind – all these factors accumulated and led to an insufficient take-off performance for a safe take-off.

The pilots were very experienced, and it can, therefore, be assumed that they were aware of all these adverse factors. However, they apparently underestimated the cumulative effect or overestimated the take-off performance of the self-launching glider, which can possibly be explained by their rather limited experience on this type of glider. Although they had the impression that the Arcus M did not accelerate normally to reach the necessary speed at the defined abort point, the pilots decided to continue the take-off, which was not safety conscious. A take-off abort-point, that has been briefed, is useless and ineffective if it is not implemented consistently.

The serious incident, during which a self-launching two-seater glider overshot the end of the grass runway on take-off and came to rest in a cornfield, was due to the insufficient performance of the glider for a safe take-off, given the prevailing conditions and the fact that the take-off was not aborted on time. Source/entire report: ‚SUST‚.

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