What happened
On September 9, 2012, a Robin DR 400/180 R departed from the Backnang-Heiningen (EDSH) airfield for a local sightseeing flight as part of an aviation event. The aircraft was carrying the pilot and three passengers. Shortly before the departure of the DR 40 and/180 R, an AN-2 had taken off for a similar flight.
Witnesses and video footage recorded the takeoff of the DR 400/180 R on the grass runway. After the rotation phase, the aircraft suddenly entered a continuous roll around its longitudinal axis at a very low altitude, reaching a bank angle of approximately 90 degrees. The aircraft subsequently struck the ground approximately 55 meters beyond the end of the runway, causing a fire. The impact resulted in three fatalities and one person was seriously injured, with one of the injured later passing away in the hospital. The aircraft was destroyed.
The investigation
The BFU investigation focused on the sequence of events and the aerodynamic forces acting on the aircraft. The investigation included complex mathematical modeling and flight testing conducted by the DLR (German Aerospace Center) to simulate the wake turbulence of the preceding AN-2.
Researchers analyzed the decay of the vortex circulation and the sinking rate of the wake. Flight tests conducted in 2014 using smoke generators and a test aircraft confirmed that under nearly windless conditions, the wake from a large aircraft like an AN-2 can remain concentrated and sink to a level where it poses a significant danger to smaller aircraft. The investigation also examined the pilot's credentials, noting that the pilot's license lacked a required signature and a valid extension entry, which technically rendered the pilot unauthorized to fly under the event's specific permit. Additionally, the investigation found that the pilot had withheld medical information regarding the use of blood-thinning medication from his aero-medical examiner.
Findings
- The primary cause of the accident was the aircraft encountering the wake turbulence of the preceding AN-2 during takeoff.
- The roll moment generated by the concentrated vortex of the AN-2 was too powerful for the pilot to compensate for using the DR 400/180 R's ailerons.
- Near-windless weather conditions allowed the wake to remain stable and sink into the flight path of the following aircraft.
- The pilot was flying at minimum takeoff speeds without sufficient airspeed reserve.
- The pilot's license was not valid due to missing administrative entries and signatures.