What happened
On 18 July 2016, a Piper L18C Super Cub, registration G-AXLZ, was conducting a training flight from Shoreham Airport. During the takeoff roll on Runway 24, the pilot noted that the grass runway surface was particularly uneven and bouncy. The aircraft became airborne before reaching its standard takeoff velocity.
Shortly after liftoff, the aircraft experienced an initial left wing drop, which the pilot successfully corrected. However, the right wing subsequently dropped. Despite the application of full corrective aileron, the pilot could not prevent the wing from striking the ground. The aircraft's fuselage made contact with the surface and the plane rotated 180 degrees before coming to a stop. The pilot sustained minor injuries, while the passenger was uninjured. The aircraft was destroyed.
The investigation
The investigation established that a Robinson 44 helicopter had performed a hover-taxi across Runway 24 approximately two minutes before the takeoff of G-AXLZ. Investigators examined the aerodynamic implications of the runway conditions and the behavior of wake vortices in light wind conditions.
It was noted that the low wind speed present during the event would have prevented the rapid dissipation of the wake vortex generated by the preceding helicopter. This allowed the vortex system to remain active and drift across the path of the departing aircraft. Furthermore, the investigation reviewed the risks associated with operating from uneven grass surfaces, noting that such conditions can cause an aircraft to become airborne at speeds lower than normal, reducing the margin for aerodynamic recovery.
Findings
- The uneven runway surface likely caused the aircraft to become airborne at a speed lower than its standard takeoff velocity.
- Light wind conditions contributed to the prolonged existence and movement of the wake vortex across the runway.
- The aircraft encountered a wake vortex system produced by a helicopter that had previously hover-taxied across the runway.
- The pilot's attempt to correct the wing drop using ailerons was insufficient to overcome the aerodynamic forces exerted by the vortex at low airspeed.