What happened
On 26 July 2020, a Robinson R44 Raven II, registration G-WTWT, was conducting a private flight near Herne Bay, Kent. After an initial flight to a friend's property, the pilot took off with three new passengers. During the climb to 400 ft, the pilot noticed that the left front door was not fully latched, leaving a small gap. An attempt by a passenger to secure the door resulted in the door opening further, prompting the pilot to decide on a precautionary landing in a nearby field.
While attempting to manage the door issue, the pilot's attention was diverted from the aircraft's flight path. During the descent, the helicopter experienced a sudden and significant increase in the rate of descent that could not be arrested through increased collective input. The aircraft struck the ground on its right skid and rolled over. The pilot and two passengers sustained minor injuries.
The investigation
The AAIB examined the flight path using GPS data from a tablet-based navigation app. The data revealed that as the helicopter descended below 100 ft, the airspeed had dropped below 15 kt while the rate of descent exceeded 1,000 ft/min. The investigation also noted that the pilot, while focused on the door, had inadvertently turned the aircraft to the east, resulting in a downwind approach to the landing site.
Findings
- The pilot was unaware that the aircraft had transitioned into a downwind approach during the distraction of the door malfunction.
- The aircraft likely entered a vortex ring state, an aerodynamic condition caused by the combination of power being applied, a high rate of descent, and low airspeed.
- The loss of control responsiveness is consistent with the onset of vortex ring state, which prevented the pilot from effectively using cyclic or collective inputs to level the aircraft.
- The incident highlights the risks of passenger distraction and the difficulty of verifying aircraft security when passengers board while rotors are still turning.