What happened
On 17 September 2020, a Westland Scout AH1, registration G-CIBW, was conducting a private flight near West Dean, Salisbury, Wiltshire. While the aircraft was maintaining a straight and level flight at roughly 100 KIAS, the pilot heard a sudden, loud bang. This sound was immediately followed by the realization that the fixed upper window of the left-hand door had become detached and fallen out of the aircraft.
The pilot attempted to locate the missing component by performing a brief search of the airspace above the fields below, but the window could not be found. Following the incident, the crew of three—consisting of the pilot and two passengers—returned the helicopter to Middle Wallop Airfield. There were no injuries to any persons on board, and no further operational issues were reported during the landing.
The investigation
Following the event, the AAIB examined the door and window frame of the aircraft to determine how the window had become dislodged. The investigation focused on the condition of the rubber retaining grommet, which is responsible for securing the window into the frame using a system of ridges and channels.
Upon inspection, investigators discovered that the rubber grommet had perished. Specifically, a section of the central ridge, measuring approximately 200 mm, had separated from the rest of the component. This degradation meant the grommet could no longer provide the necessary tension to hold the window in place, allowing the airflow to loosen the window until it eventually exited the frame.