What happened
On 27 June 2019, an Agusta A109E, registration G-ETPI, was performing a post-maintenance flight near Seaton, Cornwall. The flight was intended to conduct a track-and-balance check of the main rotor. The aircraft departed from Liskeard Heliport with the commander in the right seat and an engineer in the left seat to record flight data.
As the helicopter accelerated toward 140 kt, the crew heard a loud bang accompanied by a significant increase in wind noise. The commander immediately realized that the left cockpit door window had separated from the aircraft. There was no accompanying vibration or loss of tail rotor control. The flight was terminated, and the aircraft returned to L/L Liskeard Heliport to land safely. No injuries were sustained by the crew.
The investigation
The investigation examined the maintenance history of the aircraft, which had recently undergone a full external repaint. This process required the removal and reassembly of all cabin and cockpit doors and windows. Investigators reviewed the maintenance worksheets, which documented the reinstallation of the doors and windows but lacked specific completion dates for individual sub-tasks.
Physical inspections of the aircraft revealed that the left cockpit door window aperture showed no residual adhesive on the door structure. A greasy residue was also noted around the area. In contrast, the right cockpit door window was found to have the correct amount of adhesive present. The investigation also looked into the methods used to install the seal filler strip, which is a critical component of the window's retention system.
Findings
- The primary cause of the window separation was the insufficient application of adhesive to the door side of the rubber retaining seal.
- The use of a highly concentrated soap solution to assist in fitting the seal filler strip likely contributed to the failure. The excessive amount of solution migrated into the seal grooves, leaving a soapy residue after the water evaporated, which acted as a lubricant.
- The aircraft's ventilation system was set to maximum, creating a cabin pressure differential that may have helped push the window outward.
- A lack of effective supervisory oversight meant that no staged inspections were performed to verify the presence of adhesive during the installation process.
- Maintenance documentation lacked sufficient detail, making it difficult to track the specific timing and personnel involved in individual tasks.