What happened
On 7 January 2022, an AW189 helicopter, registration G-MCGV, was returning to Lydd Airport following a search and rescue training mission. During the transit, the crew activated the cabin and cockpit heating system. Approximately 6 nautical miles from the destination, crew members in the rear of the cabin alerted the pilots to floating fibres in the air that were causing respiratory irritation.
In response, the flight crew deactivated the heating and ventilation system and utilized onboard protective face masks. To minimize time in the air, the commander performed an expedited approach and a running landing at Lydd Airport. While the pilot-flying remained largely unaffected, the non-flying pilot experienced mild respiratory irritation. Following the safe landing, the aircraft doors and windows were opened to ventilate the cabin and clear the airborne contaminants.
The investigation
An examination of the aircraft revealed that the aft heating duct had collapsed, which allowed insulation material to be pulled into the ventilation system. This incident followed similar failures on other aircraft in the operator's fleet, specifically G-MCGU and G-MCGT.
Investigators found that G-MCGV had undergone a service bulletin modification only 71 flying hours prior to the failure. This modification was intended as an interim measure to improve the duct installation. Laboratory analysis of previous failures indicated that non-uniform adhesion on the bonded surfaces between the rigid and flexible sections of the duct was a factor. Furthermore, while the duct was qualified for use on the AW189 by similarity to the AW139, the AW189 operates with higher air pressure and flow rates, which likely increased the stress on the bonded joints.
Findings
- The failure of the heating duct allowed insulation fragments to contaminate the cockpit and cabin.
- Non-uniform adhesion on the bonding surfaces between the flexible and rigid duct sections contributed to the failure.
- The existing service bulletin modification failed to prevent the recurrence of the issue on G-MCGV.
- Higher air pressure and flow rates within the AW189 system compared to similar aircraft models likely placed excessive loading on the duct joints.