What happened
On 4 March 2021, a Leonardo AW189, registration G-MCGU, was returning to St Athan following a search and rescue training mission near Porthcawl. After the crew activated the cockpit and cabin heating, light-green foam debris began emerging from the vents under the windscreen and on the left side of the cockpit.
Although the crew deactivated the heating system, they soon detected a smell of burning and observed thicker grey smoke emanating from the vents. The commander issued a MAYDAY call to Cardiff Radar and performed an emergency landing in a field near a road. While the aircraft landed without further incident and no fire was found by emergency services, the crew had been exposed to debris and smoke. Subsequent inspections revealed that a heating duct in the rear fuselage had collapsed, causing insulation material to be ingested by cockpit fans and two Auto-Transformer Rectifier Units (ATRUs).
The investigation
Investigators examined the failed duct and found that the external insulation was missing from a significant section of the component. Laboratory analysis of the failed joints compared to intact joints revealed that the adhesive was primarily present only on the internal surfaces of the flexible sections.
Testing showed that the rigid sections of the duct had an uneven surface with various voids between fibre bundles. In the failed joints, there was a notable absence of the mechanical interlocking (cubic-shaped reliefs) that typically enhances adhesion. Furthermore, the presence of bubbles in the adhesive suggested that the mating pressure may have been lost during the bonding process before the sealant solidified, creating discontinuities in the adhesive layer.
Findings
- The heating ducts failed due to non-uniform adhesion at the joints where flexible and rigid sections of the duct meet.
- The uneven surface of the rigid sections, characterized by voids and resin-filled areas, prevented consistent mechanical interlocking.
- The presence of adhesive bubbles and thin films reduced the overall effectiveness of the bond.
- The failure resulted in the discharge of insulation fragments and smoke, causing respiratory irritation to the crew.