What happened
On 24 April 2005, a BN2-A Mk III-2 Trislander, registration G-BEVT, was performing a passenger flight from Alderney to Guernsey. During the takeoff ground roll at approximately 60 kt, the aircraft commander heard a loud bang. A passenger in the cabin reported that a window had been broken, and it was subsequently discovered that a de-icer boot had separated from the left propeller and entered the cabin. The aircraft returned to Guernsey Airport after approximately four minutes of flight. While there were no injuries to the nine passengers or the crew, the separation of the boot caused damage to the aircraft window.
The investigation
Investigators examined the detached boot and the propeller, part number HCC3 and serial number CK3663A. Laboratory analysis revealed that the failure occurred between the adhesive and the boot itself, rather than between the adhesive and the propeller blade. The investigation also reviewed the overhaul history of the propeller, noting that the boots had been replaced approximately 175 hours prior. The investigation found that the overhaul process had omitted the required application of a filler material at the root of the boot, which is intended to protect the bond from environmental and physical damage.
Findings
- The separation was caused by peel stresses generated by forces on the propeller blade.
- These stresses were able to exploit a small disbonded area at the root of the blade, which had developed because the required filler material was not applied during the overhaul.
- Physical or environmental damage to the adhesive bond at the root allowed the disbond to grow under mechanical stress until the boot separated.
- The investigation noted that while the adhesive bond can be made robust through meticulous adherence to procedures, minor deviations in application, such as incorrect drying times or improper surface preparation, can significantly reduce bond strength.