What happened
On 23 July 2004, a Fairey Britten Norman BN2A Mk III-2 Trislander, registration G-BEVT, was performing a scheduled flight from Guernsey Airport. Shortly after takeoff, while climbing through 500 feet, a loud impact was heard within the cabin. A de-icer boot from the left-hand propeller had detached and struck a cabin window, causing it to shatter. The impact resulted in two injuries to passengers, one of which was classified as serious.
Following the incident, the commander immediately initiated an emergency return to Guernsey Airport. The aircraft landed safely at 0641 hrs, and emergency services met the aircraft on the runway. Upon inspection, the pilot discovered that pieces of the detached de-icer boot had entered the passenger cabin through the broken window.
The investigation
Investigators examined the mechanical failure of the propeller de-icing system and the maintenance history of the aircraft. Laboratory analysis conducted by QinetiQ revealed that the failure was characterized by brittle fractures in the de-er lead strap, consistent with a high-strain impact. The investigation focused on why the adhesive bond between the boot and the propeller blade had failed.
It was established that while the installation was certified as compliant with the required manuals, a specific filler material mandated by the Hartzell manual had not been applied at the root of the de-icer boot. This omission left a small unbonded area near the lead strap. This gap likely allowed moisture and other contaminants to enter, progressively degrading the adhesive bond over time. The investigation also noted that the propeller had recently undergone maintenance to replace a harness guard and restrainer strap, which may have contributed to initial bond damage.
Findings
- The primary cause of the accident was the separation of a de-icer boot from the left propeller during the takeoff phase.
- The separation was driven by peel stresses acting on the adhesive bond.
- These stresses were exacerbated because the required filler material was missing from the root of the boot, allowing for contamination and bond degradation.
- The aircraft's certification at the time did not require passenger protection against debris from propeller components.