What happened
On 17 April 2011, a Britten Norman Islander VP-MNI was performing a commercial passenger flight from Antigua to John A Osborne Airport in Montserrat. Following a standard touchdown on Runway 10, the pilot discovered that the right brake pedal provided no resistance. To prevent the aircraft from overrunning the runway end—which features a significant vertical drop—the pilot applied the left brake to steer the aircraft onto the grass. During this maneuver, the aircraft struck a raised embankment approximately 150 meters from the end of the runway. The impact caused the nose landing gear to collapse and damaged the nose structure and the left wing tip. There were no fatalities and no injuries among the seven passengers and the single crew member.
The investigation
The AAIB investigation focused on the mechanical failure of the braking system and recent maintenance activities. It was established that a replacement of an O-ring seal on the right brake assembly had been performed prior to the flight. While the engineer replenished the hydraulic fluid and performed a bleeding procedure, the investigation found that the process was not completed in accordance with the aircraft maintenance manual (AMM). Specifically, the bleed screw at the top of the landing gear leg had not been opened, which allowed air to remain trapped in the hydraulic lines.
Furthermore, the investigation identified discrepancies in brake bleeding protocols. The manufacturer provided different procedures for the Islander and Trislander models, and various maintenance organizations were using different methods, some even employing hand pumps to ensure all air was expelled.
Findings
- The loss of right-side braking was caused by trapped air within the hydraulic lines.
- This air was likely introduced during the recent replacement of a brake O-ring seal.
- The maintenance procedure performed did not fully adhere to the requirements of the aircraft maintenance manual, specifically regarding the upper bleed screw.
- Discrepancies exist between the manufacturer's manuals for different aircraft models with similar braking systems.
Safety action
Following the investigation, a Safety Recommendation (2011-093) was issued to Britten-Norman Aircraft Ltd, requesting a review of the various brake bleeding procedures for Islander and Trislander aircraft to develop a single, effective, and standardized procedure for publication in maintenance manuals.