History of the Flight
Following an uneventful flight from VC Bird Airport, Antigua, the aircraft made a normal approach and touchdown on Runway 10 at John A Osborne Airport, Montserrat. When the pilot applied the brakes, the right brake pedal offered no resistance. The pilot attempted to pump the brakes without effect. To avoid departing the end of the runway, the pilot applied left brake and allowed the aircraft to veer left onto grass beyond a taxiway exit. The aircraft struck a raised embankment about 20 m north of the runway edge and approximately 150 m from the end of the runway. The impact occurred at an estimated speed of 10 kt. Damage included the nose structure, collapsed nose landing gear, left wing tip leading edge, and left propeller. All seven passengers and the pilot exited via the main door without injury.
Aircraft Examination
An engineering representative examined the aircraft. No brake fluid leaks were visible. The left brake pedal operated normally but felt slightly 'spongy'. The right brake pedal deflected fully with no resistance and remained depressed; tapping the pedal returned it to its normal position. Slow pumping increased pressure and the brake began to operate. The parking brake held for 10 minutes with no leakage. Rudder and nosewheel steering cables were correctly connected and routed. The engineer concluded that trapped air in the hydraulic lines caused the right brake failure.
Maintenance History
On a previous landing at VC Bird Airport, the pilot noted the right brake pedal required near-full deflection to obtain pressure. The local engineer found a leak at the right outer brake assembly and replaced the piston O-ring seal. During the replacement, a small amount of fluid was lost. The engineer replenished the right master cylinder reservoirs and bled the right brake system with the pilot's assistance by opening bleed screws on both right inner and outer brake assemblies. After bleeding, the brakes were tested and functioned normally. Approximately 45 minutes to one hour later, the pilot taxied for departure and confirmed both brakes were operating normally.
Brake System and Bleeding Procedures
The Islander has four hydraulic brake units, one per main wheel, with master cylinder reservoirs at each toe brake pedal. Bleed screws are located on top of each main gear leg and at each wheel brake unit. The aircraft maintenance manual (AMM) specifies a bleeding procedure that involves pumping brake pedals with bleed screws open in a particular sequence. The manufacturer also provides a different procedure for the Trislander (same brake system) and a production bleeding procedure that differs from the AMM. The engineering representative for the owner reported that the AMM procedure did not always expel all trapped air and advocated an alternative method using a hand-operated oil pump. Two UK operators contacted indicated they used variations of the Trislander procedure or a hand pump to clear air. The investigation noted that the right brakes had not been bled in accordance with the AMM after the O-ring replacement.
Pilot Licensing
The pilot held a JAA Commercial Pilot's Licence issued by the UK CAA with a Montserrat Certificate of Validation, and a JAA Class 1 Medical Certificate. Aviation activities in Montserrat are regulated by Air Safety Support International (ASSI) under the Air Navigation (Overseas Territories) Order.
Probable Cause
The loss of right braking was attributed to trapped air in the hydraulic lines, which was probably introduced during the right brake O-ring seal replacement prior to the accident flight, and the brakes were not bled in accordance with the aircraft maintenance manual. One Safety Recommendation has been made.