What happened
On 27 August 2001, a Pierre Robin R1180T, registration G-ROBN, was performing a private flight at Boscombe Down. Following standard pre-flight procedures, the pilot released the parking brake by depressing the brake pedals before beginning the taxi. The aircraft proceeded to the threshold of Runway 05, a journey of nearly two nautical miles that lasted approximately 10 minutes. During this period, the engine was maintained at 1000-1200 rpm, and the brakes were rarely used.
As the aircraft approached the holding point, the pilot observed smoke resembling dust emanating from the left wheel area. The volume of smoke increased rapidly as the aircraft came to a stop. The pilot instructed the passenger to evacuate the aircraft while she performed shutdown procedures. Due to the intensity of the smoke and concerns regarding potential fuel tank ignition, the pilot did not attempt to use a fire extinguisher. After contacting the Airfield Fire Service and police via mobile phone, flames became visible around the left landing gear leg. The fire was subsequently extinguished by the fire service.
The investigation
Investigators examined the aircraft and found significant damage to the underside of the left wing, including wrinkles in the upper wing skin that indicated the main spar had been distorted. The damage pattern suggested the leg fairing acted as a chimney for the fire. The glass-fibre fairing and wheel spat were burned, and the investigation found that a failure in the hydraulic pipe attachment to the calliper, caused by heat, likely released hydraulic fluid which assisted the fire.
An inspection of the braking system revealed that the left-hand valve seal in the brake valve was softer than the others. This led to the possibility that the seal remained on the valve seat when the brake was released, preventing pressure from escaping the lines. However, applying the toe brakes would have dislodged the seal. While some grooving was noted on the cams, the depth of these grooves was considered insignificant. No other defects, such as blocked reservoir vents or stiff linkages, were identified.
Findings
- The extended taxi duration allowed for a significant build-up of heat.
- The left brake was dragging during the taxi.
- The wind direction, coming from the right rear quarter, may have masked the dragging brake or caused the pilot to apply light left brake pressure to maintain the aircraft's track.
- The wheel spat may have inhibited the dissipation of heat from the braking component.