Incident Overview
On 17 May 1998 at 1319 UTC, a privately operated Socata TB10 (registration G-BHER) was involved in a fire incident after landing at Biggin Hill Airport, Kent. The pilot, the sole occupant, was uninjured. The aircraft sustained substantial damage to the right wing and right main landing gear leg.
Flight History
The aircraft had conducted a short flight to Redhill Aerodrome followed by several local flights, then returned to Biggin Hill. After a normal landing, it was taxied at normal speed via a grass taxiway and perimeter track to the parking area, where it was parked between two other aircraft. The taxi duration was approximately five minutes.
Fire Detection and Response
After engine shutdown and opening the left door, the pilot smelled smoke. He completed his paperwork and tidied the cockpit, then noticed a stronger, more acrid smoke. He vacated the aircraft and walked around to check the right door. As he moved to the left side, he saw smoke emanating from the front of the right main gear tyre. Within seconds, the smoke increased considerably and flame appeared. Approximately three minutes had elapsed since engine shutdown.
The fire was initially difficult to extinguish due to its enclosure within the wheel spat. The Airfield Fire Service quickly extinguished the blaze, but not before the right-hand fibreglass wheel spat and the hydraulic flexible hose to the wheel brake were destroyed, and the lower wing skin above the right wheel was melted.
Parking Brake System
The parking brake system comprised a knob on the instrument panel attached to a push/pull cable connecting to the midpoint of a transverse rod. Outer ends of this rod operated separate push/pull cables connected to sprag plates acting against the piston rods of left and right wheel brake master cylinders; the piston rods were connected to the brake pedals. The maintenance organisation reported that the system was inherently prone to sticking, and the cable runs made it difficult to manually free the mechanism if stuck. The pilot stated that the parking brake switch was known to be unserviceable, but there was no indication of brake binding on the incident day or when the aircraft was last operated seven days previously. After the incident, the AAIB found the parking brake mechanism in the 'brakes released' condition.
Investigation of Fire Cause
Due to extensive damage to the brake unit and wheel assembly, the exact cause of the fire could not be positively established. The flying club instructor attributed the fire to dried grass contacting a hot brake disc, as dried grass was found in one of the other wheel spats. However, the AAIB noted that hydraulic fluid can ignite spontaneously under certain conditions when in contact with hot surfaces at temperatures of 270°C or higher. A period of fluid contact with the hot surface, restricted ventilation, and sufficient oxygen mixing are necessary. The incident conditions—a slightly sticking wheel brake causing an abnormally hot brake disc and the protected environment within the wheel spat—may have provided conditions for a hydraulic fluid leakage to result in spontaneous ignition shortly after stopping.
Aircraft and Pilot Details
The aircraft was a 1980 Socata TB10 with one Lycoming O-360-A1AD piston engine. The 37-year-old commander held a Private Pilot's Licence with an IMC Rating and had 238 flight hours (129 on type), with 7 hours in the last 90 days and 6 hours in the last 28 days.