What happened
On 22 May 2007, a Hawker Hunter T7, registration G-BVGH, was performing a private flight from Exeter Airport. The takeoff roll on Runway 26 began normally, with the pilot reporting no initial directional control issues. However, as the aircraft reached the later stages of the takeoff run and the nosewheel lifted from the ground, the aircraft suddenly swung sharply to the right.
Despite the pilot applying full left rudder and left-wheel braking, the aircraft could not be corrected. The pilot subsequently aborted the takeoff, but the aircraft departed the paved runway surface to the right. During the deceleration process, the pilot deployed the brake parachute. The right main landing gear failed under the stress, causing the right external fuel tank to strike the ground and rupture, resulting in fuel spraying across the fuselage. The engine was shut down, and the aircraft came to a halt. The two crew members escaped the aircraft without injury.
The investigation
Following the incident, the aircraft's maintenance organisation conducted an initial inspection, noting evidence of overheating in the right brake unit. Witnesses also observed smoke emanating from the right wheel just before the loss of control. The AAIB examined the right brake selector unit and the Maxaret anti-lock braking system.
While the Maxaret system was found to be functioning correctly, the investigation focused on the brake selector unit, which regulates pressure to the main wheel brakes. Disassembly of the unit revealed that the right brake valve plunger had been subjected to wear, evidenced by marks on its shaft. Measurements indicated that the sleeve for the right valve had an excessive clearance of 0.06 mm.
Findings
- The right brake valve plunger became jammed in a position that kept the valve partially open.
- This mechanical failure allowed continuous hydraulic pressure to be applied to the right brake unit regardless of pilot input.
- The resulting friction caused rapid heat build-up, leading to the seizure of the right brake unit at the moment the nosewheel lifted.
- The excessive clearance in the valve sleeve allowed for a rocking motion that eventually caused the component to jam.
Safety action
- The maintenance organisation has implemented routine spectrographic oil analysis of the hydraulic fluid in the brake system to detect early signs of component wear.