What happened
On 25 July 1999, a Hawker Hunter F6A, registration G-KAXF, was returning to Cranfield from the Fairford Air Display. While positioned downwind at Cranfield, the pilot attempted to extend the landing gear but found the system unresponsive. Despite normal hydraulic pressure, the landing gear indicators remained unlit and no movement was detected. After attempting several procedures from the pilot's manual without success, the pilot decided to use the pneumatic emergency extension system.
Upon activating the emergency system, the nose and right main gear extended, but the left main gear remained stuck in a partially extended position. Due to concerns regarding fuel levels and the lack of fire cover at Cranfield outside of standard operating hours, the pilot diverted to London-Luton Airport. During the flight to Luton, the pilot attempted to use side-slipping and gear cycling to force the left leg down, but the gear remained stuck.
The pilot proceeded to land at Luton at approximately 140 knots. The aircraft touched down on the right side of the runway. While the left wing was supported by the underwing fuel tank, the aircraft eventually lost directional control at low speed, swinging onto the grass. During the excursion, the left underwing tank ruptured, causing a brief flash fire. There were no fatalities and no injuries to the pilot.
The investigation
Investigators examined the landing gear mechanism, which utilizes an electro-hydraulic valve controlled by cockpit switches. They also performed an engineering examination of the aircraft's pneumatic and hydraulic lines. The investigation focused on why the emergency system failed to fully extend the left main gear despite the presence of sufficient pressure in the emergency reservoir.
Findings
- An intermittent fault was discovered in the microswitch within the common earth line of the electro-hydraulic valve. This fault prevented the valve from responding to cockpit commands.
- A significant leak was identified in a pneumatic pipe joint located in the left wing.
- Although the wire locking was intact, the wire had pulled through a joint nut, causing the connection to loosen.
- The pneumatic leak depleted the high-pressure air supply before the emergency cycle could complete and prevented the oil jettison valve from opening.
- This caused hydraulic fluid to remain trapped in the retract side of the left main gear actuator, effectively jamming the leg in an intermediate position.