What happened
On 21 June 2003, a Jabiru SK, registration G-JABA, was conducting a test flight near Popham Airfield in Hampshire to renew its Permit To Fly. The aircraft, carrying a pilot and one passenger, had been refuelled to a weight of 430 kgs. Following a successful engine power check, the aircraft departed from Runway 08.
As the aircraft reached approximately 300 feet, the pilot noticed a vibration that intensified over roughly 30 seconds. The vibration became so severe that it appeared a cylinder had ceased firing. Due to the loss of power, the crew could not maintain altitude or return to the runway, necessitating an emergency landing in a nearby field. Upon entering the field, the aircraft encountered tall crops which caused the wheels to sink, leading the aircraft to pitch forward. The nose landing gear was detached, and the aircraft slid for approximately 20 yards before coming to rest upside down. Both occupants sustained minor injuries.
The investigation
The AAIB examined the engine and the propeller, noting that one propeller blade had splintered, though the propeller remained free to rotate. An inspection of the cylinders via borescope showed no signs of internal damage, seizure, or scoring. However, further investigation of the engine components revealed that the inlet valve in the forward left-hand cylinder was not moving.
Technical examination focused on the valve clearance adjuster. It was discovered that the adjuster had become loose and migrated from its intended position, creating an excessive valve clearance of approximately 0.3 inch. The mechanism relies on a lock nut to secure the threaded adjuster rod in place. Investigators found that while the lock nut was present, it was loose and failing to prevent the rod from rotating.
Findings
- The primary cause of the engine malfunction was insufficient torque applied to the valve clearance adjuster lock nut.
- The engine cylinders had recently been removed for rework and reinstalled prior to the test flight, a process requiring the resetting of valve clearances.
- The failure of the lock nut to secure the adjuster allowed the clearance to increase significantly, leading to the loss of engine power.