What happened
On November 8, 1999, a Steen Skybolt private biplane, registration EI-CIZ, departed from Abbeyshrule Airfield in County Longford. Shortly after takeoff, while the aircraft was climbing at an altitude between 200 and 300 feet, the engine abruptly lost power.
Recognizing the emergency, the pilot identified a suitable field located at a 90-degree angle to the flight path. The pilot executed a turn and a side-slip maneuver over a hedge to reach the landing site. Upon impact with the ground, the aircraft's undercarriage collapsed, and the plane skidded approximately 30 meters. The pilot sustained minor injuries, specifically a small bump to the head, and was able to exit the aircraft promptly. There was no fire following the impact.
The investigation
Investigators from the AAIU examined the aircraft and the fuel system to determine the cause of the power loss. During the field investigation, the fuel selector lever was found in the "Off" position. Subsequent testing of the engine revealed that while the engine could run at lower power settings, it would cut out after approximately 32 seconds when the fuel selector was moved to the "Off" position, matching the timing of the accident.
Laboratory analysis of the fuel selector valve revealed that the component was extremely stiff to operate. The investigation focused on the possibility that the valve's internal ports were not properly aligned during the pilot's pre-flight checks. Further consultation with the aircraft kit manufacturer confirmed that the specific type of "brass to brass" valve used in this build was prone to binding and could fail to align ports correctly.
Findings
- The engine failure was caused by a restriction in fuel flow due to misaligned internal valve ports.
- During pre-flight checks, the pilot likely moved the fuel selector to verify its position, but the stiffness of the valve meant the cockpit indicator may have suggested the valve was in the "Main" position when the internal ports were actually partially blocked.
- This partial blockage allowed enough fuel for taxiing and initial takeoff, but the increased fuel consumption required at full power exceeded the restricted flow, leading to a lean engine cut.
- The aircraft sustained considerable damage, including a broken engine bearer arm, a cracked exhaust pipe, and a collapsed undercarriage.
Safety action
- A recommendation was issued to the Society of Aircraft Constructors (SAC) to advise members against using fuel valves with "brass to brass" bearing surfaces in aircraft fuel systems due to their tendency to bind.