What happened
On 3 October 2010, an Avid Mk. IV Flyer, registration EI-HAM, was conducting a private local flight near Leegane, Co. Wexford. While flying at approximately 1,000 feet, the pilot observed the engine temperature rising. Shortly after, a sudden release of pressure caused coolant vapor to rapidly flood the cockpit, completely obscuring the instruments and the external view through the glazed surfaces.
With visibility lost, the pilot attempted a precautionary landing in a nearby field while the engine was still producing power. Although the aircraft cleared nearby trees, it struck the ground with significant force on rising terrain. The impact caused the left main undercarriage to detach and the propeller blades to disintegrably under power. Both the pilot and the passenger sustained serious injuries during the high-deceleration event.
The investigation
The AAIU examined the wreckage and the cooling system components. Investigators found that the engine's thermostat was failing to close fully when cold, which had caused the engine to run much cooler than normal during previous flights. To counteract this, the pilot had covered approximately 80% of the radiator surface with grey duct tape to restrict airflow.
Technical analysis of the cooling system revealed that the engine reached an over-temperature state, causing the coolant to boil. This increased pressure forced coolant through the engine's pressure cap into the expansion bottle. The pressure then caused the seal on the expansion bottle's vent cap to fail, releasing the vapor into the cockpit. The investigation also noted that the expansion bottle was positioned in the aft cockpit area, which contributed to the vapor's path into the cabin.
Findings
- The primary cause of the engine overheating was the taping of the radiator surface to prevent over-cooling.
- A faulty thermostat allowed excessive coolant flow, leading to the temporary use of tape to manage operating temperatures.
- The failure of the expansion bottle cap seal allowed pressurized vapor to enter the cockpit and obscure all visual references.
- The high impact forces were a result of the aircraft hitting rising ground during the forced landing.
Safety action
Following the investigation, a recommendation was made for the Irish Light Aviation Society to advise its members regarding the risks of performing unauthorized modifications on aircraft operating under a Flight Permit.