What happened
On June 10, 2017, an Aero AT-3 R100, registration SP-ACN, operated by Aeroklub Wrocławski, was performing a repositioning flight from Pruszcz Gdański [EPPR] to Wrocław-Szymanów [EPWS]. During the cruise phase, approximately 2 NM northwest of waypoint EPBYO, the pilot observed a significant increase in cylinder head temperatures, moving from the yellow warning range into the red critical range. Simultaneously, the pilot noted a drop in oil pressure and a rise in oil temperature, accompanied by the smell of coolant.
In response to the developing emergency, the pilot elected to perform a precautionary landing at the nearest suitable aerodrome, Bydgoszcz-Sustwerowo [EPBY]. The aircraft landed safely at 17:03 LMT without further incident. During the descent, the pilot observed that engine temperatures and oil parameters were gradually returning to normal levels at reduced power.
Upon post-flight inspection, the pilot discovered a leak on the external fuselage skin caused by a crack in the plastic cylinder head coolant expansion tank.
The investigation
The investigation, conducted by the aircraft manufacturer under the supervision of the PKBWL, examined the engine cooling system and the condition of the expansion tank. The aircraft had a total time in service of 122 hours and 04 minutes at the time of the event. The investigation focused on the sequence of thermal changes and the physical failure of the expansion tank component.
Findings
- The primary cause of the incident was the clogging of the vent hole in the expansion tank filler cap, which was approximately 80% obstructed.
- This blockage disrupted the cooling system's functionality, leading to the boiling of the engine coolant.
- The resulting increase in internal pressure within the expansion tank caused the plastic tank to crack.
- The failure of the tank led to the subsequent rise in cylinder head temperatures and the fluctuations in oil pressure and temperature.
- The pilot's qualifications, experience, and decision-making regarding the emergency landing were found to be appropriate.