Incident Overview
On 5 February 2020, an AT-3 R100 aircraft (registration SP-CLB) operated by ATO PWSZ in Chełm was involved in an incident during a training flight at Mielec (EPML). The aircraft experienced exceedances of cylinder head temperature (CHT) and oil temperature deviations. No injuries were reported, and the aircraft was not damaged, although the engine was later sent for inspection due to repeated CHT exceedances.
Flight Details
The flight was a training sortie as part of a professional license course (task IV/8). The crew consisted of a student pilot (holding a PPL(A) and undergoing CPL training) and an instructor (holding a CPL(A) with instructor rating). The day's flight plan included three legs: EPCD–EPLL, EPLL–EPML, and EPML–EPCD. The incident occurred during the third leg after departure from EPML. The operation was conducted under Visual Flight Rules (VFR) in daytime weather conditions; weather factors were assessed as having no influence on the event.
Sequence of Events
After engine start and achieving minimum oil temperature, the crew taxied to Runway 36 at Mielec. The engine cooling and oil radiator shutters were closed to expedite oil temperature rise. The voice warning system continuously indicated an oil temperature caution (yellow range). The crew interpreted this as temporary. Operating under time pressure due to approaching sunset and the destination airport (EPCD) closing for night operations, the crew took off immediately after receiving takeoff clearance. During departure, the instructor focused on monitoring the student and scanning for traffic in the busy airspace.
While leaving the traffic pattern, the instructor noticed the voice warning system indicating coolant temperature issues. The engine monitor (MVP-50) still displayed the start screen (showing only a brief user guide) instead of individual cylinder temperatures. Upon switching off the start screen, they saw CHT readings of 154°C (red zone) and above limits. The crew immediately opened the cooling shutters; within 20–30 seconds, temperatures returned to normal (green range). However, oil temperature began to steadily decrease even though the oil radiator shutter remained fully closed, as confirmed by voice warnings. The crew decided to return to EPML, landing on Runway 27 without further incident.
Analysis
The investigation determined that the crew had not deactivated the engine monitor start screen before takeoff, depriving them of continuous CHT information. Additionally, the cooling air shutter was left closed, accelerating coolant temperature rise. The crew omitted relevant checklist items: opening the air intake shutter and disabling the start screen. The subsequent oil temperature drop despite a closed oil shutter was attributed to the cooling system design, where the open air shutter affects oil temperature.
Operational experience on this aircraft type indicated frequent audio alarms (especially during start), with varying severity—from trivial (e.g., low oil temperature after start) to critical. Both yellow and red conditions triggered the same alarm, leading to alarm fatigue and potential masking of critical warnings.
Cause and Contributing Factors
The direct cause of the incident was the crew's failure to complete all mandatory checklist items before takeoff: the air intake shutter was not set to the open position. Contributing factors included a large number of audio alarms with varying criticality, which could desensitize the crew, and the failure to disable the engine monitor start screen, which prevented display of cylinder head temperatures.
Post-Incident Actions
Following the incident, the operator proposed changes to the MVP-50P engine controller settings and installation of thermostats in the cooling and oil systems. The manufacturer (AERO AT) issued Service Letter ATB3.56 L on 24 June 2020, specifying modifications to the electronic controller and proper use of air intake and oil radiator shutters. The modifications introduced a yellow-range audible and visual warning from 110°C CHT (previously green directly transitioned to red). After implementation, no further CHT exceedances were reported. The manufacturer also provided guidance on locking the air intake shutter in the open position and stated that thermostat trials for the coolant system were ongoing, but no thermostat was planned for the oil system.
No safety recommendations were issued by the Polish State Commission for Aircraft Accident Investigation (PKBWL).