What happened
During a training session focused on instrument flying and single-engine operations, an aircraft departed from Mehrabad Airport at 09:09 local time. The crew included a captain acting as a check pilot, four copilots undergoing testing, and a flight engineer. Following an initial takeoff, the crew encountered issues related to the feathering and unfeathering of the propellers, prompting them to return to the apron for guidance.
A second departure was attempted at 10:48 local time to continue the necessary checks. While cruising at approximately 6,700 feet, the port propeller was feathered at roughly 11:29 local time. During subsequent attempts by the captain to unfeather the component, the port propeller experienced an overspeed condition. The crew was unable to successfully refeather or unfeather the propeller, which continued to windmill at a rate of approximately 2/1000 RPM. Due to the inability to maintain altitude, the aircraft performed a crash landing in open terrain, stopping about 1,500 meters before reaching Runway 11.
The accident resulted in six total occupants on board, with three serious injuries and three minor injuries recorded.
Findings
Investigations into the incident revealed that the aircraft's right engine failed to produce enough thrust to facilitate an emergency landing on the runway while the left propeller was windmilling. Investigators determined that there were no mechanical failures identified in the right engine or its propeller assembly. Instead, the investigation concluded that the primary factor was improper control handling by members of the flight crew.