Accident Overview
On a scheduled international cargo flight from Kabul, Afghanistan to Tehran, Iran, flight IR123 experienced a propeller overspeed condition during takeoff. The aircraft was cleared for takeoff on runway 29 at 13:13. At the start of the takeoff run, the co-pilot was in the left-hand seat and serving as pilot flying.
Sequence of Events
While accelerating, the pilot-in-command observed that the propeller of engine no. 1 was overspeeding and surging up to 3300 rpm. As the aircraft reached an indicated airspeed of approximately 80 knots, the captain assumed control. He noted that the aircraft was veering left of the runway centerline toward three runway lights embedded in a concrete footing at the left edge of the runway. To avoid a possible collision with these lights, the captain applied elevator control and lifted the aircraft off the runway. Despite following the corrective action procedure outlined in the operations manual, the overspeeding propeller condition persisted.
The aircraft's flight path deviated approximately 30 to 45 degrees left of the runway, heading generally toward the Kabul Airport terminal building. The captain attempted to turn the aircraft further left to avoid hitting the building. About 325 feet from the south edge of runway 29, the left wing contacted the ground, and the aircraft crashed.
Probable Cause
The official investigation determined that the captain failed to discontinue the takeoff when he observed that the No. 1 propeller was overspeeding, at a time when the aircraft was still on the runway.