Investigators have indicated that a runway excursion that badly damaged a Douglas DC-6A freighter in Alaska was preceded by an impact just before touchdown.

The accident occurred on 1 August as the Everts Air Cargo aircraft, arriving from Fairbanks, approached runway 20 at Candle 2 airport.

It had been conducted the approach in daylight visual conditions when the crew felt a "bump" near the runway threshold, says the US National Transportation Safety Board, although it adds that this was "not extreme".

Inspection of the runway subsequently showed that "several" piles of rocks and dirt, rising to 4ft, were positioned at the threshold.

The aircraft, with two pilots and a flight engineer, started to veer to the right of the runway as the propellers were set to deliver reverse thrust.

Although the flight engineer corrected the DC-6's path by applying asymmetric reverse – enabling it to travel 2,000ft in a straight line – the aircraft then veered "sharply" to the right, departing the runway and spinning 180°, says the NTSB. It suffered substantial damage to the fuselage.

There were no mechanical failures or malfunctions prior to the accident, the crew told the inquiry. The captain of the aircraft stated that the terrain meant the approach was steeper than normal but that the "sight picture" indicated touchdown near the threshold.

Candle 2 airport's runway is 3,880ft (1,182m) in length.

None of the three crew members was injured. The aircraft is listed by the US FAA as a C-118A variant of the DC-6A and that the airframe (N451CE) was manufactured in 1953. Everts Air Cargo operates under Fairbanks-based Tatonduk Outfitters.