Flight Overview
A de Havilland Canada DHC-4A Caribou aircraft departed Calgary en route to Ross River, Yukon, carrying two pilots, two company aircraft maintenance engineers, and approximately 5,500 pounds of aircraft parts and equipment. A refueling stop was made at Fort Nelson, after which the flight continued under an IFR flight plan. Ross River has no published instrument approach and is situated in a valley amid mountainous terrain.
Descent and Engine Failure
The flight plan required overflying Ross River, proceeding 30 miles northwest, and completing an NDB approach at Faro, where the minimum circling altitude is 2,709 feet above ground level. After descending below cloud at Faro, the aircraft was to proceed visually up the river valley to Ross River. The aircraft arrived over Faro at approximately 16:15 and attempted to descend below a cloud base reported as 400 to 600 feet above ground level. When visual contact with the ground was not established, the pilot climbed to the southeast and informed the Faro Community Airport Radio Station that the aircraft would proceed to its alternate airport, Watson Lake.
During cruise flight, the number 2 engine began to lose oil, causing the oil pressure to drop to zero. The right engine was shut down and its propeller feathered.
Approach and Go-Around
Later, the aircraft was heard flying out to the southeast for an approach to runway 26 at Ross River Airport. On approach, the landing gear had to be recycled because the nose gear did not lock down during the first lowering attempt. The aircraft was no longer in a position to land, so the captain initiated a go-around.
Accident Sequence
The aircraft passed by the town of Ross River and then began to lose altitude, descending gradually into trees. Contact with trees tore off portions of the right wing, causing the aircraft to roll to the right and nose into the ground. The aircraft then cartwheeled over an embankment and down onto the river shore 20 feet below.
Probable Cause
The Canadian Aviation Safety Board determined that a lack of communication at the company's maintenance facility led to the dispatch of the aircraft with an unsuitable gasket in the right engine, and the subsequent loss of engine oil. A single-engine go-around was attempted when the combination of single-engine performance, a low cloud base, and high terrain surrounding a confined operating area made a successful overshoot uncertain. The flaps were fully retracted before the minimum speed for single-engine flight in the zero flap configuration was achieved.