Calm Air flight 464, an ATR 42-300, suffered a runway excursion while landing at Naujaat Airport, NU (CYUT) in Canada. When the flight crew boarded the ATR 42-300 aircraft they noted, as part of their initial interior aircraft checks, that the left and right propeller feather solenoid circuit breakers were out and not collared. They called the maintenance supervisor, who informed them that the breakers had been pulled as part of a routine maintenance task that had been carried out the night before. At the request of the maintenance supervisor, the flight crew then reset the breakers as instructed. At 12:17, the occurrence aircraft departed Rankin Inlet-Sherbrooke Airport (CYRT) destined for Naujaat Airport (CYUT). On board were 2 pilots and a flight attendant, as well as 3539 pounds of cargo. For this instrument flight rules (IFR) flight, 1272 L of Jet A fuel had been uploaded for a total fuel load of 5150 pounds. The departure, climb-out, and cruise portions of the flight were uneventful. At approximately 13:21, while the aircraft was on descent into CYUT, the flight crew observed that the left propeller was operating at a lower rpm than normal, while engine parameters remained normal. They briefly discussed the situation and made various attempts to troubleshoot and identify the problem, but did not consult the Quick Reference Handbook (QRH). They were unable to identify a specific malfunction and did not take any further action. The captain considered his options: returning to CYRT, shutting down the left engine, or both. However, there was a crosswind at CYRT and he did not want to land on a runway with compacted snow in a crosswind or initiate a long return flight to CYRT in arctic conditions, particularly with only 1 engine operating. The captain believed that the propeller indications were related to the maintenance task that had been performed prior to the flight and did not discuss the options with the first officer. The flight crew continued the approach and monitored the situation. During the approach, the captain advanced the condition levers to see if he could achieve 100% propeller rpm. This was successful at first, but as engine torque was reduced on descent, the left propeller rpm started to decay. On short final, the left propeller rpm continued to decrease, and the aircraft was becoming noticeably more difficult to control. During the landing on runway 34 at CYUT, the aircraft initially touched down 750 feet past the threshold and the pilot flying immediately selected reverse thrust on both power levers; however, only the right propeller went into reverse. Almost simultaneously with the selection of reverse thrust, only the “LO PITCH” light for the No. 2 engine illuminated, which went unnoticed by the flight crew. At 13:26, shortly after touchdown, the aircraft swerved to the right. In an effort to prevent a loss of control, the captain attempted to use asymmetric thrust and nose wheel steering. Although directional control was maintained momentarily for approximately 40 feet, lateral control was lost and the aircraft exited the right side of the runway in a north-easterly direction. The aircraft travelled about 500 feet through the snow adjacent to the runway before coming to rest. At some point during the runway excursion, the captain’s safety belt released, and his head struck the forward cockpit area. The aircraft came to a stop approximately 108 feet from the edge of the runway, at which time the flight crew shut down the engines with the fire handles because the condition levers were jammed. The captain received serious injuries and the 2 other crew members received minor injuries. The aircraft was substantially damaged. The emergency locator transmitter did not activate. Findings as to causes and contributing factors 1. A contaminant inside the left propeller’s high-pressure pump caused its pressure relief valve to fail. As a result, the propeller entered a pitch-lock condition and remained in that condition until the aircraft landed. 2. Because there is no indication in the cockpit of a pitch-lock condition in flight, the flight crew were not aware that the propeller had entered a pitch-lock condition, and they continued the flight to Naujaat Airport (CYUT), Nunavut, without discussing any options. 3. Immediately on touchdown, reverse thrust was selected by the pilot flying without confirmation that both LO PITCH lights had illuminated. With the left propeller in a pitch-lock condition, the selection of reverse thrust resulted in the aircraft entering an asymmetric thrust state. 4. Due to the asymmetric thrust, directional control of the aircraft could not be maintained. As a result the aircraft exited the landing surface of the runway, travelled across rough terrain adjacent to the runway, and was substantially damaged. 5. For undetermined reasons, the captain’s safety belt buckle released during the runway excursion and the captain’s head struck the forward upper area of the cockpit, resulting in serious head injuries.