Northern Thunderbird Air - NT Air

Findings as to causes and contributing factors:
During routine aircraft maintenance, it is likely that the left-engine oil-reservoir cap was left unsecured.
There was no complete preflight inspection of the aircraft, resulting in the unsecured engine oil-reservoir cap not being detected, and the left engine venting significant oil during operation.
A non-mandatory modification, designed to limit oil loss when the engine oil cap is left unsecure, had not been made to the engines.
Oil that leaked from the left engine while the aircraft was repositioned was pointed out to the crew, who did not determine its source before the flight departure.
On final approach, the aircraft slowed to below VREF speed. When power was applied, likely only to the right engine, the aircraft speed was below that required to maintain directional control, and it yawed and rolled left, and pitched down.
A partially effective recovery was likely initiated by reducing the right engine’s power; however, there was insufficient altitude to complete the recovery, and the aircraft collided with the ground.
Impact damage compromised the fuel system. Ignition sources resulting from metal friction, and possibly from the aircraft’s electrical system, started fires.
The damaged electrical system remained powered by the battery, resulting in arcing that may have ignited fires, including in the cockpit area.
Impact-related injuries sustained by the pilots and most of the passengers limited their ability to extricate themselves from the aircraft.
Findings as to risk:
Multi-engine−aircraft flight manuals and training programs do not include cautions and minimum control speeds for use of asymmetrical thrust in situations when an engine is at low power or the propeller is not feathered. There is a risk that pilots will not anticipate aircraft behavior when using asymmetrical thrust near or below unpublished critical speeds, and will lose control of the aircraft.
The company’s standard operating procedures lacked clear directions for how the aircraft was to be configured for the last 500 feet, or what to do if an approach is still unstable when 500 feet is reached, specifically in an abnormal situation. There is a demonstrated risk of accidents occurring as a result of unstabilized approaches below 500 feet above ground level.
Without isolation of the aircraft batteries following aircraft damage, there is a risk that an energized battery may ignite fires by electrical arcing.
Erroneous data used for weight-and-balance calculations can cause crews to inadvertently fly aircraft outside of the allowable center-of-gravity envelope.

Findings as to Causes and Contributing Factors:
1. The aircraft was flown up a narrow canyon into rapidly rising terrain for reasons that could not be determined. The aircraft’s proximity to terrain and the narrowness of the canyon precluded a turn, and the aircraft’s climb rate was insufficient to clear the rising terrain.
2. The pilot decision-making training received by the crew members was ineffective because they were unprepared for the unique hazards and special operating techniques associated with flying low in mountainous terrain.
Finding as to Risk:
1. The company operations manual (COM) gave no guidance to the crew for the operation of a visual flight rules (VFR) flight, except for the provision that it should not be conducted closer to obstacles than 500 feet vertically and horizontally.

The following findings were identified:
- The ceiling and visibility were reported to be below published minima for landing when the approach was initiated.
- The Flight crew carried out a non-standard abbreviated approach.
- The Flight crew descended below the published minimum altitude of 1,620 feet MSL.
- The Captain was at the controls throughout the approach. The approach was continued below the authorized minimum altitude for undetermined reasons.

The accident was the consequence of the following factors:
- The pilot continued the flight along a narrow valley into deteriorating weather conditions below the altitude required for safe terrain clearance.
- The pilot's forward visibility was hampered by an ineffective windshield wiper system.
- The pilot, after losing visual references, attempted to obtain safe terrain clearance by climbing into the cloud layer.
- The pilot for undetermined reasons, did not initiate or maintain a maximum rate of climb to clear enroute terrain.
- Accurate and up-dated presentation of forecast or actual weather conditions in the accident area was not available to the pilot prior to his departure from Prince George due in part to the lack of reporting points.