Lingering Effects

By Richard N. Aarons
Source: Business & Commercial Aviation

Studies have established the relative risk of road accidents involving cannabis-impaired versus sober drivers as “odds ratios.” A blood delta9-THC concentration of 6 to 8 ng/ml correlated with a blood alcohol level of 0.05%, and an odds ratio of 1.5 to two times the risk of accident for a sober person. Drivers who were under the influence of cannabis tended to compensate consciously by operating more cautiously.

The duration of THC effects is variable, subject to a number of conditions. Generally, after a single dose of marijuana, there will be some impairment for up to 6 hr. Experiments have suggested significant carry-over impairment in complex human/machine performance such as flying, up to 24 hr. after a moderate dose of THC via inhalation. This influence can occur after an individual ceases to be aware of any influence of the drug.

Although some clients of aviation operators in Canada require pre-employment and periodic drug and alcohol screening, said the TSB, there are no Canadian regulations requiring persons employed in federally regulated transportation industries to submit to toxicological testing. U.S. federal transportation law requires drug and alcohol testing of all employees in safety-sensitive transportation positions, including aviation.

The TSB’s Analysis

The TSB determined the airplane and its systems were working properly before the crash. What follows is from the TSB analysis of the incident:

When C-GATV departed for Lutsel K'e, the weather at Yellowknife was marginal for VFR flight. Low clouds persisted for the entire flight, which was flown at low level so the pilot could maintain visual contact with the ground. The descent during the last 2 min. of the flight suggests that the ceiling had become lower.

The conduct of the flight and the nature of the impact were characteristic of a CFIT event: the aircraft struck rising terrain under the pilot's control at cruise speed, with a wings-level attitude and a heading generally consistent with the direct track to the destination. Because no effective evasive maneuvers were made before impact, it is likely that the crest of the Pehtei Peninsula was obscured in fog and not visible to the pilot. The application of increased engine power immediately before impact was likely made when the terrain in front of the aircraft suddenly became visible.

When the pilot transmitted a position report 6 nm closer to Lutsel K'e than the actual position, it is possible that he believed that the shoreline of Great Slave Lake had been crossed and that open water at about 500 ft. ASL lay ahead. Since GPS was likely the primary navigational aide, there should have been little ambiguity in position, unless the unit was set to a waypoint associated with the RNAV approach at Lutsel K'e. However, the location of the site and the wreckage trail track indicate that the aircraft was proceeding directly to the airport. If an instrument approach had been planned, the aircraft should have been navigating toward a waypoint associated with the approach, and at an altitude no lower than 3,100 ft. in accordance with the company-published route.

A TAWS installation in C-GATV could have warned of the impending collision with the ground, possibly in sufficient time to prevent the accident.

Investigators could not determine why the pilot chose to fly the trip under VFR. Conditions were suitable to enable operation under IFR at altitudes providing safe terrain clearance. The pilot, the aircraft and the company were qualified to operate the trip under IFR. The en route weather was suitable, and with the freezing level well above the minimum IFR route altitude, icing was not a factor to preclude IFR flight. The cloud base was above the minimums required for successful completion of an approach and landing at Lutsel K'e. Before departure, the forecast weather was such that Yellowknife could be filed as an IFR alternate.

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