The TSB reviewed flight simulator experiments on the effects of THC on pilot performance. They demonstrated “that THC has wide-ranging effects on human performance, including impairment of working memory, coordination, tracking, perceptual-motor performance, temporal perception and vigilance.” The effects of impairment increase with the complexity of the task, said the TSB. A blood delta9-THC concentration over 5 ng/ml is the threshold considered to be necessary for possible impairment.

“Even allowing for a reasonable margin of error in the toxicology results, the amount of THC present in this occurrence is considerably greater than the threshold that resulted in degraded pilot performance in studies on the impairing effects of THC,” said the investigators.

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.