Cannabis test alone won't reveal if Air India pilot was impaired, experts warn

On August 4th, an Air India Airbus A320 carrying 137 passengers and eight crew members was descending toward Delhi when something went wrong. The aircraft suddenly dropped about 300 feet in altitude. Thirteen passengers and four crew members were injured in the sudden plunge. The plane landed safely, but the incident triggered an investigation that would soon raise uncomfortable questions about what a drug test can and cannot tell us.

Routine post-flight screening found that the captain had tested positive for cannabis in a confirmatory laboratory test, according to media reports citing unnamed sources. Air India has not officially confirmed the result. Both pilots were removed from flying duties. The finding seemed straightforward enough—a positive test, a serious incident, a clear line connecting the two. But aviation experts and toxicologists say the reality is far messier.

The central problem is this: a positive cannabis test proves that THC was present in someone's body. It does not prove when the drug was consumed, and it does not prove the person was impaired at any particular moment. This distinction matters enormously in aviation. Researchers have known for decades that cannabis impairs the fine motor control, attention, and complex decision-making that flying demands. A 1976 study found that pilots who smoked marijuana and then flew a simulator showed deteriorated performance in altitude control, heading, and radio navigation—effects that lasted at least two hours and generally cleared within four to six hours. But here is the troubling part: a 1986 follow-up study found that experienced pilots tested 24 hours after smoking cannabis showed significant impairment in elevator and aileron control, landing accuracy, and approach deviation. Most striking, the pilots did not realize their performance had declined. A 1991 study of nine active pilots who smoked cannabis containing 20 milligrams of THC found performance impaired at 15 minutes, four hours, eight hours, and 24 hours afterward. Seven of the nine showed some impairment at 24 hours, but only one was aware of it.

These studies are small, conducted with simulators rather than modern aircraft, and decades old. But they establish something important: cannabis can interfere with flying for hours after consumption. The problem is that THC itself leaves the bloodstream quickly after smoking—typically within hours—while metabolites, the byproducts the body creates when breaking down THC, can remain detectable for days or weeks, especially in frequent users. A 2022 clinical trial at the University of California San Diego tested 191 regular cannabis users on a simulated driving task. Those who smoked THC performed worse initially, but after about four and a half hours, their performance returned to baseline. Yet THC remained detectable in their blood. This gap between detection and impairment is the core problem. Thomas Marcotte, who led that study, explained to the BBC that unlike alcohol, where blood concentration correlates reasonably well with impairment, the relationship with THC is far more complex. The dose, method of consumption, frequency of use, and individual biology all affect how long impairment lasts and how long THC remains detectable.

Canada addressed this uncertainty by taking a blunt regulatory approach. After legalizing recreational cannabis in 2018, Transport Canada prohibited flight crews from consuming cannabis for at least 28 days before duty. The rule acknowledges that the science cannot reliably distinguish between presence and impairment, so it simply bans use entirely for a month. The United States offers a cautionary tale about reading too much into positive tests. A 2026 National Transportation Safety Board study of pilots killed in civil aviation accidents between 2018 and 2022 found that 52.8 percent had at least one drug detected in their system, with THC as the main driver of increased illicit-drug detections. But the NTSB was careful to note that the study documented drug presence, not impairment.

Investigators have sometimes gone further. In a 2011 Canadian crash, a Cessna pilot had 50.1 nanograms per milliliter of THC in his blood, and the Transportation Safety Board concluded the amount was sufficient to impair cognitive processes and likely affected the pilot's planning and conduct. But in a 2017 New Zealand helicopter crash, THC was detected in the pilot's blood two days after cannabis use, and a forensic toxicologist concluded he was unlikely to have been under its influence at the time of the accident. The lesson is not that positive tests mean nothing. It is that their meaning depends entirely on concentration, timing, circumstances, and other evidence.

In the Air India case, investigators face a three-part question, not one. First, was cannabis consumed? That may soon be answered. Second, was the pilot impaired while flying? That is considerably harder to establish. Third, did that impairment contribute to the sudden loss of altitude? Hardest of all. Air India announced it would immediately test all its pilots for banned substances, expanding beyond the existing requirement that at least 10 percent undergo random screening. But a positive test, standing alone, will not answer what investigators actually need to know.