Air India pilot’s reported cannabis test: What it can and can’t tell investigators

A positive test can show that THC is present, but not necessarily when cannabis was consumed or whether the person was impaired at the time.

Reports that an Air India pilot tested positive for cannabis after a flight sound like the sort of finding that should make an aviation investigator sit up.

The less obvious one is harder: what does a positive cannabis test actually tell us about a pilot's state of mind several hours earlier?