0.1% of airline pilots fly intoxicated, and probably many more fly hangover which is an undetectable condition.

There is speculation that in the Air France flight 447 that crashed into the ocean en route to Paris, one or the pilots only had 1h of rest because of partying the night before. Of course it’s all speculative, and however unlikely it is, eventually it’s bound to happen that we get pilots with poor mental clarity in charge of large Boeings with hundreds of lives on board. Unfortunately it only takes one lapse of judgement to compromise the flight profile of a large airliner, even if corrected after a few seconds.

https://generalaviationnews.com/2014/11/06/vanity-fair-the-h...