In which situation is a flight crew member prohibited from performing their duties?

A flight crew member is prohibited from performing their duties if their blood alcohol concentration is 0.04% or more. This regulation is established to ensure the safety of all passengers and crew on board. Alcohol can impair judgment, reaction times, and decision-making abilities, which are critical for the responsibilities held by flight crew members.

Having strict limits on alcohol consumption helps maintain a secure environment for aviation operations. The zero-tolerance approach regarding alcohol significantly reduces the risks associated with human error, which can lead to catastrophic outcomes in aviation contexts.

In contrast, while fatigue may compromise a crew member's performance, it does not constitute an absolute prohibition as the threshold can depend on recommendations and regulations regarding rest periods. Weather emergencies call for specific actions and considerations but do not inherently prevent a flight crew member from performing their duties. Lastly, a long flight may also lead to fatigue, but it is the crew's responsibility to manage their fitness for duty, making this condition subjective and not strictly prohibitive.