What is the initial action to take if the captain becomes unresponsive during a flight at FL350?

In the scenario where the captain becomes unresponsive during a flight at FL350, the most appropriate initial action is to attempt to wake the captain. This is critical because verifying the captain's condition is a key step in assessing the situation. If the captain is simply asleep or in a temporary state of unresponsiveness, waking them could quickly return the aircraft to normal operation.

Handling the situation promptly allows the first officer to gain a clearer understanding of the captain's health and the overall status of the flight. If the captain does not respond or is determined to be incapacitated, then appropriate protocols for dealing with an unresponsive crew member can be initiated.

Other options might involve wearing an oxygen mask—which could be necessary depending on the circumstances of the unresponsiveness—but the immediate priority is to assess the captain's condition to determine the best course of action thereafter. Declaring an emergency and diverting may also be necessary steps, but assessing the immediate situation should come first before escalating to such measures.