When must the take-off alternate be within a certain distance for specific aircraft?

The requirement that a take-off alternate must be within one hour at one-engine-inoperative speed is based on safety regulations and operational standards designed to ensure that aircraft have an accessible diversion airport should a problem arise shortly after takeoff. This stipulation is particularly crucial for multi-engine aircraft where the loss of one engine could affect the aircraft's performance and safety.

By ensuring that the take-off alternate is within this distance, pilots can make timely decisions and safely reach an alternate airport in the event of an emergency, such as an engine failure. This precaution is part of a broader safety protocol that acknowledges the increased risks during the early stages of flight, right after takeoff, when pilots must be prepared to react quickly to any issues that may arise.

Other options reflect different time frames and criteria that may not specifically align with one-engine-inoperative scenarios or might relate to broader operational limits not strictly related to take-off conditions. Hence, the focus on one hour specifically addresses the urgency and critical nature of situations where an aircraft may need to divert shortly after departure.