Which of the following methods is NOT used for longitudinal separation of aircraft?

The method that is not used for longitudinal separation of aircraft involves changing altitude based on visibility. Longitudinal separation primarily relies on time and fixed intervals to maintain adequate distance between aircraft traveling along the same flight path. This is essential for preventing potential collisions and ensuring safe air traffic management.

In contrast, the other methods mentioned are all effective strategies for ensuring that aircraft remain adequately separated in time. Departing at a specific time, holding at a fix until a designated moment, and arriving at a fix at a specified time all help air traffic controllers manage the flow of traffic by controlling the timing of aircraft movements. In contrast, changing altitude based on visibility does not necessarily account for the position of other aircraft or the timing needed to ensure safe separation, making it unsuitable for this purpose.