Which of the following sequences is NOT a method to achieve longitudinal separation?

Longitudinal separation refers to maintaining the appropriate distance between aircraft moving in the same direction along a flight route to ensure safety and prevent collisions. The correct answer highlights that changing speeds randomly does not establish a reliable method for maintaining this separation.

Consistent and predictable speed adjustments are essential for managing separation effectively. Randomly changing speeds can lead to confusion and unpredictability in maintaining the required distance between aircraft. Instead, established procedures typically involve defined speeds or holding patterns that allow aircraft to maintain a safe distance in a controlled manner.

In contrast, the other options are valid methods for achieving longitudinal separation. For example, allowing departing aircraft to follow enroute aircraft over a fix ensures that they are sequenced properly and maintains an established buffer. Similarly, holding at a fix until a specified time or until given clearance effectively manages the spacing between aircraft, preventing potential conflicts on approach or departure paths. These methods rely on pre-determined procedures for predictable spacing, essential for safe air traffic management.