Under what condition can departure aircraft use the 22kt rule?

The correct condition for departure aircraft to use the 22kt rule is when following an enroute aircraft that has reported over a fix. The 22kt rule is applicable in situations where a departure can maintain a safe separation distance from an enroute aircraft, allowing for a smoother flow of traffic.

When an enroute aircraft reports position over a fix, it provides the necessary information to the departure aircraft's controller to manage the traffic more effectively. This reporting allows the controller to ensure that the departure can safely increase its speed while maintaining safe separation from the enroute aircraft.

In contrast, the other conditions presented do not provide the same traffic management context or safety assurances. For example, departures from smaller airports or cruising at the same altitude do not inherently indicate the separation capabilities necessary to employ the 22kt rule effectively. Similarly, descending from a higher altitude does not guarantee that a departure can safely follow behind another aircraft without compromising separation standards. Thus, the context of following an enroute aircraft reporting over a fix is critical for safely applying the 22kt rule.