Aside from the time component, what other item should be circled in red to indicate a flight plan or estimate has been forwarded?

The correct answer is that altitude should be circled in red to indicate that a flight plan or estimate has been forwarded. In non-radar air traffic control operations, clearly marking significant components of a flight plan is crucial for maintaining accurate communication among controllers and between controllers and pilots.

Circumstances that might prompt circling the altitude in red include its importance in maintaining safe vertical separation between aircraft. It allows quick visual recognition for any altitude-related updates or changes to the flight path, ensuring that all relevant personnel are immediately aware of the altitude at which the aircraft is cleared to operate. By highlighting the altitude, controllers can emphasize its importance in the context of flight safety and operational efficiency.

Other components like the route and departure time are important but are typically not emphasized in the same way once a flight plan is forwarded. While aircraft type may be useful for operational considerations, it is not as critical as altitude, which is directly linked to safety and separation standards in air traffic control.