What must an IFR aircraft do to receive clearance through Class F airspace?

To receive clearance through Class F airspace, it is essential for an IFR aircraft to obtain permission from the user agency or have an altitude reservation. Class F airspace is generally designated for special activities or is controlled by specific agencies, and therefore, clearance is necessary to ensure safety and coordination with these activities.

This requirement is fundamental as it allows air traffic control and the associated user agencies to manage traffic within that airspace effectively. The user agency may impose specific conditions or limitations that need to be understood and followed by the pilots. The altitude reservation process allows for organized and safe passage of aircraft through regions where operations may be restricted or concentrated.

In contrast, the other options do not align with the specific procedural requirements for operating within Class F airspace. Obtaining clearance only from third parties does not ensure compliance with airspace regulations, while simply requesting clearance verbally does not inherently meet the necessary requirements for entry into controlled airspace. Maintaining visual flight rules is not applicable in this context, as IFR operations inherently involve instrument navigation and do not rely on visual references.