Where is information maintained concerning routes requiring special navigation equipment?

The correct answer indicates that information regarding routes which require special navigation equipment is maintained in the Air Carrier's Operations Specifications. This document contains critical operational details that are specific to an airline's operations, including information about the equipment and qualifications necessary for pilots and aircraft.

In the context of aviation, Operations Specifications (OpSpecs) provide the regulatory framework for different airlines and outline the types of navigation equipment that may be necessary for certain flight routes. This includes details on the regulatory requirements and operational capabilities, allowing dispatchers and flight crews to ensure compliance with safety regulations.

The other choices, while important in their own right, do not focus specifically on the requirements for special navigation equipment on flight routes. The International Flight Information Manual provides information on international flights but doesn’t specifically address operational specifications for airlines. The Airplane Flight Manual pertains more to the operational limits and performance of the aircraft rather than navigation equipment. The National Aeronautical Navigation Manual is a resource for navigation information but may not detail the specific operational capabilities required for each air carrier’s routes, especially those requiring special navigation equipment. Therefore, the emphasis on airline-specific operations in the Air Carrier's Operations Specifications makes it the appropriate source for this information.