If an aircraft has dual VOR or dual TACAN receivers, when are these systems considered reliable?

The correct choice indicates that both receivers must check within ± 4° of the designated ground checkpoint and also within ± 4° of each other to be considered reliable. This establishes a comprehensive confirmation of accuracy from both systems, ensuring redundancy and reliability in navigation.

In aviation, accuracy in navigation systems is critical for safe operations, especially under instrument flight rules (IFR). By requiring the checks to be within these specified tolerances, the systems can verify their performance against a known reference point (the designated ground checkpoint) as well as against each other. This dual-layer verification helps mitigate the risk of misnavigation due to faulty equipment.

This is not merely about meeting one set of tolerances; it emphasizes the importance of cross-checking the systems for additional safety. Systems that do not adhere to these rigorous checks may not provide a reliable navigation solution, which could endanger flight safety. Therefore, understanding the need for both measurements to fall within the specified parameters is crucial for maintaining high standards of reliability in flight operations.