Under what conditions are dual VOR or dual TACAN systems considered reliable?

Dual VOR or dual TACAN systems are considered reliable when both systems meet specified accuracy criteria when cross-checked against designated checkpoints or each other. Checking within +/- 4 degrees indicates that the systems are closely aligned with the established reference, ensuring both increased reliability and accuracy in navigation.

Having each system check within +/- 4 degrees of the designated ground checkpoint ensures that they are both functioning correctly and providing accurate navigation information independently. Additionally, verifying that both systems check within +/- 4 degrees of each other signifies that they are consistent and corroborating one another's readings.

This dual verification process enhances navigational safety by providing pilots with confidence that the information they receive from these systems is valid and trustworthy, minimizing the risk of errors in navigation that could affect flight safety.