What is the main underlying operating principle of the VOR aircraft navigational system?

The VOR (VHF Omnidirectional Range) navigational system operates primarily on the principle of a phase difference between two alternating current (AC) signals. Specifically, a VOR station transmits two signals: a reference signal that is constant and a variable signal that rotates around the station. The difference in phase between these two signals allows the aircraft's onboard receiver to determine its bearing relative to the VOR station.

By processing the phase difference, pilots can accurately determine their azimuth (direction) from the VOR station, which is essential for navigation. This functionality enables pilots to follow specific flight paths or radial courses that lead to or from the VOR station. Hence, understanding and utilizing the phase difference is crucial for effective navigation using the VOR system.

The other choices do not correctly describe the main operating principle of the VOR system. For example, a constant altitude indicator relates more to vertical navigation than to the azimuth functionality provided by the VOR. Magnetic compass alignment is unrelated, as it does not employ the phase differences that VOR relies on. Direct line distance measurement is a function more characteristic of DME (Distance Measuring Equipment) than VOR. Therefore, the correct choice pertaining to the operational mechanics of VOR is