What method indicates that a VOR station is undergoing maintenance?

When a VOR station is undergoing maintenance, the most definitive indication is the absence of the Morse code signal that normally accompanies the VOR frequency. This Morse code signal serves as an identifier and is crucial for pilots to confirm that they are tuning into the correct VOR station. If maintenance is taking place, the station will stop transmitting the Morse code. The lack of this identification signal alerts pilots that the VOR may not be operational, allowing them to adjust their navigation plans accordingly.

Other indications such as a silent station, intermittent reception, or changes in frequency may occur during maintenance, but they are not as clear-cut or reliable as the absence of the Morse code signal in confirming maintenance activity. A silent VOR could be due to multiple issues, not exclusively maintenance, and intermittent reception may simply be a result of signal range or interference rather than maintenance. Therefore, the clear indication of maintenance is the absence of the Morse code signal being transmitted.