In a precautionary landing with engine power, which radio frequency should be used to transmit a Mayday?

In a precautionary landing with engine power, one would transmit a Mayday call on 121.5 MHz because this frequency is designated as the international emergency frequency for aviation. It is monitored by air traffic control and nearby aircraft, making it the most effective choice for communicating an urgent situation.

Using 121.5 MHz will ensure that your distress signal reaches as many people as possible, increasing the likelihood of receiving assistance promptly. This frequency is essential during emergencies, as it allows pilots in distress to communicate their situation clearly and receive help from other aviators or air traffic services in the vicinity.

The other frequencies listed, while they may be used for different purposes in aviation communication (like inter-pilot communications or dedicated air traffic control channels), do not serve the same universal emergency function that 121.5 MHz provides. Therefore, it is crucial to use 121.5 MHz for any Mayday transmission to ensure that the message is received effectively and that you get the assistance you need in a critical situation.