The definition of the criteria for sending a Mayday is only if there is;
"Grave and Imminent Danger to Life, Vessel, Vehicle or Aircraft which requires Immediate Assistance"
It is only for real emergencies and should not be used in any other context.
If something has happened which is not a dire or fatal emergency, then the Pan Pan or Urgency call can be made. The Pan Pan call can be upgraded to a Mayday should it become necessary, but authorities will be very annoyed if a Mayday call is made unnecessarily when it should have been an Urgency.
In shipping, a Mayday is sent on Channel 16. If your ship is fitted with a GMDSS (Global Maritime Distress and Safety System), the first thing to do is send a digital alert by pressing the big, red EMERGENCY button, selecting the nature of distress, (if you have the time to do so) and then pressing in the same button for five seconds to transmit the electrical pulse. This will alert all other nearby ships and shore stations with a GMDSS system that a Mayday has been sent, and will also switch all of their VHF units to Channel 16 to await your verbal Mayday message. If you have no GMDSS unit, then make a Mayday call on Channel 16 using M.I.P.D.A.N.I.O
Should you hear a Mayday or Mayday Relay go out on Channel 16, then you should write it down, then listen to hear the response from the vessel or shore station taking control. If you are on inland waters, then the local Coastguard should respond almost immediately, acknowledge the message, and let boats know what steps they should take to help. Channel 16 will be under Mayday Seelonce* at this stage, and should not be used for any other traffic until you hear Mayday Feenee* (end of Mayday alert, normal conditions now apply) or Mayday Prudonce* (Thoughtful use of radio - no unnecessary calls to other boats or non-urgent traffic). Should you hear anyone abusing this, then you are within your rights to make the call Seelonce Distress (preferably in a slightly angry tone!) to let them know that there is an emergency. No one should abuse the radio by rights, as everyone should have a listening watch on Channel 16 anyway and know there is a Mayday going on.
A Mayday is a very serious situation and should never be taken lightly as a listener. People's lives are at stake and it is every sailors responsibility to do what they can to help. You never know when it might be you.
* Just as 'Mayday' comes from the french, so do 'Silence', 'Prudence' and 'Finish'. They are the only non-english words used in radio communication, english being the universal radio language.
- See M.I.P.D.A.N.I.O for more detailed instructions on sending a Mayday message.
- The source for this information is taken from the Royal Yachting Association (RYA) Short Range Communication (SRC) course which everyone should take if they are to be in charge of a vessel.