As anybody who has seen the various RNLI Saving Lives at Sea programmes will know, Mayday is only supposed to be used when there is an imminent danger to life. If your yacht's engine breaks down it is not a Mayday. If your yacht's engine breaks down and the current is sweeping you onto rocks it is a Mayday. Sending a Mayday call is asking for help, I am not sure what help can be rendered to an airliner flying at 16,000 feet, so maybe the use of Mayday by aircraft is pointless and just declaring an emergency is the correct option.