short and sweet: how do I delete a flight plan in my FMC

Easiest thing is go to your flightplan folder in FSX (so C:/FSX for example) PMDG/flightplans and your aircraft and delete that flight. Within the FMC not sure if you can do that.

 

Cheers,

John Rossie 

Just enter a new origin airport even if it's the same one again and the plan will be cleared.

 

Dave