FMC Change departure

[Makaros]

When I start a route I put a random departure on fmc so it can calculate the point of descent etc.
Unfortunately when I try to put a new departure en route, the fmc keeps the old fixes, (the random ones) and the plan becomes really messy. I guess I am missing something. Is there a way to put a new SID over another one?

[Litjan]

Hi Makaros,

I think you are doing everything correctly - when entering a new SID, the old SID should be cleared away. Of course it is very unusual to put in a random SID, usually the pilot knows very well which SID he will fly. It is a different story with the STAR, that is changed regularly during the flight.

If you have started flying your SID (or STAR) and then subsequently change it, the FMS will retain certain points of it, because you are "already using it".

Cheers, Jan