Ok Here Goes. This procedure is for Marylebone..
Thats the start sequence, now depending on how much air is in the Main Reservoir can determine how long the engine will sit at idle. The ETS needs to be running to enable the coach lighting and air conditioning and other services to function
Would I be right in saying that because it can take up to 10 minutes for the pressure to build, that is the reason that it has to idle for at least that long?
In other words, even if it is up to full in 2 minutes, it still has to run the 10 minutes because you don't know how long it's going to take until you have started the engine?
I'm not being very clear, but I hope you understand.