During a manual start on a A320, the ENG MAN START P/B opens the start valve which then runs the starter motor. During the MAN START process, at some point, the start valve closes automatically. What are the risks associated with continuing to leave the MAN START P/B ON at this point?
Well, documentation is a bit vague about this, and I suppose a technician would know more about the different things the ENG MAN START bp is doing, but from a quick glance at the FCOM there is one major difference that I can spot:
[The FADEC durring a manual start] (...) Closes the engine start valve, and cuts off the ignition when N2 reaches 43 %.
Bold emphasis is mine, no other conditions except N2>43%.
Leaving the ENG MAN START pb on means FADEC keeps assuming we are trying a manual start, and will cut off ignition anytime the N2 is above 43%. No prob as long as engines are running, right?
But that means (I assume) when you then enter turbulent/stormy weather in flight and turn on IGN/START to prevent a flameout, nothing happens as FADEC is happy that N2 is above 43% and you manual start has been ‘succesfull’.
I don’t really know if it will even attempt an auto restart when you get a flameout, as the N2 will probably keep windmilling for some time depending on your airspeed, so you could end up with a big problem.
To my understanding, the input from the MAN START switch (a ground/open discrete into the Engine Interface Unit), is only taken into account by the FADEC logic if the engine is not already running. Once the engine control logic goes into running mode (i.e. N2 rises above idle), it will stay in that mode until the engine master lever (EML) is set to Off, regardless of whether the engine is lit or not.
In other words, once the engine has started, the position of the MAN START p/b is irrelevant, even if the engine flames out (however, note that the FCOM procedures for engine failure often call for the EML to be set to Off, at which point the position of the MAN START switch comes to play in any subsequent attempt to relight the engine).
One risk I can think of is that if you are on the ground, shut the engines down, or even power off the aircraft, then when you power everything back on and attempt to start an engine (i.e. turn the engine start selector to IGN), the autostart procedure will be inhibited by the MAN START switch being already on, and the FADEC will go into manual start mode instead. If you then immediately set the EML to On (which wouldn't be a problem during a normal autostart), fuel would start to flow right away, while the engine may still be below the required minimum N2 for fuel admission (16%), therefore the engine could fail to start or worse (an engine fire).