shift+# is engine selection, meant for multiengine aircrafts. Problem is that if you are in a one engined aircraft you still can switch to only control the (non-existant) second engine, and throttle changes won't work on the engine that IS there (#1).
I don't know if it's possible to change engines in all planes or it's just a bug for a few. The one it happend in for me was the Ki-61 once in the CT, but I haven't tested it in others.
PS: Shift-E gives you control for all engines again in multi-engined aircrafts.