I am trying to operate a microwave convection oven for the first time. It is a GE Profile convection/microwave oven.  I am wondering if I am supposed to keep the glass microwave turntable and plastic microwave turntable support ring in the unit when I am operating in convection mode.  I know the first thing people will say is to read the owners manual, but the owners manual is extremely unclear.   Here is a manual I found online, which is for a model very similar to the one I have.  On page 8 of this manual, as well as on page 8 of the manual that came with my model,  they discussed that the metal shelves should not be in the unit when microwaving food, but they don’t mention anything about the glass turntable and plastic turntable support ring  being used in convection mode. You might ask: why don’t I just remove these two items anyway when using convection mode just to be safe? The answer is that the support ring sits on a piece of plastic, and that piece of plastic seemingly can’t be removed, so I will still have to worry about plastic in the unit while using convection mode. So is that piece of plastic that’s part of the unit safe for the convection oven? 
If the plastic part is non-removable, then better leave the glass plate in place. Better some spots on glass, than grease. crumbs and other stuff dripping from the convection-cooked food jamming the mechanics and stopping the turntable permanently. In this case the glass plate acts as a protective cover. In convection mode, the inside is thermoregulated (the temperature never exceeding preset on the thermostat) so the plastic will never reach dangerous temperatures; in microwave mode it's entirely transparent to microwaves so unless it's seriously dirty, it won't heat up at all.  You may consider using a non-stick baking sheet (teflon foil) on the bottom to protect it from 'drips'. Otherwise, just keep it clean.  