I am referring to the use of Airspeed hold in level flight, not whilst climbing or descending, BUT maybe airspeed hold is only designed for use WHEN climbing or descending. I don't know.
Alex, you need to do some research on the basics of aircraft operation. You cannot have Airspeed Hold while also maintaining an altitude unless you have an autothrottle. The F27 does not have an autothrottle. IAS Hold is used for climbing and descending (the autopilot using the pitch controls) with a constant power setting. This is basic stuff that would be obvious if one had done an instrument rating.
I very much doubt that it says in the manual that that mode will maintain IAS whilst also maintaining level flight.