Final answer:
To engage IAS hold, the aircraft must be at or above a certain airspeed before the IAS button can be engaged on the FD/DCP. The exact airspeed value depends on the aircraft and should be looked up in the aircraft's flight manual.
Step-by-step explanation:
The question is regarding the procedure for engaging the IAS hold function which is a feature commonly used in aircraft autopilot systems. To engage the IAS hold, the airspeed must generally be within certain parameters as set by the aircraft manufacturer or the autopilot system. While I cannot give a specific speed value without knowing the exact aircraft model, the process typically involves ensuring the airspeed is above a minimum threshold, after which the IAS button can be activated on the Flight Director (FD) or the Display Control Panel (DCP). Once the IAS hold is engaged, the autopilot will maintain the current Indicated Airspeed (IAS) at the time of engagement.
It is important to consult the specific aircraft's flight manual or autopilot guide for the exact procedures as they can differ between aircraft types and autopilot systems. Proper understanding and operation of this feature are crucial for safe flight operations, particularly when transitioning between modes of autopilot control.