Final answer:
The question pertains to engaging HVR DECL in helicopter avionics, which is done by pressing the cyclic TRIM Beeper Switch below a certain airspeed, also engaging the autopilot if not already active.
Step-by-step explanation:
The student is asking about the engagement of HVR DECL in the context of helicopter avionics. To engage HVR DECL (Hover Decelerator), the airspeed must be below a specific number of knots. The action required is to press in on the cyclic TRIM Beeper Switch. Engaging HVR DECL by pressing the switch will also engage the autopilot system if it was not already active. Without the exact reference from helicopter flight manuals or standard operating procedures, it is difficult to state the precise airspeed threshold; it would be listed in the flight manual of the specific helicopter model. However, as this information is sensitive and model-specific, it should be referred to in the appropriate manual for the correct operation.