Garmin GFC 600H Pilot's Manual page 31

While in Ground Speed Hold Mode the beep trim increments (±2 knots) or slews
(±10 knots/sec) the reference lateral and longitudinal ground speed. Ground Speed
Hold can be trimmed to maintain backward or sideward flight.
GMC 605H Annunciating Automatic Transition to Hover (Ground Speed Hold Mode - GSPD)
GSPD mode requires GPS and is available from take-off, or upon reducing the
aircraft's groundspeed below the hover transition threshold. Refer to the aircraft
specific FMS to determine the GSPD transition speed for the airframe. In GSPD
mode, the system maintains a lateral and longitudinal groundspeed reference while
maintaining a constant heading reference (with optional yaw axis). The pilot adjusts
the forward/aft and left/right GPS velocity using the cyclic beep trim.
The system automatically detects when the pilot is trying to hold the helicopter
stationary relative to a point on the ground based on GPS groundspeed parameters
and augments the control inputs to help achieve that objective.
The objective of the detent in pitch and roll is to provide a center point for the force
trim function. In attitude hold mode, this detent is a pitch and roll attitude; in a flight
director coupled mode of flight, the detent is the attitude for which the flight director
command is satisfied. The primary objective of the yaw axis is to provide coordinated
flight. The detent for the yaw axis is set accordingly.
As part of attitude hold, the system provides attitude rate and acceleration rejection
in each control axis. This stabilizes the control around the reference attitude condition.
This feature is particularly useful when augmenting the pilot controls and stabilizing
the aircraft while the pilot is hand-flying. This functionality requires the pilot to operate
the cyclic controls with a loose grip to utilize the benefit while hand-flying.
190-02081-04 Rev. A Garmin GFC 600H Pilot's Guide
HFCS
21