190-02081-03
GFC 600H HFCS Part 27 STC Maintenance Manual
Rev. 3
Page 2-3
2.2 System Overview
The GFC 600H is a digital helicopter flight control system (HFCS) which provides attitude hold, force
feedback and coupled flight to flight director modes. The overall system capabilities are dependent upon
the interfaced equipment; however, each installation offers the same core functions.
2.2.1 Attitude Hold (ATT)
The system uses aircraft attitude, rates, and acceleration data to stabilize the aircraft and hold a reference
attitude. ATT is lowest level mode of operation and ATT is the default mode when engaging the system.
When engaging the system at low speeds, the system may engage into groundspeed (GSPD) hold if the
installation is equipped with a Garmin GTN navigator. Availability of ATT is dependent upon dual AHRS
sensor availability. Air data or GPS data is required for the AHRS to initialize, but these data sources are
not necessary for continued operation in flight.
Control forces are in detent (i.e., forces are minimized on the controls) when the reference attitude is
achieved. The system provides a force feedback to the pilot in the form of a spring gradient such that forces
increase as the aircraft deviates from the reference attitude. If the pilot pushes on the controls such that the
system detects that the pilot is hands-on, an out of detent annunciation is displayed on the GMC 605H and
Garmin GDU to indicate that the system is not able to maintain its control objective because the pilot is
hands-on.
When trimmed to a level roll attitude, the system will make small corrections to maintain and stabilize an
aircraft heading.
When holding a pitch attitude at higher airspeeds speeds, the GFC 600H system will also help stabilize and
maintain airspeed.
When initially engaging the system, the GFC 600H system will set the reference attitude to the current
attitude, provided they system was not engaged through the LVL function.
The pilot can adjust the pitch/roll attitude control reference by pressing the force trim release (FTR) switch
or by using the TRIM switch.
For installations with a yaw axis, the yaw heading reference is reset when the pilot presses through the
forces on the pedals and releases. The yaw axis reference can only be adjusted in hover. When the pedals
are released, the new heading reference will be set to the current aircraft heading.
Sub-functions and capabilities of attitude hold are as follows:
Hover Assist
In hover assist, the system stabilizes the aircraft to a zero groundspeed reference. Hover assist is
automatically engaged when the aircraft groundspeed approaches zero. If equipped with an optional yaw
servo, the yaw axis will maintain aircraft heading within this flight envelope. The heading reference is
adjusted by simply flying through the pedals to a new heading.
Groundspeed Hold
Like hover assist, groundspeed hold operates in the low speed flight regime. Groundspeed hold is uniquely
annunciated as “GSPD” on the GMC 605H display. This mode allows for a longitudinal and lateral
groundspeed to be selected and maintained by the flight control system. Just like ATT, the GSPD reference
can be adjusted with the FTR switch and the TRIM switch.