question, what exactly is FCS and what does it do?! 
Azframer is correct, Flight Control System.... More specifically, it's a series of Proportional Integral Derivative (PIDs) that controls the Pitch, Roll, Yaw, Flaps and Trim systems of the aircraft. These PIDs constantly monitors what the aircraft IS doing and what the pilot WANTS to do and works to drive the difference to zero. It is our tool that allows us to mimic as close as possible the control logic of the real jet based on what we know and our math/coding abilities
......................................................Inhales...........................................................
What does it (the gauge in particular) do?.... a few things...
3-AXIS FLIGHT CONTROLWith most FSX aircraft, user input is directly translated to control surfaces (i.e. flight stick controls elevator position). For example, if you pull all the way back on your flight stick or yoke, the elevators on the aircraft will deflect all the way up. This will occur regardless of the aircraft's attitude, airspeed or configuration.
With the Hornet, things are a little different. The FCS basically asks the pilot: "What do you want the aircraft to do, and I'll move the elevators, ailerons, rudders and flaps to give you what you want." With that being said, your flight stick now controls G, Pitch Rate, AOA and Elevator Positions depending on the airspeed and aircraft configuration. For example, at 400 KIAS, you pull back on the flight stick a little. The FCS registers that position of the flight stick to equate to about 2.0 Gs. The FCS then deflects and constantly adjusts the elevators to give you the 2.0 Gs that you've "asked" for. Let's say you accelerate up to 600 KIAS, and pull back on the flight stick the same amount, the FCS again registers that you are still asking for a 2.0 G pull, but also understands that you are traveling through the air much faster and therefore will not require as much elevator deflection to give you the 2.0 Gs that you asked for, so it adjusts elevator output accordingly.
There are two primary modes, Up & Away (UA) and Powered Approach (PA). The modes are controlled by Flap Position. UA mode is entered when the flaps are placed in UP/AUTO, and PA is selected anytime the flaps are placed in HALF or FULL.
UA ModeUA mode is much like what I described above, and rings true (as close as we can tweak it anyways) for airspeeds above 310 knots, below 310 knots, the system starts blending G command with Pitch Rate command, and eventually your flight stick will control only Pitch Rate at around 250 knots and below.
There is an exception to the logic however.
-If the jet is at 22 degs AOA and above, above 22 degs AOA, your flight stick starts blending control of Pitch Rate with Angle of Attack and after a certain point, your flight control stick will command AOA only (up to about 35 degs AOA).
-Another is when
PA ModeWith PA mode, your flight control stick now tells the FCS to adjust the elevators to maintain a certain Angle of Attack.
Pitch Auto Trim
IN UA Mode, the FCS also works to automatically trim the jet to 1.0 G when no control input is detected from the pilot. Trim has to be set to neutral (0) for for this. Any nose up or nose down trim will bias the system to stabilize with greater that 1.0G (NU), or or less than 1.0G (ND) trim. In PA Mode, the FCS works to use trim to maintain the trimmed AOA.
The FCS also controls the ailerons for roll control and also the rudders to automatically keep the aircraft coordinated during turns.
FLAP CONTROLThe FCS also controls the leading and trailing edge flaps. The FCS controls their scheduling based on flap switch position, airspeed and angle of attack.
AUTO THROTTLEThe FCS.dll for this jet also controls the Powered Approach Auto Throttle logic that attempts to maintain the jet at optimal AOA/Landing Speed when Auto Throttle is engaged while in PA Mode.
NOSEWHEEL STEERINGLastly, the FCS.dll controls the Low and Hi Nosewheel steering, switching to a higher caster angle when airspeed is below 10 KIAS (i think). This allows for sharper turns while taxiing at lower speeds and more stability during higher speeds.
......................................................................exhales....................................................................
Probably more than you were looking for, but that pretty much sums up what the FCS is and what it does for our jet. All credit goes to ORION and his awesome math and C++ coding abilities for this awesome gauge!
