Flight Control Laws

Gulfstream GVII

Eddie sez:

If you don't have any Falcon or Airbus experience, the idea of flight control laws will be a foreign one. But these "laws" are nothing more than a set of software instructions that tell the Flight Control Computer how to behave under certain sets of circumstances. When everything is normal, these instructions tell the FCC to relay your inputs to the rest of the airplane and for the rest of the airplane to give you the feedback in your hands that tells you everything is normal. When things are less than normal, these instructions are designed to help you fly the airplane and, in a sense, protect it from any mistakes you might make.

In an Airbus this protection can have disastrous impact. (See Case Study Air France 296 or Case Study Air France 447 for just two examples. (There are others.)

Fly-by-wire in a Gulfstream is a fairly new concept and, as far as I can see, they seem to have it right. The documentation is sparse, so I am afraid there is some detective work involved here. Where Airbus refers to "Normal Law," "Alternate Law," "Direct Law," and "Abnormal Attitude Law," the Gulfstream approach is different. With the Gulfstream you have "Flight Control Laws" with various modes. For more about the Airbus philosophy, see: Airbus Control Laws.

These flight control laws determine which mode the Flight Control Computer is using, the "why" of the system. For the "how" of the system, see: G500 Flight Controls.

Everything here is from the references shown below, with a few comments in an alternate color.


Photo: G500 Flight Control Computer, PAS, p. 6-1
Click photo for a larger image

  • First, an Overview
  • A quick word about "Gain"
  • Then, "Normal Mode"
  • If something bad happens outside the flight control system, you could find yourself in "Alternate Mode"
  • If something bad happens inside the flight control system, you could find yourself in "Direct Mode"
  • If your FCCs become confused, if you FCCs lose communications with the REUs, or if you lose all normal hydraulic souces, you could find yourself in "Backup Mode"
  • It might be easier to see a Flight Control Mode Comparison

Last revision:


An Overview

A "law" is nothing more than a list of instructions that begin with "you must . . ." in order for things to work. The "Control Laws" are a list of software instructions that tell the Flight Control Computer (FCC) what to do with the information it receives. It gets its information from Rotary Variable Differential Transducers (RVDTs) that are essentially instruments that measure the displacement of the Active Control Sidesticks and rudder pedals, as well as the various positions of various flight controls and switches. It adds to that data from various sensors. From all of this, the FCC produces data for instruments, actuators, and to provide feedback to the pilot in the ACS.

What can be transparent to the pilot is that these control laws actually change the way they work, depending on how well the airplane is working. Under "Normal Mode," the aircraft should appear to behave normally. As things degrade, you can end up with alternate, direct, and backup flight control modes.


Photo: G500 Flight Control Laws, general, PAS, p. 6-10
Click photo for a larger image

[PAS, p. 6-10] General Info

  • Pilot control input → RVDT → FCC
  • Aircraft sensor inputs → FCC
    • IRSs
    • AHRS
    • ADCs
    • Radio Altimeters
  • FCCs contain software called Control Laws or CLAWS
    • Developed by Gulfstream specifically for G500 / 600
    • Makes the aircraft fly like a Gulfstream
    • Dampens undesirable aircraft motions such as Dutch roll
    • Several features designed for aircraft protection
      • AOA Limiting
      • High Speed Protection
      • Maneuver Load Alleviation
      • Speedbrake Auto-Retract
      • Dynamic Rudder Limiting
      • Elevator Split Load Limiting
    • 4 Flight Control Law Modes
      • Normal
        • Default Mode
        • Provides stability augmentation / envelope protection
        • Only mode aircraft can dispatch in
      • Alternate
      • Direct
      • Backup

Photo: G500 Flight Control Computers (FCCs), PAS, p. 6-7
Click photo for a larger image

[PAS, p. 6-7] Each Flight Control Computer (FCC) contains 2 channels (A and B) that contain the control laws. Normally one channel from each FCC sends inputs to the Remote Electronics Units (REUs) which average the 2 inputs then commands its actuator.

  • Each channel contains 2 lanes
    • Command Lane; Sends position commands to:
      • Control surface actuators
      • Monitor Lane
    • Monitor Lane
      • Performs same computation as Command Lane
      • Compares calculations to Command Lanes
    • Each lane utilizes
      • Same type of hardware (A or B)
      • Different software; acts as self-checking pair for error detection

    This bit is unclear the way it is written. I suspect what it means is that each channel has 2 lanes, one of which positions the actuator the other which monitors what the other channel is doing.


I think of gain as the "power steering" given to you under various conditions. Think about how little effort it takes on the stick to move a large control surface. If you had too much gain, you could break something. So gain is actually programmed to react differently at various airspeeds and control laws.

[PAS, p. 6-11] Normal Mode. Gain and control deflection scheduled with airspeed.

  • Amplification of signal to achieve desired aircraft response
  • Lower gain → Less control deflection and vice versa

In the Normal Flight Control Law, "gain" is normal. That means control deflections asked for by you (though the active control stick) or the autopilot are "scheduled" by airspeed. The amount of flight control deflection decreases with increasing airspeed. You also have a number of protections, such as low speed protection.

There are three other modes: Alternate Mode, Direct Mode, Backup Mode.

[PAS, p. 6-16] Fixed set of gains (2)

  • Flap and gear handles up → Based on 340 KCAS (low gain)
  • Flap or gear handle down → Based on 250 KCAS (high gain)
    • Extend flaps to at least 10° < 200 kts (Improves response due to high gain at lower speed)

In these modes, the variable gain is reduced to two steps. When the gear and flaps are up, gain is set to 340 KCAS. So at slower speeds, your flight controls will be comparatively sluggish. If either the landing gear or flaps are extended, the gain is set to 250 KCAS. Once again, the slower you go the more sluggish the flight controls will be.

Normal Mode

In normal mode, the flight control system allows you to deflect the flight controls commensurate with the airspeed. The system keeps you from flying too slow or too fast, keeps you from pulling too many Gs, and will even retract the speed brakes for you if that is the right thing to do.

Aircraft sensor inputs that go into the input signal manage all of the critical sensor inputs (air data, IRU) use a mid value logic strategy that average the two middle values. That is why when you are down to one ADS or IRS/AHRS there is no averaging and the system will go into alternate mode.

You only need one FCC channel to stay in normal mode.

Normal Mode


Photo: G500 Flight Control Modes, side stick, PAS, p. 6-11
Click photo for a larger image

[PAS, p. 6-11] Normal Mode

  • Gain and control deflection scheduled with airspeed
    • Amplification of signal to achieve desired aircraft response
    • Lower gain → Less control deflection and vice versa
  • High authority yaw damper
  • Sub modes of Normal mode
    • On-Ground
      • Determined by WOW or wheel speed sensors
      • Pitch Trim Indicator displays
        • Stab position in degrees Nose Up (NU) or Nose Down (ND)
        • Green Band
      • Pitch trim directly commands the horizontal stab
      • Remains until 10’ AGL after takeoff
    • In-Flight
      • Active at 10’ AGL
      • Pitch Trim Indicator displays
        • KCAS instead of stab position; 60 knot increments; 100 at top and 340 at bottom
        • AP OFF; Digital readout with pointer of KCAS aircraft is trimmed for
        • AP ON; White triangle pointer only; No digits
      • Pitch trim commands
        • Trim speed change; Max trim speed setting → Vmo (300-340 KCAS)
        • Elevator offset (initially)
      • Off-load Feature
        • Stab auto offsets to offload any persistent elevator offset
        • Elevator returns to neutral
        • Stab and elevator remain faired; Decreases drag
      • AP DISC / TSS button
        • Red button on sidestick
        • With AP Off → Trim Speed Sync (TSS); TSS button resets trim speed to current speed
    • AOA Limiting FCC AOA Limiting
      • Replaces stick pusher
      • Pitch control restricted by FCS; Helps prevent exceeding stall AOA
      • Activates approaching 0.88 or 0.93 AOA
        • Depending on rate of deceleration
        • Can occur at any AOA with abrupt control input
      • Stall Protection Active; Full aft stick Commands 0.95 AOA max
      • Exit this mode by reducing AOA
    • High Speed Protection
      • Available when AP Off; Vmo / Mmo +5 kts (earlier depending on accel rate)
      • Pitch control restricted by FCS; Helps prevent further overspeed by limiting nose down authority; Nose down authority decreased up to 75%
      • Will not protect if using High bank angles; Protection faded out > 60° bank angle
  • Additional Functions and Features
    • Pitch trim switches (3) all available: (2) Sidestick switches, Pedestal switch
    • Autopilot
    • Electronic-Feel
    • Turn Coordination
    • Auto Ground Spoilers
    • Steep Approach
    • Stick Shaker → Activates at 0.97 AOA
    • Speedbrakes with Auto Retract available
      • At high power settings → Approx 90% Thrust Lever Angle (TLA)
      • Main purpose is for a stuck / jammed handle; Momentarily set thrust levers to max to retract speedbrakes
        • Handle remains in selected position
        • Return handle to retract position to regain use
    • Dynamic Rudder Limiting; Helps prevent pilot from over stressing rudder
    • Maneuver Load Alleviation; Ailerons symmetrically deflect upwards to reduce loads
      • When pilot commands > 1.5 Gs
      • Reaches max 3° deflection ≥ 2.5 Gs
    • Elevator Split Limiting; Protects against large torque associated with a split elevator

    Photo: G500 Flight Control Modes, reset switch, PAS, p. 6-14
    Click photo for a larger image

  • FLT CTRL RESET switch
    • Located on center pedestal next to pitch trim switch
    • When pressed
      • Resets both A and B channels in both FCC’s
      • Resets all 16 flight control surface actuators
    • Perform this action when directed by a checklist
    • Recommendations for best results
      • Press during stabilized, wings level flight
      • Don’t press again within 10 secs
    • Does not work in Direct or Backup modes

Alternate Mode

You will degrade to alternate mode if you find yourself down to a single air data or inertial reference source. If that happens to you, you can try resetting the system. If you are stuck in alternate mode, your envelope contracts a bit and you lose some capability.

All that being said, it would be quite unusual to find yourself here. If a flock of birds took out all three smart probes, for example.

Degrade to Alternate Mode


Photo: G500 Flight Control Modes, degrade to alternate mode, PAS, p. 6-15
Click photo for a larger image

[PAS, p. 6-15]

  • FCS Alternate Mode (U)
    • Loss of air data
      • Less than 2 valid air data sources
    • Loss of inertial data
      • Less than 2 valid inertial reference signals
      • Min required to remain in Normal: 1 IRS agreeing with 1 AHRS, or 2 IRS’s
    • Loss of communication between FCC’s and HSTS
      • FCC’s can’t talk to REU’s controlling Horizontal Stab MCE
      • Stab Pri Trim Fail (U)
    • FLT CTRL RESET Switch may allow return to Normal
      • If degrade reason resolved
    • Probability of occurrence is < 1 per 10 million flight hours

Characteristics of Alternate Mode

[PAS, p. 6-16]

  • Fixed set of gains (2)
    • Flap and gear handles up → Based on 340 KCAS (low gain)
    • Flap or gear handle down → Based on 250 KCAS (high gain)
      • Extend flaps to at least 10° < 200 kts (Improves response due to high gain at lower speed)
  • Simplified yaw damper
    • More oscillations expected
  • Speedbrakes
    • Auto Retract not available
  • Stick Shaker earlier onset → 0.85 AOA (if AOA remains valid)
  • Loss of following features
    • Autopilot
    • AP DISC / TSS button → TSS not available
    • AOA Limiting
    • High Speed Protection
    • Turn Coordination
    • Dynamic Rudder Limiting
    • Maneuver Load Alleviation
    • Steep Approach
  • Sidesticks
    • Remain linked
    • May revert to Degraded Active Mode
  • May lose following features depending on cause of mode change
    • Normalized AOA, Stick Shaker, PLI
    • Low Speed Warning
    • Primary Pitch Trim switch on sidestick if
      • Stab Pri Trim Fail (U)
      • Utilize Pedestal Pitch Trim switch instead

Direct Mode

It is tempting to think of each of these modes as one progression to the next. You are in normal, bad stuff happens, you go to alternate, more bad stuff happens you go to direct, and so on. But that isn't really how it is. Alternate mode seems to be caused by things outside the flight control system, direct mode stems from things within the flight control system.

If the Flight Control Computer loses all of its inputs or otherwise become incapacitated, you end up in direct mode. In addition to what you've lost in alternate mode, your sidesticks are degraded and you lose a few more features (shown here).

Degrade to Direct Mode


Photo: G500 Flight Control Modes, degrade to direct mode, PAS, p. 6-17
Click photo for a larger image

[PAS, p. 6-17]

  • FCS Direct Mode (U)
    • All 4 FCC channels are invalid
      • Command and monitor lanes don’t agree
      • Different set of software is now utilized; Impossible to return to Normal or Alternate modes

Characteristics of Direct Mode

  • Flying qualities identical to Alternate mode except
    • Sidesticks → Degraded Active mode
    • Primary Pitch Trim switch will be available
  • Loss of additional features
    • Normalized AOA
    • Stick Shaker
    • PLI
    • Low Speed Warning
    • Auto Ground Spoilers
    • Roll trim
    • AP DISC / TSS button; Will not stop yaw trim movement

Backup Mode

I think you end up in backup mode when the FCC becomes confused and is no longer able to communicate with the Remote Electronic Units or if you lose all normal hydraulic pressure sources. In backup mode, you have completely separate computers, software and hardware, as well as actuators independent of the normal sources. You end up with what you had in direct mode with fewer spoilers and reduced trim capability.

Degrade to Backup Mode


Photo: G500 Flight Control Modes, degrade to backup mode, PAS, p. 6-18
Click photo for a larger image

[PAS, p. 6-18]

  • BFCU Active (U)
    • All 4 FCC channels unable to compute control law
    • System tries to go to Direct Mode before Backup Mode
    • BFCU sends commands but can’t receive REU feedback
      • Utilizes single direction bus to REU’s
      • Surface positions not displayed on synoptic pages
    • Impossible to return to any other mode
    • Probability of occurrence is < 1 in a billion per flight hour
  • BFCU talks to EBHA’s only
    • Whether EBHA operating normally or in EB mode

Characteristics of Backup Mode

[PAS, p. 6-18]

  • Similar flying qualities as Direct mode except
    • Loss of speedbrakes and ground spoilers; Inboard and midboard spoilers not available (no EBHA’s)
    • Outboard spoilers available for roll control only
    • Pitch trim available only via pedestal mounted PITCH TRIM switch
      • Moves horizontal stab only, not elevator
      • Moves at reduced rate
  • Loss of additional features
    • Yaw trim
    • Limited amount of yaw damping; IRS #3 must be working

Flight Control Mode Comparison

As presented in the source document, this table is missing the labels on top. I've added what I think should be there.


Photo: G500 Flight Control Modes, comparison, PAS, p. 6-19
Click photo for a larger image

Gulfstream GVII-G500 Airplane Flight Manual, Revision 4, August 29, 2019

Gulfstream GVII-G500 Production Aircraft Systems, Revision 3, July 15, 2019