Eddie sez:

I am not an aeronautical engineer, I just fantasize about being one. (Yes, that is weird.) For most airplanes we can get by with a very limited understanding of aerodynamics ("houses bigger, houses smaller") but this airplane is different. The airplane will do things you aren't expecting if you get it into a part of the envelope it shouldn't be. So you need to have a better understanding of your stick and rudder.

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

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Photo: A sketch of a wing in a wind tunnel
Click photo for a larger image

Last revision:

2020-07-15


Displayed versus Normalized Angle of Attack

AOA versus Normalized AOA (NAOA)

We learn early on that airspeed doesn't fly airplanes, angle of attack does. We also learn that the angle of attack is the angle between the chord line of the wing and the relative wind through which it is flying. Okay, close enough. More about AOA: Aero / Angle of Attack.

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Photo: Angle of Attack (from Eddie's notes)
Click photo for a larger image

Gulfstream makes a big deal to differentiate "normalized angle of attack" against "angle of attack" and the engineer in me appreciates it. But the pilot in me thinks this is much ado about nothing. Is "Normalized" any different than what we think of as AOA? Not really. To be technically accurate, AOA is expressed in degrees but that isn't really useful to us since the stall angle of attack changes with wing configuration. We think of AOA as from 0 to 1, where 1 is the maximum lift we can get out of the wing. That's normalized AOA in a nutshell. It is curious that none of our instruments say "NAOA" but the manuals do:

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Photo: Normalized AOA Readout on PFD, Symmetry Guide, §2B-04-00, p. 38
Click photo for a larger image

Now wait a minute. If 1.0 AOA is the maximum lift we can get out of the wing, and anything higher than that is the — cue dramatic music — aerodynamic stall, how can we have an AOA of 1.10? Well first off, there is a mistake in the Symmetry Guide illustration. It says "0.00 to 1.10 degrees" and that is wrong. Normalized AOA is a ratio and doesn't have any units at all. The actual stall angle of attack is likely to be something well above that. But I digress, what is this about a Normalized AOA of 1.10?

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Photo: "More than Max" angle of attack(from Eddie's notes)
Click photo for a larger image

The critical thing to understand is that most wings do not stop producing lift when they get to the point where they are producing maximum lift. In some ways calling this point the "stall angle of attack" does a disservice, because while the wing is beginning to stall, it is still producing lift. This effect is more pronounced with swept wing airfoils. So while the amount of lift produced goes down, the AOA can indeed exceed the stall angle of attack and the NAOA can exceed 1.00.

NAOA and the GVII

[G500 Ground and Flight Operations, p. 95]

  • The displayed angle of attack (AOA) is used to show the pilot the current lift condition. This is influenced most strongly by Mach number, flap setting, and wing contamination (icing). The digital AOA display is "normalized" (referred to as NAOA) and shows positive numbers only between 0.00 and 1.10.
  • Zero is the AOA which approximates zero lift on the aircraft and 1.00 is the maximum lift that was demonstrated to be usable during certification based on required stall margin and handling/controllability characteristics.
  • In the range from zero to one, the AOA, lift, and load factor are generally all proportional and should be thought of as essentially equivalent. In other words, the fraction (O to 1) of usable positive lift, AOA, and "g" are essentially the same.
  • I'm not sure about this as written. Perhaps: "The fraction (0 to 1) of usable positive lift, AOA, and available g are essentially the same." This becomes a useful tool. The lower your AOA is, the more energy you have available to maneuver. If you are flying at 0.33 AOA, for example, you are using 1/3 of your energy (in terms of lift) to fly but still have 2/3 left over (in terms of available g) to pull back on the stick or turn. If you are flying slower, say at 0.67 AOA, more of your energy is taken up with the task of just keeping in the air so you have less energy available to maneuver.

  • The available AOA at any given flight condition can be further limited by the AOA limiter, "g" limits, maximum control system deflections and malfunctions. Additional annunciations can also be present to include the shaker, Low Speed Awareness callouts, and increased buffet.
  • The NAOA range where these limits and annunciations occur are generally between 0.85 and 0.97, and at lower speeds where "g" limits are not rapidly approached. So, if the indicated NAOA is 0.40, then the wings are generating 40% of the usable lift, AOA, and g loading. Pulling to 0.80 AOA at constant speed would double the lift and g's.
  • The reason NAOA is displayed, rather than actual AOA, results from the dramatic reduction in available AOA in the high Mach regime -- due to shock induced flow separation on the wing, and the large change in actual AOA with flaps. The maximum achievable, non-normalized AOA values (in degrees) at high Mach and/or high altitude can be approximately one third of those in the low Mach/low altitude regime. Thus, to give the pilots the most useful indication of AOA condition and stall margin, the NAOA indication was developed.
  • In my opinion, all of this is true but not particularly relevant. The reason we have NAOA, indeed every airplane I've ever flown with displayed AOA has been this way, is that it is incredibly useful to know what your ratio of angle of attack over maximum available angle of attack is. Giving pilots the AOA in degrees over the camber of the wing is unlikely to be as readily understood.

  • Normalized AOA is calculated real time based on air data, flaps, and icing status and is the following fraction:
  • AoACurrent - AoAZeroLift AoAMaximumLift - AoAZeroLift

    These AOA values are in units of degrees and you end up with a ratio that has no units.

  • For normal operations, final approach AOA is normally in the range of 0.60 to 0.66, depending on wind and gust additives, etc. Interestingly, these AOA values roughly represent the maximum endurance and maximum range AOA's also. While the official aircraft performance data and FMS calculations more accurate, the following is useful information.
  • For max endurance (e.g. holding or loitering), the aircraft should be flown at 0.66 AOA (allowing good maneuver and stall margin) at an altitude that has the best available combination of low temperature (good) and low Mach number (also good). This generally occurs around FL350.
  • For best cruise range, the same NAOA range is flown (0.60-0.66), but at the design condition of Mach 0.85 near the ceiling altitude, to 4,000 feet below the optimum ceiling as required by cruise altitude restrictions. If these altitudes are not available, choose the next highest possible flight level. At altitudes below FL350, good range can be achieved in the 220-280 KCAS range depending on weight (heavier implies fly faster).
  • Summary of NAOA Values of Interest:

    • 0.66 —Approximate NAOA for VREF
    • 0.75 — Pitch Limit Indicator (PLI) appears, PLI removed at 0.73 (hysteresis)
    • Hysteresis is a simply lag in the system.

    • 0.85 — Stick Shaker activates for all non-normal flight control laws (Alternate, Direct, Backup), No AOA limiting present in non-normal (Alternate, Direct, Backup) flight control laws, No Auto-Pilot available in non-normal laws
    • 0.88 -0.93 — AOA limiting begins in Normal Law, based on rate of increase in AOA , CYAN CAS message - FCC AOA LIMITING
    • 0.95 — Full-Aft Stick - Maximum AOA achievable in Normal Law with pure pitch input
    • 0.97 — Stick Shaker activation in Normal Law; Normally only seen when on aft limit and attempt a large roll input or sideslip

    Anecdotal NAOA Examples

    I took the airplane around our local pattern with a video camera on the instrument panel just to have a log of what a normal airplane should look like, in anticipation of one day having the airplane not behave normally. The airplane was light: just 7,000 lbs of gas, two pilots and a jump seat photographer. But it was a gusty day and our VREF additive was 13 knots, which I carried until the flare. A side benefit is that I got a good view of the indicated NAOA. Here is what I found:

    Status Gear Flaps KCAS NAOA
    Begin takeoff roll Down 20 40 (on the "peg") 0.00
    Takeoff roll Down 20 60 0.09M
    V1 Down 20 114 0.21
    Begin rotation Down 20 132 0.24
    Gear fully retracted after takeoff Up 20 185 0.28
    Flaps fully retracted after takeoff Up 0 200 0.35
    30° bank turn, level flight Up 0 200 0.54
    10° flaps, crosswind, level flight Up 10 180 0.40
    20° flaps, final, level flight Up 20 160 0.45
    On glide path Down 39 135 (VREF+13 (for gusts) 0.55 - 0.65 (gusts)
    Begin flare at 25' Down 39 135 0.52
    Touchdown Down 39 125 0.65

Here is the video:

Gain

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Photo: Normal control law gain, Eddie's drawing
Click photo for a larger image

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] Gain and control deflection scheduled with airspeed; amplification of signal to achieve desired aircraft response, lower gain means 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.

Low Speed Awareness (LSA) System

What is it?

[G500 Ground and Flight Operations, p. 97] The low-speed awareness (LSA) display is designed to detect and prevent low speed conditions arising from a wide variety of causes. The LSA system is displayed, and is always active, except when on the ground below 100 KCAS, or when failures occur.

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Photo: Low Speed Awareness (LSA) system, Eddie's drawing
Click photo for a larger image

LSA Caution Speed (Yellow Band)

[G500 Ground and Flight Operations, p. 97] The top of the yellow band indicates the minimum speed at which a 30 or 40-degree bank turn (at a constant flight path angle) can be maintained using full aft stick, while in AOA limiting mode (0.95 NAOA). A 40-degree bank turn is normally used -- a 30-degree bank is used at FL350 and above, or in special cases such as below 50 feet AGL, or during takeoff with an engine failure. The caution speed is calculated in the FCCs using smoothed values of current speed, load factor, and NAOA. The caution speed therefore changes with Mach number, sideslip angle, flap position, and anti-ice. The caution speed does not change significantly with load factor or AOA because these two effects generally cancel each other out. This results in a stable caution speed while maneuvering vertically or holding flight path angle during turns. When the airspeed drops below the LSA caution speed (in the yellow band), the airspeed and AOA digits turn yellow and the HUD will display "AIRSPEED LOW" text with flashing airspeed and AOA indications. If the condition persists for more than 2 seconds or the airspeed drops more than 5 knots below the LSA caution speed, an "Airspeed Low!" voice prompt is initially repeated twice and then once every 5 seconds until the yellow region is exited.

[G500 Ground and Flight Operations, p. 98] The top of the yellow band will normally be about 5 knots below VREF. The yellow band may be useful to quickly estimate VREF without any reliance on pilot FMS inputs. It provides an independent verification which is helpful in detecting VREF errors (e.g. if the FMS weight is incorrect for any reason). The yellow band predicts where the red band will be (with a .02 AOA margin) during a 40 (30) degree bank constant FPA turn. . . . The yellow band indicates turn maneuver margin.

LSA Warning Speed (Red Band)

[G500 Ground and Flight Operations, p. 97] The margin between current speed and the LSA red warning speed indicates proximity to stick shaker and, thereby, proximity to stall (accelerated stall with increased load factor). The red bar moves dynamically with the PLI and provides similar information. The PLI and the top of the red band will both be "touched" when the AOA reaches 0.97 and the stick shaker activates. The top of the red band estimates the stick shaker speed (.97 NAOA) at the current conditions if only airspeed and AOA were to be changed. . . . The red band indicates accelerated stall speed margin.

[G500 Ground and Flight Operations, p. 98] The red band is normally below the yellow band, but is not necessarily so. When maneuvering requirements are beyond those required for normal operation, the red band provides immediate feedback of changes to the accelerated stall speed and proximity to it.

Pitch Limit Indicator (PLI)

[G500 Ground and Flight Operations, p. 97] The PLI (Pitch Limit Indicator) provides a conformal, dynamic display of AOA margin to stick shaker (0.97 AOA). The angle between the FPM and the PLI is AOA margin in degrees. The PLI is shown on the PFDs, HUD, and standby displays when the AOA is 0.75 or greater.

[G500 Ground and Flight Operations, p. 98] The PLI directly correlates to the Red Band and is normally not displayed. The most basic use of the PLI is noticing whether it is displayed or not. If the PLI is not displayed, the AOA is less than 0.75. Since 0.95 AOA is available with full aft stick, then at least 20% of AOA range is still available. Similarly, 20% higher load factor would typically be available at a given speed. When maximum performance is required, the PLI may be used to monitor AOA and proximity to stick shaker activation. . . . The PLI indicates AOA margin.

Auto-Throttle Speed Protection (ASP)

Note: the following sentences come directly from the cited AOM pages, but have been reordered so as to be understandable to the average English speaker.

What is it?

[G500 Ground and Flight Operations, p. 107] The Auto-Throttle Speed Protection (ASP) function uses Auto-Throttle auto-engagement and selective airspeed targeting to prevent excursions outside of the normal aircraft speed envelope. ASP includes both static and dynamic speed protection functions.

[G500 Ground and Flight Operations, p. 99] The Auto-Throttle system has a speed protection mode to automatically engage the Auto-Throttles to prevent underspeed. The Auto-Throttle Speed Protection (ASP) system will target the higher of the guidance panel (GP) speed target or Auto-Throttle (VMIN), which is calculated as 1 knot above the LSA airspeed. ASP activation is annunciated to the pilot as "VMIN" on the Auto-Throttle (or Performance) FMA.

[G500 Ground and Flight Operations, p. 107] Auto-Throttle Speed Protect (ASP), when engaged during a high-speed excursion, initially targets a speed inside the speed envelope (VMO) and then will subsequently target the speed set in GP speed window.

Static ASP

[G500 Ground and Flight Operations, p. 100] The static underspeed threshold is set to be coincident with the LSA airspeed, preventing airspeed excursions outside of the envelope that may develop slowly over time. Static underspeed protection will activate when the current underspeed threshold has been exceeded without activating the dynamic ASP function.

[G500 Ground and Flight Operations, p. 100] Static overspeed protection will activate when the current overspeed threshold has been exceeded without activating the dynamic ASP function.

Dynamic ASP

[G500 Ground and Flight Operations, p. 100] For dynamic underspeed protection, the ASP function uses a filtered CAS rate and the current CAS to determine if the aircraft will underspeed in the next 6.5 seconds. The dynamic underspeed threshold is determined by predicting if the aircraft's Calibrated Airspeed in the next 6.5 seconds will hit or the amber or red bars on the airspeed tape. The dynamic underspeed threshold is defined as 30% below the top of the amber bar. For example, if LSA was 119 and Vshaker was 104, the dynamic underspeed threshold would be 114.5 knots.

[G500 Ground and Flight Operations, p. 107] For dynamic overspeed protection (aircraft accelerating towards the limit), the ASP logic uses a filtered Calibrated Airspeed (CAS) rate and the current CAS to determine if the aircraft will overspeed within the next 2.5 seconds.

Low Speed Scenario

The AOM takes you through several training scenarios with varying conditions of autopilot and autothrottle conditions. I've combined them into one to better understand what is going to happen when you get too slow. All of this comes from [G500 Ground and Flight Operations, pp. 102 - 106. The following is under Normal Control Law.

  • As the aircraft decelerates through VREF the NAOA will be approximately 0.66.
  • The PLI will appear at 0.75 NAOA.
  • The speed tape and AOA indications will turn amber when the current speed tic mark touches the top of the LSA caution (yellow) band.
  • After more than 2 seconds in the yellow bank or more than 5 knots below the LSA caution speed, an "Airspeed Low!" aural voice prompt will be made twice and then once every 5 seconds until airspeed is above the yellow band.
  • If not engaged, the auto-throttles will activate. Once engaged, the auto-throttles respond to stop the deceleration and keep airspeed at the top of the yellow band.
  • If the auto-throttles fail and the speed continues to decay, the autopilot will disconnect at 0.93 NAOA.
  • AOA-Limiting occurs between 0.93 and 0.95 NAOA and the stick inputs are limited to 0.97 NAOA.
  • The stick shaker occurs with full aft stick and roll inputs.

Under Alternate Control Law the autopilot and auto-throttles are not available. You will have the PLI but you will not have LSA indications. The stick shaker will go off at 0.85 NAOA.

VMax Mode / High-Speed Protection

What is it?

The airplane doesn't want you to exceed VMO / MMO and will pull the throttles back or pull the nose up to keep you within the speed envelope. See Auto-Throttle Speed Protection (ASP) for details about that. The following is paraphrased from [G500 Ground and Flight Operations, pp. 107-108] to enhance readability.

VMAX Mode (Flight Director Overspeed Protection)

The VMAX Mode generates flight director commands to prevent you or the autopilot from exceeding VMO/MMO when in the VS, FPA, VPATH, or ASEL-descent modes.

This mode will activate based on predicted overspeeds and will be annunciated with an amber VMAX in the FMA vertical window (furthest right). If the autopilot is engaged it will enter the VMAX vertical mode if the system predicts it will exceed VMO/MMO + 5 knots. If the autopilot is off, the pilot will be prompted with flight director commands.

The mode will deactivate when the aircraft is below VMO/MMO and the command generated by the in-use flight director will prevent further overspeed.

High-Speed Protection (HSP)

The High Speed Protection (HSP) function of the flight control computers is active under normal control law. It works with the autopilot off and the auto-throttles on or off, at or below 60° of bank. HSP activates when airspeed is above VMO/MMO, or earlier if the aircraft is accelerating). Activation is annunciated by a cyan CAS message.

When HSP is active, an increase in airspeed above the trim speed results in a pitch-up, and the pilot's maximum nose-down command is limited. The overall effect is to increase the pitch attitude of the aircraft to slow the rate of acceleration and reduce the aircraft's speed.

HSP will deactivate when airspeed is below VMO/MMO.

AOA Limiting

[PAS, p. 6-12] AOA Limiting

  • CAS: 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 the rate of acceleration, and can occur at any AOA with abrupt control input
  • Provides stall protection by only allowing up to 0.95 AOA with full aft stick.

High Incidence Protection Function (HIPF)

What is it?

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Photo: Angle of incidence, Eddie's drawing
Click photo for a larger image

Recall that the "angle of incidence" is angle between the chord line of the wing and the longitudinal axis of the aircraft. Further defining the chord line and the longitudinal axis of the aircraft can be a messy process and really doesn't impact us here. What we want to know is why any of this matters to us. The reason for that, is that it matters to the flight control computers.

The High Incidence Protection Function (HIPF) matters to us because it will overrule our actions at times that don't seem to make intuitive sense. It isn't explained at all in the usual places and is only documented incidentally. So what follows are collected notes from various sources, as well as the few places it is actually mentioned in our manuals.

Under contaminated conditions, the HIPF reduces the angle required to get a Pitch Limit Indicator, stick shaker, and other stall prevention measures.

The only real documentation is spread through the AFM:

[AFM, §05-01-01] VSR, REFERENCE STALL SPEED: for the G500, VSR is selected to be slightly higher than the speed at which aerodynamic stall would otherwise occur in 1-g level flight. The HIPF (High Incidence Protection Function) of the G500 limits the angle of attack that can be achieved with full aft stick such that the minimum steady speed is not less than VSR.

[AFM, §05-01-40] Variations of reference stall speeds, VSR, with weight and altitude for speed brakes retracted are shown for all flap positions. The reference stall speeds presented were developed in accordance with 1G stall speed criteria. Because of variation in the stall speeds with the operating mode of the Wing Anti-ice (WAI) system, various charts are presented.

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Photo: HIPF Stall Speed Schedule Based on Flap and WAI Status, AFM, §05-01-00, figure 11
Click photo for a larger image

The AFM does not provide reference stall charts for what it calls "Pre-activation Ice Shape" so we are left to guess the stall speed goes up and assume the low speed awareness cues will give us an idea.

[AFM, §05-08-10] Buffet boundary data are presented as a function of speed, weight, and load factor. Buffet Boundary. At low speeds, the buffet boundary is restricted by the maximum achievable angle of attack at full-aft stick due to the High Incidence Protection Function (HIPF). At higher speeds, natural buffet (high speed buffet) or VMO / MMO is limiting.

The AFM gives one chart for 0° flaps that shows buffet boundary increases about 0.02 Mach (approximately 15 knots) at low altitudes with the WAI turned off.

Why does it matter to us?

Note that the HIPF assumes the wing is contaminated whenever the wing anti-ice is off and the flaps are up. That means in this condition the airplane will think it is nearing the stall sooner than is actually true and that can result in the airplane doing things you don't want it to. The Zero or Partial Flaps procedure restricts you to no lower than 200 KCAS "until ready to configure" even with the WAI on. We are taught that this is because of the HIPF but that isn't written down anywhere. The pilot who flew the zero flap tests told me the winglets tend to flutter below 200 knots until the flaps are extended and the flutter can be confused for signs of a stall. One of these days I'll watch the yellow and red low speed awareness bands near 200 KCAS and extend the flaps. Until then, I am using 200 KCAS as my minimum no flaps speed.

Maneuver Load Alleviation

[PAS, p. 6-13]

  • Ailerons deflect upwards symmetrically to reduce loads
  • When pilot commands > 1.5 Gs
  • Reach maximum deflection of 3° at 2.5 Gs

Speedbrake Auto-Retract

[PAS, p. 6-13]

  • At high power settings (about 90% Thrust Lever Angle)
  • Handle remains in selected position

Dynamic Rudder Limiting

[PAS, p. 6-13] Helps prevent pilot from overstressing rudder.

Elevator Split Load Limiting

[PAS, p. 6-13] Protects against large torque associated with a split elevator.

Low-speed / Stall Recovery

[G500 Ground and Flight Operations, p. 101] a. At first indication of impending stall or stick shaker:

  1. Disconnect autopilot and Auto-Throttle.
  2. Apply nose down pitch control to reduce AOA.
  3. Apply power.
  4. Roll wings level.
  5. Retract speed brakes.
  6. Return to desired airspeed and altitude.

Use of the Rudder in Flight

These three rules are really two rules and they come from Gulfstream. The cardinal rule in these things is to avoid rapid rudder reversals. Why? See American Airlines 587 for a case study in what can go wrong.

[G500 Ground and Flight Operations, p. 109]

  • Flight crews should use caution when operating the rudder in flight. The flight control system design protects the vertical fin for prolonged maximum rudder inputs in a single direction only. If the rudder is deflected to maximum deflection then suddenly reversed to the maximum deflection in the opposite direction, the vertical fin can be overstressed. Additionally, the vertical fin can be overstressed by a pilot "walking" the rudder either abruptly or in small increments in tune with the yaw response. The issue is magnified at high speed.
  • There are three rules of thumb to follow when using the rudder in flight:
    1. Maximum deflection of the rudder in a single direction may be used to control the airplane when needed such as in the case of an engine failure at takeoff. Do not return the rudder past neutral when completing this maneuver.
    2. Do not walk the rudder in tune with the yaw response either with abrupt or smooth inputs.
    3. If you follow the above two rules and continue to fly the airplane within the published envelope using normal airmanship, you will not overstress the airplane.

Gulfstream GVII-G500 Ground and Flight Operations, released as part of the GVII-G500 AOM, §02-01-10

Gulfstream Symmetry Flight Deck for the G500 Aircraft Pilot's Guide, Honeywell Pub. No. D201110000019--003a, October 29/18