Angle of Attack (Normalized)

James Albright

Updated: 2020-07-15

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: Angle of Attack.

Photo: Normalized AOA Readout on PFD, Symmetry Guide, §2B-04-00, p. 38

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:

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?

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:

*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 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