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AOA information on the speed tape: Benefits and caveats

Updated: Feb 12

As pilots and engineers, we know about the importance of the Angle-of-Attack (AOA) and we certainly do our best to keep track of it. Depending on the airframe, this involves more, or less cognitive effort. It is somewhat ironic that in almost every flight-test aircraft and accident report we find information about the AOA, while in so many “normal” flight decks it remains missing…

The aim of this article is not to dwell on the old “AOA-debate”, but rather to discuss the issues that may arise, if AOA-derived information is “mixed” into a speed tape on a Primary Flight Display (PFD).


The old problem

There are limitations which necessitate some form calibrated airspeed information on the flight deck. When it comes to stall-margin however, airspeed is a deceptive parameter to say the least. Stall is an AOA-problem and therefore AOA information is the way to go. It turns out that many manufacturers have opted to use some form of AOA-derived information and present it on the airspeed indication (speed tape). There are some caveats…


Normalized AOA – a useful concept

There are several manufacturers that display “Normalized AOA” (NAOA). This is very useful, as it gives transparent information to the flight crew about the “fraction” of available AOA-range they are currently using. Let us look at this in graphical form:


Figure 1: Normalized AOA concept

The above graph shows that the NAOA concept conveys in a very intuitive manner to the flight crew, how much AOA they have used and how much is still available. Implementations vary but typically consider the following: aircraft configuration (spoilers, flaps/slats), Mach number, Anti-Ice status (later aircraft) and sideslip angle [1] [2].


Where do the 66% NAOA at Vref come from?

It turns out most pilots are very familiar with regulatory approach speed factors, such as 1.23 Vs1g or 1.3 Vs for older aircraft. When asked about what fraction of the available AOA-range this equates to, the answers become less widespread. Let us look at the underlying physics:


Figure 2: Derivation of NAOA at Vref

This basic derivation assumes a linear relationship between AOA and lift coefficient, which is acceptable for normal aircraft operating regimes.

An example of NAOA being displayed on a PFD can be seen in Figure 3 below:


Figure 3: PFD showing NAOA (red circle). Credit: James Albright

AOA-derived information on the speed tape

So far, we have looked at the NAOA concept and how it is displayed. Not all flight decks show NAOA to the flight crew. Some avionics use it in the background and only display AOA-derived information on the speed tape. This gives rise to the following issues:

  • CAS information needs to be available to display AOA-related “protections” on the speed tape

  • If CAS is flagged as “unreliable”, the indication is typically removed [3]

  • In turbulence, the CAS values for these “protections” may become unsteady

  • Some form of filtering is usually applied


Speed tape example

Implementations of this vary across the industry and type-specific knowledge is usually required. Most manufacturers show some form of “maneuvering limit speed” and “AOA limit speed”. This may refer to a min. speed to conduct certain maneuvers or a speed where the flight control characteristics change and to the speed at which the maximum AOA will be reached for the current set of circumstances. As a pilot, it is important to be familiar with the intricacies of your type. Figure 4 below shows a typical example (G500).


Figure 4: Example of AOA-related information on speed tape. Credit: James Albright

As mentioned earlier, these speeds may vary depending on the type-specific implementation. The effect of turbulence on these speeds can be seen in the video below:


Video credit: James Albright


The flight crew must develop a good understanding of the data sourcing and filtering process in order to interpret and anticipate these indications correctly.


Only a pilot who understands how a system works, will be able to predict how it can fail.


Background info: AOA protection in icing conditions

Certification requirements for flight into known icing conditions have changed significantly over time. Regarding stall warning and protection systems, an important change occurred in 2000 (year of design): Since then, stall warning and protection systems need to demonstrate accuracy in icing conditions [4]. This results in many aircraft adjusting AOA limits for icing conditions or others assuming a contaminated wing for certain flight phases.


Accident case study: When protections “bite” you


Everything in this paragraph is based on the official accident report [5].


The crew was performing a private flight from Moscow to Paris. The aircraft was an Embraer Legacy 500, featuring a FBW control system with AOA protection. The protections were displayed to the crew on the speed tape as depicted below:


Figure 5: Protection speeds on the EMB-550 PFD. Source: Embraer, as cited in [5]

At Vaoa, the flight control law changes from a load-factor demand to an AOA demand.

At Vlim, the FBW system limits the AOA, regardless of pilot input.


Figure 6: Reaching Vaoa and Vlim on the EMB-550 PFD. Source: Embraer, as cited in [5]

Crucial: These limits change in icing conditions, as required by certification rules.


Before departure, a thin layer of ice had formed on the aircraft and the crew had the aircraft de-iced. Before engine start, the crew set the selector of the ice protection control panel to “all”. This triggered the A-I WINGSTAB FAIL message. The crew’s actions were against the AFM procedures, which required the selector to be in the “auto” position. Subsequent reset attempts failed, and the crew took off despite the failure message. Shortly after take-off, the STALL PROT ANTICIPATE warning appeared. This indicated that the aircraft was applying reduced AOA-thresholds for the envelope protection and thus higher approach speeds were required, even if there were no icing conditions present. The crew never understood this.


As the crew used their “normal” approach speeds, they were reaching the AOA limit during the final approach segment.


The autopilot disconnected twice during the final approach and the crew continued to touchdown. As they were flying on the edge of the red tape (but at their “normal speed”), they had no AOA margin left to conduct a flare. The result was a very hard touchdown (1350 fpm and 4 g).


Figure 7: The aircraft applied icing AOA limits, the pilots did not: Substantial damage resulted [5]

In FBW aircraft with envelope protections, it is of utmost importance for the flight crew to be aware and understand the underlying principles of these systems. Failure to do so can result in substantial damage, as shown above.


Ask yourself during the final approach: How much AOA-margin do I have left?


References

[1] Gulfstream, «GVII-G500 I1/G600 Operating Manual,» 2021.

[2] AIRBUS, «Safety First #21 "Control your speed",» 2016.

[3] BEA, «Final Report Airbus A330-203 F-GZCP AF 447 Rio de Janeiro - Paris,» 2012.

[4] FAA, «Advisory Circular 91-74B Pilot Guide: Flight in Icing Conditions,» 2015.

[5] BEA, «INVESTIGATION REPORT Accident to the Embraer EMB-550 Legacy 500 registered RA-02788» 2020.


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