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Angle of Attack Awareness

#FlySafe GA Safety Enhancement Topic

The General Aviation Joint Steering Committee’s (GAJSC) loss of control workgroup believes that a lack of awareness, with respect to angle of attack (AOA), has resulted in the loss of aircraft control and contributed to fatal GA accidents. The GAJSC also maintains that increasing a pilot’s awareness of the aerodynamic effects of AOA and available technology will help reduce the likelihood of inadvertent loss of control.

Illustration showing an AOA indictor.
An AOA Indicator Helps You Avoid a Stall and Prevent Loss of Control

What is Angle of Attack?

The angle of attack is the angle at which the chord of an aircraft’s wing meets the relative wind. The chord is a straight line from the leading edge to the trailing edge.

Illustration of AOA.
The angle of attack is the angle between the chord of an airfoil and the direction of the surrounding undisturbed flow of gas or liquid.

What’s So Critical About AOA?

Illustration of AOA.
An AOA indicator can have several benefits when installed in general aviation aircraft, not the least of which is increased situational awareness. These devices measure several parameters simultaneously and provide a visual image to the pilot of the current AOA. They also tell the pilot how close he or she is getting to the critical AOA. Increase the AOA or increase pitch to a yellow indication and lift will go up. Decrease pitch to a green indication translates to less lift being made. This is why airplanes cruise at a low AOA. However, upon slowing down, increase the AOA to compensate for the decrease in lift formed by the airflow speed. Also, every AOA equates to a specific airspeed once the plane is allowed to settle down. For each individual airspeed, a specific AOA is required to support flight.

At low angles of attack, the airflow over the top of the wing flows smoothly and produces lift with a relatively small amount of drag. As the AOA increases, both lift and drag increase; however, above a wing’s critical AOA, the flow of air separates from the upper surface and backfills, burbles, and eddies, which reduces lift and increases drag. This condition is a stall, which can lead to loss of control and an abrupt loss of altitude if the AOA is not reduced.

It is important for the pilot to understand that a stall is the result of exceeding the critical AOA, not of insufficient airspeed. The critical AOA is an aerodynamic constant for a given airfoil in a given configuration. The velocity of the relative wind does not matter; the airfoil will ALWAYS stall when the critical AOA is reached.

Please also note that the term “stalling speed” can be misleading, as this speed is often discussed when assuming 1G flight at a particular weight and configuration. Increased load factor directly affects stall speed (as do other factors such as gross weight, center of gravity, and flap setting). Therefore, it is possible to stall the wing at any airspeed, at any flight attitude, and at any power setting.

A conceptual representation of an AOA indicator. It is important to become familiar with the equipment installed in a specific airplane.

AOA in Steep Turns

Illustration of a turn.
Increased load factor can directly affect stall speed. For example, if a pilot maintains airspeed and rolls into a coordinated, level 60° banked turn, the load factor is 2Gs, and the airplane will stall at a speed that is 40% higher than the straight-and-level stall speed.

Due to the increased aerodynamic loading of the aircraft in a steep turn, the wing is much closer to the critical AOA. Here are a few things you’ll want to remain aware of during a steep turn:

  • The increase in pitch angle may be much smaller than expected to stall the wing.
  • The indicated airspeed at the critical AOA is significantly higher than in normal flight.
  • The increased load (i.e., aerodynamic loading) of the airplane requires greater lift which can be created by increasing airspeed or increasing the AOA.
  • Due to the increased aerodynamic loading, the stall sequence is condensed. The progression from indication, to buffeting, to fully stalled can be very rapid.

Make it a point to practice stalls and steep turns at different configurations during your next flight training opportunity.

AOA Indicators

Illustration showing AOA.
The AOA indicator delivers critical information visually or through an aural tone to indicate the actual safety margin above an aerodynamic stall. The more efficiently the airfoil operates; the larger stall margin that is present. Remember Green — nice and clean; Yellow — extreme caution to the fellow; and Red — your gonna bump your head.

Since we know that stall speed changes with the aircraft’s configuration (e.g., cruise, landing, etc.) and aerodynamic loads, the use of an AOA indicator can help provide a more reliable indication of airflow over the wing, regardless of its configuration. Without it, AOA is essentially “invisible” to pilots.

An AOA indicator can be used to get the pilot’s attention (via audio and/or low-cost stick shakers) even if the pilot is not looking at it. This focuses the pilot’s attention on where it needs to be to avoid the stall. It can also help when used in conjunction with airspeed and existing stall warning systems, when available.

A head-up AOA indicator.
A head-up AOA indicator in a Seawind 3000.

A New Angle on Safety

AOA systems offer many benefits to safe flying so consider looking into one for the aircraft you own or fly. And if you do install one, make sure you’re familiar with its operation and limitations. Check out this video produced by the FAA’s Center of Excellence to Enhance General Aviation Safety, Accessibility, and Sustainability (PEGASAS) that explains the concept of AOA and demonstrates how certain AOA indicators work in flight.

https://www.youtube.com/watch?v=8JcjWnAJGKQ&list=PL5vHkqHi51DT2Y54kjRtmjJ3Dgaj_Sv7V&index=66

It’s also a good idea to keep your skills sharp through practice of stalls and slow flight as well as pattern and instrument work with a flight instructor. Be sure to document your achievement in the WINGS Pilot Proficiency Program too. It’s a great way to stay on top of your game. Go to FAASafety.gov for more.

Watch: Angle of Attack Awareness in 57 Seconds

https://youtu.be/ekECV7MD-Yk

Watch: Angle of Attack Safety Minute

https://www.youtube.com/watch?v=Pf0-po4IsY8&list=PL5vHkqHi51DT2Y54kjRtmjJ3Dgaj_Sv7V&index=54&t=1s

Resources

🛩️ AC 61–67C — Stall and Spin Awareness Training (PDF download)

🛩️ FAA news release on streamlining the AOA installation process for small aircraft.

🛩️ FAA Airplane Flying Handbook, Chapter 4, Maintaining Aircraft Control (PDF download)

🛩️ “Pushing the Envelope: A Plan of Attack for Loss of Control,” FAA Safety Briefing, May/June 2018

🛩️ “A Long Term Plan of ‘Attack’”, FAA Safety Briefing, May/June 2016 p. 33 (PDF download)

🛩️ “The Alpha (and Omega): How a Small Angle Can Make a Huger Difference,” FAA Safety Briefing May/June 2014 p. 28 (PDF download)

🛩️ “AOA: More than Just a Display: Real World Uses for Angle of Attack,” FAA Safety Briefing, May/June 2014 p. 25 (PDF download)

A head-up AOA indicator.
A head-up AOA indicator in a Cessna 180.

📅 FAASTeam Events (April/May 2022)

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FAA Safety Briefing

FAA Safety Briefing

Official FAA safety policy voice for general aviation. Part of the national FAA Safety Team (FAASTeam).