Do you know that an airplane can stall at any speed, any attitude, and any power setting?
The 1G stall speed published in airplane flight manuals is only valid for unaccelerated flight (1G load factor), coordinated flight (ball centered), and at one weight (most typically max gross weight). Unfortunately these conditions are not always met on a given real world flight. Consequently, speed itself is not a reliable parameter to avoid a stall. Again, an airplane can stall at ANY speed.
The Angle of Attack – AOA – is a better parameter to use in avoiding a stall because for any given configuration, the airplane will always stall at the same angle of attack, also known as the critical angle of attack. This stall angle of attack does not change with weight, temperature or density altitude. AOA indicators can help pilots detect this otherwise invisible airfoil position and avoid a stall.
Here are some basic points about AOA indicator use:
An AOA indicator is a useful reference when used in conjunction with airspeed and existing stall warning systems.
AOA system/indicator alerts when entering stall parameters.
Other benefits include more precise and efficient flying (Vy, Va, Vx, and Vref AOA are always the same).
Source: FAA Safety Briefing
Photo Credits: Open EAAgles