Avgeek Alert: What Is an Aircraft´s ´Angle of Attack´?


In order to make an aircraft as efficient as possible, so that its aerial maneuvers are perfect, there are a series of aerodynamic factors that pilots – as well as aeronautical engineers – must take into account.

Among them is something called the angle of attack (AOA), that is, the angle between the wind passing over an airfoil and its chord, which as we explained in another recent post is the imaginary line that goes from the leading edge (the point where the wind comes into contact with the wing profile) to the trailing edge (the point where the upper and lower surfaces merge and leave contact with the wing). The relative wind is the direction of movement of the atmosphere with respect to the airfoil. And the angle we´re speaking of is that which the profile ¨attacks¨ the air.

The Lift Coefficient

As every aviation geek knows, lift is one of the four forces that make flight possible, and it´s opposite to that of weight (that is, the force with which gravity attracts objects). The lift force has to be identical to the weight of the ship to maintain its height.

The lift coefficient is closely linked to the angle of attack, and without getting into complex mathematical formulas, suffice it to say that former depends directly on the latter.

As the angle of attack increases, so does the lift coefficient, and together they would form on a graph an ascending diagonal line. But only up to a point, known as the maximum coefficient of lift. At that point, the angle of attack at which the maximum lift coefficient occurs is known as the critical angle of attack.

The Critical Angle of Attack

Let’s say the aircraft is efficient, as well as its angle of attack, up to that point, where the air stops flowing smoothly over the outside of the wing and separates from the upper surface. As we´ve have seen, this is the angle that produces the maximum level of lift, usually falling between 15º and 22º.

The objective is to fly at a speed that gives the aircraft a margin of safety with respect to this critical AOA.

How Is It Determined When the Critical AOA Is Reached?

To find out, we need to look at aircraft speeds – and especially ¨stall speed¨. This is the minimum velocity at which an aircraft is able to fly while maintaining lift equal to its weight,  below which the ship would lose altitude.

To summarise, at higher velocities a lower angle of attack is needed. So in an aircraft at a high speed, the angle of attack will be low, but the speed needs to be reduced, the AOA will have to be increased to compensate, and it can be done until reaching the critical AOA that we´ve already seen.

This is in theory. In practice there are other factors (weight, load factor, etc.) which influence the speed at which the critical angle of attack is reached.

In other words – and as with other vehicles – the speedometer which indicates the different stall speeds is indicative, since these figures are only given under certain conditions. As soon as only one of them changes, the stall speed will be another. For example, if the ship is carrying less than the maximum weight, the stall speed will be less.

Of course, the critical angle of attack will always be the same, regardless of weight. This is important for the safety of the aircraft: if we know precisely what the angle of attack is, we´ll know at all times what its safety margin is with respect to the critical angle of attack. And that´s where the AOA indicator comes in.

What Is the AOA Indicator, What Does It Do, and How Does It Work?

This instrument installed in the cockpit allows pilots to measure angle of attack with respect to the critical AOA at all times (although not exclusively, since there are other important angles of attack, such as the approach or glide).

An AOA indicator ensures that the aircraft will always fly with the optimum angle of attack in relation to the maximum value of lift/drag. It´s also used for other values that occur at determined angles of attack, including the best climb rate, the best climb angle, maximum autonomy, and maximum range. Remember that without it, weight must be taken into account, but with it this isn´t necessary.

The device comes in various models of different shapes and forms, but essentially uses the same colour code, arranged from top to bottom and interpreted the same way:

  • When blue is lit, this indicates that the aircraft is flying at the optimum angle of attack.
  • Green indicates a cruising AOA, i.e. relatively low.
  • Red indicates a critical AOA.
  • Intermediate colours are also present: yellow between green and red as well as orange between blue and red.

Finally, the indicator functions using three types of sensors:

  • One measuring differential pressure, the difference in relative wind dynamic pressure using two apertures through which air enters.
  • A transducer, which moves according to the direction of the relative wind (in effect a kind of small weather vane).
  • One measuring inertial reference, meaning the flight path and the pitch attitude of the aircraft.