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How much the pitch (horizontal orientation) can differ from the angle of attack? I am trying to understand the claim that "angle of attack indicator was unfortunately not available", contributing to problems during Air France Flight 447. Attitude indicator most likely was available?

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angle of attack is in relation to the flight surfaces (wings) while pitch is the entire craft –  ratchet freak Mar 14 '14 at 21:17
yes, but since the "fligh surfaces (wings)" are fixed in one place to the "entire craft," AOA can be measured anywhere on the aircraft, and in fact AOA probes are not attached to the wings at all –  rbp Feb 4 at 23:42
The pilot would have had to combine the information from the artificial horizon, forward airspeed and rate of descent to guess the angle of attack. Not so easy in a stress situation. –  copper.hat Feb 5 at 6:24
@JanHudec: The AoA (along with pitch, roll, control deflection, etc.) is the solution of a differential equation and is certainly not a simple function of airspeed and weight. In steady state conditions the AoA may be a expressed function of airspeed (depends on the equilibrium manifold), but it is not a simple relationship. For example, AoA can be changed quickly due to either pilot input or external conditions while the airframe dynamics are much slower to respond. –  copper.hat Feb 7 at 0:16

3 Answers 3

up vote 5 down vote accepted

The angle of attack is the angle between the wing (wing chord to be precise) and the direction of travel (undisturbed airflow). The angle of pitch is the angle between the main body axis and the horizon. The difference can theoretically be any angle, but during normal flight it will be limited to about 15 degrees.

The reason that the angle of attack sensor was inoperative was due to low airspeed. Below 60 knot IAS the indication is unreliable and therefore the indicator is inhibited. This also inhibits the stall warning. This lead to the confusing situation that lowering the nose to correct the stall increased the airspeed beyond 60 knots, thereby reactivating the stall warning.

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The Angle of Attack indicator was not inoperative, because there is no such thing. The Angle of Attack sensor was operative, but it's output is only used for the alpha-protection (which was inhibited due to lack of airspeed) and stall warning (which was inhibited below 60 kias), but it is not shown in cockpit. It was shown, there would have been no reason to inhibit it; pilots know it is not useful on the ground. –  Jan Hudec Feb 6 at 13:48

It can be. Remember that the angle of attack is the angle between the chord line of the airfoil and the relative wind. Imagine if the plane is level with the horizon with zero airpseed. It will fall straight down, putting the angle of attack close to 90 degrees with the pitch close to zero.

That said, during normal flight it's not likely that pitch and angle of attack will be excessively different. But honestly the two aren't closely related: You can exceed the critical angle of attack and stall at any pitch, bank, or yaw angle.

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Just watch this video of a looping gone bad during a flight display. At 1:40 into the video it becomes painfully obvious by how much both can diverge in extreme situations.

The flight path angle is the difference between pitch attitude and angle of attack. If pitch attitude and angle of attack would be equal, the airplane could only fly straight ahead at the same altitude. Once it climbs, it has to increase pitch attitude at constant angle of attack. With enough thrust or speed, both can be 90° apart.

Now consider flying inverted: Both are almost 180° apart.

In a dive, again the difference will become large because the flight path angle takes on negative values.

The more fun it makes to fly an aircraft, the more both angles diverge. Only boring aircraft will keep both of them at similar, low values.

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