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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4$\begingroup$ angle of attack is in relation to the flight surfaces (wings) while pitch is the entire craft $\endgroup$– ratchet freakCommented Mar 14, 2014 at 21:17
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3$\begingroup$ 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 $\endgroup$– rbpCommented Feb 4, 2015 at 23:42
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$\begingroup$ i277.photobucket.com/albums/kk76/batcave777/… $\endgroup$– copper.hatCommented Feb 5, 2015 at 6:20
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1$\begingroup$ 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. $\endgroup$– copper.hatCommented Feb 5, 2015 at 6:24
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3$\begingroup$ @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. $\endgroup$– copper.hatCommented Feb 7, 2015 at 0:16
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5 Answers
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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.
answered Mar 14, 2014 at 21:34
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4$\begingroup$ 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. $\endgroup$ Commented Feb 6, 2015 at 13:48
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1$\begingroup$ Sure there are AoA indicators. What have you the impression they don’t exist? $\endgroup$ Commented Nov 25, 2018 at 21:06
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1$\begingroup$ BEA acknowledged the unavailability of AoA indicators in the cockpit could have been a contributing factor, and recommended to assess the need to add them in the future. AoA indicators were only optional, and AoA information was accessible to the crew only after a voluntary disconnection (or failure) of the 3 ADR (BUSS demonstration on Youtube). $\endgroup$– minsCommented Nov 26, 2018 at 22:15
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$\begingroup$ @CarloFelicione, some aircraft do have AoA indicators, but A330 is (or was at the time) not one of them. $\endgroup$ Commented Nov 27, 2018 at 22:24
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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 want to add the image.
Angle of attack is relative to the direction of relative wind (which is equivalently relative to the direction the plane is going, if in still air and the angle of incidence = 0, i.e. the wing is mounted parallel to the longitudinal axis of the plane), while pitch is relative to the horizontal axis. Angle of attack depends on the pitch, current velocity of the aircraft and the wind. They can be different.
Angle of attack is important to the aerodynamics (amount of lift, drag, etc.), while pitch tells you the aircraft's orientation relative to the ground.
Assumptions for the image: still air and angle of incidence is 0.
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4$\begingroup$ +1. It's nearly correct, but the AoA isn't between the trajectory of the plane and the longitudinal axis. You need to add the angle of incidence. The chord line is not parallel to the longitudinal axis. This incidence angle is the mounting angle of the wings, and can be about 5°. $\endgroup$– minsCommented Nov 27, 2018 at 12:02
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$\begingroup$ in addition to what mins said, you also have to consider that the relative wind might come froma direction slightly different from the plane's trajectory, as there might be actual wind to be summed to the aircraft velocity vector. $\endgroup$– FedericoCommented Nov 27, 2018 at 13:21
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1$\begingroup$ @mins, actually, it depends. Angle of Attack of the wing is measured from the chord line, but given that many aircraft have twisted wings, it seems pretty common to measure it from the longitudinal axis for the airplane as a whole. $\endgroup$ Commented Nov 27, 2018 at 21:51
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$\begingroup$ @JanHudec: Wouldn't that be better to use the MAC rather than the longitudinal axis? $\endgroup$– minsCommented Nov 27, 2018 at 23:21
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$\begingroup$ @mins, better for what? The zero lift line does not coincide with the MAC anyway, so you'd still have some $C_{L_0}$, so it makes more sense to just use the longitudinal axis and skip the addition of angle of incidence. $\endgroup$ Commented Nov 28, 2018 at 14:54
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Just before the ground impact
Just watch this video of a looping gone bad during a flight display. At 1:30 into the video it becomes painfully obvious by how much both can diverge in extreme situations (shown above).
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.
answered Feb 4, 2015 at 22:49
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$\begingroup$ "Now consider flying inverted: Both are almost 180° apart."-- what do you mean by this? When I think of flying inverted the first thing that enters my head is sustained inverted flight. Let's say the aircraft is designed for aerobatics and has a symmetrical airfoil with zero incidence. What would a typical angle-of-attack be in sustained inverted flight, maybe minus 10 degrees? $\endgroup$ Commented Nov 29, 2018 at 3:07
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$\begingroup$ Whatever the number is, wouldn't the pitch attitude just be that same number times minus one (so positive 10 degrees in this case, because the aircraft is pitched 10 degrees nose-to-sky.) So that's a difference of 20 degrees-- nowhere near 180 degrees. Even if we double the angle-of-attack to 20 degrees we just get a difference of 40 degrees, nowhere near 180 degrees. $\endgroup$ Commented Nov 29, 2018 at 3:07
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$\begingroup$ @quietflyer: Guess what definitions are for! They are meant to clarify what we are talking about. You are free to define your pitch angle differently from everyone else, but then please accept that your opinion will lead to different results. And the results of others aren't necessarily wrong only because you choose to pick a different definition. $\endgroup$ Commented Nov 29, 2018 at 15:39
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1$\begingroup$ Kampf: can you clarify your comment? I'm not following. How am I defining pitch attitude different from everyone else? For example, what would be the pitch attitude according to the standard definition, of the aircraft in sustained inverted flight as described in my comments above? $\endgroup$ Commented Dec 1, 2018 at 14:05
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How much the pitch (horizontal orientation) can differ from the angle of attack?
270 degrees appears to be the maximum angle that the pitch attitude can differ from the angle-of-attack.
Some examples of extreme differences between pitch attitude and angle-of-attack:
Jet fighter or aerobatic airplane or aerobatic glider in a prolonged vertical climb. (May be a steady-state situation or the aircraft may even be gaining airspeed or it may just be a "zoom" climb where airspeed is exchanged for altitude; obviously only the latter is possible with the glider.) For simplicity assume a symmetrical airfoil. Pitch attitude is 90 degrees, angle-of-attack is zero degrees, for a difference of 90 degrees.
Now the throttle (if present) is pulled back to drop the engine power to zero, but the nose is kept pointing straight up until the aircraft starts to tailslide backwards. Pitch attitude is still 90 degrees, but angle-of-attack is now 180 degrees, for a difference of 90 degrees.
Now imagine the tailsliding aircraft experiences a slight variation in angle-of-attack-- perhaps due to a horizontal wind gust striking the aircraft-- that changes the direction of the relative airflow by one degree, so that the relative airflow is aimed slightly toward the top surface of the wing, rather than aimed directly at the trailing edge. Now the angle-of-attack has changed from 180 degrees (which also could be called minus 180 degrees) to minus 179 degrees. Now the difference between angle-of-attack and pitch attitude is 269 degrees.
Flat spin with flight path approximating a vertical descent. Angle-of-attack may be close to 90 degrees, but pitch attitude may be close to zero degrees. Actually on the retreating wing it would seem the angle-of-attack may go beyond 90 degrees (wing actually moving backwards relative to the airmass, so the local airflow comes partly from behind), in which case the difference between angle-of-attack of that wing, and pitch attitude of the aircraft, would also go beyond 90 degrees.
answered Nov 29, 2018 at 2:58