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In this comment on this question about stall warning systems, it was stated that:

You can stall at any airspeed (see this question and this one) and in any attitude, only the AoA is important because that's what causes the airflow to separate from the wing.

Why, then, does the Cessna 172's POH (Page ii or 2 of 422) list stall speed, not stall AoA?

http://i.stack.imgur.com/CZutr.png

I can understand how flying slower than the listed speeds would cause a stall, but the comment above seems to imply that one could stall the plane by flying at 60, 90 or even 120 KCAS but at a high AoA.

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  • $\begingroup$ Could you please tell me what KCAS is? $\endgroup$ – Madhav Sudarshan May 20 '15 at 18:17
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    $\begingroup$ @MadhavSudarshan "Calibrated AirSpeed expressed in Knots" $\endgroup$ – Federico May 20 '15 at 18:19
  • $\begingroup$ @FreeMan in the questions linked in your quote, see this answer at the point "level flight" $\endgroup$ – Federico May 20 '15 at 18:45
  • $\begingroup$ If, for instance, you were flying at 100kts and you suddenly decided to over bank the wings and yank back on the stick you'd likely separate the flow and induce a so called "accelerated stall". You'd still be well above published stall mins, but wouldn't have any laminar flow. Assuming you didn't overstress your aircraft, the fix is simply to push on the stick and unload the aircraft. This instantly reduces AoA and you'd probably find that you were no longer stalling. $\endgroup$ – Rhino Driver May 20 '15 at 18:54
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    $\begingroup$ @FreeMan exactly, yes. $\endgroup$ – Federico May 20 '15 at 19:06
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Those stall speeds are derived for specific conditions. In the case of that POH those speeds assume level unaccelerated flight, 2550 lbs gross weight, standard atmosphere and no wind. They are given as a speed rather than raw AoA because most non-aerobatic small airplanes lack AoA indications in the cockpit, while all have airspeed indicators. A non-aerobatic airplane also spends most of its time in the part of the flight envelope where the assumptions needed to list a stall speed tend to be valid.

The stipulation of level flight means the relative wind is fixed, thus the AoA depends only on the pitch attitude above the horizon. For an given weight there is a relation of pitch attitude required to maintain level flight for a given airspeed. Given these specific stipulations, you can determine the airspeed that results in the critical AoA being exceeded.

If any of the criteria listed above are not true, then those published stall speeds are not meaningful to you. You can stall at any attitude and at any airspeed.

To see the true relation between airspeed and stalling you need to consult a Vg diagram.

enter image description here

The curved lines starting at 0 MPH and 0 load factor represent the stall speed for a given load factor. The stall speed listed in PoH you quote is specifically the point noted "Normal stall speed" on this chart (this chart isn't for the same airplane, so the numbers will differ).

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  • $\begingroup$ I think you've restated what Federico linked to in his comment. $\endgroup$ – FreeMan May 20 '15 at 19:05
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    $\begingroup$ @FreeMan So a person goes out of there way to post a detailed answer to a question you asked, and the only thing you can do is complain to them that they didn't click and read through every link in the comment section? Unreal. $\endgroup$ – Rhino Driver May 20 '15 at 19:10
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    $\begingroup$ @RhinoDriver, I can see now how it may be taken that way, but that's not the way it was intended. My intent was to get a confirmation of what I said, not to complain about it. Apologies to any who may have been offended. $\endgroup$ – FreeMan May 20 '15 at 19:15
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Most small planes do not come with an AoA sensor or display, so giving the stall AoA isn't useful to the pilot.

The bulk of flying is done in the vicinity of level flight, no roll. When in that configuration, there is a simple relationship between airspeed and angle of attack. So the airspeed provides useful information about how the wing is likely to be doing. It must be understood by the pilot when that relationship is no longer valid.

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  • $\begingroup$ In the (currently) accepted answer to the referenced question is a picture of a Cessna with an AoA sensor/reed. I am not plane spotter enough to know if the pictured plane is a 172, or not, but it is a small Cessna. Therefore, I'm not immediately inclined to accept Most small planes do not come with an AoA sensor without further substantiation. Your answer does, otherwise, make sense. $\endgroup$ – FreeMan May 20 '15 at 18:25
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    $\begingroup$ When I way no "AoA sensor/gauge", I mean a display in the cockpit that shows the current AoA (so you can see how far you are from it). The stall warning system activates only after you exceed a critical AoA. $\endgroup$ – BowlOfRed May 20 '15 at 18:35
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    $\begingroup$ The reed is binary. Either you are exceeding the angle of attack it is calibrated for, or you aren't. It doesn't show trends or how far from critical AoA you are, both of which are very useful information. For that, you need a more sophisticated AoA indicator, like this, which tend to only be on more sophistacated aircraft. $\endgroup$ – Brian May 20 '15 at 18:36
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    $\begingroup$ But the pictures there don't show an 'AoA sensor' in the sense of an AOA indicator in the cockpit where an actual gauge or meter visually displays the current angle of attack. Those pictures show stall warning indicators, which only sense a particular AoA and provide an audible warning in the cockpit. There's no use for the numeric value of the critical AoA if you don't have an instrument that displays numeric α values. $\endgroup$ – Zach Lipton May 20 '15 at 18:38
  • $\begingroup$ @BowlOfRed, that makes more sense. You said sensor I thought "the device in the wing", while you meant "an indicator in the cockpit". $\endgroup$ – FreeMan May 20 '15 at 19:07
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Cessna 172s don't have an AoA indicator or gauge and even today most light aircraft still don't, although you can add them to some models and the FAA would like more aircraft to have them. So in the absence of an AoA instrument, airspeed is used as a rough proxy, assuming 'normal' flight conditions.

Crudely put, if your C172 is flying at a more or less steady 110kts then the AoA is low and you have a lot of 'reserve' before you stall; if you're flying at 60kts then it's high and you have only a little reserve. This is why watching your airspeed closely on final approach is essential.

But because airspeed is only a proxy for AoA, it isn't a completely accurate way to know how close you are to a stall. Changing the load factor (roughly, the G force on the aircraft) changes the stall speed but not the critical AoA, which always remains the same (18° is commonly mentioned for light aircraft, but that's an approximation). The diagram in @casey's very useful answer shows this clearly.

Finally, note that 14 CFR 23.49 says that light aircraft must stall at a calibrated airspeed of 61kts or less, there's no mention of AoA.

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    $\begingroup$ "light aircraft must stall at ... 61kts or less..." Interesting side note to this, larger aircraft DON'T have a single stall speed listed in the flight manual as light aircraft do, because of the effect of large changes in operating weight. In a light aircraft, the published stall speed typically assumes worst case (i.e. max certified takeoff weight), and other cases (single pilot, no cargo, less fuel) will lower the stall speed slightly. Large cargo aircraft can nearly double their empty weight with cargo+gas, so the range of stall speeds is too broad for 1 value to be useful. $\endgroup$ – Ralph J May 20 '15 at 20:24
  • $\begingroup$ @Pondlife - the actual wording of the regs is single engine airplanes and multiengine airplanes of 6,000 pounds or less maximum weight that cannot meet the minimum rate of climb specified in §23.67(a) (1) with the critical engine inoperative, rather than all light aircraft (MTOW 12,500 lbs. or less). I'm guessing the reasoning behind this is that they want to make sure that the airplane is still controllable at a speed that allows the pilot to deadstick the airplane and have a chance of slowing down to where impacts are generally still survivable by the occupants. $\endgroup$ – habu May 21 '15 at 14:20
  • $\begingroup$ @habu That's correct: I was using the term "light aircraft" in a loose sense, just to make the point that there is a specific regulation related to the question. $\endgroup$ – Pondlife May 21 '15 at 14:29
  • $\begingroup$ @Pondlife - fair enough. just wanted to point it out to avoid any confusion :) $\endgroup$ – habu May 21 '15 at 14:39

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