The Angle That Matters
The angle of attack is the angle between the wing and the air actually flowing over it. Not the angle between the wing and the ground, and not the angle between the nose and the horizon. Those are attitudes, and they are what you see out of the window. The wing cannot see out of the window.
Picture the chord line, a straight line drawn from the leading edge of the wing to the trailing edge. Now picture the direction the air is arriving from, which in a descent is somewhat from below and in a climb somewhat from above. The angle between those two is the whole story. A wing gliding down towards a field can be angled steeply into its own oncoming air while the nose sits below the horizon, and a wing in a fast climb can be at a gentle angle while the nose points at the sky.
This is the trap that catches every beginner. The instinct is to read the angle off the horizon, because that is the picture in front of you, but the wing is working against the air arriving at it, and that oncoming air swings around as the flight path changes. Two aeroplanes held in identical nose-up attitudes can be doing completely different things, one flying happily and one about to fall out of the sky, and the only difference between them is where the air is coming from.
A Straight Line, Then a Cliff
Here is the part that makes the angle worth caring about. For most of its useful range the wing is beautifully well behaved: raise the angle a little and you get a little more lift, raise it more and you get more still, in a straight and predictable line. The wing feels like an obedient machine because across this range it is one.
Then, at one particular angle, the line does not simply bend. It falls off a cliff. The airflow that had been hugging the top of the wing detaches, breaks up into a churning wake, and the lift that was climbing so obediently collapses. That angle is called the critical angle of attack, and for a given wing it is essentially fixed, somewhere near fifteen degrees for a typical aerofoil. Reach it and the wing stalls, every single time, with no exceptions and no negotiation.
The word to hold on to is fixed. The critical angle does not care how heavy the aeroplane is, how fast it is flying, whether it is climbing or diving, or which way is up. It is a property of the shape of the wing, decided when the aeroplane was designed. Everything unpredictable about stalling comes from the fact that pilots watch speed, which varies constantly, instead of angle, which does not.
Why Airspeed Gets the Blame
If the wing answers only to angle, why does every pilot learn a stall speed, a number in knots below which the aeroplane is said to stall? Because in one narrow and very common situation the two line up. Straight and level, at a steady weight, the angle needed to hold the aeroplane up depends on speed alone, so the critical angle arrives at a repeatable airspeed, and quoting that speed is a convenient shorthand.
The trouble is that the shorthand lies the moment the situation changes. Load the wing up in a turn or a sharp pull and it has to make more lift than the plain weight of the aeroplane, which means a larger angle for the same speed, which means the critical angle arrives early, while the airspeed indicator still reads comfortably above the number. The real stalling speed quietly climbs and the dial says nothing.
This is how an aeroplane can stall at well above its book figure, and how accidents happen with the airspeed needle sitting reassuringly in the green. The pilot was flying the number instead of the wing, and the number was describing a situation that had stopped being true.
Flying on the Back of the Curve
There is a strange region of flight that lives on the far side of the angle where drag is least, and it teaches the lesson better than anything. Fly slowly enough, at a high angle, and the aeroplane enters what pilots call the back of the drag curve, where going slower actually demands more power rather than less, because the wing is working so hard that the drag it makes is climbing steeply.
In this region the ordinary rules feel inverted. Add power and the aeroplane climbs without speeding up. Raise the nose to climb and the speed bleeds away and the sink gets worse instead. The controls have not stopped working, but the aeroplane responds to them in an order that feels backwards until you think in terms of the angle rather than the attitude.
Every approach to land finishes in this region on purpose, flown slow and high-angled so the aeroplane arrives near the bottom of its speed range. It is also where a heavy aircraft dragged in too low and too slow gets into trouble, sinking towards the ground with the nose high and no quick way to arrest the descent. The wing is close to its limit and simply has nothing left to give.
Reading the Wing, Not the Dial
Because the angle matters so much and is so hard to see, aircraft go to real lengths to show it to the pilot. The simplest tool is a small hinged vane on the leading edge of the wing that swings to point into the oncoming air. When the angle grows too high it trips a switch, and a loud horn or a shaker on the control column warns the pilot the wing is near its limit, regardless of what the speed is doing.
Fast jets take it further with an angle of attack indexer, a set of lights or a gauge that shows the angle directly, and carrier pilots fly their approaches to that gauge rather than to airspeed, because the angle is what the wing cares about and the angle is what puts the aeroplane on the deck at the right attitude to catch a wire. Some airliners feed the same measurement into a system that physically lowers the nose if the angle climbs too far, taking the decision out of human hands.
For a beginner none of this hardware is the point. The point is the habit of mind. When the aeroplane feels mushy, when the controls go soft, when a buffet begins to rumble through the airframe, the answer is almost never about speed and almost always about angle. Ease the wing back towards the air, give it a smaller angle to work with, and it starts flying again. The whole of Glide, and every careful touchdown in the games here, rewards a pilot who has learned to feel that angle instead of chasing a number.