One angle, and everything else is bookkeeping.
Lift is the lift coefficient times the dynamic pressure times the wing area. The wing area is bolted on; the dynamic pressure is density and speed; and the lift coefficient is very nearly a straight line in angle of attack — the angle between the wing and the air it is flying through — right up to the point where the flow separates and the line collapses. That collapse is the stall, and it happens at the same angle every time. It does not care about your mass, your bank, the density altitude or how loudly the airspeed indicator is reassuring you.
Airspeed became the standard proxy because it is easy to measure and, at one specific condition, it maps onto the angle exactly. That condition is the placard: maximum weight, one g, wings level, unaccelerated. Change any of the four and the stall speed moves, while the stall angle does not. Fly a lighter aeroplane and the wing needs less lift, so it reaches the critical angle at a lower speed. Pull, or bank, and it needs more, so it reaches that angle sooner — at a speed the placard never mentioned.
The arithmetic behind the accident report.
The relationships are square roots, which is why the numbers surprise people. Stall speed scales with the square root of the load factor: a 45° level turn is 1.41 g and multiplies the stall speed by 1.19; a 60° turn is 2 g and multiplies it by 1.41. It also scales with the square root of the mass — which is exactly the same law that governs a glider taking water ballast.
Now put that into the base-to-final turn. A pilot overshoots the extended centreline, tightens the turn, adds a little bottom rudder to hurry it round, and holds the nose up to protect the height. The bank went up, the load factor went up, the stall speed climbed toward the approach speed — and the approach speed, chosen from the manual at maximum weight, was never the margin they thought it was. The airspeed indicator was telling the truth about speed the whole way down. It simply was not being asked the right question.
One gauge that never has to be re-learned.
Set a comfortable 110 km/h and then move only the mass slider. Watch the red stall-speed figure walk from 63 up to 92 km/h while the needle climbs the same fixed scale toward the same fixed mark. That is the entire argument for the instrument: the number you must not exceed is printed on the dial, once, and it never changes. Now add bank and watch how quickly a comfortable margin evaporates in a steep turn.
Three ways to measure an angle.
A small blade on a bearing, free to weathercock into the local flow. Direct, cheap, honest — and exposed to ice, ground handling and the wing walker who grabs it.
Two ports facing at different angles into the stream. The ratio of the two pressures is a clean function of the flow direction, with nothing moving and nothing to bend.
From airspeed, load factor and mass, an AHRS-equipped system can infer the lift coefficient and therefore the angle — no extra hole in the airframe, at the price of depending on what it was told about the aircraft.
All three share one requirement that no amount of engineering removes: calibration on the aircraft itself. The relationship between the probe reading and the angle the wing actually sees depends on where the probe is mounted, what the fuselage does to the flow around it, and what the flaps are doing. An AoA system that has not been flight-calibrated on your airframe is a decorative gauge.
The approach that is right at any weight.
The operational payoff shows up on final. Flying an approach by angle rather than by speed automatically produces the correct speed for the weight you happen to be at today — light with one pilot and low fuel, heavy with four aboard, or a glider that still has half its water. Naval aviation adopted this decades ago for exactly that reason: an aircraft returning to a deck has burnt an unpredictable amount of fuel, and the on-speed angle is the same regardless.
The second payoff is audio. A margin you have to look at competes with the runway for your eyes; a tone that changes character as the angle rises does not. This is why the best AoA installations are heard rather than watched, and why the gauge is the training aid while the sound is the safety device.
What it does not fix.
An angle-of-attack indication tells you how close the wing is to its limit. It says nothing about whether you have the height to complete the turn, whether the rudder input you are making will drop a wing when the stall does come, or whether the approach was set up badly two minutes ago. The gauge is a margin meter, not a judgement engine, and every honest instrument article ends in the same place: the instrument buys you information, and the information only pays if it changes what you do.
Fly the angle, know why the speed moves, and the stall stops being a number you memorised and starts being a limit you can see coming.
The Airplane Pro sensor units carry a dedicated angle-of-attack pressure sensor alongside airspeed, AHRS and 25 Hz GNSS.
Attitude, airspeed and altitude from one calibrated sensor block — the panel that shows you the margin, not just the number.
For gliders the same physics arrives as wing loading: the vario that knows your ballast state and what it does to the polar.