It is not necessary to empirically determine a specific airplane's stall speed in order to operate it safely. It's not required in the US, we just use the number the manufacturer publishes.

It's normal for airplanes of the same model to fly differently: I fly a little fleet of six Citabrias, and their stall characteristics are radically different. You'd expect more uniformity from a modern aluminum airplane, but still: nobody should be flying an airplane like this so close to the edge the exact stall speed needs to be known numerically within one knot.

The 40lbs of gas I burn flying for an hour decreases the stall speed by more than 1mph on those Citabrias I fly.

EDIT: I was mistaken, this isn't a requirement in Europe either.

Soapbox: Stall speed is an approximation. It's baffling that GA aircraft don't have one of the most safety-critical measurements: AOA. Stall airspeed varies with a number of factors; this includes the mods described in the article, and weight change from burning fuel, passengers, payload etc. AOA is more invariant to that as a metric for choosing stall speed, speed down final etc.

Non sequitur from non-pilot: I was once in Duluth, MN in the bitter cold and watched a Cessna with skis (for landing on the frozen lakes of the Boundary Waters) land at the airport. It was the utterly bewildering to see how slowly it was going in the air. And how little distance it took to stop. Short Landing Kit I assume. I've seen ducks and geese come into land on lakes at higher speeds!

>“I let it lift off by itself. It was well-trimmed and it lifted off normally by itself.”

It sounds like the pilot wasn't fully prepared and engaged to compensate for propeller torque at the moment the aircraft left the surface of the water. At full takeoff power in a single engine aircraft this can be very intense and jarring, particularly with a high pitch ascent and full prop pitch. All it took was a momentary lapse in keeping the wings level to stall out at that speed.

>The indirect causal factor was the pilot’s lack of experience with stalling the aircraft. He told the investigation that he had never stalled the aircraft, which meant that he was unable to recognise the stall during the take-off.

It's this lack of stick and rudder skills at the root of the incident.

Just so we're clear, there's practically no such thing as an "unstallable plane". If some pilot believes that then their license should be revoked. Even jet engines can experience stalls internally on the compressor blades, and even helicopters can experience stalls on their retreating blade. I would compare it to someone believing that their car cannot possibly lose traction.

Exceptions, which of course must exist, include some fly-by-wire setups which limit the actuation of flight surfaces so that it should be theoretically impossible to put an aircraft in that situation, and rumored properties of some abnormal constructions like the An-2. Although even there you should repeatedly get comfortable with what happens in/around a stall, at least in simulators.

The fact that air started to separate and the end of the wing, and not at the root, is scary. It means the pilot wouldn't get the normal warning in the form of airframe shaking. Bad modification.