When an aircraft stalls, do all or most lift forces abruptly disappear or is this transition continuous? Will the aircraft drop from the sky like a brick or can this (losing lift and altitude) happen so gradually that it will be entirely unnoticed from the crew and passengers?

I'm troubled by this sentence on Wikipedia referring to the AF447 disaster: "The aircraft remained stalled during its entire 3 minute 30 second descent from 38,000 feet (12,000 m)" (source: https://en.wikipedia.org/wiki/Air_France_Flight_447#Accident) It seems the crew were unaware of the stall condition. (But let's not discuss the reasons here)

If lift abruptly disappears when entering a stall, wouldn't people notice a considerable loss in perceived body weight at least as long the aircraft accelerates downward and builds up vertical speed downward. I expect that the airframe experiences enormous drag in the vertical direction once downward speed is being built up and it probably reaches an equilibrium condition limiting its fall rate.

But if lift disappears abruptly during the stall, then you should have a sensation of free fall (no weight) at least during a short time. Is this the case or does lift diminish continiously during a stall?

PS: While writing the question i have searched and found those Lift-AoA diagrams (like this: https://www.grc.nasa.gov/www/k-12/airplane/incline.html) that should be able to answer the question. Basically my question is: How quickly does Lift drop off to the right of the diagram. All diagrams i found did not in fact go much beyond the stall condition.

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    $\begingroup$ This article might help give you a better understanding of stalls. Long story short: you won't fall out of the sky. $\endgroup$
    – Jimy
    Sep 4 '17 at 12:59
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    $\begingroup$ "wouldn't people notice" you'll be surprised to know what people do not notice. To the point, there is quite some literature (and accidents) about senses deceiving pilots. $\endgroup$
    – Federico
    Sep 4 '17 at 13:01
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    $\begingroup$ @Federico indeed and there is quite a recent video in Skybrary (can be found here ) explaining that problem, called sensory illusions $\endgroup$ Sep 4 '17 at 13:08
  • $\begingroup$ Lift does not disappear at the onset of a stall. Find a pilot to take you up and demonstrate a stall, in a lot of aircraft it is a gentle break and simple recovery. This is one of the things I don't agree with in the new airman certification standards, they teach recovery before the stall (fine) but never fully stall the airplane. $\endgroup$
    – Ron Beyer
    Sep 4 '17 at 13:11
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    $\begingroup$ It's worth noting that a Cessna stalls in a very different manner to a large swept-wing aircraft $\endgroup$
    – Ben
    Sep 5 '17 at 7:41

You make some valid observations: not all lift disappears after the stall, and at the event of stall there is a downward acceleration that may or may not be perceived.

  1. Let's start with what happens after the stall. Measurements on airfoils have been taken over 180°, and they show that after the stall, lift drops back until flat plate lift is achieved at the same angle of attack. Increase AoA further, and lift increases again. An example for a relatively thin airfoil from this document:

    enter image description here

    Pull the aircraft back far enough to 45°, and the $C_L$ is even higher than when it stalled! Of course, the drag will be huge compared to normal operating circumstances, and any fixed wing other than a military jet fighter or a VTOL won't have enough power installed to fly at such angles of attack.

  2. Isn't there a perceived loss of body weight during the acceleration to stall. Yes there is, but it does not last long, is not necessarily very powerful, and could be referenced to other factors than stall, such as turbulence. Once the $C_L$ - $\alpha$ curve hits the trough at 15°, the acceleration stops and weight perception returns to normal. And it's sensory perception, the body only really understands what is happening if all sensory organs are aligned (vision, motion, sound), and can only react in a composed manner if it anticipated the cues and is trained in how to respond. Otherwise we just fight or flee.

You mention the case of AF447. It was in a fully developed stall and descending with on average 10,000 ft/min, but the onset to stall was different. It was very gradual, and it was a high speed high altitude stall: there simply is not enough air up there to provide enough lift for the weight of an airliner. It can stall at AoA of zero, then start to fall, as a result of which the AoA changes but the pitch attitude of the aircraft does not, and your body is still aligned with gravity in a dark cockpit where the horizon cannot be seen. No physical cues on what is happening whatsoever!


As an addendum to Koyovis' answer as concerns AF447:

If you look at some of the parameters below, taken from the BEA accident report the second graph shows the normal acceleration (up and down) in G's. As you see, during the initial upset the passengers would have felt some spikes in G's up to about 1.75G upward and 0.5G downward. This is in the range of heavy to severe turbulence. They would notice. Anyone standing up might even have been tossed around. After that it settles down to around 0.75 to 1.25 G's. So there was no feeling of freefall.

Several places I've read assert that the passengers probably did not know anything was wrong. I strongly disagree with that. Aside from the G forces during the initial part of the accident, the pitch attitude fluctuated between 20° nose up and 10° nose down. The aircraft was also rolling back and forth, sometimes up to 30°. Since it was night IMC they may not have known the seriousness of what was happening, but I can't imagine those kinds of pitch and roll motions would seem normal to anyone.

enter image description here

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    $\begingroup$ That first patch of 1.5 g combined with pitch angles of 15 deg, was that directly after the AP disengaage when the pilot pulled on the stick? $\endgroup$
    – Koyovis
    Sep 5 '17 at 23:34
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    $\begingroup$ @Koyovis Yes. That initial pitch up is the first 20 seconds after AP disconnect $\endgroup$
    – TomMcW
    Sep 5 '17 at 23:39

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