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So basically, a stall from a high AoA to the point that the entire plane just falls with the rear end pointing straight down?

Because if wind is no longer passing over the wings, then this includes the elevator, and the elevator can’t deflect the wind with no forward speed?

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    $\begingroup$ If your aircraft is aerodynamically stable, the slightest perturbation from this rear-first flight will tend to turn your aircraft front first $\endgroup$ – Manu H Dec 6 '19 at 11:06
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    $\begingroup$ If you drop a jet fighter from a hovering helicopter, this is pretty much what happens. (Or a toy jet fighter from your hand) $\endgroup$ – user3528438 Dec 6 '19 at 17:37
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    $\begingroup$ If you drop your jet rear-end first, there will be airflow over the wings, just in the opposite direction. $\endgroup$ – John Dvorak Dec 6 '19 at 21:23
  • $\begingroup$ Do you really mean "rear end pointing straight down", or just pointed downwards in general? Many of the answers below reference a condition where the aircraft is pitched up, leading to insufficient speed to provide sufficient control authority to change the attitude of the aircraft. This condition is not referencing an attitude with 90 degrees of pitch (ie tail pointing straight down), but I think it meets the intent of your question. $\endgroup$ – tmptplayer Dec 7 '19 at 15:11
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Yes it's called a Deep Stall, and is mostly a problem with T Tail aircraft, especially jets with Supercritical Airfoil wings (like the CRJ Regional Jet line).

Such wings stall from the leading edge and the stall's flow separation spreads rapidly and completely across the entire wing all at once, so there is very little residual nose down pitching moment. Plus the T tail ends up in a spot where it's in the wake of the flow from the wing, and gets blanked out, and loses its ability to pitch the nose over with positive lift since it's sitting there in the turbulent wake of the wing.

So the plane just mushes downward in an unrecoverable, stabilized, well, mush, and will pancake into the ground like that. A CRJ200 test aircraft was lost in development testing when it got into a deep stall and IIRC, the stall/spin recovery parachute in the tail failed to deploy (or it didn't have one at the time; I forget which).

Such airplanes require, in addition to the usual stick shaker, a stick pusher to force the nose over, by shoving the control column for you if you do nothing while the shaker is going off, before the natural stall can occur, since the natural stall can be unrecoverable. Generally if an airplane has a stick pusher system it means it has a deep stall mode.

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    $\begingroup$ Some T-tail gliders are also affected. It's particularly bad because gliders are often normally flown in near-stall conditions. Still, as they are agile enough to spin (unlike jetliners), you can usually go from a deep stall into a spin, and recover from there (but you lose a lot more altitude with that than in case of a simple horizontal stall recovery) $\endgroup$ – vsz Dec 7 '19 at 12:49
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    $\begingroup$ You mean it needs an MCAS?? ;-) $\endgroup$ – Peter - Reinstate Monica Dec 7 '19 at 17:07
  • $\begingroup$ @vsz: No, that would be a regular stall. Deep stall is a very different affair. $\endgroup$ – Peter Kämpf Dec 8 '19 at 9:20
  • $\begingroup$ Why would a supercritical airfoil facilitate super stalls? The first case had conventional airfoils, as did most other cases. Once the wing is fully stalled, the details of the upper wing contour don't count anymore. Tail engines which shift the cg (and thus the wing) closer to the tail and a T-tail are mostly to blame. $\endgroup$ – Peter Kämpf Dec 8 '19 at 9:44
  • $\begingroup$ @PeterKämpf : I wouldn't say it's a regular stall if the elevators aren't working at all in the stall. The only way in that particular case is to struggle with the ailerons in the hope one of your wings drops and you start entering into a spin. It takes time and you lose a lot of altitude. Compare it to a regular stall from which you can easily recover in a typical glider with a simple push of the stick while not losing much more than 50 meters of altitude. (and the instructors did call it deep stall) $\endgroup$ – vsz Dec 8 '19 at 9:49
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What you describe is a tailslide, as another answer has noted -- but there is a condition in which the wing is stalled and the normal recovery method (apply down elevator and wait for the nose to drop and airspeed to build) can't be used.

It's called a "deep stall" and is only a problem with certain layouts of flying surfaces. One of the best known is a T-tail in which the spacing between wing and horizontal tail is such that it's possible for the turbulent wash from the stalled wing to completely blanket the stabilizer and elevators, making it impossible for those surfaces to overcome the drag pushing the nose up. This can make a deep stall unrecoverable in certain T-tail aircraft (the F-104 was infamous for this, and it also affects some sailplanes).

Worthy of note is that deep stalls (commonly called a "falling leaf") were a common maneuver in the days of fabric covered biplanes; many of these had enough elevator authority to hold the stall, and enough rudder to keep the stall "straight ahead" rather than letting it turn into a spin. They were used as an alternative to a slip, to dump altitude without letting airspeed increase excessively. Most monoplane designs (that I'm familiar with) don't have enough pitch authority to hold a deep stall (part of making aircraft spin resistant is making them stall gently), so the maneuver has fallen out of general familiarity.

The difference between a controlled deep stall and an unrecoverable one is loss of pitch authority due to blanketing of the horizontal tail.

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    $\begingroup$ The falling leaf is not a deep stall en.wikipedia.org/wiki/Stall_(fluid_dynamics)#Deep_stall. A deep stall is by definition unrecoverable. You can fly a lot airplanes with full aft stick and the thing shuddering and shaking and bobbing with bits of rudder to keep it from rolling off. I do it in mine from time to time. The outer wing is still unstalled and even if the tail gets blanked off there is still significant pitching moment in the wing to drive the nose over. It's only a deep stall when there is no tail authority and there is minimal or no pitching moment present. $\endgroup$ – John K Dec 6 '19 at 16:28
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    $\begingroup$ The answer is a little misleading at the beginning, but he does not actually say that a tailslide is a deep stall. I agree, a “falling leaf” stall is not a deep stall. $\endgroup$ – Mike Sowsun Dec 6 '19 at 16:40
  • $\begingroup$ @JohnK The definition must have changed. I learned (long ago) that a "falling leaf" was in fact an artificially maintained deep stall, just as the "dethermalizer" on a free flight model induces a deep stall, but (if R/C DT) the aircraft will recover if the 30-45 degree decalage is restored to normal value. $\endgroup$ – Zeiss Ikon Dec 6 '19 at 17:01
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    $\begingroup$ Yes you are "deeply stalled" but the tail is still generating downforce to oppose the pitching moment (remember that during the stall the CP moves aft, increasing pitching moment putting more downforce demand on the tail) there is kind of an oscillation between more or less stalled as the tail loads and unloads. However if letting go of the stick results in recovery, it's not THAT deep stall, where the tail is not able to generate any pitching moment either up or down and there is little pitching moment from the wing itself. $\endgroup$ – John K Dec 6 '19 at 17:40
  • $\begingroup$ The characteristic of a deep stall is a second pitch stability point at high angle of attack. Yes, this can be achieved with a huge negative deflection of the whole horizontal tail surface (elevator control isn't sufficient) as done in free-flight models. Now the tail will double as a speed brake and lose all pitch authority, so the aircraft drops out of the sky. No manned glider that I am aware of is capable of this kind of descent. $\endgroup$ – Peter Kämpf Dec 8 '19 at 9:52
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Yes. Moving CG farther and farther back will eventually cause the plane to be directionally stable falling backwards. Abusing rearward CG limits is contributory to this condition.

Secondly, poor design of horizontal stabilizer, particularly, lacking sufficient "weathervaning" area, will cause an aircraft to be more susceptible to the unrecoverable "deep stall". With models, this is tested by holding the plane horizontal to the ground and releasing it with no forward motion. The relative wind, being 90 degrees to the wing and tail, means they are both stalled, but the pitch torque from the DRAG on the horizontal stabilizer, the rear fuselage and the trailing edge of the wing should flip the nose down and unstall the plane.

Higher aspect wings and/or a shorter fuselage requires greater tail area for the same pitch torque, AND, a larger weight to surface area ratio (bigger plane) also requires a greater tail/wing area ratio.

Placement of the all important horizontal stabilizer can also affect its performance. If it is in the "shadow" of the wing, as with T tails, a very high angle of attack can limit its ability to create pitch down torque. Downwash from the wing can also affect a "low" Hstab. Lengthening the fuselage is a remedy for not only wing airflow effects, but also increases the pitch torquing lever arm of the aircraft, enabling the same size Hstab to be more effective.

Thrust angling is also a key factor to assisting in pitch down torque. Many aircraft have their thrust line angled down a few degrees, which helps control pitch up tendency as the plane accelerates.

Choice of sound and proven design is important, as well as keeping CG within limits.

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    $\begingroup$ As a comment on abusing C of G, in hang gliding it's not uncommon for pilots to misjudge and flare with too much airspeed, with the inevitable result of zooming as much as 15-20ft up. The hang glider flare is essentially pushing your C of G all the way back. It's essential to hold the flare if this happens, so the glider tailslides down stably - you'll come down hard, but it'll usually be OK. If you panic and let it go though, the glider immediately goes nose down to try to recover the stall, which typically would take around 100ft or so. At 15-20ft off the deck, this is not a good thing. $\endgroup$ – Graham Dec 9 '19 at 1:15
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No, it is not possible that the nose will “refuse to go down” with “the rear end pointed straight down”.

If the aircraft is going down tail first, then there IS airflow over the wings. It may be briefly in the wrong direction, but the center of gravity and placement of the wings will soon have it pointed in the correct direction. With the nose pointed down again, the wings may still be stalled, but there will be airflow over the wings, and the stall will be recoverable with the correct control inputs.

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    $\begingroup$ I have a friend who experienced an actual natural stall on a CRJ200, when the flow was tripped by extruded sealant on one leading edge, and the wing stalled before the normal pusher firing point during a test. What saved them was I believe the fact that only one side let go and it rolled off knife edge and fell off that way more or less as a lawn dart, so that the vertical fin would get the nose pointed back down again and he was able to roll it straight and recover from the dive. Caused by releasing the a/c before the LE gap sealant had fully cured. That's all it took. $\endgroup$ – John K Dec 6 '19 at 17:47
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If a conventionally laid out aircraft is flown into a tail slide, it's unlikely that it could maintain that attitude for long. The tail has low mass relative to the rest of the aircraft and a substantial moment between the tail surfaces and aircraft center of mass. As negative airspeed (falling down tail first) builds, aerodynamic forces on the tail will cause it to flip around. Angular momentum will continue the rotation until there is enough opposing aerodynamic force to stop it. While a deep stall may be unrecoverable, a tail slide should be recoverable, given a sturdy airframe, enough altitude and the correct control inputs properly timed.

Hypothetically:

Given an aircraft of conventional layout, if it was released at zero airspeed and level attitude, the greater surface area of the tail as well as the greater surface area of the wing aft of its center of gravity will create a force that will rotate it nose down. What distinguishes this sort of scenario from a deep stall is that the condition is unstable and robustly forcing the airframe towards a normally-oriented airflow.

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