High speed stall

[ATC Watcher]

Situation: light business jet cruising at max speed indicated ( say > 0.8 M ) at maximum altitude ( say 40.000 ft) encountering suden (wake) heavy turbulence.
Aircrfat will most probably bank heavily 70 degrees and stall.

But what would be the likely follow up ?
Spin ? if yes will it be easily recoverable ? bank stall = chances to go inverted ? idea about the subsequent loss of altitude ?

Please this is not for sensational news but to provide correct info during a debate on wake turbulence at high altitudes with VLJ. Thks in adavance .

[Miles Magister]

A small business jet flying at 0.65m or greater at height will probably be through most wake turbulence before there is any great effect. I have been through it in a small Citation on several occassions and just experienced a minor bump.

It is impossible to predict the severity of any wake turbulence at height. The probably effect of a serious incident would be a high 'g' encounter in the form of a big 'bump' with a possible 'g' overstress.

If an upset was the result the cew should lock the controls and let the aircraft fall out before recovering from the subsequent unusual position once flying speed and control is regained. The height loss or gain is difficult to predict but could be within the 300ft avoiding a level bust or in extreem cases be as much as 1000'. However, as a matter of opinion only, this is very unlikely but not impossible.

A high speed stall is a very different set of circumstances to a wake turbulence upset but one could experience some buffet as part of the experience.

MM

[By George]

Sadly during our basic training we only do low speed stalls. Without moving into a delicate area I wonder if one of the 737 'rudder hard over' cases could have been saved if the crew were more aware of the high speed stall. It was some years ago in my previous company I attended a 'Jet Upset' course and I was suprised at how little I understood the high speed stall. Natural instinct when the window is full off ground is to pull back, but when you release the back pressure and unload the wing you can recover in some cases. I found it was a gap in my knowledge that was quite disturbing. One of the best courses I ever did. The ex-military people seem to have a better understanding and it's something the civilian trained pilots need to address. It's the angle and loading that hurts, not the speed.

[Intruder]

Stall is unlikely, unless the pilot or autopilot overcontrols the airplane.

Since wake turbulence is a vortex, there will likely be more rotational component than any other. If the roll limit of the autopilot is reached and it disconnects, the pilot will have to manually fly the airplane out of the vortex. Since the airplane is banked already, continuing the turn to get outside the lateral extent of the vortex may be the best approach.

So, at "maximum" altitude and 45+ degrees of bank, holding altitude and airspeed will be impossible. Stay out of the stall regime, and continue to fly the airplane. If you do get stall warning (buffet, stick shaker...), use your airplane's stall recovery procedure.

In the cited conditions, altitude loss is very likely. If you allow the airplane to stall, the altitude loss could well be several thousand feet. However, if you fly it out of the vortex, the loss will be minimal.

[chornedsnorkack]

A sudden updraught might cause high g-s up. But could a sudden updraught cause the AoA to increase past the stalling angle of attack before the plane can accelerate up into the updraught?

[ATC Watcher]

Interesting , thanks. we were wondering if a situation , like the China 747SP had in 1985 over the Pacific (see China Airlines B747SP Loss of Power and Inflight Upset (http://www.rvs.uni-bielefeld.de/publications/Incidents/DOCS/ComAndRep/ChinaAir/AAR8603.html) could occur with a VLJ banking abruptly following a wake or severe turb encounter at very hight altitudes.

[canadairguy]

"Sadly during our basic training we only do low speed stalls."

To their credit, Transport Canada introduced the requirement for cruise stall training at 75% of max certified altitude or above. This came after the Pinnacle Airline CRJ high altitude stall and it's excellent training.


Commercial and Business Aviation Advisory Circulars (http://www.tc.gc.ca/CivilAviation/commerce/circulars/AC0247.htm)

[JABBARA]

Hi, all

As an ex-airforce pilot I can explain what high g stall is. There are two other names for this kind of stall: Accelerated Stall or High Speed Stall.

D= Density
V= Speed
Vs Stall speed
Cl= Lift Coefficient
Clmax = Max Lift Coefficient (where the stall occurs)
SQR = Square (2nd exponent)
SQROOT= Square root
Vs = 1 g stall speed
Vsn = Stall speed when n times of g is applied

In fact it is very simple:

L=1/2 x D x SQR (V) x S x Cl

If airplane is flying level unaccelerated (1g flight), when the speed is low enough so that when Cl=Clmax, that is the time of stall and that speed is the stall speed (Vs), so we can write:

L= 1/2 x D x SQR (Vs) x S x Clmax (1)

Now if airplane is maneuvering with a higher g, (lets say ng, n= number of g), practically L should also increase "n" times (= n x L). So we can write the above equation(1) like this:

n x L = 1/2 x D x SQR (Vsn) x S x Clmax (2)

Here Vsn is the stall speed when the airplane is maneuvering with n times of g (e.g 3 g).

Now if we divide equation (1) and (2) together, we will get the following:

Vsn= Vs x SQROOT (n)

A solid example, if 1 g stall is 100 kts, then 2 g stall is,

Vs2g = 100 x SQROOT (2) = 141 kts.

So what happens with high g stall: As soon as you relieve g, stall recovers.

For spin there are two requirements :
- Stalled airplane
- Sideslip (whatsoever the reason, either created by misuse of rudder or naturally happens)

Unless these two are not there, spin is impossible. Simply overbanking does not cause stall. Stall only occurs when level flight is tried to be maintained with that bank, as applying more "g". Inanother word the main reason for stall is "g". I.e zero g equals to zero Lift. If Lift is zero stall speed is zero as well. If somehow airplane is stalled, now at least we should avoid from sideslip to avoid from spin. That is why when trying to recover airplane upset, it is always recommended to be very careful while using rudder.
But I believe, today`s design technology and computer systems are good enough to prevent an airliner from spin, both aerodynamically and electronically.

[Intruder]

nteresting , thanks. we were wondering if a situation , like the China 747SP had in 1985 over the Pacific (see China Airlines B747SP Loss of Power and Inflight Upset could occur with a VLJ banking abruptly following a wake or severe turb encounter at very hight altitudes.

Upset and stall are 2 VERY different things. Upset is a sudden change in pitch or roll attitude beyond defined limits. Stall is increase in angle of attack beyond a defined limit. Either one may occur without the other.

Indeed, an upset will be the likely result of such an encounter with wake turbulence. While such an upset may be violent enough to cause injury to unrestrained people (or to restrained people colliding with other unrestrained objects), it will not necessarily result in a stall or prolonged uncontrolled flight.

[AirRabbit]

I think that if you go back and look at the China Airlines B747 accident report you’ll find that there wasn’t a vortex or turbulence encounter. If memory serves, the autopilot was engaged and one of the outboard engines failed. The flight crew either didn’t notice the failure or were so preoccupied with what caused the failure they didn’t recognize that the autopilot was trying its best to keep the airplane where it was supposed to be. When the autopilot ran out of capability, it disconnected and left the aircraft in a horrendous state of trim complicated by the asymmetrical thrust. From there it was most probably pilot control issues that exacerbated the problem.

As Miles suggested in his post, above, a clean airplane simply plugging along will usually not generate a huge vortex, and what there is generated will likely be more rotational than anything else. Additionally, unless the encounter is parallel with the aircraft leaving such a vortex, virtually all of those kinds of encounters would be cross-track and very short lived – but, again, as Miles suggested, it could be a very noticeable “bump.”

The B737 “rudder hard over” cases were a matter of not sufficiently understanding a phenomena known as “cross over speed,” and really had nothing to do with high speed or low speed stalls. In these B737 situations there was something that caused a significant rudder displacement when the aircraft was either descending to enter the traffic pattern or was already in the traffic pattern. With the roll tendency, naturally the pilots attempted to counter the roll with aileron input and attempted rudder input as well. It is believed that the crews were not aware of the fact that at slower speeds the rudder is significantly more powerful than the ailerons and there was simply insufficient aileron power to stop and reverse the effect of the displaced rudder. Had the crews accelerated the aircraft up to and past this “cross over speed,” where the ailerons become more effective and can be used to overpower the effects of a displaced rudder (even a rudder fully deflected), there is significant chance that the aircraft could have been saved – at least from that initial encounter.

[safetypee]

‘High speed stall’ is usually associated with transonic characteristics and thus is only loosely associated with ‘stalling’. Spinning is not a great concern, as indicated above.

The effect of high altitude turbulence on a VLJ should not be any greater than for other aircraft excepting any major aerodynamic differences. I suspect that VJLs might be more responsive than heavier aircraft, but likewise the control response should be quicker. There is little reason for any aircraft to stall in turbulence as all certifications require an aerodynamic control margin for such eventualities.

Re wake turbulence, then normal ATM procedures should take minimise the most severe encounters.
All ‘following traffic’ will have distance margin, and the preceding wake turbulence will be descending and disipating. Any following traffic 1000ft below might encounter some effects, but where this is detected a small track offset can be used. Similarly, crossing traffic should not suffer any marked effects.

[BOAC]

It looks to me as if the OP may have confused transonic lift degredation ('shock stall' - aren't they all?:)) with the 'normal' understanding of 'high speed stall'?

Trying to keep things incredibly simple, without recourse to any equations, the 'traditional' 'high speed stall' is when a wing is taken to or past its stalling angle at a speed above the straight and level stalling speed. Normally this will occur when manouevring. It is (or was) - and should be - included as an exercise in all basic flying tuition. Such a 'stall' is easily recovered by reducing the angle of the wing to below that stalling angle, normally by relaxing the tailplane/elevator input that has caused the angle change.

[chornedsnorkack]

Trying to keep things incredibly simple, without recourse to any equations, the 'traditional' 'high speed stall' is when a wing is taken to or past its stalling angle at a speed above the straight and level stalling speed. Normally this will occur when manouevring.
Such a 'stall' is easily recovered by reducing the angle of the wing to below that stalling angle, normally by relaxing the tailplane/elevator input that has caused the angle change.

So, let´s go back to the case of high speed stall considered in this thread!


So, an aircraft flying at high TAS and Mach number but at low IAS (because at a high altitude close to its ceiling) and high AoA close to stall already (perhaps heavily loaded) encounters, at previously unchanged pitch attitude and control positions, a sudden updraught. Which might be caused by wake turbulence, or by CAT around a jetstream, or mountain waves, or, say, occur around a thundercloud that the plane is trying to fly above.

Suppose that the updraught increases the AoA right through the stalling AoA so that Clmax is passed and lift actually falls below 1g. What next?

And you could have bank or spin because one wing met an updraught while the other found a downdraught.

As for business jets, note that they usually have rear engines. While some of them (Jetstar, Falcon, a few others) have +-tails, many have T-tails. A known deep stall hazard, and stick pushers would not have time to react to a sudden updraught.

For example, a Russian Tu-154, struggling at a heavy load and tail heavy CoG to fly over a thundercloud near or above its ceiling entered a flat spin that brought it right into the thundercloud it had been trying to avoid - and right through the cloud into Ukrainian soil. Could a wake turbulence likewise blast a plane into a flat spin?

[ATC Watcher]

Thanks a lot for the answers so far, I already understand the situation better.
2 remarks :
Re wake turbulence, then normal ATM procedures should take minimise the most severe encounters.
All ‘following traffic’ will have distance margin,
Not at high altitudes. For the moment the curent ICAO recommendations only applies to departure and arrival phases . There are no ATM procedures for wake turbulence separation minima to be applied en route.
( hence our current discussion)

As for business jets, note that they usually have rear engines. While some of them (Jetstar, Falcon, a few others) have +-tails, many have T-tails. A known deep stall hazard, and stick pushers would not have time to react to a sudden updraught.


On the engines : Most VLJs ,but not all, have rear engines, some are single engine, but most are T tailed indeed, but a few are even V tailed..(From my air-force time in the Fouga Magister, I remember that stalling it in a turn resulted always in inverting the aircrfat , but I cannot say this was tail-related, probably not.)
Seen their price tags, I doubt they have stick shakers/pushers.( but I do not know)
The ability /experience of the pilots , as most will be ex- turbo-props light twins owners, is also a worry , but not for this discussion.

[BOAC]

So, let´s go back to the case of high speed stall considered in this thread!

So, an aircraft flying at high TAS and Mach number but at low IAS (because at a high altitude close to its ceiling) and high AoA close to stall already - and the point I was making is that THAT is not a 'high speed stall', is it?!!! Hence the confusion. It is actually a 'low speed stall'.

Regarding 'updrafts', they are normally so short-lived that any significant change in AoA would be over before there was any response. Also any pilot 'worth his salt' would not be flying in those conditions at those speeds.

For the OP, as Intruder said, we are looking primarily at 'upset' recovery, not stall/spin. Normal 'upset' recovery would be to 'nurse' the a/c back to where you wanted it.

[safetypee]

ATC Watcher just to avoid any misunderstanding re:- ‘following traffic’ will have distance margin, and your reply “Not at high altitudes.”
My points, perhaps by implication, were that following aircraft at the same altitude will have some distance margin, and even if this is small (as with crossing traffic) a wake encounter (or the effects) should be minimised because the wake descends.
With aircraft below, which could encounter a descending wake, they should not experience a major encounter as during the time taken for the wake to descend by 1000ft, it has dissipated somewhat.

chornedsnorkack the outcome of the scenarios which you envisage are most unlikely. The certification requirements of CS 25.253 ‘High-speed Characteristics’ provide significant margin for normal operations. There are similar requirements for gust loads on the structure CS25.3--, and for stalling CS 25.205/207.
(a) Speed increase and recovery characteristics. The following speed increase and recovery characteristics must be met:
(1) Operating conditions and characteristics likely to cause inadvertent speed increases (including upsets in pitch and roll) must be simulated with the aeroplane trimmed at any likely cruise speed up to VMO/MMO. These conditions and characteristics include gust upsets, inadvertent control movements, low stick force gradient in relation to control friction, passenger movement, levelling off from climb, and descent from Mach to air speed limit altitudes.
(2) Allowing for pilot reaction time after effective inherent or artificial speed warning occurs, it must be shown that the aeroplane can be recovered to a normal attitude and its speed reduced to VMO/MMO, without – … etc, etc.
(3) With the aeroplane trimmed at any speed up to VMO/MMO, there must be no reversal of the response to control input about any axis at any speed up to VDF/MDF … etc, etc.
(4) Adequate roll capability to assure a prompt recovery from a lateral upset condition must be available at any speed up to VDF/MDF.
ect, etc,
A point of debate might be whether VLJs will be certificated to the same standards as commercial aircraft flying at high altitude.
Some issues have already arisen elsewhere, suggesting that as the skill level of VLJ pilots need not match those of commercial operations, then the certification aspects applied to larger aircraft, or in this instance the airspace, might be invalidated.

[boofhead]

Pushing the discussion a little:
An airplane wing will stall when the angle of attack exceeds the critical angle.
If the g load is zero the stall speed is zero also.
Once below the stall speed, an airplane can still be "flown", in that the wings can be rolled, yaw applied and pitch adjusted. It was a fun thing to do in small jet trainers when manoeuvering below the normal stall speed, such as a low speed loop. At speeds close to the normal 1g stall speed, it was important to reduce the elevator force, using the natural stall buffet as a guide. But once below stall speed, full elevator could again be used to adjust the pitch and the wings kept level as the airplane continued ballistically over the top of the loop. Approaching the normal stall speed reduced elevator force was again required to avoid a stall.
My question from all this is what happens to the airflow when below stall speed? I figure the stream lines re-attach to the wing and there is no turbulent flow or break-away since there is no real low pressure or high pressure areas, all the pressure differentials are zero and the controls work only by Newtonian principles, like the rudder does on the initial takeoff roll.
Or to put it another way the wing is not stalled when speeds are below stall speed.

When a wing stalls it does not lose all lift. There is still some lift being developed and in fact the reason the nose drops is not because of the loss of lift. The airplane will start to fall since the total lift is less than the weight. The tail will then act as a drag on the upward flow of air over the airplane and, since it has a longer moment, force the nose down. The T tail can be in the disrupted, turbulent flow with the result that this natural nose-down force is not available and the stall will not self-recover. Neither will the elevator work, since it is also in the turbulent airflow.

In severe turbulence, hold attitude steady and all will be well. Over-control and when the airflow re-attaches another stall or perhaps structural failure will result.