New (2010) Stall Recovery's @ high altitudes

[BOAC]

HN - Dan W has summed up the 'buffets' nicely. There is no way that the 'routine pilot' should get anywhere near the often misnamed 'coffin corner', and as Dan says providing the pilot is aware of what he/she is doing vis a vis pitch attitude there should be no great difficulty in distinguishing between the two. As Dan says they are different, but I would not expect a 'diagnosis' based on 'what it feels like' to take place. We are well-provided now with target pitch attitudes for different stages of flight and should know which we are feeling from that.

Dan - I am completely with you on the correct sequence of stall recovery ie pitch then level wings, but you do need to be very careful with aileron even on a swept wing. These do not exhibit the classic straight wing stall and aileron is useable while the a/c is in buffet BUT if you look at the FDR from PGF you will see the classic 'Stalling II' demonstration of use of aileron increasing wing drop on a fully stalled wing. That is how they got to those bank angles. I suspect that your successful use of aileron at 'the point of the stall' in a Hawk was not actually at the 'stall' - the Lightning, which as I have said, spent much of its time in pre-stall buffet would respond readily to aileron until it didn't and then you were in a spin - and it was that quick, and the key was to prevent yaw into the dropping wing which always leads to autorotation. I have several (unexpected) 'flick rolls' in my log-book to prove that. The lesson has to be driven home - forget aileron UNTIL you have unstalled the wing. It can and does kill. That is all I am trying to say. It is of concern that people seem to either forget or not be aware.

[HazelNuts39]

Dan Winterland;

Thanks for your explanation on buffet.

regards,
HN39

[TheChitterneFlyer]

What a convoluted thread this has turned-out to be! I think everyone will agree that the most dangerous phase of flight to experience the stall, or the approach to a stall, is the intermmediate/final approach phase where the loss in altitude (during recovery) will 'hurt' if we don't get it right.

Whilst that we're all professionals and that we all (I hope) understand the theory behind these discussions, it's true to say that the regulators do, sometimes, have to 'spell things out' to those who don't. The Training Manual; part of the Ops Manual, must provide a standardised procedure for stall recovery technique. Hence, the basic concept for stall recovery (when within the area as stated above) is to reduce alpha. The application of thrust alone will innevitably increase alpha. I believe that this is more of an 'awareness' issue; in practice, you'd no-doubt lower the nose and increase thrust whilst also 'controlling' the tendancy for thrust induced pitch-up.

We're all running around in circles and getting 'bogged-down' with semantics. If we were all to be in the same room and having the very same discussion we'd no doubt all agree with each other... this ain't rocket science!

TCF

[BOAC]

Originally Posted by TCF

whilst also 'controlling' the tendancy for thrust induced pitch-up.

- yes - in my opinion I would have preferred the manufacturers to have focussed on 'unusual attitudes'+pitch/power couples rather than fiddling with basic stall recovery techniques. We are seeing many examples of failure to control this, and 'tweaking' the stall recovery will not stop these a/c losing control in a g/a. A few years back Boeing pushed out a series of 'U/Ps' plus notes and videos for a recurrent sim exercise.

Personally if I am stalling near the ground I want and need power.

The procedure (as described by Airbus) is now to SMOOTHLY INCREASE THRUST AS NEEDED but only when out of stall.

- hmm!

As with others here, I am still unclear what this amendment means by the STALL in terms of recovery - stall warning or full stall?

[Dan Winterland]

Hi BOAC. It's the stall warning. The Airbus Flight Crew Operating Manual 3 defines the point at which recovery should be made, the new procedures are an amendment to the current procedures and deal purely with the recovery actions. However, you will only get a stall warning in a reversion flight control modes.

The FBW Airbus will normally be flying in Normal Law where stalling is theoretically impossible. The protections will not let you do it. If there are flight computer degradations or an upset which has not been prevented by the normal protections, then you may be in Alternate Law. If you lose more computers or ADCs, then you may be in Direct Law which means that the control surface deflection is proportional to stick movement - i.e. like a conventional aircraft. The aircraft can be stalled in Alternate or Direct Law, but there is a very definate stall warning associated with his. In Normal and Alternate law, the aircraft will not pitch up with power application. Only in Direct law.

So the instances where these new memory items may be required will be very rare. I have never been out of normal law in 4000hrs of Airbus flying. My company has a fleet of 30+ and we have had one case of Direct law in the last 5 years - casued by a double RAD ALT failure where the aircraft reverts to Direct Law when the gear is lowered.

And as for the new procedure, I suggest any pilot would apply a healthy dose of airmanship if required. I think if I was to stall at 200' on approach, I would use some power.



As for the use of ailerons, I don't remember exactly at what stage of the stall we used them down to while flying the Hawk- I will ask someone who has flown it more recently. As for the Airbus procedure, it's in the memory item procedure therefore, it is approved by the manufacturer. Perhaps it's because in Alternate and Direct law that VSW (Velocity Stall Warning) is somewhat higher than the actual stall speed and the ailerons are still safe to use and limiting the load factor by reducing the angle of bank is the main priority.

I'm in agreement about the different characteristics of swept wing stalling. Earlier types tended to pitch up as the tips stalled first and the centre of pressure moved forwards. Two of my previous types exhibited this trend (Victor and VC10) and as both had T tails, this was definately not a good thing! Some other types exhibit a classic nose down pitch. The B747 was originally designed with no stall warning or protection at all. It was only the British ARB which demanded a warning system which led to a stick shaker being fitted. Which is why the B747's device looks like an afterthought - an electric motor with an eccentric weight attached, and clamped to the control column.

The VC10 has a duplicated shaker and warning system. The pusher actuates before the stall and the system has a 'rate computer' which takes a feed from the AoA sensor and triggers the system earlier if there is a nose up pitch detected. The Victor had no stall recovery aids - except for deploying the brake chute in the vague hope it would pitch the nose down before the weak link broke. More than one Victor met it's demise in stalling.

[CONF iture]

Originally Posted by Dan Winterland

However, you will only get a stall warning in a reversion flight control modes.

Except in Perpignan where one AoA sensor was considered as damaged by the system, even if actually it was the only one to tell the truth but the system didn't deem as necessary to advise the crew of the actual AoA DISAGREE.

When you know the Major role that Airbus gave to those AoA sensors in their Protection features, I think it was absolutely necessary to advise the crew that something was wrong with the readings.

To me, clearly, the Airbus philosophy is ALSO to be blamed in the Perpignan accident.

[411A]

I wonder...all this discussion about stalls and using rudder (or not) or ailerons (or not) during recovery.
Has anyone here actually stalled a large swept-wing airliner?
Is so, what were the results?

Having discussed this subject with Lockheed production test pilots, whom have actually intensionally stalled an L1011 during acceptance tests, rudder is never ever used to 'lift a wing' when the airplane is fully stalled, either in the clean, approach or landing configuration.
To use rudder to 'lift a wing' in the fully stalled condition, is to invite a rather abrupt up-side down condition, IE: inverted flight.
Not good for keeping ones breakfast down, me thinks.

Personally, I have completed acceptance tests with L1011 aircraft where we approached the fully stalled condition.
Rudder was never used during recovery...stab (the L1011 has an all-flying stab, unique to civil jet transport aircraft), ailerons and power, only.
No rudder.

[Tee Emm]

forget aileron UNTIL you have unstalled the wing.

And providing you have instantly pushed the stick forward when the stall is detected, then that takes less than two seconds to unstall the wing. So would it be correct to say that for all intents and purposes the stick forward and aileron as needed to roll wings level is simultaneous.

On the other hand, to take this rudder thing to a ridiculous degree I was astonished to have a senior flying school instructor tell me that on final approach in a Cessna 152 which has a Vref of 54 knots,his students are taught never to use the ailerons below 60 knots IAS to correct bank angle. Instead they must ensure the wheel is central and control bank angle by carefully "skidding" the wings level using rudder in case they are close to the stall.

When a CFI of a flying school teaches that to his student instructors on an instructors course, then no wonder the standard of the flying industry where I come from is appalling. The regulator,in the form of local flight ops inspectors, have no idea this goes on under the very noses, because they are too busy travelling around the countryside trawling through flying school paperwork audits and writing audit reports criticising the operator for using non standard sub-paragraph format in their Operations Manuals. OK - that is a generalisation; but believe me its not far from the truth all the same.

[PantLoad]

How I love to spin!!!!!

Let's see, if I were to try to get an aircraft to spin, how would I go about it????



Fly safe,

PantLoad

[BOAC]

Originally Posted by TM

So would it be correct to say that for all intents and purposes the stick forward and aileron as needed to roll wings level is simultaneous.

- absolutely - unstall the wing and ailerons are back on the menu - but the problem is that there were pilots (now dead) who tried full aileron to pick up a wing on an Airbus in 'flight' at less than 46kts and thereby managed an extreme bank angle.

Yaw towards a dropped wing is, of course, an excellent way to get the nose back down in an otherwise uncontrolled power/pitch situation (and as taught for upset recovery) providing height is sufficient, but alpha must be kept under control.

[Airclues]

Has anyone here actually stalled a large swept-wing airliner?
Is so, what were the results?

In the early 80's I was co-pilot on several C of A air tests on the Boeing 747 when a full stall was completed (I believe that the UKCAA was the only authority that required this).
Although there was a significant nose drop, I can't remember there being any wing drop. There was a severe buffet which made it difficult to read the instruments (I don't know how the test engineers came up with their figures). The height loss was about 5000ft. In the mid 80's the UKCAA fell into line with the rest of the world and cancelled the full stall requirement.

Dave

Thread drift;

On 3/8/83 we carried out an air test on G-AWNB. We were given permission to do the stalls in the Portland Danger Area so that we could be sure that there was no traffic below us. When we had completed the stalls we were invited to carry out a GCA approach at Portland N.A.S. Was anyone there at the time? If so, do you have any photos?

[Goprdon]

Stall recovery--any height.

To the following CAA Flight Crew Training Notice I would add:
Do not try to pick a wing up with rudder, it may work on some aircraft but on others you will spin.Do use the rudder to prevent yaw.
Do move the control column forward to lower the nose and reduce the AoA.
Do follow the manufacturer's instructions regarding stall/spin recovery.
Do re-read PantLoad's post above at Post69.
Now for the notice:

SAFETY REGULATION GROUP FLIGHT CREW TRAINING NOTICE
01/2010
Applicability: RETRE, TRIE, TRE, SFE, TRI, SFI Effective: Immediate
STALL RECOVERY TECHNIQUE
1 Recent observations by CAA Training Inspectors have raised concerns that some instructors (both SFIs and TRIs) have been teaching inappropriate stall recovery techniques. It would appear that these instructors have been encouraging their trainees to maintain altitude during recovery from an approach to a stall. The technique that has been advised is to apply maximum power and allow the aircraft to accelerate out of this high alpha stall-warning regime. There is no mention of any requirement to reduce the angle of attack – indeed one trainee was briefed that “he may need to increase back pressure in order to maintain altitude”.
2 It could be argued that with all stall warning devices working correctly on an uncontaminated wing, such a recovery technique may well allow the aircraft to accelerate out of danger with no height loss at the lower to medium altitudes. The concern is that should a crew be faced with anything other than this idealised set of circumstances, they may apply this technique indiscriminately with potentially disastrous consequences.
3 The standard stall recovery technique should therefore always emphasise the requirement to reduce the angle of attack so as to ensure the prompt return of the wing to full controllability. The reduction in angle of attack (and consequential height loss) will be minimal when the approach to the stall is recognised early, and the correct recovery action is initiated without delay.
NOTE: Any manufacturer’s recommended stall recovery techniques must always be followed, and will take precedence over the technique described above should there be any conflicting advice.
4 Any queries as a result of this FCTN should be addressed to Head of Flight Crew Standards
Captain David McCorquodale Head of Flight Crew Standards
21 April 2010

[PBL]

Originally Posted by Dan Winterland

PBL. Thanks for the diagram. I had to stare at it for a long time before I could work out what it was trying to tell me. I thinks it's what I always understood - that lift decreases with compressability.

What I was hoping it would tell you is that C_L_Max occurs at different angles of attack depending on Mach number. Which shows (if you take C_L_Max to represent the point of stall) that the point of stall is dependent on Mach number, contrary to your suggestion that the "notion[.] ... that stall is speed dependent .... ha[s] been eradicated years ago"

BOAC seems to want to maintain the view that this stuff is "theory" (whatever that may mean; we are talking about the physical characteristics of the airfoils you chaps fly, as demonstrated by wind tunnel data). I find that a bit odd, since Boeing Aero is a magazine which is designed to address practical issues for pilots, so one may take it that Boeing thinks this "theory" is relevant to flying their airplanes.

BOAC says

Originally Posted by BOAC

from a piloting point of view- compressibility etc has no relevance on stalling

I can't see that. The diagrams tell me a different story. Not to you?

Originally Posted by BOAC

the driver nowadays has guidelines of 'normal' pitch attitudes at different altitudes and does not need to know or care about shock separation,

Shock separation occurs with overspeed. We were talking about stall.

Originally Posted by BOAC

.....As long as he/she can differentiate between high-speed and low-speed buffet and carry out the appropriate recovery action, that is enough.

With respect, no it's not enough. There are two different kinds of high-speed buffet: that caused by approach to stall and that caused by overspeed. The "appropriate recovery actions" are different in those two cases and the pilot needs to distinguish them. Anecdotal simulator evidence, from a reliable source, suggests to me that not all line pilots can.

BTW, I am curious as to what the new Airbus procedures are for stall warning at cruise altitudes. It used to be to reduce back-pressure on the stick.

Also BTW, I am surprised that some people are suggesting you can't stall an Airbus (say, an A330) in Normal Law. You can, obviously, if you encounter a strong-enough gust. You can also stall it in Alternate Law, for the same reason. And in Alternate Law, in case of an ADR DISAGREE, high-AoA protection is lost in any case, so you can stall it without a gust in that situation.

PBL

[Capt Pit Bull]

Been chewing over this one a bit.

One thing that occured to me is that if I tallied up all the time in the sim spent with stall warning / stick shake / stick push going off, on 95% of the time the exercise was windshear recovery at fairly low level, with large back forces on the control column and overpowering the stick push.

(several differnt types involved).

Now, regardless of what I know intellectually, you have to wonder what the muscle memory will make of it all.

pb

[john_tullamarine]

If anyone has made the observation earlier in the thread, my apologies.

As far as I can see, PBL has been pretty much a lone voice in the wilderness extolling the influence of M. Certainly chaps such as JF and MFS, naturally enough, will be well aware of the following ..

For routine activities we tend to think of lift and drag only being functions of alpha and that works fine and beaut for most low speed activities in typical aircraft.

However, for many circumstances one needs to delve just a little deeper to get the story. If one digs into any of the standard engineering texts, one finds that the variables include -

(a) alpha

(b) a length measure of some sort, usually taken to be mean chord

(c) density

(d) dynamic viscosity

(e) local velocity

(f) sonic velocity

If one plays with some dimensional analysis sums, and that's always good fun, this list can be simplified to CL and CD being dependent on

(a) alpha

(b) Re (Reynolds Number)

(c) M (Mach Number)

For most cases, we can ignore the effects of Re and M and simplify to the usual CL = etc...

However, if there is any significant variation in Re or M throughout the flight regime, then one ought not to be surprised if the simplified sum gives a dodgy answer ...

It's not helped by pilot training only talking about the simplified sum without qualifying that story with the usual engineering caveats.

As we so often find, a little knowledge can generate the most stimulating of spirited discussions.

[mad_jock]

For those made of very stern stuff...

Navier-Stokes Equations

The thought of that exam still sends shivers down my back.

[error_401]

5 cents worth into the thread.

I've been an observer on an acceptance flight for an ATR72 in Toulouse with test pilot from ATR on right hand seat and flight test engineer on jump.

Aircraft is kept in level flight with a bit over flight idle power until:

1st stick shaker activates
2nd stick pusher activates

I wanted to know about the margins and this is what I've been told: There should be about 2 degrees AoA more before the wing actually stalls. The stick pusher will activate while the wing is still stable to fly.

IMHO: I would always go for pitch and then power. At height I would accept an altitude loss. I hope NEVER TO FIND myself in a FAR/JAR 25 aircraft near stall and then experience a wing drop because it has fully stalled!
I would always refrain from using rudder else as to minimize sideslip and then only after the wing is flyable again.

What do we learn:
- Modern aircraft even ATR with conventional controls have no pronounced low speed buffet therefore stick shaker
- Stick pusher activates WELL BEFORE entering a wing stall else it would be pointless to have it but is based on AoA.
- Nevertheless I agree with BOAC/PBL when you leave normal flight envelopes you're out on your own and it may stall before stick pusher.

[rudderrudderrat]

Hi JT,

Forgive my ignorance - but shouldn't there be an "area term" amongst the variables?
If you include chord, then don't you need span?

[BOAC]

Originally Posted by error 401

and it may stall before stick pusher.

- leaving aside system failures and icing which introduce a whole new 'ball game', I maintain that a properly designed a/c should not exhibit that characteristic - after all, stall warning computations should be corrected for Mach in modern a/c.

"I hope NEVER TO FIND myself in a FAR/JAR 25 aircraft near stall and then experience a wing drop because it has fully stalled!" amen to that, but it is apparent that people are.

rrat - I venture to suggest that JT is talking about 2-D flow?

m_j - many thanks for revisiting the scene of the odd recurring nightmare

jt - "It's not helped by pilot training only talking about the simplified sum without qualifying that story with the usual engineering caveats." agreed, but it should be remembered that not all pilots are BSc level aeronautical engineers - some are just capable of the 3 times tables. Surely the simple lessons are:

Stick to the limits from the manufacturer - IAS/Mach/Altitude
If something untoward happens when you do 'X', undo 'X'
Learn the correct recovery actions if the problem persists.

One would hope this would work across the airline pilot spectrum.

[mad_jock]

There isn't an area term because the foil is presumed to be infinately long so you don't have issues with air movement in the z axis.

So one of the boundary conditions is that the length considered is of unit length. So as the area equals chord x length it simplifies it down to just chord.

2D analysis of this stuff (which is what we are doing by stipulating unit length) is number crunching intensive. 3D anaylis will blow you and you computers socks off. If you then include time as well as turbulent flow you will decide sod this for a game of solders those fluids boys are off there heads I am sticking to structures and thermo things made out of steel. That fibre stuff is a pile of cack as well.

and BAOC I resent that comment, BEng Mech can do at least up to ther times 12 table.