Minibus (A320) Stall Recovery

[TSIO540]

Hi folks,

I've searched the forums, but can't find discussion on my particular query.

For the A320, can anyone shed light on why: if in clean configuration and below 20,000ft, select flap 1 is considered only AFTER: flight path, recover smoothly

Surely selecting flap 1 after the fact is a bit moot..

[compressor stall]

It's not to get out of the one you were in, rather to stop you getting into it again.

[Fly3]

Flap 1 will reduce Vls considerably and hence aid the stall recovery. Could even make stall recovery unnecessary.

[vilas]

Fly3
stall is an angle of attack phenomenon so recovery is to be effected through pitch down and subsequent thrust increase. So flaps are to be used only after recovery as compressor said.

[stilton]

I thought the Bus was stallproof :E

[ok45]

Extension of slats will let you stall at a higher angle of attack. So IMHO it will help on the recovery as well as not getting into a secondary stall. Of course the primary method to get out of stall is to reduce the AoA. Hence it comes first..

[tom775257]

I suspect there is an aspect of almost muscle memory with this also.... you hear STALL STALL STALL you lower the nose. Then after that you can have a think about what comes next including your altitude, what was the flap altitude limit etc.. which will take a few more seconds to think about.

[EGPFlyer]

I thought the Bus was stallproof



Only in normal law. In alternate/direct law you can stall like any other aircraft

[CaptainProp]

Oh dear, here we go agan....

EGPFlyer, stilton was being sarcastic and I think he knows something very important, that you (?) and many others seem to have misunderstood - The airbus can stall, in any flight control law, in any configuration, at any altitude.

It's an aircraft. Aircraft have wings. Wings can stall. End of.

[subsonicsubic]

I fly baby planes. They only have trailing flaps - no leading edge droops.

But surely setting flap 1 in a 320 will immediately reduce the angle of attack.

Feel free to "attack" me if my supposition is incorrect.

[vilas]

Capt prop
"It's an aircraft. Aircraft have wings. Wings can stall. End of". Sorry it's not end of it at all

[CaptainProp]

Yes it is. At least as far as commercial aircraft like the Airbus, Boeing, Embraer etc goes.

The airbus control laws provide early warning and limited protection against high load factors, AOA, high speed etc etc.

The flight control laws do not make the aircraft immune to stall.

From the Airbus Flight Crew Training Manual:

Protections are intended to:

• Provide full authority to the PF to consistently achieve the best possible aircraft performance in extreme conditions
• Reduce the risks of overcontrolling, or overstressing the aircraft
• Provide PF with an instinctive and immediate procedure to ensure that the PF achieves the best possible result.

Not immune to stall.

Here, a fully serviceable airbus 319 stalled:

Chaotic CRM blamed for Chinese airbus a319 stall incident during approach (http://news.aviation-safety.net/2013/12/02/report-chaotic-crm-blamed-for-chinese-airbus-a319-stall-incident-during-approach/)

Whatever anyone tells you, it can stall in normal law, all it needs is to be misshandled or get a little "help" from a wind shear or downburst just as you reach a-max/a-floor.

[vilas]

Capt Prop
You are not contributing anything to the original question. I read the CAAC report. Since stall is AoA phenomenon, rapid changes due severe environment can do anything with any aircraft but aircraft response is not the same in Boeing and AB. In AB FBW aircraft computers act towards recovery. Stall recovery cannot be practiced in normal law by mishandling or any way.

[rudderrudderrat]

Hi TSIO540,
Surely selecting flap 1 after the fact is a bit moot..

Your first actions must be to get out of the stall "(no longer stall indications):" by reducing the AoA.

The other actions are listed to aid recovery to your original flight path and are done sequentially and without any significant delay.
If you received a stall warning at 2,000 ft (say whilst trying to intercept the ILS) then you must recover without contacting the ground. Selecting F1 will aid in minimum height loss whilst avoiding any "stall stall" caused by the +ve delta g you will need to pull (which is not felt in the sim) in order to change the trajectory.

"THRUST
SPEEDBRAKES
FLIGHT PATH RECOVER SMOOTHLY
• If in clean configuration and below 20 000 ft:
FLAP 1 SELECT
Note: If a risk of ground contact exists, once clearly out of stall (no longer stall indications), establish smoothly a positive climb gradient."

[TURIN]

I'm getting worried now.

Are there still pilots out there that think an Airbus cannot be stalled?
In any control law state?

Really?

After everything that has happened over the last few years? :suspect:

[CaptainProp]

Vilas, the problem is that "qualified" and experienced airbus pilots are flying around thinking the aircraft is immune to stalls.

With regards to selecting Flap 1, and as already mentioned by previous posters, the FCOM only recommends selecting Flap 1 When out of stall.

From the FCTM:

STALL RECOVERY - The immediate key action is to reduce AOA: The reduction of AOA will enable the wing to regain lift. This must be achieved by applying a nose down pitch order on the sidestick. This pilot action ensures an immediate aircraft response and reduction of the AOA. In case of lack of pitch down authority, it may be necessary to reduce thrust.

Simultaneously, the flight crew must ensure that the wings are level in order to reduce the lift necessary for the flight, and as a consequence, the required AOA. As a general rule, minimizing the loss of altitude is secondary to the reduction of the AOA as the first priority is to regain lift. As AOA reduces below the AOAstall, lift and drag will return to their normal values.

- The secondary action is to increase energy:

When stall indications have stopped, the flight crew should increase thrust smoothly as needed and must ensure that the speed brakes are retracted. Immediate maximum thrust application upon stall recognition is not appropriate. Due to the engine spool up time, the aircraft speed increase that results from thrust increase, is slow and does not enable to reduce the AOA instantaneously. Furthermore, for under wing mounted engines, the thrust increase generates a pitch up that may prevent the required reduction of AOA. When stall indications have stopped, and when the aircraft has recovered sufficient energy, the flight crew can smoothly recover the initial flight path.


Here some comments on the subject from two very experienced guys, an interview with a Terry Lutz, airbus experimental test pilot and Boeing deputy chief 777 test pilot Van Chaney, last year in Flight Global:

.....pilots have been incorrectly trained for years to concentrate on maintaining height. "You must be willing to trade altitude,"

Pilots need to identify the risk of stall at high altitude, he says, recognise the onset of buffet and memorise appropriate pitch and power settings. But they must also know how to break the stall, with nose-down input, and - crucially - be patient. "Airspeed build slowly. Altitude must be traded for airspeed," he says, adding that engine thrust might be ineffective.

Chaney says thrust demands particular attention, particularly during low-altitude recovery. "I caution our pilots to be very careful with thrust application," he says. Restoring normal pitch and roll, he stresses, are "of secondary importance", particularly in a turning stall, for which he advises a two-step recovery - lowering the nose before smoothly rolling back to the horizon as airspeed increases. Simultaneous dual-axis recover is "not desired", he says, because it risks higher tail loads and reduces the effectiveness of control surfaces, delaying recovery.

"You fly into the stall, and fly out of the stall," he says, cautioning that rapid reduction of the angle of attack risks dropping the horizontal stabiliser into the wing vortices.

[stilton]

'Are there still pilots out there that think an Airbus cannot be stalled?
In any control law state? '



There were three on AF447.

[Genghis the Engineer]

Joint Boeing / Airbus view on the subject...

Royal Aeronautical Society | Aeronautical Journal | Stalling transport aircraft (http://www.aerosociety.com/News/Publications/Aero-Journal/Online/1051/Stalling-transport-aircraft)

I can read it where I am, although some IPs may not.

Just picking a useful paragraph from it...

Typical stall characteristics of transport aircraft in 1g non-accelerated flight (ref FAR Part 23.201) begin with the onset of initial buffet. This is best described as light airframe buffet which begins a few knots prior to stick shaker. As the aircraft approaches CLmax the level of buffet generally increases and can become severe to deterrent in nature. It is not uncommon to see buffet with a repetitive load factor of ±1g in the vertical direction and ±·5g in the lateral direction (see Fig. 20). It feels similar to driving an automobile across railroad ties. Buffet on large aircrafts tends to be much greater than experienced in smaller aircraft. This is due to wing airflow separation and turbulent airflow vortices which produce a strong excitation forcing function on the wing. This excites the fundamental frequency of the fuselage leading to large vertical and horizontal deflections. It can be very evident on the flight deck, where anything not securely tied down, such as an errant water bottle, can get hurtled into the air.

Stall identification is deterrent buffet for most recent models in the clean wing configuration. With flaps down, however, stall identification is either full column deflection to the control stop for two seconds, with no further pitch increase, or a nose down pitching moment that cannot be readily arrested.

Followed a bit later by..

Recovering from a stall is straight forward and is in fact nearly identical to that used in general aviation aircraft. First and foremost the angle-of-attack must be lowered using elevator. During recovery the buffet level can momentarily increase, however, this tends to be transitory in nature. Engine thrust can also aid in stall recovery but, the timing of its use is absolutely critical. If thrust is added too soon, the upward pitching moment of under wing-mounted engines may cause an increase in the angle-of-attack. Under certain conditions it may even be necessary to reduce thrust to prevent the angle-of-attack from increasing (Ref. 3). Regardless of when or if thrust is used, the altitude cannot be maintained and should be of secondary importance to reducing the angle-of-attack with the elevator (Ref. 2). Also, of secondary importance, is the restoration of normal pitch and roll attitudes. Flight testing has shown that a properly conducted stall recovery at low altitude using the elevator as the primary control typically results in minimal altitude loss.

[vilas]

Now there is Stilton who has no idea what is he talking about and yet would like to comment. In normal law this aeroplane has been designed to avoid stall in average circumstances and pilot cannot override them. But if it encounters environmental factors beyond those parameters surely it can. Yet those protections make the aeroplane safer than those without it, no mistake about it.

[Microburst2002]

Airbus "don't stall" in normal law because its FCS is designed not to allow that to happen. A different thing is that Airbus FCS is failure proof or perfect.

By the way: What happens if you fly at VLS at level flight, stick loose, with CONF 3, and then you select flaps zero? clac, clac. Ooops!! Seriously. What will happen? Will the FCS pitch the a/c down to get the AoA to alpha prot? Will it just switch to ALTN law, or abnormal attitude law. If still in normal law, Will you get the stall warning or will it be a "silent" stall?

Of course airbuses can physically stall.

The stall recovery procedure is very old. It used to be hidden in the SUPs, then things happened and it was promoted (embarrassingly) to the "front page", that is, the QRH.

[rudderrudderrat]

Hi Microburst2002,
If still in normal law, Will you get the stall warning or will it be a "silent" stall?
According to page 18 of the accident report http://www.bea.aero/docspa/2008/d-la081127.en/pdf/d-la081127.en.pdf "At 15 h 45 min 05, the aeroplane was at 2,910 ft altitude and a speed of 99 kt. Pitch angle was 18.6 degrees. The stall warning sounded."

They were in Normal Law, but received false AoA info from frozen sensors.

Those of us with a healthy disrespect of "In normal law this aeroplane has been designed to avoid stall ..." would not go near a pitch of 18 degrees in level flight and IAS of around 100 kts - because it doesn't look right and AoA probes have been known to fail.

[Gysbreght]

Will the FCS pitch the a/c down to get the AoA to alpha prot?Yes, that's what I think it will do. Why would it switch to ALTN or Abnormal Att law if everything else is normal? The flaps will take some time to move, there would be no stall.

RE stall warning: it is not inhibited in normal law, but the threshold is set so high that it will normally not be reached.

Crawling back under my rock ...

[WhyByFlier]

Normal law, unreliable airspeed caused by blocked pitots (the ASI behaves like a VSI) - just after take off, as the aircraft climbs away, the airspeed unreliably indicates a rapid increase which in turn causes the overspeed protection - a PROTECTION thus implying the aircraft is in normal law. Once in the protection you ain't getting out without crashing or forcing it into alternate law. The aircraft, being in normal law and trying to protect from the overspeed will keep raising the nose, into the stall, and then roll on it's back. To force into alternate law consider switching off 2 ADRs.

[mcdhu]

As Rudderrudderrat intimates, apart from on T/O, you should not be above 10 degrees NU. If you are, be very scared and do something about it - quick!

[Sidestick_n_Rudder]

By the way: What happens if you fly at VLS at level flight, stick loose, with CONF 3, and then you select flaps zero? clac, clac. Ooops!! Seriously. What will happen?

Alpha Lock function will inhibit slat retraction.... :8 Since VLS for Conf3 is higher than Vs for Conf1, you won't stall. The low speed protection might however kick in...

Normal law, unreliable airspeed caused by blocked pitots (the ASI behaves like a VSI) - just after take off, as the aircraft climbs away, the airspeed unreliably indicates a rapid increase which in turn causes the overspeed protection - a PROTECTION thus implying the aircraft is in normal law. Once in the protection you ain't getting out without crashing or forcing it into alternate law. The aircraft, being in normal law and trying to protect from the overspeed will keep raising the nose, into the stall, and then roll on it's back. To force into alternate law consider switching off 2 ADRs.

It won't stall - AoA protection has priority over all other protections....

[drfaust]

The topic has veered off plenty from the original question, but it is an interesting discussion. I don't know about anyone else, but I try to view all those Airbus protections with what I consider to be healthy suspicion.

These computers are slow. In a dynamic environment, it's good to remember that physics is the only thing that keeps you alive. If the air stops flowing over the wing, you are ****** and need to act yesterday. I'm not waiting for any computer to kick in when my and others lives depends on it. Although I am sure that all those Airbus engineers are brilliant, airmanship is why we get paid.

In my humble opinion, of course.

[fdr]

The AI FBW logic acts to provide protection away from the stall, but it is just another system and as such has limits that may exceed the design criteria. At that point, even without system failure, the aircraft may achieve a stall. At that point, the fact that it is such a rare and unexpected outcome, and indeed "trained" out of the normal expectation of the crew, results in potential issues arising in the recovery.

The basic recovery for any airplane achieving a loss of control of flightpath remains simple, and is the same for an F4, MiG21 B777 or a C150, or a Pitts S1... aircraft are inherently recoverable without pilot intervention once the thrust pitch couple is removed, except where the FBW has acted in such a manner to put the controls in a non neutral position... i.e., sensor failure, software faults. In the latter case, the pilot may well need to defeat the logic to achieve the inherent longitudinal stability of the airplane. (true to the point of initial departure... ).

I routinely (poor choice of words) stall large and small aircraft in test, and the factors I emphasise in the test brief and test cards is ensuring that the recovery is not complicated by power additions, and that the crew terminate stab trim before reducing speed below Vref. We also conduct test profile training in simulators, however the stall conditions of the simulator are not necessarily indicative of the aircrafts actual dynamics and performance. The ride in the cockpit of a large aircraft in the stall is rather more interesting than the simulator, vertical and lateral accelerations at the seat are considerable.

If you are comfortable with the assumption that your aircraft cannot stall, so be it. Be prepared to recover when it does happen, and ensure that you do not forget the thrust couple and the stab trim position, particularly when you are now trained by AI design to not use a trim system as a matter of routine. At least the B777 and B787 still keeps a thumb in the loop to some extent... FYI, I prefer AI FBW laws when operating normally over TBC's but, when they go squirrelly, there is a simplicity to the TBC product that is nice in a time of high cognitive load.

cheers.

[shorthauler72]

In the OP it was suggested that selecting F1 after the fact would be moot. Now am I correct in assuming that selecting F1 --during-- the stall might make things even worse because the airflow that should generate lift has already seperated and hence the flaps are only increasing drag?

[vilas]

drfaust
Some of us in our faith or hostility to protections are forgetting that they come in to effect beyond the normal flight envelope. A professional pilot should never experience these protections through acts of commission or omission. It's like money in fixed deposit which you are not going to break but only take comfort from the fact that it is there. so like the SFO accident "Yah the speed was falling but we thought ATHR will correct" is plain bull****.

[vilas]

shorthauler
F1 there is no flap. It is only slat. The point is it doesn't help recovery from stall. The basic technique before they got obsessed with preventing loss of height, reduce AoA.

[WhyByFlier]

It won't stall - AoA protection has priority over all other protections....

So FCOM says. In the sim it would have stalled, for sure. Why? Because HIGH ANGLE OF ATTACK PROTECTION ASSUMES RELIABLE SPEED INDICATIONS. It all occurs at the PFD speeds (black and amber pole and the red pole) V alpha prot, V alpha Max and V alpha floor all exist at the BOTTOM of the speed scale and this is an over speed situation. Though it's an alpha protection, it's triggered by FAC calculated speeds based upon current weight and configuration. The aircraft never gets close to stalling on the PFD! Get 2 ADRs off and get it into alternate law very quickly or stall.

Just remember, you're flying an aeroplane not a text book.

[Sidestick_n_Rudder]

@WhyByFlyer

I am in no way saying that the Bus can't be stalled. Laws of physics apply equally to all aircraft.

However, regarding your scenario:

The AoA protection (as the name implies) is based on angle of attack, not on airspeed (regardless of whatever the FAC calculates and presents on PFD) and it should be able to ovrride the high speed protection. What happened to you in the sim is (hopefully!) just a simulator glitch. If the real a/c behaves the same way, then we have a serious problem...

I do agree however, that switching off 2 ADR's is the quickest way to get into ALTN Law, anytime the FBW system decides to go AWOL...

[Microburst2002]

Lots of very interesting posts by knowledgeable people.:D

[vilas]

Side stick
You are correct. AoA info of two ADRs is averaged and used for calulation of Valpha prot and VMAX and converted to speed for display and it definitely has priority over everything else. Also it is phase advanced to cater for rapid increase in angle of attack so you don't have to think yesterday but still in dynamic environment it may be found short.

[DozyWannabe]

Here, a fully serviceable airbus 319 stalled:
Not quite - they got as far as Stall Warning, but that is fired before entering the stall regime itself. Of course you'll get the warning in the event of unusually changeable wind conditions if you let the AoA deteriorate to that extent.

Are there still pilots out there that think an Airbus cannot be stalled?
In any control law state?
I would hope not, because the FCOM is very clear on the matter, and always has been.

Normal law, unreliable airspeed caused by blocked pitots (the ASI behaves like a VSI)
UAS condition takes at most 1 second for the EFCS to detect, and will cause instant reversion to Alternate Law and suspension of hard protections.

These computers are slow.
Compared to modern tech yes, but the CPU types involved range between 10,000 and 25,000 operations per second, which is perfectly adequate - and then some - for the kind of calculations we're talking about. In High AoA Protection Mode the systems are also designed to log trends, which in effect provides a crude "look-ahead" function to orient the aircraft accordingly.

All this is a bit of a sideline though - the first two answers pretty much nailed it.

[WhyByFlier]

What if all ADRs give the same wrong indication after the pitots have been left covered from maintainance or hitting a flock of birds? Then the system assumes all are correct, incorrectly.

[vilas]

DOZY
Some of your posts give real insight into AB FBW design of flight controls. off course it takes a software expert like you to understand it so well. According to me since it was conceived in the 80s Airbus wanted to keep it simple to use like a calculator but in a dynamic environment it is very difficult to decide how much is too much or too little. Since every one is not a software expert it leaves certain areas which are not well understood or for that matter cannot be completely understood which gives rise to fear resulting in suspicion of the systems and lack of faith. That is what I see in numerous posts.I had flown B707, B747, A300, A310 before I flew the A320 and I did enjoy the FBW.

[Zaphod Beblebrox]

I have not read all the replys, sorry.

The reason you select Flaps 1 below 20,000 ft and not above is that the aircraft is not certified for any flap or slat extension above 20,000ft.

It's that simple.

[WhyByFlier]

Perhaps if you'd read the replies you'd have credited type rated A320 pilots with a little more knowledge.

[DozyWannabe]

What if all ADRs give the same wrong indication after the pitots have been left covered from maintainance or hitting a flock of birds? Then the system assumes all are correct, incorrectly.

OK - if the pitot tube covers have been left on by maintenance then you won't see the airspeed come "alive" during takeoff roll, which to the best of my knowledge results in a mandatory RTO/"STOP". The same should apply if any one of the airspeed indicators disagree on the roll.

The odds of a birdstrike blocking all three pitot tubes simultaneously and with the same density is infinitessimally small, and in any case you'd notice a very definite change in the reading - even in the case of AF447 there was enough time gap inbetween the three pitot tubes clogging for the systems to detect UAS DISAGREE and drop to Alternate Law. Remember the systems are checking and cross-checking tens or hundreds of times per second.

a software expert like you
Heh. I do (a much simpler kind of) Software Engineering for a living - I wouldn't call myself an expert by any means!

but in a dynamic environment it is very difficult to decide how much is too much or too little.
That's why they spent tens of thousands of hours flight-testing the systems and tweaking them in the years before the A320 was put into service!

Since every one is not a software expert it leaves certain areas which are not well understood or for that matter cannot be completely understood which gives rise to fear resulting in suspicion of the systems and lack of faith. That is what I see in numerous posts.
Understood, but the truth is that pilots have been reliant on engineering calculations since the days of the Wright Brothers, and the systems were growing in complexity continuously as aircraft started going higher and faster. The FBW systems were just the latest iteration of that trend.

[sonicbum]

Hello folks, regarding the UAS by 2 completely blocked pitots at the same time I understand that the ADR disagree might not be triggered as after the 3rd source has been rejected the remaining 2 (wrong) won't have any discrepancy (in the sim, in reality I believe it's almost impossible). In that scenario the airplane will remain in normal law till it hits the high speed protection as it was mentioned before and eventually approach the stall by the constant pull up of the protection. Will then the high AOA be triggered and override the high speed protection ? In the SIM if you have tried it before it doesn't.

Cheers and thanks.