Response of FBW aircraft in updraft

[Mr Optimistic]

Curious as to how automated a/c respond to a severe convective updraft. If this stalls the wing killing the lift, would the a/c respond with ND and descent? Assumed trimmed for cruise and 'uniform' updraft over all the airframe.

[CaptainProp]

Yes, because in your scenario I'm assuming the airspeed would also drop and the short version is that the A/P disconnects (@ alpha protection +1 degree for airbus). If speed decreases fast, and below alpha protection, and beyond the ALPHA FLOOR threshold, the A/THR commands TOGA. In this scenario the autopilot could of course also disconnect earlier due to the turbulence caused by the updrafts but that would ultimately not change how the FBW would react to the loss of airspeed.

There are lots of variables affecting the various speeds, thresholds etc etc, and it could be discussed and debated forever, but that's the simple version.

As always with the FBW threads you can count on this one to also turn in to pages and pages of Boeing vs Airbus and at least 200 versions of what would happen and how the aircraft would react in case of, and what if etc etc.

[TyroPicard]

If you mean airbus FBW, AP OFF, side-stick left at neutral....
The FBW will try to maintain 1.0g in pitch. It has no pitch, speed or altitude targets to aim for, only load factor.
If Alpha exceeds Alpha Prot it will try to maintain Alpha Prot.

If the ATHR protection (Alpha Floor) engages as well, then the a/c will end up maintaining Alpha Prot with TOGA thrust.

[Mr Optimistic]

Thanks. Wasn't intending to challenge the automatics, real interest is on altitude, with neutral manual control inputs would the initial motion be to climb, descend or hold, or is this a hopeless question because of other factors?

[bex88]

In Normal law the aircraft will not provide a progressive pitch down. This is speed stabilities which are active in alternate law with protections only. It occurs between 5-10kts above the stall. In this case at alpha prot plus 1 the AP disconnects. At alpha floor toga is applied and as said before the aircraft will with a neutral side stick maintain alpha prot. To exit alpha prot the auto thrust must be disconnected and the angle of attack reduced.

That's the theory anyway and it works. However every aircraft and system has its limits and if the downdraft was strong enough insufficient lift would be the result in all cases.

[Mr Optimistic]

OK, thanks to all.

[CaptainProp]

would the initial motion be to climb, descend or hold, or is this a hopeless question because of other factors?

In theory it could be any of the above as the target will be to maintain Alpha Prot.

[FullWings]

...in your scenario I'm assuming the airspeed would also drop
I’d have thought it would have been the opposite.

When you encounter an strong updraft, the *immediate* effect is for the IAS to increase along with the effective AoA.

This can have several results. If the resultant AoA is high enough, the wing section will stall (but still be generating significant lift). If it doesn’t stall, then there will be a marked increase in lift which may overstress the structure, depending on the airframe limiting load factors described by the flight envelope.

[CaptainProp]

When you encounter an strong updraft, the *immediate* effect is for the IAS to increase along with the effective AoA.

Correct. But I would assume that if caught in a prolonged updraft (recent AirAsia crash?) the airspeed will start to decrease rapidly as the relative speed of the shear to the aircraft is decreasing.....?

[Mr Optimistic]

Just been reading other threads about alpha prot and alpha floor: certainly a fertile area of discussion! It says in an AB doc that alpha floor is purely a thrust mode but then goes on to say its activation not only gives TOGA but also a pitch up. Rather glad I don't have to do this for a living..

[FullWings]

Correct. But I would assume that if caught in a prolonged updraft (recent AirAsia crash?) the airspeed will start to decrease rapidly as the relative speed of the shear to the aircraft is decreasing.....?
For an airframe with intrinsic speed stability, the normal response to a step change in relative airflow from a strong updraft would be an increase in g followed by an eventual return to steady state. That is with no constraints, such as trying to maintain an altitude.

Throw in FBW and pilot inputs and what happens? How does a FBW Airbus deal with rapidly increasing airspeed and AoA at the same time?

[tubby linton]

The FBW protections have a prioritisation logic, the foremost of which is the high alpha protection.
I have not seen the reference to a pitch up with alpha floor in the FCOM. The aircraft is still in Normal Law and it resists pitch changes with large changes of thrust.
The Airbus is not immune to the forces of nature because it has protections. It will try and resist them but if there was a sufficient atmospheric disturbance you will stall it. A normal stall recovery would then be required. A lot of pilots don't seem to appreciate this.

[phiggsbroadband]

Surely the remedy to most Airbus crashes (Paris Airshow, XL888t, AF447...) is for the Pilot to use the Pitch Trim Control manually.


However this is the last control the plane wants the pilot to touch, it is imbedded under several layers of FBW technology. It is even painted Black and White, and will take your fingers off, like a circular saw, if you get too close to it.


There are only a few aircraft with Anhedral wings that actually need FBW to fly, all others have Dihedral wings, C of G, Centre of Pressure, and inbuilt stability in their airframes.

[Gysbreght]

The response of the Autpilot, Autothrust and FBW Flight Control System of an Airbus A320/A330/A340 to an updraft in cruise will depend on the updraft velocity and rate-of-buildup. If the updraft is strong enough to trigger the high-angle-of-attack protection (alphaprot) of the FBW system, the autopilot disengages. In the absence of pilot control inputs on the sidestick, the FBW system will tend to maintain the angle of attack at a fixed value equal to alphaprot. The airplane will then enter a steep climb at the expense of airspeed up to a certain altitude, and will then start to descend. The speed at the apogee can be well below stall speed, but the airplane will not stall because alphaprot is less than the stall angle of attack.

There have been at least two incidents of this nature to A330/A340 airplanes, one of which is reported here:
Incident Report Airbus A340-314 F-GLZU 22 July 2011 in cruise at FL350 (http://www.bea.aero/docspa/2011/f-zu110722.en/pdf/f-zu110722.en.pdf).

Alphafloor is an autothrust function that is triggered at an angle of attack greater than alphaprot. If triggered, it will increase thrust to maximum takeoff thrust. However, since takeoff thrust at cruise altitude is not much greater than normal cruise thrust, the effect of alphafloor activation would be relatively insignificant.

[FullWings]

The airplane will then enter a steep climb at the expense of airspeed up to a certain altitude, and will then start to descend.
Comparing that to the limited data we have on the Air Asia, there are some similarities. The low GS at altitude (close to an unaccelerated stall when IAS is back-calculated) especially...

[CaptainProp]

Fullwings, when I was talking about "prolonged" updraft I was referring to the type of event(-s) Gybreght is referring to in his post.

I was perhaps not that clear (laying by the pool as I'm writing :} ) but there will eventually decrease in airspeed as you gain altitude with the updraft and with increased altitude, close to normal/max cruise altitudes, the stall speed will also increase.

As mentioned already by tubby universal laws of aerodynamics apply also to the Airbus and it WILL stall if the meteological forces at work are strong enough. I have been shot down here before for trying to tell people that the Airbus can and will stall given the "right", wrong, circumstances.

[phiggsbroadband]

Towards the end of Gysbrecht's A340 F-GLZU article, it mentions that the aircraft had also encountered an increase of headwind of +25knts, which would have contributed to the overspeed and high positive rate of climb.

[ironbutt57]

The protections protect the aircraft from violent pilot inputs, not mother nature if she chooses to be violent

[Gysbreght]

As mentioned already by tubby universal laws of aerodynamics apply also to the Airbus and it WILL stall if the meteological forces at work are strong enough.With certain failures of system components the FBW system cannot function normally. It then reverts to Abnormal Law without full flight envelope protections and then it can be stalled.

In NORMAL law that is extremely unlikely, and can only occur transiently while the airplane is already on its way towards an unstalled condition.

[CaptainProp]

There have been more than a few incidents where the Airbus stalled in normal law with no failures. I provided links here before but it was still not "accepted" as enough evidence so I will leave this part of the discussion for now.

[Gysbreght]

CaptainProp,

Apologies for sloppy terminology. I should have written:

In NORMAL law without failures ...

[Mr Optimistic]

TL, it was in FAST-20 pg 7 but I misread it, the sub-para was an example on terrain avoidance with full side stick applied. Apologies.

[No Fly Zone]

No endless A vs. B debate from this reader! My only question is why so many (most, active pilots on one or the other types), so not instantly know the answer. The proverbial question, "What is it doing now???" is one that I do not like to hear. Regardless of the platform flown, IMHO the occupants of both seats 0A and 0B should already know those details.:ugh: Thanks and HNY.

[DeafOldFart]

Without FBW systems, gusts are dealt with by reducing alpha to recover from stall/overstress events. Talk to a glider pilot who has encountered extreme turbulence in stubble fires! Paradoxically, whether in up or down draught, the nose ends up pointing down a little, to maintain airspeed... and keep alpha sensible.
The FBW system does what the programmer tells it to, and there's the rub....
Up in the pointy end of the envelope, control deflections must be kept below any level that will cause structural pain, and this will come into direct conflict with the movement required to give an effective control of attitude in severe turbulence. Hence the need to go slower in rough air....
When the envelope is breached, priority should be given to avoiding stress by excessive control deflections, and hopefully the ups and downs may cancel themselves out in a reasonably short time scale.
Huge cu nim have descending air round the outside, and high speed jets in the middle... any speed can result in large stresses being imposed. Gliders have been spat out in pieces, and considering they enter at around 50 knots, it doesn't take too much imagination to figure out that 450 is too fast...
FBW is maybe not quite the right description, Fly By Computer should be employed. Maybe the data available needs to be researched further..

[Intruder]

The response of the airplane depends on the airplane and the modes of autopilot and autothrottle engaged when the updraft is encountered.

In normal high-altitude cruise, VANV will effectively be in an altitude-seeking mode, and the MCP cruise altitude will be the target. The FMS or MCP Mach will also be the target of the autopilot and autothrottles. I don't think it matters whether it's Boeing or Airbus at this point; manual flight is a different story, though.

So, in a severe updraft, the pitch will initially go down and the throttles retarded in an attempt to maintain target altitude and Mach. If the throttles get to idle, pitch will continue to seek target altitude until limit Mach is reached. At that point a climb at limit Mach will commence.

If the updraft abruptly ends, the throttles will advance (likely to the CLB or CRZ limit) and pitch increased to attempt to maintain altitude. Altitude will be sacrificed to keep airspeed above the low-speed limit.

Again, with autopilot and autothrottles engaged, it doesn't matter if it's FBW or conventional, Airbus or Boeing. Response will be essentially the same.

[RetiredF4]

Again, with autopilot and autothrottles engaged, it doesn't matter if it's FBW or conventional, Airbus or Boeing. Response will be essentially the same.

And will autopilot and autothrottle stay engaged?

[Microburst2002]

Airbus FBW is a path stable rather than a speed stable airplane, as conventional airplanes are. This is achieved by a C* pitch control law, which achieves and keeps a target value of g load (blended with pitch rate). Boeing FBW is very similar but it has added a speed maintaining element, and it is referred to as C*U law. So, Boeing are still speed stable airplanes.

It is important to understand that stability, and the tendency of an airplane to maintain angle of attack, or airspeed or whatever (flight path in the case of the bus) is just that: a tendency.

Therefore, as a result of a gust, updraft or downdraft, the airplane can end up in a slight climb or descent. It can¡t be predicted. It will depend on the gust, on how many, how quick and how intense they are, how long they last and in what sense they go.

The pitch normal law of an airbus is not like an ALT HOLD mode of an AP. It will resist to flight path change, but this does not mean that it will succeed in doing so. It doesn't have to. Flight path will change. All we need is that it resists to change. That's enough. That allows us to easily fly the airplane, just like "normal" longitudinal stability helps in conventional airplanes.

If you fly level and an updraft hits you, the airplane will pitch down (pretty conventional right?) But it is likely that some height is gained. Nothing in the control law makes it recover that, nor even to assume V/S zero. It tries to keep the flight path constant. But… Which flight path? As it changes, the reference to be maintained changes, too. Therefore, if after the upset the airplane it in a shallow climb, that is then the new flight path. If the pitch down reaction was a little "late" and "excessive" this may put the airplane descending, but not to recover an altitude.

If you understand this you will fly the FBW better in gusts. All you have to do is to let the airplane resist for a sec before you make inputs to put her the way you want. If you do it simultaneously you will start the "dance".

[vilas]

Tubby and capt prop
The moment you say airbus FBW cannot be stalled some people counter it by saying under certain circumstances it can. Let us understand what it exactly does. You cannot bank Airbus more than 67 degrees means exactly that you cannot. But that doesn't mean in violent storm it cannot be turned upside down. Similarly stall is an angle of attack phenomenon which you cannot achieve with side stick in normal law. If it happens due to severe atmospheric conditions the computer will try and reduce the angle of attack to Alpha Max which is not the case in conventional aeroplane. When you enter an updraft the angle of attack will increase the auto pilot will try to maintain the altitude by pitching down which will increase speed etc. How ever unlike normal aircraft if the angle of attack exceeded alpha prot it will latch and aircraft will maintain that angle of attack even after autopilot trips. Whether aircraft can pitch up in alpha prot yes it can. This protection is designed to pitch down to maintain it when you are increasing the angle of attack but when angle of attack is changing due to updrafts it will maintain the latched condition by doing whatever it takes even by climbing unless pilot pushes the stick forward and unlatches it. It has happened and caused Air prox incidents.

[tubby linton]

Vilas in your bank example I presume the aircraft will transition into abnormal attitude law (Bank>125deg).
This is an unusual law as I don't think that any sim is certified to operate to parameters where it could be shown.

[vilas]

Tubby
In the sim you can not only bank 120 degrees but you can do a barrel roll also off course in alternate law. It will go in abnormal attitude law. You can recognise it by aircraft staying in alternate law instead of direct law after gear is put down. Landing becomes a bit tricky as there is no flare mode. If you reset ELACS you can get direct law.

[tubby linton]

Vilas I was always taught not to exceed the normal flight envelope in the sim as the sim has not been certified for such manoeuvres, this includes full stalls.

[safelife]

The A320 does spin very nicely as well in the sim.
Not sure that is a realisticly simulated characteristic.

[gums]

Just asking, and the FCOM is no good. Gotta talk with folks that have been there and have the tee shirt.

Seems to me that the pitch correction to the basic one gee command could make for a rougher ride than a simple gee command ( not attitude command, and no correction for pitch attitude). So a severe updraft has the plane trying to reduce the gee and vice versa for a downdraft.

The "pure" gee command ( yeah, some rate and gains, but that's all) I flew with 35 years ago had the effect of dampening out the turbulence at altitude and same for the thermals we had flying real low over the desert. The tiny jet felt like it was much bigger and heavier. I had a joy ride in a 'vark one day, and that's a good comparison.

I would bet that the 'bus smooths out turbulence faster and better than we humans. Gotta get some feedback from real 'bus drivers.

PJ2???? You on here?

[vilas]

Tubby and safe life
Nobody teaches aerobatics in the SIM. But if you want to experience the implications of abnormal attitude law I showed you how to do it. Take it or leave it. SIM will do what is programmed and cannot trusted for unknown contingencies. Abnormal laws are known parameters.

[roulishollandais]

I would bet that the 'bus smooths out turbulence faster and better than we humans. Gotta get some feedback from real 'bus drivers. That is of course the best side of these algorithms. But devil is in details of the hard limits (F16, Airbus, but not Boeing). You kept control of your aircraft in overspeed by modifying Nz, but Airbus has been built for "concierges" supposed to be ignorant of physic, in fact emerging countries airlines supposed to be led by ignorance, religions, strange traditions and "culture" if I read well the many posts about asian culture that I cannot share. So buses don't have such additional actuators to control the effective system in that case who only creates climb until physic laws get master again and drop the plane.

[roulishollandais]

A case of missing observability -which is the other system design worst fault- can be pointed in the missing AoA under 60 KTs in AF447.

Remember : it is MANDATORY -state of art by serious guys- that any system be always
1. Observable
2. Controllable

[A33Zab]

So buses don't have such additional actuators to control the effective system in that case who only creates climb until physic laws get master again and drop the plane.


A case of missing observability -which is the other system design worst fault- can be pointed in the missing AoA under 60 KTs in AF447.

And how is that handled (prevented) by the other brand?

[roulishollandais]

First quote : concerns the loss of controllability reaching the limitation/protection. Boeing are not hard limited. Airbuses and F-16 are hard limited.
Second quote : concerns the certification of the AF-447 AoA sensor from the report and our threads i.e.

[A33Zab]

First quote : concerns the loss of controllability reaching the limitation/protection. Boeing are not hard limited. Airbuses and F-16 are hard limited.


IMO nothing to do with behavior of 'response of FBW in updraft'.
In case of sensed overspeed both Airbii as Boeing FBW raise their nose to arrest the exceeding airspeed.
Both needs pilot intervention to restrict the induced climb.


Second quote : concerns the certification of the AF-447 AoA sensor from the report and our threads i.e.


The other brand has it's certifications too, being 30kts for ADIRU and 60kts for SAARU.

[roulishollandais]

Hi A33Zab,
You are pointing other systems' faults... If sensors are missing for any reason and observability cannot be reached in the closed loop you must modify or add sensors, perhaps threads of wool on the frame! Engineers will be able to find something if the certification guys get able to verify observability and controllability.
And the same with controllability and missing actuators. Pilot intervention is possible only so long you have an actuator to jump in the loop again, creating a system interruption and doing the correction quickly enough...and adequatly, if it is possible (the 30 seconds don't fit, too slow)
We are in the time to reject every fault in systems, from A, B or C.

[Gysbreght]

In post #14 I wrote that an A320 will not stall when the high-angle-of-attack protection (alphaprot) of FBW system is triggered by an upward gust.

An Emergency Airworthiness Directive was recently issued in response to an occurrence where an Airbus A321 aeroplane encountered a blockage of two Angle of Attack (AoA) probes during climb, leading to activation of the Alpha Protection (Alpha Prot) while the Mach number increased. The EAD requires amendment of the applicable AFM to advise the flightcrew of emergency procedures for abnormal Alpha Prot. The new procedure starts with:

“At any time, with a speed above VLS, if the aircraft goes to a continuous nose down pitch rate that cannot be stopped with backward sidestick inputs, immediately:
Keep on one ADR
Turn off two ADRs”.

Suppose the event mentioned in post #14 occurs, and is erroneously associated with the new emergency procedure introduced by the EAD. Would switching off two ADRs reconfigure the FCS to Alternate Law, and the high-angle-of-attack protection would be lost?

[vilas]

Gysbreght
The very idea of switching of two ADRs is to get in Alternate law. That is the only way of getting out of the strangle hold of the hard protections as they are now triggered by erroneous AoA data. So you will get reduced protection as is in alternate law which will also be erroneous but can be over ridden.

[Goldenrivett]

vilas,
So you will get reduced protection as is in alternate law which will also be erroneous but can be over ridden.
So if your are subjected to up and down drafts and turbulence, and your only working ADR is receiving erroneous info from a frozen pitot (due icing), and the stabiliser trim is still auto trimming, how does the Airbus now fly a safe trimmed AoA (speed)?

[vilas]

Goldenrivett
As you know stick free Airbus trims for 1g and not for speed. AoA or speed data is not needed for that.The AD is for frozen AoA and not for frozen Pitot. These have different implications. In frozen Pitot case one ADR is kept for stall warning which comes from AoA probe and not for pressure data. However in frozen AoA case stall warning will also be false. This AD is not very comprehensive and doesn't cover frozen Pitot and can mislead the pilot if Pitots are also blocked. A320s which have MOI153528/P12909 the FPV is modified to calculate FPA from hybrid GPIRS(mixed IRS and GPS position). So you can use it in this case. But without this MOD bird can only be used provided Pitot/static is not blocked.

[TyroPicard]

Goldenrivett
So if your are subjected to up and down drafts and turbulence, and your only working ADR is receiving erroneous info from a frozen pitot (due icing), and the stabiliser trim is still auto trimming, how does the Airbus now fly a safe trimmed AoA (speed)?
No problem .. fly level, and from the pitch attitude and config you can deduce whether your IAS is higher or lower than the recommended value, just like every other aircraft on the planet.

[vilas]

Tyro
You have not understood GR's question. how does the Airbus now fly a safe trimmed AoA (speed? Airbus never trims for angle of attack.

[Goldenrivett]

Hi TyroPicard,
No problem .. fly level, and from the pitch attitude and config you can deduce whether your IAS is higher or lower than the recommended value,
If you fly level during an updraft, you will initially observe your pitch attitude is lower than recommended and be tempted to reduce power. If the updraft is large, you will be able to fly level when the aircraft's rate of descent matches the updraft; and if you manage to set your recommended pitch attitude, that's only achievable when you're flying slow.

Thanks vilas, you recognised what I was getting at.

[Gysbreght]

Further to an earlier post
here (http://www.pprune.org/rumours-news/553569-air-asia-indonesia-lost-contact-surabaya-singapore-68.html#post8812749),

I have recently learned from a discussion on another site that the following picture shows one of the last ADS-B transmissions received from Air Asia flight QZ8501 and that the data shown are from GPS. The missing data are all from the relevant ADIRU. Since a total failure of an ADIRU (ADR and IRU) may be considered unlikely, it is possible that it was switched off.

http://031c074.netsolhost.com/WordPress/wp-content/uploads/2015/01/image002.jpg

[vilas]

Gysbreght
It appears to me that two issues are getting mixed up. In the first case the aircraft is in normal law and there is no icing of any AoA sensors or Pitot tubes. But due to change in AoA externally without any action on side stick by the pilot the alpha prot protection is activated. This will cause speed(AoA) to latch and the aircraft will normally descend but if AoA decreases because of outside airflow on the wings the aircraft will climb. This can be unlatched by pushing the stick forward. The second case is AoA probes are frozen at low speed (high AoA). When the aircraft accelerates to higher Mach the alpha prot threshold is decreased as the CL max reduces and that triggers erroneous alpha prot which cannot be unlatched. In this case you put the aircraft into alternate law and override the protection. This case is not going to put the aircraft in serious dive but normal descent at constant speed as it is latched. At lower altitudes at equatorial region temperatures ice will melt and situation should become normal.

[Gysbreght]

It appears to me that two issues are getting mixed up.Sure they are. That's my point. They can be mixed up.