Airbus envelop protection (I am curious)

tspear

tspear

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Timothy
Last week, I had to take a commercial flight on an A319.
The first flight, on take off the pilot pitched up from the runway very quickly and the left wing dropped a fair amount, then the pilot over corrected and the right wing dropped followed by the nose dropping a bit.
Basically it felt like a student pilot who pulls the nose up too high and gets to the start of a stall.

There was no wind to speak up, I pulled the METAR to check since I was curious.

So here is my question; I thought the Airbus fly by wire system was supposed to prevent getting this kind of behavior. Or how does it work?

Tim
 
tspear said:
Last week, I had to take a commercial flight on an A319.
The first flight, on take off the pilot pitched up from the runway very quickly and the left wing dropped a fair amount, then the pilot over corrected and the right wing dropped followed by the nose dropping a bit.
Basically it felt like a student pilot who pulls the nose up too high and gets to the start of a stall.

There was no wind to speak up, I pulled the METAR to check since I was curious.

So here is my question; I thought the Airbus fly by wire system was supposed to prevent getting this kind of behavior. Or how does it work?

Tim
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Without knowing some hard data from the airplane at the time, those questions are hard to answer and if so, is only based on speculation.
 
Doc Holliday said:
Without knowing some hard data from the airplane at the time, those questions are hard to answer and if so, is only based on speculation.
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Oh I agree. I just just more interested in the basics of how it is supposed to work.

Tim
 
tspear said:
Oh I agree. I just just more interested in the basics of how it is supposed to work.

Tim
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It gets quite involved understanding the systems and protections. Trying to describe it in a forum post would only lead to more questions.

Try googling Airbus 320 system protections.
 
1) Do you know if there is some sort of noise abatement procedure?

2) Did you listen to the ATC to see if there was a heading change given, either by tower or departure?

3) An Airbus captain explained how the trim works to me. Basically, it's either auto trim or manual trim - and manual trim is only used on the ground or in an emergency. There's no electric trim button like on many of our planes, or even a non-FBW plane like a 737. Instead, the aircraft figures out the trim needed and sets it based on the current attitude. That said, just like when were trying to figure out where we want the trim on our planes, the Airbus has to figure out the best setting and may feel like it is trying to "find" where it wants to be in that specific pitch and roll attitude.

You were probably feeling a combination.
 
...aaaaand we're off. Without knowing what the pilot was doing and what was around it's impossible to tell. If any implementation of auto trim causes a wing to drop 'a fair amount' trying to find level the airplane should be downed and the issue investigated.

Nauga,
licking the envelope
 
Doc Holliday said:
Another situation: Several times I’ve departed behind a heavy and got his wake turbulence after rotation, and got thrown around.
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I've felt wake, confirmed by the flight crew, in a 757 following an A319. The crew said the A319 slowed up really fast and got dirty right at the wrong time. It can happen even when you don't expect it.

And don't get me started on departing in a light single behind a Super King Air. They are worse than most corporate jets.
 
As a general rule, Airbus protects the plane from the pilot (eg prevents what the flight control laws consider over banking) while Boeing protects the pilot from the plane (eg bank as much is needed to perhaps avoid, or maybe create, a disaster). If you know how it works, you can adjust.

Cheers
 
Landing Fees said:
Can you tell us if the plane was in direct law or normal law at that moment?
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any time the aircraft downgrades to direct law you are in an emergency situation. direct law is a awe s..t moment. even alternate law, which is above direct law is a emergency condition. if all goes well a bus driver should go his/her entire career and never see direct law in the aircraft.
because the control system is different from a cable driven system, you are not asking for a control deflection amount like you are in a cable system. you are asking for a change in loading. it is a very sensitive airplane, it takes some getting use to. It is however, a very nice aircraft to hand fly once you get the feel of it.
 
The Airbus uses a dead stick concept which is the cheapest method for flybywire. The stick actually has no feel at all. It feels exactly the same sitting on the ground with no power as it does inflight. You are moving the stick against springs. More modern Flybywire aircraft like the A220 and several business jets use a sidestick that incorporates feedback as most military systems due. Considerably more expensive but provides a much nicer flying aircraft. No bumping the stick as most due in the Airbus for small corrections.
 
unsafervguy said:
any time the aircraft downgrades to direct law you are in an emergency situation. direct law is a awe s..t moment. even alternate law, which is above direct law is a emergency condition. if all goes well a bus driver should go his/her entire career and never see direct law in the aircraft.
because the control system is different from a cable driven system, you are not asking for a control deflection amount like you are in a cable system. you are asking for a change in loading. it is a very sensitive airplane, it takes some getting use to. It is however, a very nice aircraft to hand fly once you get the feel of it.
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However, in ground mode and for a bit after takeoff, the controls are essentially direct because of the relationship of the stick to the movement of the control surfaces.

What the OP fails to understand is that going from left wing down to right wing down is not indicative of the lack of envelope protection but the aggressive movement of the controls. Since the A319 was probably rotating at around 135ish and stall is closer to 100ish that would probably not be indicative of a stall either but somebody who chased the flight director too far up and then back down.

I vote we delete this entire thread until we get some real info here. I did my ATP in an A320 sim (that’s right 12 hours of Airbus Nintendo).
 
Landing Fees said:
However, in ground mode and for a bit after takeoff, the controls are essentially direct because of the relationship of the stick to the movement of the control surfaces.

What the OP fails to understand is that going from left wing down to right wing down is not indicative of the lack of envelope protection but the aggressive movement of the controls. Since the A319 was probably rotating at around 135ish and stall is closer to 100ish that would probably not be indicative of a stall either but somebody who chased the flight director too far up and then back down.

I vote we delete this entire thread until we get some real info here. I did my ATP in an A320 sim (that’s right 12 hours of Airbus Nintendo).
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I’m betting the wing dipped due to a crosswind, and the nose down was due to the acceleration altitude being reached.
 
Jeff767 said:
No bumping the stick as most due in the Airbus for small corrections.
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Funny, I never "bumped the stick" nor did I see anyone else use that method. Never felt that the side controller needed feed back as well, never even thought about it.
 
Kritchlow said:
The gear lever comes up as soon as you have positive rate. That’s pretty much immediately.
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Oh I see. As Airbus people say “watch the magic happen” but I don’t recall the gear lever moving on its own.
 
Landing Fees said:
Oh I see. As Airbus people say “watch the magic happen” but I don’t recall the gear lever moving on its own.
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Where was it implied that the gear comes up on its own?

Positive rate, gear up. Just like every other jet.
 
X3 Skier said:
As a general rule, Airbus protects the plane from the pilot (eg prevents what the flight control laws consider over banking) while Boeing protects the pilot from the plane (eg bank as much is needed to perhaps avoid, or maybe create, a disaster).
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That's all I need to know, give me Boeing. (Except the Max, what happened there?)
 
dtuuri said:
That's all I need to know, give me Boeing.
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As a red blooded 'Murican my inner eagle wants to agree with you. And if **I'm** the one flying.. the sure, yes. But the thing with computers is.. they don't get fatigued, they don't have bad days, they're not beholden to an IMSAFE checklist.. they just do their thing

As a passenger I find the newer generation A320/321 also much more comfortable.. cabin is slightly wider and far quieter. I also know that unless the plane is in direct law it's going to work very hard to not kill me.. even if the pilot has a brain fart

This video was quite impressive to me.. they cut an engine just after rotation.. fast forward to around 4:50.. dude takes his hands off the controls and cuts the engine. Impressive stuff. Anyway, they take the plane through all sorts of maneuvers.. other favorite maneuver they do is the overspeed around 8:10.. worth a watch

@tspear the guy also does a decent job of describing the FBW
 
tspear said:
Last week, I had to take a commercial flight on an A319.
The first flight, on take off the pilot pitched up from the runway very quickly and the left wing dropped a fair amount, then the pilot over corrected and the right wing dropped followed by the nose dropping a bit.
Basically it felt like a student pilot who pulls the nose up too high and gets to the start of a stall.

There was no wind to speak up, I pulled the METAR to check since I was curious.

So here is my question; I thought the Airbus fly by wire system was supposed to prevent getting this kind of behavior. Or how does it work?

Tim
Click to expand...
It could have been very gusty at takeoff. The airplane didn't stall, did it? Envelope protection only limits the extent of the excursions, it doesn't prevent them.
 
Jeff767 said:
The Airbus uses a dead stick concept which is the cheapest method for flybywire. The stick actually has no feel at all. It feels exactly the same sitting on the ground with no power as it does inflight. You are moving the stick against springs. More modern Flybywire aircraft like the A220 and several business jets use a sidestick that incorporates feedback as most military systems due. Considerably more expensive but provides a much nicer flying aircraft. No bumping the stick as most due in the Airbus for small corrections.
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When you move the stick on an Airbus you are making rate commands.. so, in effect, the force will be consistent throughout the flight envelope since input X will always yield rate Y. You don't actually *need* feedback in that type of application

On different aircraft (other FBW and non) input X will yield different results depending on speed, aircraft weight, atmospheric conditions, etc.. so the feedback is more valuable since at 120 knots you'll need a different input than at 250 knots.. on an Airbus the computer is doing all that work for you, it's just asking the pilot "tell me where you want to go and at what rate" - outside of Airbus most modern FBW are basically Airbus-direct-law.. with a GFC700-esq envelope protection. The Airbus FBW is an entirely different kind of flying

With the type of FBW envelope protection Airbus uses I don't see how force feedback would be an aide, seems it would be a hinderance as now the pilot and computers are somewhat at odds. Remember the Airbus is always trimming itself for 1G flight..

Given everything that went into the Airbus fly by wire I highly doubt the one area they decided to go "cheap" on was their FBW. Those jets you mention don't have the same type of FBW system.. so in those applications force feedback is critical for the pilot

PS - I've never watched felt, seen, or heard of Airbus pilots having to "bump" the controls
 
Kenny Phillips said:
Envelope protection only limits the extent of the excursions, it doesn't prevent them.
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From the video I posted above it seems certain excursions may in fact be prevented. Guy had the stick back full and plane did not stall. However he was able to overspeed and overbank the plane. It's pretty cool stuff honestly what they've done. And their safety record would seem to prove it.. and even when "real pilot skills" are required the FBW does not appear to be any hindrance.. like Sully in the Hudson or the Russian guys that just did the dead stick gear up landing in the field. I'm sold. The Air France A330 was more the result of lack of training, CRM, and failure to fly pitch and power basics when they started losing systems.
 
Tantalum said:
From the video I posted above it seems certain excursions may in fact be prevented. Guy had the stick back full and plane did not stall. However he was able to overspeed and overbank the plane. It's pretty cool stuff honestly what they've done. And their safety record would seem to prove it.. and even when "real pilot skills" are required the FBW does not appear to be any hindrance.. like Sully in the Hudson or the Russian guys that just did the dead stick gear up landing in the field. I'm sold. The Air France A330 was more the result of lack of training, CRM, and failure to fly pitch and power basics when they started losing systems.
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he was not able to overspeed or overbank the plane. the system allows a certain level of overspeed or over bank to remain in the envelope it is trying to maintain as the protections kick in. it will allow an overspeed if it will keep the load factor within the envelope. but it will reduce it as soon as it can to get in the envelope. it is deciding that the overspeed or overbank is the lessor of the evils that it must choose from and will allow it.
it will not allow you to over bank, or pitch with full stick deflection it will reduce the angle to the programmed levels.

you are right, the air france aircraft, due to system failures, was not in normal law. once the system downgrades to alternate law or direct law the protections are lost and you can stall the aircraft and thats what happened. It is very apparent from the screens as to which law you are in. anytime you have the green tick marks on the ai you are in normal law with protection.

also you are right, about training. when flying the bus you need to know what modes it is in at all times. otherwise it will do something that you are not expecting it to, or not do something you are expecting it to do. it also knows when to give up and ask for some of that "pilot S..t" when it goes into abnormal alternate law, it says, i cant handle it its up to you do something he only protection you have is loadfactor protection.
 
Found video of the actual event.
 

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tspear said:
@Landing Fees
Correct, I have no real clue how the FBW works in the Airbus. Why I asked the question.

Tim
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I don’t either, never flown the real thing. I did nudge the stick and maybe even bumped it a bit in the sim. I do know that it will fly (sim) pretty crazy single engine if you over-control it.
 
On the ground during the takeoff roll, the flight controls are in Direct Law. Flight control deflection is proportional to stick deflection. After getting airborne, over a period of about 5 seconds (I'd have to look it up), it goes to Normal Law. The airplane is now pitch and roll stable. Wherever the pitch and roll are when you let go of the stick is where the plane stays until it runs out airspeed or redlines. During the transition from Direct to Normal if you let go of the stick it will pitch down. You wouldn't normally do that, but who knows what was happening in the cockpit.

Under Normal Law, the side stick commands a G loading. Neutral stick is 1G. Full aft stick is 2.5G (or whatever the load limit is.. again I'd have to look it up) or max alpha, whichever you get to first. Pushing forward will take you to the negative limit. The roll commands a roll rate in normal law. This makes the plane feel the same whether you're flying at 135kts or 335kts. If you aren't down low making large corrections for wind whatnot, you fly the thing with two fingers and barely move the stick.

The joke is hold the stick like you're touching someone else's d**k. Touch it as little as possible.

It really is an elegant design and gives the pilot full authority to throw the plane around as much as he wants without stalling or breaking it. It does limit pitch and bank, but those values are well beyond anything one would encounter in anything remotely considered normal flight. If you hit an upset and get rolled upside down or otherwise into an unusual attitude, you have full control available to get out of it.

As far as the OP, the rolling could have been anything from wake, bird avoidance, or just a little over controlling for some reason.
 
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Rex Kramer said:
On the ground during the takeoff roll, the flight controls are in Direct Law. Flight control deflection is proportional to stick deflection. After getting airborne, over a period of about 5 seconds (I'd have to look it up), it goes to Normal Law. The airplane is now pitch and roll stable. Wherever the pitch and roll are when you let go of the stick is where the plane stays until it runs out airspeed or redlines. During the transition from Direct to Normal if you let go of the stick it will pitch down. You wouldn't normally do that, but who knows what was happening in the cockpit.

Under Normal Law, the side stick commands a G loading. Neutral stick is 1G. Full aft stick is 2.5G (or whatever the load limit is.. again I'd have to look it up) or max alpha, whichever you get to first. Pushing forward will take you to the negative limit. The roll commands a roll rate in normal law. This makes the plane feel the same whether you're flying at 135kts or 335kts. If you aren't down low making large corrections for wind whatnot, you fly the thing with two fingers and barely move the stick.

The joke is hold the stick like you're touching someone else's d**k. Touch it as little as possible.

It really is an elegant design and gives the pilot full authority to throw the plane around as much as he wants without stalling or breaking it. It does limit pitch and bank, but those values are well beyond anything one would encounter in anything remotely considered normal flight. If you hit an upset and get rolled upside down or otherwise into an unusual attitude, you have full control available to get out of it.

As far as the OP, the rolling could have been anything from wake, bird avoidance, or just a little over controlling for some reason.
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Flown the airplane for years... never knew the airplane was in direct law during takeoff. One of the things we look for is to make sure it’s in normal law. Perhaps it indicates normal law but is really in direct?
I will ask our training dept about this.... I’m not convinced.... yet. I do know there is a ground mode. Perhaps that’s it, but it does not display direct law.
 
Kritchlow said:
Flown the airplane for years... never knew the airplane was in direct law during takeoff. One of the things we look for is to make sure it’s in normal law. Perhaps it indicates normal law but is really in direct?
I will ask our training dept about this.... I’m not convinced.... yet. I do know there is a ground mode. Perhaps that’s it, but it does not display direct law.
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Sorry I should have been more precise. It is in Normal Law on the takeoff roll, but in Ground Mode, which is under Normal Law. The end result is the same as direct law. Sidestick controls flight control deflection proportionally. After liftoff, over a period of about 5sec, it transitions from Ground Mode to Flight Mode. Stays in Flight Mode until Flare Mode.

You're right, the plane is always in "Normal Law" unless computers start breaking. I should have been clearer in that.

For anyone reading this, it might sound convoluted, but the end result is seamless and the plane flies very nicely... :D
 
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