AF 447 report out

[HazelNuts39]

RetiredF4;

As discussed earlier, the C* control law doesn't change at low speed, so the SS g rate command doesn't change to a pitch rate command.

The following quote from section 2.2.5 of the final report may be relevant to your questions regarding the change of gains with speed:

In the specific case of alternate 2B law, some coefficients used in the longitudinal flight control law become speed-independent and are set for the maximum speed for the aeroplane configuration (330 kt in clean configuration). This hardly modifies the behaviour of the aeroplane in comparison to normal law, but can nevertheless induce an unusual response dynamic when the aeroplane has an abnormally low speed for the configuration.

EDIT:: Another quote that may be helpful is from C. Favre's FBW - The Airbus Experience:

A homogenous law, ensuring aircraft behaviour independent of the flight conditions and, in particular, independent of the centre of gravity location, is achieved by tabulating the gains as a function of the computed airspeed, high-lift configuration and centre of gravity location.

[Lyman]

Quote:
In the specific case of alternate 2B law, some coefficients used in the longitudinal flight control law become speed-independent and are set for the maximum speed for the aeroplane configuration (330 kt in clean configuration). This hardly modifies the behaviour of the aeroplane in comparison to normal law, but can nevertheless induce an unusual response dynamic when the aeroplane has an abnormally low speed for the configuration.

So when the THS started moving, NU, it was at this time that both PF and OTTO were commanding a flight path into STALL. For different reasons? Or were both trying to maintain "1G"?

Does that constitute the "Unusual Flight Response" ? BEA are saying the unusual response was that of the aircraft?

Is that what that sentence is saying about Alternate Law 2B?


Quote:
A homogenous law, ensuring aircraft behaviour independent of the flight conditions and, in particular, independent of the centre of gravity location, is achieved by tabulating the gains as a function of the computed airspeed, high-lift configuration and centre of gravity location.


Was 447 trying to "Land"? Did she confuse "high lift configuration" with 'high drag configuration'

[HazelNuts39]

Originally Posted by Lyman

So when the THS started moving, NU, it was at this time that both PF and OTTO were commanding a flight path into STALL. For different reasons? Or were both trying to maintain "1G"?

The THS doesn't move on its own, it is slaved to the elevator. The elevator moves in response to the PF's sidestick, whose 'demand' is interpreted by the flight control law. If there is no demand from the sidestick, the flight control law maintains essentially 1g. In short, the PF commanded the flight path into stall. OTTO disconnected at 02:10:05.

As I read the quote from the final report, I would interpret 'unusual response dynamic' as 'sluggish', but I may be wrong.

[Lyman]

The THS doesn't move on its own, it is slaved to the elevator. The elevator moves in response to the PF's sidestick, whose 'demand' is interpreted by the flight control law. If there is no demand from the sidestick, the flight control law maintains essentially 1g. In short, the PF commanded the flight path into stall. OTTO disconnected at 02:10:05.

In the absence of PF input, NU, the a/c still trims for 1g. This means UP elevator.
With or without PF input, the a/c Stalls. Autopilot is not the issue? Neutral stick the a/c Stalls independently.....

You say, the elevator moves in response to the pilots input. It doesn't, it moves as a result of input by the computer. The computer commands NU, to provide 1G. The pilot provides a collateral input, NU. There is essentially a consonant, or "dual" input.

Because the THS is moving to follow elevators, who wants to parse who is who; they both are commanding the same thing, irrespective of the other's command.

All along, in this flight Law, this is what is happening, referenced to G.

No?

I think it is possible that the pilot was mimicking the flight path set by the computer, when he sensed less than 1G, he input NU. And he would be sensing less than 1G, whenever the a/c started to ND, both the pilot and computer have been adding up elevator. Now you could say he instigated the flight path that he ended up chasing up into the Stall, but he was terrified of descent, and "plus vitesse". Dropping airspeed was his goal, and climb was his result. Not cute, but he was in a corner......and his accomplice was the computer, for a different, and preprogrammed reason....

[RetiredF4]

HazelNuts39
As discussed earlier, the C* control law doesn't change at low speed, so the SS g rate command doesn't change to a pitch rate command.

Wait a minute, i object.
The pitch command changes gradually from a pure g command in high speed to a pitch rate command in low speed. The changeover point being around 210 Kts. Meaning the SS orders a special amount of g change per amount of SS deflection above 210 kts and blends in pitch rate below that speed, SS ordering a amount of pitch change from existing pitch per amount of SS deflection. That does not change the Nz-law, without outside input the aircraft maintains 1 g flight in low and high speed and is therefore flightpath stable.

HazelNuts39
The THS doesn't move on its own, it is slaved to the elevator. The elevator moves in response to the PF's sidestick, whose 'demand' is interpreted by the flight control law. If there is no demand from the sidestick, the flight control law maintains essentially 1g.

Let me try to be more specific. To maintain 1g flight the computers have to use the elevators in short term and THS trim in long term to counter any speed deviations. That is especially true, when autothrottle is not available and when speed therfore is decreasing or increasing over more than a short turbulence induced period. Like in AF 447 case once placed into the climb, SS neutral would try to maintain 1 g / would try to maintain this climbing flightpath. The speed would decay further and without pilot interaction by either adding enough power or reducing the flightpath significantly with SS ND the aircraft will stall, assumed no protections are available.

In short, the PF commanded the flight path into stall. OTTO disconnected at 02:10:05

True, if you reference that statement to the initial NU SS input after AP disconnect. The timeframe Lyman seems to be questioning and what the discussion turns round since few pages is after stall warning 2, when the THS trim started to move NU. And this movement would have happened also with SS neutral, as in NZ-Law the aircraft tries to maintain that 1 g (tries to maintain the flightpath.
Question remains though, would it have moved in the same timeframe without the help of NU SS input? Was elevator travel already maxed by the task to maintain 1 g without SS input or did the SS NU input aggravate the situation.
Therfore my question concerning the gains.

[Lyman]

Indeed so... Neither the PF, nor the Flight Law, was paying any attention to the Stall Warning. There was no Alpha Prot, at this stage, so the computer is exonerated? PF was sure the SW was spurious, but he is not excused?

I suppose....I cannot disagree.....

quote...Franzl...
Question remains though, would it have moved in the same timeframe without the help of NU SS input? Was elevator travel already maxed by the task to maintain 1 g without SS input or did the SS NU input aggravate the situation.

I think yes, of course, the elevator has lost authority to climb further, so THS comes in, not to trim, but to command

It was 48 seconds after Stall that the Nose finally fell, thirty five degrees.
that is quite a potent control, that THS and elevator partnership..

Stall @ 2:10:57, Nose drops, 2:11:45....

[HazelNuts39]

Lyman,

In the absence of PF input, NU, the a/c still trims for 1g. This means UP elevator.

If the flightpath is one of decreasing airspeed, yes.

With or without PF input, the a/c Stalls.

Unless the PF commands a downward change of flight path.

You say, the elevator moves in response to the pilots input. It doesn't, it moves as a result of input by the computer. The computer commands NU, to provide 1G. The pilot provides a collateral input, NU. There is essentially a consonant, or "dual" input.

This is getting into semantics.

RetiredF4,

The pitch command changes gradually from a pure g command in high speed to a pitch rate command in low speed.

The response of the airplane changes. For example, in a steady state pull-up at 1.5 g, the pitch rate is 5.5 deg/sec at 100 kTAS, and 1.4 deg/sec at 400 kTAS.

The changeover point being around 210 Kts.

It was 400 ft/s in the first application, but may vary as required in other applications. At the changeover speed the nz contribution to the feedback is equal to the pitch-rate contribution. Using that value in the above example, the pitchrate contribution would be 70% at 100kTAS, and 37% at 400 kTAS.

That does noit change the Nz-law, without outside input the aircraft maintains 1 g flight and is therefore flightpath stable.

That is essentially correct except that, if the airspeed changes and the flight path is not level, maintaining 1g means that the flight path changes (slightly).

To maintain 1g flight ...

Agreed.

The timeframe (...) is after stall warning 2, when the THS trim started to move NU. And this movement would have happened also with SS neutral, as in NZ-Law the aircraft tries to maintain that 1 g (tries to maintain the flightpath.) Question remains though, would it have moved in the same timeframe without the help of NU SS input?

Probably yes, but much more slowly. At 2:10:49 the pitch attitude was 6 degrees and the rate of climb 1100 fpm. Without SS input that RoC would have been maintained until the airplane stalled.

[BOAC]

I think we need to have a serious thought here about this 'progress' to software control. How many AB pilots would understand/anticipate EXACTLY what a movement or relaxation of the SS input would do in this situation? Is it reasonable to expect a stressed human to work this out, especially one that may well have been inculcated with the 'AB will look after you' mantra? Agreed, 'nose down' is the sensible thing to do, but..............have we actually reached the limit where a human can reasonably be expected to work out what is happening to what and how and why? Surely the cry for a drastic overhaul in both philosophy and content of FBW training needs to become very strident?

Now, Alt2B - let me see C* - is that 1g or flight path, or is it perhaps constant pitch attitude......? Do I have alpha protection in this mode or not? I cannot recall the groundschool - Oh! - what was that noise?

[RetiredF4]

HazelNuts39
The response of the airplane changes. For example, in a steady state pull-up at 1.5 g, the pitch rate is 5.5 deg/sec at 100 kTAS, and 1.4 deg/sec at 400 kTAS.

Thanks HN, we are in the same game again.
Are those values real? Can we compute the values for the speeds for the times when SS was held NU and when later on SS was ND for some seconds?

Would that tell us, that during the period of unreliable low speed the pitch rate was at the high end, thus SS input caused rapid pitch change?

And when speeds became valid again at the initial higher speeds the pitch rate would have been smaller?

Could you work those values into one of your marvelous graphs from the respective time period?

HazelNuts39
Probably yes, but much more slowly. At 2:10:49 the pitch attitude was 6 degrees and the rate of climb 1100 fpm. Without SS input that RoC would have been maintained until the airplane stalled.

I see that point, but the normal acceleration (smoothed) was below 1g 13 seconds later and stayed below 1 g for over 1 minute. throughout the timeframe, the THS was moving. If the pull would have increased the travel rate, shouldnīt the normal acceleration then be above 1 g?

[HazelNuts39]

Originally Posted by RetiredF4

the normal acceleration (smoothed) was below 1g 13 seconds later and stayed below 1 g for over 1 minute.

13 seconds later, at 2:11:02, the airplane was stalled, beyond maximum lift, unable to provide the LF demanded by the sidestick, hence elevator and THS continued to move NU, attempting in vain to accommodate the SS demand.

[Lyman]

BOAC

"- Oh! - what was that noise?"

BUFFET......

HN? Semantics? No, I think the computer was in control, regardless the Inputs of PF....

At the time of lost authority elevators, when the THS started to crank up again, the Stall was delayed, and for this reason, it can be said, that the actual Stall was caused not by the pilot, but by Autotrim...

Without Autotrim, the elevators can no longer keep the Nose UP, and thence, MUSH.... PF was ignoring trim, as he was taught to, he should not have. He was focused on overspeed, and he should not have...

He was flying G, on his lonesome, and he should not have, for the aircraft was duplicating his NU, degree by degree, and it should not have. Left alone, w/o THS, he experiences Nose Down on the natch, with more energy on hand, and less AoA. Does he recover the a/c? As yet, unknown. Or, does he eventually Stall, even without the THS.

I am going to say. No, for although at altitude, he is replicating a recovery from approach to STALL at low level, and with power, and reasonable AoA, he may mush along until he susses that he truly is not overspeed, and releases pressure on the stick, with engines blazing, until he recovers straight and level, without sink.

Can we say that the THS causes the aircraft to violate its type certificate?

IMO, Yes.....

[ap08]

May I ask a stupid question...?

What is the purpose of alternate law? Why is it not enough to have just Normal law for normal flight and Direct law for emergencies?

[DozyWannabe]

@ap08:

Graceful degradation based on maintaining familiarity. If you throw a pilot into a scenario where he or she will have to repeatedly perform an action that is not part of their day-to-day experience (such as manually trimming a FBW Airbus), then it adds a risk factor because they may not get it right. As such, Alternate Law exists when a partial systems failure means that Normal Law can no longer be sustained, but not all the benefits need be lost.

For a (very) rough analogy, when driving your car and the ABS system fails, you don't want it to take out the power steering and cruise control as well if the latter two systems are not showing any indications of failure.

Direct Law should only be reached in the air when the failures are significant enough that none of the FBW assistance can be relied on.

It's worth bearing in mind that despite all the in-depth discussion going on here, most FBW Airbus line pilots will never experience Alternate (or for that matter Direct Law in the air) outside of the simulator.

Originally Posted by BOAC

I think we need to have a serious thought here about this 'progress' to software control. How many AB pilots would understand/anticipate EXACTLY what a movement or relaxation of the SS input would do in this situation?

It doesn't matter what aircraft you fly - it's expected that a pilot should understand that pulling back to maintain 15 degrees pitch up when at cruise level is a very bad idea!

Is it reasonable to expect a stressed human to work this out, especially one that may well have been inculcated with the 'AB will look after you' mantra?

That's an oversimplified line propagated by the press - I'd be very surprised if conversion training didn't include the caveat "as long as everything is working - if not you need to take extra care".

Surely the cry for a drastic overhaul in both philosophy and content of FBW training needs to become very strident?

I think that restricting an industry-wide review to FBW would be too narrow a focus. As PJ2 pointed out in Tech Log, the real problem is that from senior management on down, the peculiarities of the aviation industry is not well-understood anymore. In short, an MBA is not enough to run an airline safely.

Now, Alt2B - let me see C* - is that 1g or flight path, or is it perhaps constant pitch attitude......?

Doesn't matter - the only mantra you need to remember is that outside of Normal Law, extra care must be taken.

Do I have alpha protection in this mode or not?

Outside of Normal Law, no. Also not particularly difficult to remember.

[RetiredF4]

DozyWannabe
If you throw a pilot into a scenario where he or she will have to repeatedly perform an action that is not part of their day-to-day experience (such as manually trimming a FBW Airbus), then it adds a risk factor because they may not get it right. As such, Alternate Law exists when a partial systems failure means that Normal Law can no longer be sustained, but not all the benefits need be lost.

Well, it didnīt help this crew and their pax and payload. And while a risk of not being able to trim manually may be present , which is teached in basic aircrew training ab initio, they had to deal with an autotrimming function not limitied by protections as it is present in the day to day AB flying.
Guess what i myself and some pilots of old school would prefer more.

DozyWannabe
Doesn't matter - the only mantra you need to remember is that outside of Normal Law, extra care must be taken.

It at least should be known, what kind of extra care must be taken. To know that, it is mandatory to recognize the system limitations asociated with the special circumstances. As we (as retired or non AB flyers on this thread) can recognize here and as my own feedback with active AB-pilots shows, the knowledge about downgraded systems is way of the scale we oldheads had been checked about. That may be a management failure or AB philosophy. Your statement shows, that you are either ignorant or not sensible enough to the same theme here.

[DozyWannabe]

Originally Posted by RetiredF4

Well, it didnīt help this crew and their pax and payload.

Wouldn't you say it's a fairly minor issue compared to pulling 15 degrees nose-up when you're already at your cruise level?

And while a risk of not being able to trim manually may be present , which is teached in basic aircrew training ab initio

So is detection and recovery from stall, which they didn't manage.

they had to deal with an autotrimming function not limitied by protections as it is present in the day to day AB flying.

It at least should be known, what kind of extra care must be taken. To know that, it is mandatory to recognize the system limitations asociated with the special circumstances.

The extra care can be boiled down to a single rule - stay within the flight envelope outside of Normal Law!

Guess what i myself and some pilots of old school would prefer more.

It's an opinion you're welcome to, but to me it seems like an attempt to pin blame on an aspect of the design because of a pre-determined conclusion that you don't like the design.

As we (as retired or non AB flyers on this thread) can recognize here and as my own feedback with active AB-pilots shows, the knowledge about downgraded systems is way of the scale we oldheads had been checked about.

Most of the in-depth knowledge was held by the Flight Engineer in the old days. I suspect that were you to show an old FE the layout of the Airbus systems they'd consider it child's play compared to what they had to deal with.

That may be a management failure or AB philosophy. Your statement shows, that you are either ignorant or not sensible enough to the same theme here.

Not at all - what bothers me is that those harping on about the level of complexity either haven't flown the FBW Airbus, or have flown it despite having a pre-existing dislike of the design. There appear to me to be just as many current and former FBW Airbus pilots who don't find the design overly complex at all. They don't post as frequently, but they're definitely around.

[RetiredF4]

Originally Posted by RetiredF4
the normal acceleration (smoothed) was below 1g 13 seconds later and stayed below 1 g for over 1 minute.

Answer HazelNuts39
13 seconds later, at 2:11:02, the airplane was stalled, beyond maximum lift, unable to provide the LF demanded by the sidestick, hence elevator and THS continued to move NU, attempting in vain to accommodate the SS demand.

Lets be friends in thaught. I value your judgement, therefore please donīt evade the key question:

Even a 1 g demand with SS neutral would have stimulated the automation to order an NU demand to the elevators and the THS.

What difference does it make, if a SS NU command in whatever magnitude is made under circumstances, when the the overall outcome of this command is less than 1 g concerning the elevator and THS movement? SS neutral would have forced the identical elevator and THS NU order from the automation to achieve and maintain the "SS neutral 1 g command". This 1 g could not be achieved in the relevant aerodynamic and energy available environment, neither with SS NU command nor without NU SS command.
So what did the SS NU command change bottom line? It only shows, that the PF and the PNF didnīt grasp the correct situation and therefore made the wrong SS input. But this input didnt worsen the situation imho.
Only ND input would have had any positive effect, as we know in hindsight and some of us may have known from the beginning.

[ap08]

If you throw a pilot into a scenario where he or she will have to repeatedly perform an action that is not part of their day-to-day experience (such as manually trimming a FBW Airbus), then it adds a risk factor because they may not get it right.

They may or may not get it right - no one knows for sure. We may suspect there is a risk, but in absence of real data supporting one point of view or the other, they are both just speculation.
AF447 is a real-life example of a situation when Alternate law did not save the day, and possibly even contributed to the events that led to the crash. Are there real-life examples of the contrary: situations when the plane suffered malfunctions and reverted to the alternate law, and in that condition successfully brought back to a runway in one piece?
EDIT: never mind this post, I now realize that the same situation happened each time there was unreliable airspeed.

[Lyman]

"Only ND input would have had any positive effect, as we know in hindsight and some of us may have known from the beginning."

Are we sure that it required actual input or was the airframe paid out with elevator only, and that the Nose would come down on her own without the renewed action of THS movement? She had energy, had not succumbed to partial ballistic flight (momentum only....BEA).

Earlier on, how did she get to +1.65 G, whilst in load factor 1G?

[DozyWannabe]

Originally Posted by ap08

Are there real-life examples of the contrary: situations when the plane suffered malfunctions and reverted to the alternate law, and in that condition successfully brought back to a runway in one piece?

Over thirty of them relating to this specific UAS problem alone!

Originally Posted by Lyman

Are we sure that it required actual input or was the airframe paid out with elevator only, and that the Nose would come down on her own without the renewed action of THS movement?

The THS movement was as fast as it was because of continued input after the apogee - As far as I understand, neutralising the stick at apogee would have meant a slower movement, and a positive level-off at apogee would have stopped movement dead at the current position.

In reference to system behaviour only (to avoid argument), the sim experience indicated that it took 6-8 seconds to roll the THS back to neutral with full forward stick applied, and with full forward stick the trim wheel very rapidly responded to the command (at most 1 second or two).

Judging by the DFDR traces even with full-NU THS, the nose did drop when it became slow enough - the problem was because of the THS and elevator position, it came straight back up again as soon as there was enough speed to do so. I don't buy the idea of autotrim in isolation making a recovery harder, because it was only doing what was asked of it. With the correct inputs it would have been a significant help.

Earlier on, how did she get to +1.65 G, whilst in load factor 1G?

The system allows up to 2.5G in Normal Law when manoeuvering. 1G is not a hard and fast limit, it's just part of the state the system tries to maintain when there's no input requesting differently.

[RetiredF4]

DozyWannabe
Wouldn't you say it's a fairly minor issue compared to pulling 15 degrees nose-up when you're already at your cruise level?

All pilots make mistake, some die in the course of making mistakes because they are not able to recognize this and therefore canīt correct them. Therefore i cant agree to the term "fairly minor issue". Letīs say, if the pullup wouldnīt have occurred, we would have no reason to talk about it. But when **** happens, you should be able to recognize it and get the appropriate action going to save the day. Afterwards go to have coffe without cookies and tell others about your dumbness in hope, that they learn from you.

DozyWannabe
So is detection and recovery from stall, which they didn't manage.

Believe me, thatīs a completely different horse. The stall in the P149 and later in the T37 and in the T38 felt all different to the stall in the F4E, that one was different to the non-slatted RF4E, and the F4C with the inboard LE-Flaps again felt different. To recognize an stall when you dont expect it is a very challenging task. See the amount of LOC-events and then tell me otherwise.

DozyWannabe
The extra care can be boiled down to a single rule - stay within the flight envelope outside of Normal Law!

Thatīs a oversimplified version of cure. As i assume, that you drive a car, it would equal an advice to stay on the street to avoid hitting a tree along the road. Not very helpful despite being absolutely true.

DozyWannabe
It's an opinion you're welcome to, but to me it seems like an attempt to pin blame on an aspect of the design because of a pre-determined conclusion that you don't like the design.

Just stop the blaming thing, i didnīt blame anybody or anything and i do not intend to. I have no axe to grind over Ab or to favour B. iīm not on the paylist of either one. I donīt care who is manufacturing the aircraft which brings myself or my loved ones to Sardegna or another place of my choice, but i expect to reach my destination i paid for. As i was involved in training and evaluation of aircrews as well as contributing in accident investigations for the greatest part of my military career my interest lies in developping an understanding of the happenings, which led to this tragic accident. I do that by discussion and not by blaming.

DozyWannabe
Most of the in-depth knowledge was held by the Flight Engineer in the old days. I suspect that were you to show an old FE the layout of the Airbus systems they'd consider it child's play compared to what they had to deal with.

I agree, and it would be bad if it would be otherwise. The more i wonder, how little knowledge is present by the averageAB pilot (might also be the B pilot) concerning the basic stuff, we had to know in our times.

DozyWannabe
Not at all - what bothers me is that those harping on about the level of complexity either haven't flown the FBW Airbus, or have flown it despite having a pre-existing dislike of the design. There appear to me to be just as many current and former FBW Airbus pilots who don't find the design overly complex at all. They don't post as frequently, but they're definitely around.

Well, you havenīt flown it either, so we are even on that one. But what bothers me is, that there are thousands of AB pilots around and only less than a handfull still flying ones participated in this thread from the beginning. But those had been attacked as AB haters the same like those without or with limited AB expierience. The AB haters spelling is a defense line against any discussion sensible to the matter and is brought up not by the people attacking AB (there are a few), but by the self declared defendents of any system built into an AB aircraft. You may count anybody to the AB haters, but please donīt count me in.