rudderrudderrat

Hi Clandestino,

Yes you do. But suppose you are "overloaded" by warnings as you struggled to maintain wings level. Consider these two alternatives.
1) In Direct Law, the stab trim would not have moved. If PF relaxed the back pressure, the aircraft would have nosed down naturally whilst it attempted to regain it's trimmed speed.
2) In ALT LAW, when he relaxed back pressure, the aircraft maintained it's nose up attitude as the speed washed off.

In which Law do you think is easier to actually stall?

@AlphaZuluRomeo,
The IAS was grossly under reading. The acute angle of attack to the pitot tubes rendered them useless. The fact that the aircraft is airborne should be sufficient logic to maintain stall warnings and Angle of Attack.

Clandestino

Well, duh. It is supposed to be that way. I don't mind writing for a zilionth time that DP Davies has it all neatly explained.

Just when did he relax the backpressure? You are acquainted with sidestick traces from 3rd intermediate report, aren't you? A couple of times stick goes forward but quickly gets behind neutral again and stays there before elevators even have the chance to get to neutral.

airtren

A33Zab,

The better understanding of possible problems later in the life of a complex system is somewhat contradictory with discounting the possibility of uncovering flaws later in the life of such a complex system. The latter calls for some reminders:

- the probability of uncovering all flaws/shortcomings/bugs during tests, or in the first year or years of operation of a system, is decreasing with the complexity of the system.

- a shortcoming may stay hidden for a good amount of time, as long as its hosting path in the system's algorithms is not operated, or fully operated - the type of AF 447 Stall is not happening often.

Can you elaborate on "THS inhibit at high AOA", in terms of description, and the problems incurred? I am not sure if you refer to the elements that were the object of several earlier posts on this thread, but either way, elaborating would be helpful, and would be appreciated.
Are there any reasons for the BUSS not to have been adopted? particularly for a route that has a higher risk? like the AF 447's?

rudderrudderrat

Hi Clandestino,

Yes thanks.

See Page 74.
"The PF copilot said “I have the controls” and made rapid and high amplitude lateral, almost stop to stop, inputs. He also made a nose-up input that increased the airplane’s pitch attitude
up to 11° in ten seconds……."

The nose up request on the side stick rotated the aircraft by about 1 deg per second. If they had been in Direct Law, as the speed washed off, the nose would have wanted to drop and the back stick pressure would have been less effective. The aircraft may have stabilised at a speed above the stall whilst still holding the same back stick pressure.

Do you think it is easier to stall in Direct or Alternate LAW?

Clandestino

Great. Now we'll deepen our acquaintance with sidestick and controls positions on page 108 which are raw data behind words on page 74 that you cling to. You have certainly noted that despite right sidestick hovering around halfway nose-up input, elevators that are delivering that one degree per second pitch-up are barely off neutral. What do you think would happen if FBW went into direct law and tried to move elevators to half-deflection at 0.8 Mach? I don't know whether it would be even faster zoom climb to stall at higher pitch, dynamic stall or overstress but it would certainly be uglier than 11° in 10 seconds pitch-up.

I have seen no valid argument on which to base my decision on the matter, therefore I'm undecided for the time being.

rudderrudderrat

Hi Clandestino,
I don't know either - but I bet they would have felt it.
Instead of an unnoticed, increasing pitch up - all he would have had to do was let go and the aircraft would have naturally nosed down to recover any lost speed. It would still have been trimmed to fly at M.08.

airtren

Clandestino, Ruderrat,

An interpretation that the pilot has intentionally, in full knowledge of the situation entered NU commands, calls for a reminder of the confusion shown by the CVR transcript in the BEA report.

Furthermore, there is additional data to consider - such interpretations were advanced before on the AF 447 threads, but do not recall them being documented this way.

The combining of the Pitch and Roll control BEA graphs - see bellow - show a significant coincidence and persistence of Pitch and Roll commands.

As I think it is normal that not every Pitch command required a Roll, and vice-versa, this can be interpreted as:

a). a lesser familiarity with the separation of the two types of commands on the stick, and furthermore:

b) a lesser familiarity with the Neutral position of the stick.

Consequently, it seems that what would have been a perceived Neutral position of the stick for Pitch and for Roll commands, was in reality a OFF Neutral position, in reality a NU command, and respectively a Roll command.

In conclusion, this can be interpreted that the persistent NU commands were not intentional, but just a simple consequence of the handling of the stick.

If the NU commands were not intentional, than the Autotrim of the THS to max NU was not intentional.

Furthermore, the THS was positioned to the max NU position during the Stall Warning time interval, contrary to what the computer logic would have asked for, if the two logical paths (Stall, and Autotrim) were linked.

Note: if we expect the pilot to conclude that in STALL a NU commands is a NO, NO, why shouldn't we expect the computer to arrive at the same conclusion???

It is possible that the BEA work on the Human Factors and a future report will add more analytical elements, interpretations, conclusions, and recommendations that may intersect with elements presented in this post

Lyman

Clandestino

It is NOT 'intended' to compromise the elevator's authority. Its intent, as you well know, is arguably out of place in Alternate LAW 2, when Pitch is unprotected, and the Pilot hasn't a clear knowledge of the actions of the THS.

Further, in a recovery, what the THS does more than anything, is FRUSTRATE the Elevator's authority, and you also know that.

To Stall in DIRECT? or ALTERNATE? Like clandestino, a toss up.

This I do know. To recover from a STALL? given 447's data, DIRECT. WHY? Because without a fully up THS, the elevators could not have kept her in the STALL (caveat: pending a final decision by BEA re: CG). She'd have dropped her nose, something these gents were not granted as a cue that STALL was underway, (and just broken).

If, after the nose dropped, the PF kept holding full back, a slow repetition of the same thing. Then if he eventually did not figure things out, perhaps the rollercoaster may have prompted one or the other worthies to sit down.

airtren

Clandestino,

I read your post as saying that "an active TRIM is supposed to be absolutely compromising the elevators authority".

Can you explain that in your own words, interpretation and logic?

mm43

The question should really be, "Did the elevator have ND authority during the period when the aircraft was on the wrong side of the stall warning?" The answer is yes it did, and indications are that it would have had control up to around 180 KCAS. If a Side Stick max ND command had been made and held for about 2 seconds, the THS would have started neutralizing the elevator demand. Any further ND would have provided increased control as THS followed and airspeed ramped up. The whole process could be sped up by use of the Manual Trim Wheel.

The only real problem I see in the way the THS operates is, the use of the Manual Trim Wheel appears to have been absent from the training regime, or at least discouraged. Why has this happened? The THS is a major control surface, and where it is positioned should be part of the instrument scan. OK, in Normal Law it is taken care of automatically, but in Alt Law the situation changes. Did the PF ever understand how the THS operated in Alt Law? We will never know, but if you don't understand a STALL WARNING, nor understand that consistently pulling the nose toward the heavens is not going to help you on this planet, then what chance does a trim wheel turning constantly for a minute within your periphial vision have of getting noticed.

As for the THS doing its 3°NU to 13.6°NU thing; it had nothing to do with 'autotrim'. It was operating in an integrated demand/time manner to neutralize the elevator input demand - nothing more. The elevator demand was wilful.

The A310 TAROM approach zoom climb/stall/recovery incident at Orly on 24 September 1994 provides some insight into the movements of the THS. Though I note the PNF made some manual adjustment to the THS following the stall recovery while setting up for their second approach, and without the PF being advised. That doesn't mean that the PF was unaware that the THS was moved to provide better trim. Also, the incident was marked by NO stall warning, and the reason given was that AoA SW thresh-hold had not been reached before the IAS had dropped below 60 KTS and the control logic inhibited it. Sound familiar?

Essentially the same logic with minor changes has been around for a long time. Unfortunately it requires incidents/accidents before everyone realizes how some of this control logic works.

Clandestino

Yes.

Trimmable horizontal stabilizer, on any such equipped aeroplane, is always the most powerful control surface and easily overpowers the elevator.

Get this book:



and read from page 35 onwards for details.

RetiredF4

What does change in Alt Law?
According to my references and also posted here from others in Alt Law the autotrim is working as well.
What are you getting at with "the situation changes"?

jcjeant

Hi,

Very little change ..
No more pitch protection
Autotrim is working as well .. but the change is that the elevators are now commanded by the pilot SS .. no the autopilot
As we had seen in the AF447 case .. this can make a difference

Lyman

No, jcjeant, the ss' inputs go to the computer, which controls the elevators. You are describing PITCH DIRECT, not part of AL2. PITCH DIRECT would make possible a lomcevak, perhaps, as the elevators are not "protected".

airtren

Hello mm43,

I've noticed in your post a number of differences in the use of words, that are perhaps just semantics? There are also a number of adjustments to be called. For instance:

Your "THS followed" .... is the THS being moved NOT MANUALLY, but under electrical control by the A/C's PRIM computer controls - which I would call - following the documentation - "autotrim".

In Alternate Law, the "autotrim" is still active. See page 75 of BEA Report (English version), second paragraph of Phase 3 Section, in two sentences: "THS began a movement... It should be noted that in Alternate Law the auto trim is still active."

The NU move from -3 to -13 degrees was controlled electrically, as the Autotrim was active - see the BEA Report, same section, and more.

You're using the word "neutralize", while some documentation is using the word "compensate".

As the THS's role is in stabilizing the pitch attitude that has been reached by means of the Elevators - that's why the name - perhaps "compensate" is closer to the definition of the function? although I can see how "neutralize" can be used with the same meaning in mind.

Once the THS is in a NU at a certain angle, a further Elavator NU at a certain angle is summing the NU effects of the two, while if the Elevator goes ND at a certain angle, the effect is a difference as long as the THS is at an opposite (pitch, or angle) position.

It is generally accepted that the CVR transcript shows confusion, which puts a certain doubt if such a demand was in full awareness of the situation, and with a clear defined goal.
I am familiar with the Tarom case, as well as some other successful Airbus Stall Recoveries, as I've read several reports, watched 3D renditions, but a while ago.... For the Tarom case, the BEA English version of the report, which you've pointed to, has the CVR transcript in a mix of French and English - with several instances of "alarme decrochage-cricket" for "Stall Warning" at page 54.

Despite their mistakes, they've shown their airmanship, and pilot talent, achieving quite a recovery from only 4100ft to 800ft (250m), 60 degree AOA and 30 knots Stall, landing shortly after.
That's so true...

HarryMann

Yes, they are (potentially dangerous), which is no doubt why RRR mooted a stick shaker, and I (knowing someone would bite) extended the argument to a stick shaker... in a specific context.. that is where a plane can easily be held in a stall by the crew with a neutrally stable trim - AF447

Also bear in mind that D. P Davies was writing quite a while ago now... technologically speaking.

JenCluse

Thank you, A33Zab, for that detailed info on the ISIS. My reservations are disarmed, with just one small niggle: one wonders why yet another generation of standby instruments don't maintain an internal backup power source, good for 10, but ideally 15-20 mins of self sufficiency.

But as the question relates to AF447, this little device should have been more than adequate to coarsely validate any suspicions or doubts concerning the primary displays.

Thanks again.

DozyWannabe

Yes, and he made a weary aside in the book that some pilots were complaining about the presence of stick-pushers as potentially unsafe and unnecessary technology usurping their authority.

Then, a month after the final edition of the book was published a Trident crew comprised of a senior Captain and two relatively junior co-pilots (both S/O) on climb out from LHR retracted the droops too low and slow, approached stall and inhibited the stick pusher. The result was a full deep T-tail stall, pancaking into a field in Staines and 118 people dead.

[EDIT : On a quick fact-checking browse, I decided to have a look at Wikipedia's entry on BEA548/G-ARPI, and I have to say it's probably one of the most impressively researched articles I've seen on the site.

Now, superficially there are at least as many glaring differences as there are similarities between AF447 and BEA548 - the command gradient was very similar, and both incidents involved a loss of control following inappropriate handling, however the former was cruising, the latter was on climb-out, and while the Trident was probably the most advanced airliner in service at the time, the A330 is something else entirely. However what struck me when looking at the Flight articles in the wake of BEA548 was how similar the questions being asked and the debate over automation and warning systems is.

Check this out (apologies for the C&P, but I'm truly gobsmacked by the similarities - emphasis mine):

From 1972 | 3235 | Flight Archive

Here's one for the force-feedback/straight to Direct/Manual Trim supporters to think about, also note the trim situation :

And from 1973 | 1189 | Flight Archive :

Plus ça change indeed! ]

airtren

Clandestino,

This succinct rephrasing of the sentence I asked about reads and means different, as you are well aware.

In the rephrasing, your wording "overpowering" is an element that is at the core of the reasoning behind why to have, or not to have an automatic move NU of the THS during Stall, when Elevator action need be immediately fully efficient.

I should clarify that I was looking for your own words, and your own supporting explanation, as short statements alone are often too brief to be a platform for a discussion, and so can be a book reference.

This last book reference is illustrative - you may find an appropriate appreciation. Unfortunately, I don't have the technical means to share the collection of excellent documentation on theoretical and practical aspects of the workings of the THS and Elevators which I have, for being able to reciprocate.

USMCProbe

AlphaZuluRomeo;

Thanks for the info I didn't know that the FPV would go away, but it makes some sense, or at least did to some design engineers.

I do have a few questions about this. If the FPV gets "kicked out" by the system, how long does it take to come back?

The constraints that you wrote were that one of those parameters hat to have been met, by ALL ADRs, at the same time. The 60 knot parameter should have only been met spuriously, and transiently. The 20,000 fpm would have also been transient only, as the average was less than 15000 fpm.

Was there any indication about how long the FPV was unavailable?

Thanks