AlphaZuluRomeo

Hang on, I never said it was a strict quote (hence my use of the "+/-"), but my understanding of your PoV (which I understand & respect). Something that bring confusion is dangerous in a cockpit.
I may differ of your PoV in a broader picture:
- I'm not sure Alt laws bring that much confusion
- I'm not sure Direct law is always the best fail mode (in AF447, it may have been, as you very well demonstrated; but this is one accident, what about other events/situations?)

I don't know if you read french? I do, and that's simply my reading of the CVR transcript around 2 h 13 min 30.

You said "IMO there was not any panic on board".
I agree that no mention of "shouts" or breakdowns are on the CVR. I agree that the crew tried until the last second to do its job.
The fact that they "tried everything", without much logic, without agreement between crew members, with the serious concern of altitude passing FL100, with the PF & PNF "fighting" for the controls, with rambling, chopped talks... is IMO symptomatic of at last some kind of panicked state. Or should I say deeply concerned?

CONF iture

The laws and protections on the Airbus are very much dependant on the reliability of the numerous probes and sensors. Airbus has opted to keep as much as automation as possible depending of the circumstances. My proposition would be different : As soon there is a doubt on any data, first advise, second degrade to the basic.
That way, any Airbus simulator training would invariably lead to direct law with manual trim in case of manual flight and the crews would be used to that known and unambiguous state.

In Perpignan, the only reliable AoA data was silently discarded by the system, the crew proceeded with the test at an unreasonable low altitude, but knowing what we know now on AF447, nothing positively indicates that the test would have ended differently if the crew had tested at the minimum recommended altitude …

The simple and logic thing would have been to merely advise the crew of the loss of certainty regarding the data : AOA DISCREPANCY
My bet is that the crew would not have proceeded with the test at all as the test was specifically based on the accuracy of the AoA readings.
And why not reverse to direct law with still the capability for the AP ?
Many airliners fly around that way, is it a problem ?

Lets keep the magic of the Airbus when everything is known at 100% not lower.

CONF iture

The BEA transcript is not :
Ça ne servira à rien

The BEA transcript is :
(* fera rien)
on (fera /verra) rien

with the BEA notes :
( ) Words or group of words whose meaning has not been identified with certainty. The “ / “ symbol gives various proposals.
(*) Words or groups of words not understood

Lonewolf_50

Conf, I am not sure I agree with you on the all or nothing point you are making.

Let me compare what Airbus does with the cascading control laws when various inputs go wrong to what Sikorsky does with the levels of flight control automation for a Seahawk helicopter. (Note: Not FBW in a general sense).

When all of the buttons are green, you have AFCS which includes the trim, SAS and boost features. You can disable AFCS (which allows for a variety of "hold" autopilot features, like altitude hold, airspeed hold, heading hold, etcetera) and keep the SAS, boost, and trim. You can also slowly but surely disable or lose those servos and their features as well, and end up with a "boost off" flying mode. This is a challenging but flyable mode. You are your own SAS system, and your touch on the stick (cyclic) has to be very light, even though your feet (rudders) and left hand are working very hard. (Aside: "boost off" is a slight misnomer in the Seahawk, since there is hydraulic power still driving the gross inputs into the rotor system). The inputs into the flight controls that feed the rotor system controls have a boost piston that can be diabled if there are problems.

Given a choice, I would prefer not to go from "all systems on" to "boost off" in instrument condiditions where a chance of keeping a few features, depending upon failure mode, could be worked in.

Granted, not the same kind of aircraft.

AlphaZuluRomeo

I totally agree with the first item. This one is lacking (or at last was on june 1st, 2009).
For the second one, I still am not sure (I insist: I'm not saying "alt law is good", nor "alt law should be discarted"). There are pro and cons (e.g.: Lonewolf_50's answer). There are different situations/failures/combinaisons of those.
While I understand your reserves (in AF447's scenario), Airbus people chose to make an Alt law, they are better informed than I'm. Why is interesting. But I, as an outsider, prefer to restrain from an all-or-nothing position.
In french: Il ne faut pas jeter le bébé avec l'eau du bain.


Yes, I'm aware of that. Once again, that's simply my reading (my understanding/guess) of the meaning of the captain's words.
Do you have another (better?) explanation of the meaning of those words? As for myself, I see no other possible meaning of these words. It's not (and cannot be) a proven fact, as he's no more here to explain us what he meant.

OK465

Agreed.

This doesn't sound right (bolding), but I may not understand what you're getting at.

Amount of elevator deflection for a given SS longitudinal input is essentially the same for 330K in Normal Law, 330K in ALT2(B), and, for example, 200K in ALT2(B), an abnormally low speed for the clean configuration.

At 200K in Normal Law, elevator deflection for the above same SS input is about twice that of the ALT2 deflection.


As a result, I would think you would not ultimately achieve the same peak G value in ALT2 as in Normal, ALT2 value would continuously be lower over the period of the input and never catch up. Holding a less deflected elevator longer isn't going to eventually increase G....and the aircraft response difference would simply be one of the further reduced pitch rate for that speed. Longer time for a given FPA change, which is what a pilot would sense even though the SS 'spring' feel might be associated with different expectations.

My point is couldn't that paragraph in the report just refer to this directly? In the sense that the aircraft is even 'more sluggish' than 'normal sluggishness' at these speeds, I guess truly that is an 'unusual' response dynamic. But given that one is not normally at these speeds clean anyway, I would imagine a pilot assessment of 'unusual' behavior would be a bit superfluous.

gums

Good point Okie.

The thing about a smaller and less rapid rate of elevator when on the backup gains is that HAL will keep trying to achieve the commanded gee. So after a minute of back stick, the THS starts trying to help, and as the smash runs out both elevator and THS keep moving to get to the gee command.

We dinosaurs didn't quite understand this aspect of our FBW control laws until we had line pilots get into the "deep stall" ( not what AF447 had). Same scenario...... relatively steep pitch and losing energy real fast. Elevators are trying to make the jet get to one gee ( corrected for attitude). If the time constants and such are not aggressive, you wind up with a high AoA that the control surfaces cannot handle.

So with the "standby gains" active, and below the default value with everything smoothly running, you are further screwed. The control surfaces don't move as fast or as far to keep you away from the stall AoA values and such.

To all here: I fully realize that the AF447 scenario will likely NEVER BE REPEATED. However, it should serve as a very serious lesson to all the "professional pilots" here that fly planes with FBW systems.

Owain Glyndwr

@OK465

I'm not sure if I can explain it easily, but essentially what I am saying is that the time to achieve the desired level of steady state 'g' will not be radically different whether the C* gains are high speed or low speed values. What will be different is the time history of how you get to that state.

With C* operative you cannot simply relate SS movement and elevator deflection in any dynamic sense. What is true is that for any given set of weight/CG/speed conditions the SS movement, elevator deflection and steady state load factor are uniquely related and independent of whether it is normal or alternate law in operation. In other words the steady state elevator angle/g required is independent of C*.

What does change with C* law changes is how the elevators are moved between initial and final states. Suppose one has an aft CG condition where the steady state elevator angle/g is only say 2 degrees/g and you want an increment of 0.25g (these are just for instance numbers OK?). If you just apply 0.5 degree elevator you will eventually arrive at the new trimmed state, but 0.5 degree elevator isn't going to set the world alight in terms of pitch acceleration, so it will take a long time to get there.

To avoid this C* does what I think pilots would do instinctively in these circumstances - overdrive the elevators to get the aircraft moving and then back off to avoid any excessive overswing [correct me if I am wrong] and let the aircraft come gently to the final trimmed state.

In normal law, or alternate law with standard gains the aircraft response would be a fairly rapid g response, followed by a modest overswing and a damped recovery to the desired steady state g. I suggested that this whole process would take about 6 seconds, but that was a notional value and could be anything form 4 to 7 secs depending on aircraft.

In Alt2B with the default gains the elevator overdrive would be less, the initial pitch acceleration would follow that and there would be no overswing, just a gradual build up of 'g' to the final value. From reports I have read where C* gains have been varied (on large aircraft) this final state is arrived at in about the same time as with the 'normal' laws. The pilot's perception of the dynamic response however would be very different.

In this respect I agree that the dynamic elevator deflection in normal law could be twice what it is in Alt2B.

For the reasons given above I can't agree that the 'g' in Alt2B will never catch up or that holding a less deflected elevator longer won't eventually increase 'g'. There would be a slightly longer time for a given FPA change, but (question) does the pilot "see" FPA change first as a 'g'?

Agreed

Owain Glyndwr

@ gums

Gums. maybe it is just me, but when you write this I keep on thinking that you are inferring that the aircraft is not actually flown to 1g.

If you consider the whole range of genuine 1g level flight conditions the aircraft attitude could be anything from (say) 2 degrees to 6 degrees. An accelerometer fixed to the airframe could read between 0.9994 and 0.9945. If you are going to use that accelerometer to maintain 1g level flight then you must adjust the accelerometer output to correct for those attitudes.

Is this what you meant?, because if so the reference to attitude correction is superfluous as the aircraft is actually flown to 1g period.

gums

Spot on, OG, that is exactly what I am saying based upon the documents provided by several here.

So if I am at 30 degrees of pitch, the actual Airbus command would be 0.87 gee. This is different than the Viper control law. Ours commanded "absolute" gee according to how we had the jet trimmed ( - one plus a bit or so, and + 3.5 gees). We had no bias for pitch attitude, so at an extreme pitch attitude, the jet would try to pull up to maintain the trimmed gee, which was mostly 1 gee.

The result of this control law was if you released the side stick the system would keep cranking in elevator to reach the one gee "command" that most of us used. The Airbus doesn't allow a gee command to be manually trimmed, and it looks to me that it always tries to achieve one gee corrected for pitch attitude. This also plays on the apparent speed stability, as the system has zero stick pressure changes to compensate for AoA changes due to speed changes - the basic "feeling" most of us had with the conventional systems and the original systems in the Chipmunk, Cub, Luscombe, et al.

Just some thots from an old FBW veteran.

OK465

Thank you OG & gums.

I wish you guys had taught my initial 'flight control' ground school, instead of CBT.



(BTW: those elevator deflections were compared at the same weight/CG)

CONF iture

Lonewolf50

Thanks for the analogy you bring forward.
If I get you right, what you describe on the Seahawk helicopter is related to hydraulic issues and other servos malfunctions. An Airbus or any other more conventional aircraft is as much vulnerable to hydraulic issues. A dual HYD failure would trigger alternate law with fewer usable flight control surfaces and to switch straight to direct law for such malfunction is not the purpose of my proposition.

What I am after is everything related to probes and sensors, on which the Airbus is so dependable for its normal operation – This includes the law in force, the THS operation, the protection availability …
It is for that type of malfunctions known as unreliable data, that I would suggest a healthy degradation.

The guys are talking about ALT2, C*, one G command and so on, it is all very much interesting to try to understand what’s happening behind these words, and we all take such opportunity to educate ourselves, but in my book, direct law is the most understandable thing for a pilot who has to act now. For a given displacement of the stick there is a proportional displacement of the elevators. If not gentle enough the load factor will remind the pilot how fast his aircraft still is. If a large stick displacement don’t produce much, his aircraft is probably a lot slow.

Owain Glyndwr

Hi gums,

That is just my point! The system tries to achieve one gee. The pitch attitude input to the system is only to account for the fact that in 1g level flight the accelerometer doesn't read 1.0 and that the difference depends on attitude.

Or maybe its me again

gums

The Nz command I am talking about would be that from a "strapdown" style sensor referenced to the aircraft, not a space-stabilized inertial sensor such as used for navigation and weapon delivery.

The difference between my FBW system of old and that in the 'bus is we did not correct for attitude WRT the Earth coordinate system. Ours was and is still body-oriented for the Nz command. Because the 'bus corrects for attitude, it appears to command an attitude. At small climb or descent angles, it's a small correction. Ours seemed the same when close to cruise attitudes, but we flew at extreme pitch attitudes, so our "one gee" command could help us get into the "deep stall" if we relaxed pressure on the stick when zooming up at 70 or 80 degrees. I don't see this as a factor with AF447.

The point I keep trying to make is that the 'bus appears to command an attitude, but it's a gee/rate command with bias for pitch attitude.

My second point is "hands off" and even at small pitch attitudes like 10 or 15 degrees, that trying to maintain a constant Nz will cause the elevator to trim nose up, and then the THS. Because the 'bus has more drag than our Viper, speed/energy decays quickly, and it is possible to reach a stall AoA even with "hands off" ( as BEA report asserts). With no AoA protections, you're on your own. Holding the stick back for minutes doesn't help at all.

Lastly, the "feel" of non-FBW planes with positive static stability is helpful, as trying to fly slower than the trimmed speed/AoA requires more back stick. Trying to fly faster requires forward stick pressure/movement. OTOH, most FBW systems provide "neutral speed stability" regardless of the AoA, cee gee and/or static stability margins. So we don't get that feedback like in the old days, even with fully hydraulic control valves and no artificial "feel" such as the B777 has.

Pretty good discussion on this aspect of the control laws, way I see it.

Owain Glyndwr

OK, A330 has a strapdown sensor so we are talking same language

That is an important difference, but I don't see how that implies that the 'bus commands an attitude. I see the pitch correction as a technical requirement brought about by the laws of physics, not a deliberate attempt to manipulate flight path. [Which is why I think that the phrase "tries to achieve one gee corrected for pitch attitude" might give the wrong impression].

This is the point I don't get. The AI system, as I understand it, seeks to maintain a commanded gee in earth axes not body axes as in your Viper. With that assumption the pitch attitude term is a necessary correction feature but not a command. The basic system is a simple gee demand with gee/pitch rate feedback arranged to optimise transient response throughout the envelope. [For completeness, the THS becomes the integral part of a standard P/I control system]

Agreed, except to remark that in A330 terms 15 degrees is not a "small" attitude. One needs to add a proviso though - the speed divergence only kicks in when the (thrust - drag) vs speed curve slope goes negative and even then the initial rate of divergence would be very low, although it gets more exciting as one nears stall conditions. The argument that the A330 system could take the aircraft into stall even "hands off" is a valid one, but really only becomes significant (in AF447 terms) because the aircraft was put in the vicinity of stall in the first place.

Agreed

Yup!

HazelNuts39

You mean in level flight?

Owain Glyndwr

@HN39

Yes of course - any gee puts the CL and Cd up and makes deceleration more rapid.

syseng68k

CONFiture, #182
I tend to agree with that, though perhaps not to that extreme. However, it would have the desirable effect of forcing airlines to put more emphasis on manual flying skills, which seem to have become neglected, fwics.

Simplify, simplify, where is my razor ?...

syseng68k

Owain Glydwr, #195
That sounds like a serious fault in the design, in that under such circumstances, the effect is a positive feedback loop that might be unexpected, not trained for and thus very difficult to recover from.

No info as to where it would get it's acceleration / g info from, but would assume the INS, which has a triad of accelerometers. It wouldn't be enough to just take the G (vertical) value alone, as the value would affected by both pitch and roll. The assumption would be that they are using the processed outputs, via a bit of trig etc, to resolve earth axis vertical from 2 of the 3 accelerometers.

Just edited to add some content, rather than the usual hobby horse criticism about fragile at the edges, which i'm still uncomfortable with :-)...

Keep it up - some of the best techlog/af447 stuff i've seen for months...

PJ2

Yes, excellent discussions!

Owain Glyndwr, twenty years ago when checking out on the A320, one of the instructors observed that because the system maintains 1g, that it would gradually increase elevator deflection in the climb to cruise altitudes (where gravity is very slightly less, was the claim...), to maintain 1g and that a tiny ND input was required during the climb. I did a lot of thinking about the implications but never found a clarification. It seemed logical enough but as a pilot I nevertheless doubted that the statement was true. Is there any clarification that might put this notion to rest? Thanks...