Lyman

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.


When the THS started to trim NU along with the pilot's NU command, can it be said that for whatever reason, they were both trimming into STALL?

Pilot error, and, what..... Program error? Blame aside, they both had their reason, both faulty.

roulishollandais

In the crash of Air Inter in the Vosges (near the Mont Sainte Odile on the mountain BLOSS) there has never been a FMC-based mode confusion. The family of victims would have wanted it would be said to get more money... from AIRBUS of course. I showed the prosecutor the two approach charts problems (Bloss altitude decreased to 2500 FT, and "FAF" missing) with copy to BZ !

You see I am independant from the A/B fight !

DozyWannabe

@roulishollandais:

The approach chart problems probably negated a legal avenue for compensation, but the fact is that the incorrect mode *was* set, and Airbus/Honeywell changed the FPA/VS display on the FCU panel accordingly.

Of course, that wasn't the first time that particular problem had arisen (although it was the first fatal accident) - the first time it reared it's head almost resulted in an A320 on approach to Gatwick landing on Crawley High Street!

The point I was trying to make was that there is a very important distinction that must be made between automation (i.e. autopilot/autothrust/FMS/FMC) and FBW - and that extends to modes. In the case of the Indian Airways accident, the mode problems were related to autoflight (incorrect FMC settings for approach resulting in autothrust going into Open Descent rather than the managed approach path) and not the flight control computers/FBW aspect, which is under consideration in this accident.

Believe it or not, I have no dog in the A. vs. B. hunt either - I just don't like opinion masquerading as fact, and I especially don't like lazy assertions that have no basis in fact (examples of which include "The [Habsheim] A320 thought it was landing", "The FBW Airbus design process excluded pilots", "FBW is the first step towards pilotless airliners" and "Boeing's latest models do not rely on computers for control") being repeated in the public domain.

rudderrudderrat

Hi DozyWannabe,
It depends on how you define rely. They can be flown in "Direct" without any PFC computers. Airbus only allows Direct Law when the gear is down.

"What should be noted, however, is that none of these features limit the action of the pilot. The 777 design utilizes envelope protection in all of its functionality rather than envelope limiting . Envelope protection deters pilot inputs from exceeding certain predefined limits but does not prohibit it. Envelope limiting prevents the pilot from commanding the airplane beyond set limits. For example, the 777 bank angle protection feature will significantly increase the wheel force a pilot encounters when attempting to roll the airplane past a predefined bank angle. This acts as a prompt to the pilot that the airplane is approaching the bank angle limit. However, if deemed necessary, the pilot may override this protection by exerting a greater force on the wheel than is being exerted by the backdrive actuator. The intent is to inform the pilot that the command being given would put the airplane outside of its normal operating envelope, but the ability to do so is not precluded. This concept is central to the design philosophy of the 777 Primary Flight Control System."

"Direct
—In the ‘‘Direct” mode, the ACEs do not process commands from the PFCs. Instead, each ACE decodes pilot commands directly from the pilot controller transducers and uses them for the closed loop servo control of the actuators. This mode will automatically be entered due to total failure of all three PFCs, failures internal to the ACEs, loss of the flight controls ARINC 629 data busses, or some combination of these failures. It may also be selected manually via the PFC disconnect switch on the overhead panel in the flight deck. The airplane handling characteristics in the “Direct” mode closely match those of the ‘‘Secondary” mode."

http://www.davi.ws/avionics/TheAvion...ook_Cap_11.pdf

DozyWannabe

Interesting stuff rudderrudderrat!

My knowledge of the B777 is very patchy, so that's one for the memory banks. Having said that, I should have made clear that I was referring to a specific conversation I was involved in (many moons ago!) where one of the guys asserted that Boeing never implemented FBW at all, and that the 777 used higher-tech versions of the electromechanical connections in the 75 and 76.

It appears that Boeing opted for defence-in-depth at the electro-mechanical level, whereas Airbus opted to do it at the software level (2 separate and distinct "clean room" implementations per computer etc.). In layman's terms, Boeing opted to have a mode that bypasses the computers entirely in the case of a triple failure, whereas Airbus opted to make it harder for the computers to fail in the same way. Both completely valid ways of solving the problem - and it's a testament to both builders that neither scenario has been put to the test.

CONF iture

Airbus, with reason, put in place some restriction to not autotrim all the way when everything works fine, but decided autotrimming all the way was the way to go when data acquisition was known as deficient …

I don't see what's graceful in such degradation ?

I question Airbus on such 'logic' ?

But more than anything, I do question the BEA on their apparent lack of curiosity ?

RR_NDB

Quote:

I don't see what's graceful in such degradation ?

It seems to me this important System design principle was not properly used.

Logic?

My boss always told me: Not just bureaucrats. Low level bureaucrats.


Who cares?

It seems, just the pilots.

The UAS incidents before F-GZCP were registered. 50+ OR MUCH MORE

RR_NDB

Quote:

...whereas Airbus opted to make it harder for the computers to fail in the same way. Both completely valid ways of solving the problem...

Question:

Graceful degradation is the same in both?

FM (frquency modulation) has the so called "threshold effect". Up to a certain level, everything is fine (excellent Signal to noise ratio).

Below a certain level worms, dominate the scene. So aviation uses AM.

Doppler effect also was a factor in the selection

DozyWannabe

Incorrect. Autotrim is never inhibited or restricted directly (on the A330 at least), it is subject to the protection limits in the same way that autoflight or human pilot input is in Normal Law. Take away those protection limits (i.e. in any law below Normal) and any of the aircraft's controls (including trim) has full authority.

That's because it's what you want to see. You're so invested in the idea of Airbus's logic being wrong and that their intent was to restrict pilots that you can't see the wood for the trees - in this case the design is intended to give full authority to the pilot in every axis precisely because "data acquisition was known as deficient".

If the pilot mishandles the aircraft outside Normal Law then the chances are good that something's going to go amiss, but that's true of every aircraft flying, not just Airbus.

I repeat - autotrim only worked against this crew because their control inputs were inappropriate, and if they'd realised their situation and applied corrective inputs then autotrim would have helped them get stable - until quite late in the sequence.

Now - I can see either putting an extra alert or (in extremis) limiting the autotrim angle being a possible option, but as studi pointed out:

this comes with a risk - especially when such a change is in response to a single incident that was way outside the bounds of acceptable aircraft handling.

And again - just because it does not mention a specific item in the report in the way you want it to (because trim response to SS input is covered in the sequence in some detail) does not mean that the investigators didn't look into it.

They can't be, because the control philosophy is different. Airbus used advances (in 1982 terms) in technology to rework the concept of an airliner cockpit from the ground up. Boeing used the fact that their technological starting point was later (about 1988-89) to create a more complex system that restores a more traditional feel through artificial means.

I'm a little lost by this - if we're talking code complexity here, the Airbus logic is actually incredibly simple (in software terms), even by the standards of the time.

TTex600

Do you mean statements like this? FMC/FMS is not a fancy term for an advanced A/P, and they are not the exclusive domain for the "what's it doing now" problem.

BTW, why will my MCDU not accept the speed I program in it for a apeed/altitude crossing restriction? Sometimes it does, sometimes it doesn't. Why not? For example, a fix (jingl for example) must be crossed at 280 kts/17000ft but the FMC will not accept 280kts. I enter 280/17000, but it only gives me 275/17000.

Why will the 320 fly 260kts when the FCU is selected to 250kts in managed speed? (it's not a weight issue)

NOTE
Edit: the last sentence should reference selected speed not managed. That's what I get for posting after a red eye trip.

LandIT

LonewolfI agree forcibly. The only thing I can expand on this is if the two pilots have different ideas (which I think we see in AF447), the odds go up even more!

CONF iture

That specific item is a flight control in a stall scenario.
It deserves a paragraph in Aircraft information 1.6 ... where is it ?
It requires also a full analysis of its operation during the event ... where is it ?

... that's what I say, you too need a good presentation on the THS operation ... Ask the BEA.

DozyWannabe

FMS/FMC is the automation component regardless of FBW - "advanced A/P" (when compared to, say, the old Sperry and Smiths units) is as good an analogy as any.

I'm perfectly happy to be corrected by the way - can you name an incident where "what's it doing now"? was related to the FBW and not the automation?

I can't help you there I'm afraid - however there are many on here who can (I'd suggest A33z or rudderrudderrat).

My guess would be an external factor in the managed flightpath - does it maintain 250 in selected mode?

TTex600

You're incorrect and I've already given the reason. An FMC/FMS is not an advanced autopilot.

I refuse to argue from your position. If you want to discuss in context, 'll be glad to do so. Actually, I'll help you - again. The context was this: pilots do not always know exactly what an airplane is doing. Go back and read post 649 from the "AF447 Report Out" thread Rumours and News. Here, I'll make it easy for you We could probably get along well if you could find it in yourself to answer what I write instead of trying to use my words tomorrow further your jihad against what you judge to be misinformation.

Regarding the questions I asked. I know the answers......because.......... I actually operate the airplane.

wozzo

So you know the answers, but you won't tell us, because Doze was mean to you?

DozyWannabe

You've yet to tell me *why* though - it'd be very helpful!

Thanks - OK.

So, my position - which happens to be fact - is that FBW is not automation. Never has been. FBW relates purely to the link between the control inputs (whether they be manual or automatic) being digital in nature and being passed through a computer, rather than through electro-mechanical or pure mechanical means - to the flight surface actuators.

To my knowledge, the "what's it doing now?" incidents (i.e where "pilots do not always know exactly what an airplane is doing") are FMC/FMS rather than FBW-related. If you disconnect the FMC/FMS and fly an A320 manually, the computer will do everything you ask of it - up until the point where alpha max is reached or the G loading exceeds the level where structural damage is likely.

The only instance of a grey area that I can think of was the Habsheim crash, where it was not so much a case of "what's it doing now?" as "why won't it give me more nose-up?". The answer to that question was "because if it did, you'd have stalled - and you disabled autothrust so alpha-floor can't help you".

So - as to your earlier point:

These are questions relating purely to automation - FBW doesn't come into it.

Believe me I'm trying my best - it does none of us any good to be combative!

I'm just trying to set a baseline for discussion based on what I know and that is that FBW and FMC are completely separate and distinct systems.

I'd be really happy to know - do tell!

Mean? I've never been anything less than honest, enthusiastic and hungry for information! If I've come across as mean at any point, please accept my humble apologies...

As I said before, the flight control computers have no concept of "stall". They "know" about alpha max, however. In Alternate Law the computers allow a pilot to exceed alpha max (as well as the load limits) if sufficient control force is applied.

Come on - you're well-read on Habsheim. When the Captain tried to pull past alpha-max the computers didn't simply disregard the pitch command, they held the aircraft just shy of alpha-max, including ND elevator commands where necessary. The reason for this is that the hard protections aren't part of the system which controls the surfaces, they are a separate system that monitors the aircraft's position relative to those limits and will maintain those limits until such a time as the command in the opposite direction is given.

Think of it as a bounding cuboid around the aircraft, constantly changing in dimension based on the aircraft's pitch, bank, AoA and speed data. In Normal Law, there isn't a bit of logic that says "Don't move THS/elevator/aileron beyond this position" - the logic says "if these limits are reached or exceeded, use all the flight controls to remain stable at this limit". A drop from Normal Law to Alternate 1 replaces the "do not exceed" logic with "return to this limit when controls are released". A drop to Alternate 2 and beyond removes it entirely.

CONF iture

DW has been answered on that point already after he made similar comments ...

DozyWannabe

Gah - semantics! FMC/FMS relates to automated flightpath management whether A/P is engaged or not.

It's an advanced FD/autopilot combination - is that any better?

In any case, the point is that it's a separate and distinct system to FBW, which relates solely to digital interpretation of flight control input.

TTex600

Thanks CONF iture.

And I apologize to everyone. After reading tens of thousands of posts, I guess you all already know the foibles of the main characters. And I don't intend to become known as just a pedantic jackass. You men obviously don't need me to point out others issues, sorry and I'll try to avoid getting pissy in the future.

CONF iture

And your point is ... ?
Does it disqualify the THS as a flight control surface ?
What is the THS operation under Normal, ALT1, ..........................., ALT2B Law ?



Please, Habsheim for another thread ... when time permits.