jcjeant

Hi,

DW
Because
CONF
Logic .. no ?

CONF iture

Why not Doze ?

DozyWannabe

Because it adds extra workload, requires ambidextrous manipulation of controls and undermines the concept of the systems design in the first place. If a majority of Airbus pilots shared your view then it might be worth revisiting, but as that seems to not be the case - for all my opinion's worth - I don't think it is.

rudderrudderrat

Hi DozyWannabe,

The first two reasons you quote would make every conventional aircraft impossibly difficult to fly.
The real reason is as Tyropicard pointed out a long while back - How would you know when and in which direction to trim, if you are in Normal or Alternate Law?"

CONF iture

Sorry, I almost forgot you actually don't have practical experience of piloting.
Better understand now why guys on Boeing and every other brands must be more qualified ...

DozyWannabe

Not at all - it's just that autotrim can help take the load off and give you hands on throttle and stick if necessary.

Same way as in a conventional aircraft, by looking at the trim wheel indicators.

Airliners no, but I've done some single-engined prop flying in my youth.

How does that work? Just because autotrim is present doesn't mean that pilots should not be trained how to trim manually - as I've said from the start, if airlines want to use the automatics so much that manual flying skills are eroded, then they must beef up their manual training programmes to compensate.

CONF iture

Then time to practice again maybe ... just to remind how you don't need to "look at the trim wheel indicator" to actually trim.

DozyWannabe

I know you don't *need* to, but it's one way to quickly verify what your trim settings actually are. This isn't the place for another go on the "tactile feedback" hamsterwheel.

Machinbird

Dozy
What ran the trim up at the stall was the Airbus trying to effectively hold an attitude and being unable to do so without adding more THS trim.

Who really knows why the PF decided the nose needed to be 15 degrees up, but the aircraft was trying to make it happen.

If the nose had not been prolonged up by the addition of THS trim, it would have fallen through at the stall like most other aircraft.
In Normal law, additional nose up trim is disabled by angle of attack protection:In Alternate law, it appears that all use of AOA is discarded. No doubt the engineers had their reasons, but I would be real interested in knowing what they were.
Instead, in Alt 1, you have Low Speed Stability which is derived from Airspeed indications:

DozyWannabe

It "tried" to hold that attitude because that was the attitude commanded and *held*. You can't have it both ways - heaven knows the FBW Airbus has come in for some stick for "limiting" pilot input - in this case it was doing exactly as it was told, as would any other aircraft.

Because Alternate Law is by definition a degraded systems status. Any and all stability protection in Alternate is "soft" and as such can be overridden by pilot input, because the design assumes that the pilot knows more than the systems at that point. Even if AoA protection was operative in Alternate Law, the PF's inputs continued to demand nose-up for the majority of the way down and would have overridden it.

The rules in a crisis situation are Aviate, Navigate, Communicate in that order, and whether through poor training or otherwise, this crew sadly failed at the first hurdle. Regardless of the THS angle, all that needed to happen was for one of them to push the nose down in response to the Stall Warning alarm that was going off for nearly a minute and the result would have been a scary ride for the passengers and coffee without croissants for the flight crew after landing in Paris.

infrequentflyer789

Do we know that it's THS or just speculate ?

I don't think the trim goes up much until well into the stall - and by the time it reaches max NU, AOA is already well over 30deg.

What is clear (to me) is that any C* law aircraft will respond to any nose-down stall break with control inputs to counter it - and therefore the pilot may never see the break. I think C*u (i.e. 777) will do the same, but not entirely sure what influence the speed term will have. Once upon a time I might have known how to work it out...

It will, however, respond to the break with elevators, not THS - which will follow slowly on the bus.

Looking at the report, at around 2.11.00, AOA around 10deg, pitch starts to vary and elevator - but sidestick input is all over too, so not clear as to cause/effect. I think they are stalled there though. About 45secs later, elevators and THS hit the stops and suddenly theres a big downward pitch. The stall break finally wins ?


Or, short form, you're right, but it's elevator and THS that stop it - not THS alone.

I don't know but I have some thoughts / speculation. For another post, no time right now.

Machinbird

It is simply logical considering the mechanics of the situation. The aircraft in pitch is effectively bar on a pivot (a seesaw if you will). The THS additional incidence had the effect of applying additional downforce on the tail. Without that additional downforce, the elevator by itself would have met its match long before and the nose would have fallen.

You can see a pitch oscillation as the aircraft begins to stall with PF actively countering and the THS angle continually increasing.

When the THS trim could go no higher, PF's strategy changed from some nose up, some nose down to steady nose up-he was trying to make up for the additional THS trim angle he no longer was receiving.

Dozy
Conventional airplanes recognize aerodyamic reality and drop their nose in the stall (assuming they are not T-tails)
FBW aircraft continue to follow their instructions and in the case of the A330 can grab additional resources (THS nose up trim) that no sensible pilot would apply in a stall.

Sounds to me that they threw out the baby with the bathwater. If AOA had stopped the THS trim at 3 degrees or so, AF447 would have likely begun to bob up and down like a Cherokee or Cessna 150. Don't you think one of the guys in the front office would have gained a clue?

The one problem with AOA is that the A330 AOA envelope reduces at higher Mach.

Probably all they needed to do was use the Mach at dual ADR loss for setting autotrim upper limits as in Normal law. If you really needed more trim later, you could do it manually.
Dozy, I wasn't there to see what they were up against nor were you, but it is one thing to put a foot through the bars of the tiger's cage and another thing entirely to open the door and walk in. If the trim had stopped earlier, the probability of successful outcome would be much higher.

airtren

No Dozywannabe. The PF acted on the elevators, not THS. If the PF acted on the THS, that would have been Manual THS Control.

It was an algorithm in the FBW system that decided to move the THS to max UP position, during the exact period of time in which the STALL WARNING was ON, in spite of the elementary rule, which now is recognized straight forward by Airbus that the STALL is an AOA issue, which is addressed by reducing the AOA.

The algorithm used at the time of the AF 447 accident is incorrect; it has contributed to the Stall. Airbus has recognized the need for corrections.
.....

There is also another thing you may want to come to terms and recognize. It is the importance of a "direct information transfer" between the PF and PNF regarding the position, and actions on the side stick.
,
Perhaps there is a need for a call onto the computer/software expertise, which you've mentioned on a number of occasions, which should be of help in recognizing the qualitative difference between "direct" and "indirect" "information transfer".

Old Carthusian

Gentlemen
The debate about trim, types of laws etc and whether the aircraft has an influence on the accident is sterile and irrelevant. The simple principle here is 'Know your machine'. It doesn't matter what the aircraft does or how it behaves if you know what the machine does. This is the question that everyone should be focusing on - why didn't the pilots know their machine and it isn't a problem with the machine.

Lyman

Also, well before the THS acted UP automatically, the Stall would have occurred if the a/c had not been moderated by g prot. G protection (limit to 1.65+) allowed the a/c to Pitch up longer, delaying the STALL and Nose Drop. With STALL earlier, and at higher velocity, the PF would have experienced the Drop, and could have acted accordingly. As it was, the a/c STALLED later, slower,because it was "Protected" from attitudes that would have caused the STALL to be pedestrian, rather than exotic.

Machinbird

OC
OC, There are several levels of knowing your machine. Because I was mechanically inclined, I got into the maintenance manuals and knew my machines better than most, but that was not a 100% guarantee that I knew what these aircraft were doing at every single moment.

Since then, the level of complexity of aircraft has gone up by an order of magnitude or two. I remember a number of years ago that Boeing and the airlines decided that it wasn't practical to teach aircrews everything about their aircraft and to only teach procedures (About the time that the 747 first came out as I recall). With computerized aircraft and pages and pages of logic diagrams, you will run out of time before you really understand how your aircraft works.

Now take an aircrew that knows their procedures pretty well, put them several hours into a routine flight, then suddenly pull the props out from under them in the wee hours of the night in turbulence. Before they can even begin applying procedures, they need to understand what they are facing. If they begin poorly, they can make seriious mistakes. Do we really need to pull all the props out?

Lyman

Pulling all the props (prots) out is exactly what should have occurred. That is the manual a/c being proposed under these conditions. can't have it both ways. Having it every which way or trying to, is what caused the crash, if one is dependent on the mode (decision) loop explanation.

RR_NDB

When facing important failures (design flawns, algorithms, etc.) you must be able to aviate and "accomplish your mission". A pilot MUST know how to, INDEPENDENTLY of the degree of TYPICAL degradation in the a/c.

(Typical: The Probable and even the Possible in practical terms)

The best way is to deliver (after the degradation) to the crew a basic machine which they (MUST) know VERY WELL in order to "complete the mission".

The "graceful degradation feature" of the a/c never could create complex scenarios difficult or impossible to be "solved" in time by the crew.

And to allow the crew to aviate, the design (law reversion, human machine interface, etc.) must be "State of the art". To allow a "basic" simple operation. A K.I.S.S. one. When mostly needed.

Obs. Obviously, i am not speaking about maintenance related failures or other "external" degradation factors.

DozyWannabe, I will answer ASAP important considerations of your recent posts.

Just a preliminary remark: Obviously the "mechanisms" involved in a modern airliner are growing in complexity. When i mention the K.I.S.S. concept i am not speaking about what is inevitable. Einstein put this very clear. There is a threshold.

Old Carthusian

It is precisely a matter of knowing your machine, nothing more and nothing less. Knowing what it does, knowing what happens if the various flight states are in operation. It's really a simple thing - it's called professionalism. If the machine is complex then one needs to be able to work on a different level but one needs to prepare. It's called flying and it seems it isn't encouraged these days. But the aircraft is not responsible for this - the computer systems aren't responsible for this, the manufacturer isn't responsible for this. The airline is. Isn't it time to stop chasing after red herrings? The accident is the responsibility of the pilots flying and the airline which put them in the situation. It is not due to the complexity of the machine or even the interface but a failure in procedures, training, psychology and CRM. Human factors, gentlemen - focus on these.

mm43

Thank you! I have absolutely no doubt that you are right.

The aircraft was never involved in teaching its masters - it was the other way. Its masters were 'meant to know' the aircraft's limitations.