DozyWannabe

Cheers.

I thought that was the case, but wanted to make sure.

So - with that checked, your assertion about the discrepancy between the selected VS and actual VS during the descent phase of the sequence is correct (i.e. the FDs could well have been commanding a high nose-up pitch angle). However, during the descent phase with the aircraft stalled, the PF would have had a job keeping the nose consistently above 0 degrees, and chasing a positive FD command then would have been practically impossible.

What's interesting to me is what happened in the climb phase, and for me it's vital to keep these two phases distinct in terms of analysing FD behaviour and correllating it with the PF's actions. In both cases of the FDs' reappearance during the climb phase the discrepancy between the pitch angle, selected V/S and actual V/S should never have indicated an instruction to put the nose up to 15 degrees, but that's what happened.

Picking this apart is a complex endeavour!

rudderrudderrat

Hi DozyWannabe,

Please see post #113 by A33Zab.
I can't explain the initial climb without flight directors (perhaps work load, surprise, CBs, Lightening flashes, smell of ozone, noise on the cockpit from hail, AP disconnect wailer, ECAM dings, ALT Alert warning, "Stall Stall" SV, and calls from the cabin crew, etc. played some part.)
However A33Zab's picture seems to correlate quite well a bit later.

OK465

OG:

This is somewhat of a conundrum in the sense that when you reach -10 degrees, as you say a long time before actual recovery, to hold that pitch attitude do you...

1. release the SS, i.e. FCS positions the elevator to hold what?

Still stalled, It's likely the elevator would go right back to full ANU to hold ~1G and the existing flight path which is quite a bit steeper downhill than 10 degrees at this point. Rule out SS release?

2. or is the SS manipulated, i.e. what is necessary, considering FCS elevator response, to hold a constant pitch attitude while stalled? If not continuing full forward, how do you do this? Some occasional aft SS necessary? (rhetorical )

The FCS has a pretty big say in what you actually get here in practicality, although the theoretical side is certainly sound and I understand that's as far as you intended to take it or that anyone could take it.

In practicality, I wouldn't come off the forward SS stop (with whatever extended nose down pitch rate was actually available) until I knew that I was 'flying' again. This approach, of course, may change some of the values.

DozyWannabe

I hope we're looking at the same post - I'm seeing one with this image, which deals almost exclusively with the climb phase...

In fact I've (roughly) highlighted the areas where the timeline says the FD was available and in which mode:



In the first case, I'm not sure what ALT CRZ* might have presented, but in the second case, even with +6000fpm selected, the FDs engage with an actual VS in excess of that. In that case, would the FD have commanded nose down?

HazelNuts39

You definitely would have to apply forward SS until starting to pull up. How much SS I don't know. This post has a graph that shows the normal acceleration NZw.
Another post shows it for another trajectory.

N.B. In these graphs NZw is the acceleration normal to the flight path, whereas it is usually measured normal to the airplane's longitudinal axis.

Owain Glyndwr

Perhaps I should have written "assumed to be flown at a constant pitch attitude"
Even in the stall the aircraft always responded to elevator is a conventional manner, and I didn't really anticipate any difficulty in using the stick to maintain constant pitch. So I didn't give the practical piloting technique a lot of thought

But as you say, I didn't intend it to be taken too far - I was just curious to know whether there was, in fact, any chance of recovery.

BOAC

It doesn't really matter. What many of us are saying is that FD 're-enable' should either be inhibited without a deliberate selection by crew OR, less desirable, it should re-engage in 'acquire selected altitude' mode, although this latter could cause problems if slow and below that altitude. Whoever decided it should just 'accept' the existing VS? Did anyone ask a pilot?

Presumably if the A/P was not inhibited also it could just re-engage and rear up to pitch for 6000fpm up? How sensible. Now, wait a minute, that rings a few bells..................

PJ2

OK465;
In the sim exercises, for recovery the SS was held full nose-down from the beginning of the stall warning at about FL360 until the wing was unstalled at about FL250, about 40 seconds total time, with a maximum achievable ND pitch of about -12 deg with an average of -10deg. The thrust levers were in the CLB detent and the THS was initially at 13.6deg and was returning to the normal cruise setting.

With the FPV symbol available, the FPA could be observed just above initially at -40deg, (pitch -10deg).

It began to move, initially very slowly up, about 15 seconds after full ND SS;

- at 29 seconds after full ND SS, the FPA had moved from -40deg, (pitch at -11) to -25deg, (same pitch);

- in the next 5 seconds it moved from -25 to -15, (FL257);

- at 38 seconds after full ND SS the FPA was -9deg, (pitch -5deg) with the wing unstalled and the CAS at 255kts.
Absolutely correct!

DozyWannabe

Presumably - given the presence of pilot engineers in the people required to sign such behaviours off - yes.

Which is exactly why, unlike the FD, the AP *is* inhibited under such circumstances.

PJ2

Hi Dozy;

Re the discussion with RRR on the FDs...just to clarify, when they re-engage, they synchronize with the current V/S or FPA aircraft parameter, and not with anything that is selected.

DozyWannabe

For reference - what RRR said was:

We really need to be sure about this, because it will have had a massive impact on what the FD may or may not have been displaying.

JD96

One thing I haven't seen mentioned on here (only reading the latest thread), is that it took 40 seconds to call the captain after autopilot kicks off, and another minute for him to arrive. I'm not a pilot, but it seems it should be policy for the captain to be called immediately in that circumstance.

OK465

We are sure...

The select VS window on the FCU will NOT be open (i.e. the display will be blank) until the FD's return. There is no active mode until FD's are available.

The display window will then 'open' and will, on the FCU panel, display the existing VS at FD recovery, as will the FMA, i.e. +1400 VS. The FCU value & the FMA value cannot be different.

edit: BTW, in ALT2 the A/P is inhibited permanently for the remainder of the flight.

PJ2

Dozy;
I clarified the FD matter in post #119 with help from various documents at my disposal. While complicated to explain, the FD operation is fairly straightforward after training and some experience with the system.

It works extremely well, and there is a very good reason why the UAS memorized drill requires that they be turned off.

I suspect you're well aware that building in an "auto-shutoff", like the oft-cited issues with the stall warning NCD matter and the movement of the THS, may create new, separate problems with their own extremely-rare-though-possible risk exposures, and isn't as straightforward a solution as it may seem. It was stated many threads ago that this is a performance accident not a technical one. The issue here is performance, not system characteristics in extremely rare circumstances, for no designer can possibly expect to anticipate and then counter all possibilities and permutations of degradation and system behaviour. As I have observed many times, this is, in essence, sadly, tragically, not a complicated accident.

OK465;
Precisely.
Which clarifies BOAC's concern that the AP would engage at "+6000fpm"; It wouldn't, in ALT2B.

The AP would engage in Normal Law but, under normal protections/limits it would also ensure a smooth reduction from an obviously-too-high VS to a stable level-off or gentle climb.

In any case, (because of the Madras accident), as speed reduces, VS reverts from VS to OPEN CLIMB and the thrust reverts to SPEED.

BOAC

PJ - out of interest, how did you get the sim to an AoA of 40degrees? Starting recovery 'at the stall warning' would not achieve that?

CONF iture

When and How long are PRECISELY known, where have you been ?
Also FD commands have been recalculated by Airbus, or call it BEA if you like.

Everything is presented on a video animation we will not see.
The animation presented during the press conference has no interest.

PJ2

I think the AoA was closer to 30deg, (-40 FPA, -10 Pitch), but I held the stick fully back until the stall warning and kept it back. The sim pitched down to about -2 or 3 degrees, even with power on and the THS moving towards -13.6deg. The stick was then placed fully-forward and held until it was clear the wing was flying again.

In this image, the stall warning was constantly sounding. The image of one of the recoveries may help envision the attitude, rate of descent and recovering speed even while still fully stalled:





I posted a schematic a while back and it may help as a refresher:

jcjeant

If this video exist .. maybe this will be seen on the judicial process ...
That can be required by some parties

Clandestino

Dozy, recalculation of F/D commands is covered in section 1.16.5.4, graph is on page 96 of the English edition. Values on graph are shown relative to aircraft symbol on ADI, positive is nose-up, negative nose-down.

Since the stakes are pretty high, I bet video will leak, as will CVR track, eventually.

BOAC

There seems to be some departure from 'actual' parameters in that sim (as expected), as you describe a pitch down below the horizon with full THS and power. 447 managed to hold a high nose attitude in that situation. How much stick effort was required to hold the nose down against the thrust couple? Full forward, half?

I should clarify, to calm DW's fevoured brow, that I do KNOW the A/P would not 're-engage' - I was attempting to transfer the apparent FD 'logic' to the A/P as an example, in my opinion, of misguided design. After all, we do have enough examples of the AB A/P ACTUALLY causing a dramatic 'uncommanded' climb.