HazelNuts39

Have you read the BEA report?

nikplane

HazelNuts39

The final report Bea, I’ve printed 224 pages, with my printer, I did not deliberately read yet. But, I’ve read the other Af447 Bea report.

HazelNuts39

nikplane,

if you've read the 3rd interim report, you should know that the pitot-tubes did not act like an altimeter. The final report has a figure 64 that shows the vertical and horizontal movements of the air.

CONF iture

Switching off 2 FCCs came pretty late, they were trough 10000 feet already.
But "on comprend rien on a tout tenté" came much earlier, close to 2 minutes earlier, in fact it was part of the briefing that the PNF gave to the captain at his return.

There is some history on that especially at AF which was one very early operator and at great scale of the new protected FBW technology.
Came Habsheim (I don’t want to discuss it here) but with Habsheim came already suspicions of computer interference. Then came in France the air inter accident where again the investigation was lacking of transparency, FDR was labeled as destroyed … QAR data were not extensively published, and multiple time references were used indicating partial 'loss' of CVR data.

Also the FCOM has this interesting note :



Did it happen already at AF on the early days of the 340 … I’m not sure but I’ve heard something in that direction where the only way to get rid of unwanted protections was to force direct law by switching some FCC off.
Was it unofficially briefed at AF during sim training ... ?

jcjeant

Concerning the AF447 and BEA final report .. it's unfortunate (a shame ?) that the complete FDR LISTING is not public

AlphaZuluRomeo

CONF iture: In the said case (re: your #164), switching off one of the two consistent-but-wrong ADRs would be enough?

CONF iture

Turning one of those ADR off could be one way to deactivate the protections, but turning all FCC but one off would be a radical way to switch to direct law and deactivate the protections as the same time.

Such initiative to switch 2 FCC off without any ECAM command did not come out of the blue. I am questioning the BEA how they did not investigate this episode to find out what may be hiding behind …

This action also shows IMO there was not any panic on board as many like us to believe.

bubbers44

Pitching up to 15 degrees at FL350 indicates panic to me. No airliner can do that without stalling.

Owain Glyndwr

As I understand the system it doesn't work quite like this. The BEA report says:

From all the diagrams I have seen describing how C* works the pitch rate/g mix is a feedback term. S/S movement always commands a load factor as the steady state output just as the BEA report says. What the pitch rate/g term does is change the shape of the transient response between initial stick movement and achievement of the final desired 'g'. The origins lie way back in aircraft handling research when observers noticed that pilots tended to base their opinions on what constituted 'good' behaviour on how the pitch rate transient varied at low speeds and how the 'g' transient varied at high speed. Then somebody thought if that is what they rate as good why can't we give it to them?

So when in Alt 2B the system gains default to their 330 kt (Vmo) values the effect would be that the system would be trying to achieve the 'ideal' transient response (remembering that the Airbus intent was to make the pitch response the same at all speeds) using high speed 'g' base gains but at much lower speeds. This is what I think BEA are getting at when they say the aircraft has an unusual response dynamic in this situation.

Since the transient 'g' response to elevator movement would be much crisper at high speed I think this would lead the system to apply less elevator (to drive the pitch acceleration) than it would normally use at these low speeds, so the aircraft response would be more sluggish.

Anyway, that is how I see it working

RetiredF4

@Owain Glyndwr

Thank you for this input, there was an argument about this some posts back and thankfully we have straightened that one out. Your explanation is sound and easy to understand ( sorry that i couldn´t explain it that way), and now the result of using those fixed high speed gains gets clearer. This sluggish behaviour in pitch would be a new expierience to PF, especiually when the roll channel behaved quite differently (more agile).

Interesting would be, at what point those default values come into action. Is it the point where the speeds are lost or is there some time-delay / internal checking before this changeover takes place? And is this changeover sudden or gradual? Could the initial NU input be under the false speeds ( agressive) and the later correcting attempts under the default gains (sluggish)? When would the default gains be replaced again by the actual speeds?

AlphaZuluRomeo

Sure, and I understand you would prefer that (IIRC you are +/- advocating the Alt laws are useless, if not dangerous).
My point was: it's not the only way.

Perhaps the BEA didn't investigate enough. Or perhaps they hadn't found anything consistant to write about it.
In intermediate report 2, the BEA wasn't sure if the PRIM1 and SEC1 were commanded off (by the crew) or faulted off (by the "system"). Ref IR2 §1.16.2.4.1.
With the benefit of the CVR transcript, one can conclude that PRIM1 & SEC1 were commanded off:

2 h 13 min 28,2
PNF : essaye de trouver ce que tu peux faire avec tes commandes là-haut (my note: the PRIM/SEC commands are on the overhead panel)
2 h 13 min 30,4
PNF : les primaires et cetera (my note: "primaires etc." may refer to the PRIMary and other computers)
CPT : (* fera rien)
2 h 13 min 31,5
CPT : on (fera /verra) rien

And then:
2 h 13 min 45 F/CTL PRIM 1 FAULT
2 h 13 min 51 F/CTL SEC 1 FAULT


I disagree. The transcript show the PNF to want to "try" something with the "primary" (computers?) then the captain answering it will be no use, then the PRIM1 & SEC1 to be switched OFF.
This crew did enough "strange" actions for me to believe they were following a really logical path. It seems to be a case of "what about trying that?" (even with no clue/agreement) meaning: they didn't understand, they tried this or that, because they didn't realised the root cause of their incapability to control the aircraft (i.e. the stalled condition).

Lonewolf_50

Owain: thank you, a light went on as you described the gains and input relationships, and responses.

Retired F4:
Some years ago, I experienced a somewhat disorienting moment in the cockpit when the SAS and trim in the lateral channel went awry while the pitch channel remained just fine. We disabled AFCS and flew AFCS and SAS off (which required a gentle touch ... but is also something we used to practice!) once we figured out what was wrong. It "felt" wrong to say the least at onset.

I suspect that PF in AF447 encountered a similar "wrong" feeling, if the graphs depicting his efforts in the roll control are any indication.

OK465

"What we have here is...(a) failure to communicate."

Lyman

One would want to check the trace record of elevators position at Law change with PF's first inputs in Pitch. With very low indicated airspeed wouldn't the response of elevators via FCC be extreme? relative to .82Mach? The aircraft exhibited a brisk increase in VS just at handover. Also just prior to the First Stall Warn, the PNF says: "What was that?". Could that have been a sudden reaction of the a/c to inappropriate "response"? Here, as in other areas, the CVR would be helpful, but is not published. The explanation for the comment was offered that PNF was noticing the SW, certainly, but it is not proven.

"Unusual Response-sluggish" Or "Unusual Response-Sudden"

BEA could clear this up with a memo, rather than relying on someone's best guess?

One does not expect a Stall Warn from a pull on the stick that is modulated through a computer to correct for load? Sluggish? NOT

Can this be related to Airbus OEB not to reselect A/P they released after 447?

Were they fearful the ap would "Climb Uncommanded?" with low speed inputs in this control law? Alternate Law2B? why wouldn't the ap be locked out by the system when UAS obtains? Not to mention Flight Directors....

Lyman

lyman #151

re: "HazelNuts39" and "sluggish"
"If the responses were sluggish, would that not possibly explain PF's dogged pursuit of some G? Might he have been seeking the twitchiness in Pitch that he found in Roll? If he thought the aircraft was unresponsive in Pitch, would he not command as much as he could get? If he sussed sluggish, and got no immediate "response" would he be tempted to trade rate for continuation of input?"

i consider this comment pertinent to Pilot's possible reactions derived from known or unpublished evidence. iow Human Factors...

"Pilot continued his NU commands, and the aircraft 'started to climb...' " from BEA, IR3....

OK465

HN39 & OG may be entirely correct here.

However, from my experience, the 'usual' response of aircraft at abnormally low speeds for the configuration IS sluggishness.

Unusual would be 'responsive' to a higher degree, much like 'stick force lightening' in some aircraft.

Lyman

OK465

"OK465 "HN39 & OG may be entirely correct here."

"However, from my experience, the 'usual' response of aircraft at abnormally low speeds for the configuration IS sluggishness.

Unusual would be 'responsive' to a higher degree, much like 'stick force lightening' in some aircraft."



447 was not at abnormally low speed. She was moving right along. Her indicated airspeed was erroneously low.

So it would not be unusual for her to respond with a too-far excursion of elevator If at high speed but the computer sensed low? Something is behind Airbus' concern for uncommanded climb.

Again, this should have been addressed by BEA...

Owain Glyndwr

@Retired F4

I'm entirely with you regarding the relative changes in lateral and pitch control, although I believe the change from S/S demanding roll acceleration instead of roll rate to have been the important factor. But pilots who have tried it say that although the aircraft is more sensitive in roll it is by no means difficult - just like a non-FBW aircraft in fact, and the fact that the AF447 pilot got it sussed in 30 seconds or so seems to confirm that.

'Sluggishness' in pitch is a relative term and it it always difficult to find words that give the desired impression. Based on experience with other large aircraft I was thinking of numbers like these for cruise conditions (the times are approximate indications only)

For a demand of Xg
Normal: after 2 seconds Xg; 4 seconds 1.2Xg; 6 seconds and onwards Xg
Sluggish: after 2 seconds 0.7~0.8Xg; 4 seconds 0.9Xg; 6 seconds and onwards Xg

In other words the initial response would be less rapid but achieving the final desired 'g' not appreciably different.

I don't know of course, but I would think the default gains kicked in at once when the internal checking logic said go to Alt2B. That would place the whole sequence with the default gains. I can't see any change to these gains unless and until Alt2B got replaced by some other law - which didn't happen.

CONF iture

Where did I mean 'dangerous' – Any quote ?
Alternate law on the pitch but direct on the roll brings confusion.
Yes, direct law all the way, that crew would have been better served for all the reasons mentioned earlier.

The BEA did not positively report as such.

Asking questions and trying something is not my definition of panic.

gums

Thank you, OG. A great explanation of some fine points of the control laws.

As a matter of credibility, I was one of the pilots 33 years ago that talked with the engineers and flew the simulators that had different control laws and such. In our primitive system, we had a very clear "warning" that we were in "standby gains". If we were at 400 or 500 knots CAS, then the thing was sensitive! Once below 350 knots or so, the gains felt good.

Most of us would have liked a blend of AoA and gee command, with pitch rates to prevent overshoots. But the AoA sensors were less reliable if our speed got too slow. Nevertheless, above 120 knots CAS or so there was no problem.

In normal gains, we only saw a significant dampening of pitch rates when AoA was approaching the max limit. At low AoA we could get unbelieveable rates until we got to 20 degrees or so of AoA, regardless of the gee command. The so-called "bat turn" at 9 gees. Remember that we could maintain 25 degrees per second of sustained turn rate. The "corner" was at about 360 knots CAS, with 9 gees available and AoA of about 15 degrees.

Again, than you OG for a great explanation of the gains.