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AF 447 Thread No. 12

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AF 447 Thread No. 12

Old 24th Jan 2015, 15:54
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Hi Clandestino,
...could you please provide an example of an accident/incident where application of "Approach to stall recovery" instead of "Stall recovery" was a factor?
"The trimmed position of the stabiliser, combined with the selection of maximum thrust, overwhelmed the available elevator authority."
http://www.aaib.gov.uk/cms_resources...OF%2006-09.pdf
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Old 24th Jan 2015, 19:07
  #922 (permalink)  
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gums, re, "I like the 'bus clontrol laws in "normal" . They are about what I would expect for the mission and the mechanical design limts of the jet. So my problem is with reversion laws and lack of indications that the jet has reached trim l;imits or AoA limits or.......

- My comment about continuing a climb with stick in "neutral" stands. Until the Alpha stuff comes into play, PLZ show me where the jet will nose over or act like the "old" ones most of us flew years ago. In other words, if energy and AoA allow, the jet will continue to climb after you relax the stick. It will also trim the THS as long as it can. "

The continued climb is the normal result of increased pitch and added thrust, (MACH/SPEED A/T mode when autothrust engaged) to maintain speed - little if any autotrim due speed being maintained. The Alpha stuff doesn't come into play under these circumstances.

In Alt I & Alt II laws, alpha floor is inactive as we know. If the speed reduces as per the scenario you describe, the pitch is gently reduced with reference to speed, (not AoA), as the aircraft approaches within 5 to 10kts of Vs. The pilot can override this pitch down demand. At the same time, the autoflight system reverts to Direct Law.

It would be an odd, though not an unknown set of circumstances for a pilot to increase pitch, or not increase thrust / reduce pitch in response to decreasing airspeed, (see Goldenrivett's link), but just leave the airplane to its own devices, (ie., permitting speed loss, as in the linked incident and for the AMS B737 fatal accident). A pitch-up order from the SS or AP/AT for the A320/A330/A340 series would result in increased thrust in Normal law because the AT's would be in either MACH or SPEED mode. If the AT is disengaged for some reason, (that would be rare/odd - Alt I & II Laws, etc or someone just hand-flying), then the above nose-down demand applies.

Regarding the incentive to learn about one's airplane, I couldn't agree more strongly. However, there seems a "sea-change" in attitude regarding the knowing-of-one's-airplane where NTK has overtaken N&B's learning. Don't know how prevalent such surface knowledge is regarding knowing the machine that is always trying to kill one, but I'm not impressed with some of the incidents seen in that their "character" has changed from weather/navigation/system-failure/mid-air to LOC & CFIT, both of which have robust prevention systems, providing one knows one's airplane and one's craft.

Last edited by PJ2; 24th Jan 2015 at 19:27.
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Old 24th Jan 2015, 21:19
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@vilas

Thanks for that very nice explanation Captain !

"I didn't explain 1G fully. In conventional aircraft you directly move the elevator that creates some load factor to raise the nose to 10 degrees of pitch and then trim yourself to keep it there, if you didn't trim and left the yoke aircraft will pitch down as it is not in trim."

Understood, like the 737. Thanks !

"Maintaining 1G is same as maintaining same degree of pitch what ever you set it. So it trims to maintain the pitch but when the thrust is not sufficient only then FBW will keep pitching up to maintain 1G flight path, otherwise the nose will drop resulting in some minus G."

Understood I think, can you correct me if I'm wrong, so maintaining 1G is pretty the same as maintaining same degree of pitch that we set. To take an example, if we encounter some turbulence and that this turbulence decrease a little the pitch, the g will also decrease in the same time so aircraft will pitch up to regain 1g (to regain the pitch of before the turbulence) thanks to THS/elevators.

"The advantage of treating side stick movement as a load factor demand is that when you move the stick back one inch the load factor ordered is same at all speeds but the elevator movement is varied to give the load factor, thus the aircraft response to the movement of the side stick is same at all speeds unlike normal aircraft where you have to pull the stick more or less to achieve the same degree of pitch depending on the speed of the aircraft"

Also understood, with conventional aircraft, control surface deflection is directlyproportional to control yoke deflection meaning that we have to pull/push more in low speed while on highspeed you have to be more soft, whereas with Airbus control surface deflection is not proportional to sidestick deflection. Meaning that with the same ss deflection, the aircraft control surface will be large at low speed and small at highspeed.

As far as 2.5g is concerned it is achieved in a level turn at 67 degrees of bank so if you reduce bank you can pitch up otherwise not. You are flying a commercial jet and not a combat aircraft. Conventional aircraft how much it can pitch up depends on speed at higher speed you can pitch it up beyond its structural limitations causing structural failure. In FBW you cannot."

Roger, thanks captain !

@PJ2

Thanks Captain, that's right "Airbus is just another airplane and can be flown just as any other aircraft."

@Clandestino

Thanks a lot also for these detailled answer !

"If the stick were held at neutral, FCS would try to maintain constant vertical flightpath which would tend to exacerbate the stall but FCS does not use elevators alone to achieve demanded G, it uses THS full time - unless THS inhibition threshold is reached (e.g. less than 0.5G actual vertical acceleration)"

So by saying " would try to maintain constant vertical flightpath " we can translate it to try to maintain a pitch the aircraft trim back, true ?

"False. Push forward and FCS moves THS and elevator towards nose-down position to comply with G demand."

Understood, thanks !

"It's true 67 degrees banked coordinated turn demands 2.5 G so no pitch up authority is available but high positive pitch does not automatically imply 2.5G. It's about pitchup rate, not angle.

Say, whaddaya need 67 deg banked turn on transport aeroplane for?"

As said vilas, I undestood that also.

Thanks again !

Last edited by TC-DCA; 24th Jan 2015 at 21:37.
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Old 24th Jan 2015, 23:02
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@Winnerofer

I saw that article it's also well detailled but don't really talk about the 1g rule.

And I also want to add something, I just spoke with an EZY pilot and he told me that there will be normally not a pitch up during a stall in alt. Law and that it´s an requirement of certification by EASA (cs-25) that no abnormal nose up pitching will occur in a stall ?
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Old 25th Jan 2015, 01:25
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Originally Posted by TC-DCA
As far as 2.5g is concerned it is achieved in a level turn at 67 degrees of bank so if you reduce bank you can pitch up otherwise not.
No. The bank angle limit of 67 degrees at full stick deflection is a completely separate definition from the 2.5g limit. Obviously, the closer one is to 67 degrees of bank, the more conservative the system will be when it comes to applying pitch input, but at no point will the system prevent response to pitch input entirely.

...I saw that article it's also well detailled...
It also contains blatant minsinformation.

e.g.
...Shortly after the accident, the crew and Air France are singled out without any concrete evidence...
A complete fabrication - at least in terms of the official investigation. The BEA are prevented by their own charter from apportioning responsibility or blame in any for their reports. Obviously the press can say what they like, but that doesn't really hold any weight.

All this helps explain so completely rational rise initially to recover its altitude
There's no way that a temporary drop in indicated altitude of 300-350ft would require a nose-up command of 15 degrees to correct.

...Airbus serve only send "requests" of the pilot to computers, applications accepted or not.
Incorrect. In Alternate Law, the human pilot's commands will override every protection mechanism there is by design.

How to explain that we have put almost two years to find a wreck that was less than 12 km from its last position?
Because deep-ocean search operations are extremely difficult!
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Old 25th Jan 2015, 02:42
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Hi,

Dozy
Because deep-ocean search operations are extremely difficult!
Certainly it's difficult
But the first search phase was made in a zone under the LKP but unfortunately not with all the assets ( Investigation underwater gear like Remus) to make a research with the greatest chance of success
So ... in this case this further increases the difficulty !
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Old 25th Jan 2015, 03:38
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Right, but that doesn't imply any wrongdoing. Sometimes it just works out that way. Famously, the joint IFREMER/Woods Hole Titanic expedition of 1985 started with the IFREMER team getting massively strong responses on their high-resolution sonar equipment. They assumed (reasonably) that the equipment was faulty, pulled it up and did a full diagnostic and repair. They then spent many weeks trawling the depths with no success. Dr. Bob Ballard and the WHOI team returned and used their US Navy-funded video sled (Argo) with only a few days to go, and elected to start from the original start position - and in doing so found the Titanic's wreckage within a day or two. As it turns out, the initial response on IFREMER's equipment was not a malfunction - but they assumed it was, because they didn't dare to hope they were as close as they turned out to be.

The point being that this kind of deep ocean search is not an exact science, even with the best minds and equipment available assigned to the task.
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Old 25th Jan 2015, 04:40
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DozyWannabe
When you hit the stick to one side and hold you keep 67 degrees bank and if you are holding your altitude all the while then you see on the system display above or below the clock 2.5g. Now there is no way to increase pitch without exceeding 2.5g. unless bank is reduced. Off course this is another way of reaching g limit which can be reached simply by pitching with wings level. But 67 degrees bank limit is not in the design by accident.
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Old 25th Jan 2015, 05:17
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Maybe so, but as I understand it there's enough "wiggle room" to add a pitch component at 67 degrees of bank (even if only a small pitch component) - just as you can command a small (but effective) amount of bank while commanding Alpha Max.
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Old 25th Jan 2015, 06:35
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Dozy,

I am very sorry .. but those who have done the research in the first phase have done a nonchalant way and was as expected a failure
They did not have adequate resources (and those were available) which was postponed two years the discovery of the wreck
It was not only the scrap that one is looking .. but also the bodies of victims expected by families
Everything had to be implemented .. this was not the case
Two years of waiting .. not to mention the money wasted in vain other research ....
That's my opinion
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Old 25th Jan 2015, 06:52
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DozyWannabe
At 67degrees when you maintain level you reach 2.5 g where is the question of any wiggle without exceeding 2.5? At Valphamax it is different it is not a wiggle but you can bank up to 45 degrees for short term. Unlike 2.5g Valphamax is not a fixed but variable value.
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Old 25th Jan 2015, 13:28
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Are Airbus a/c ergonomically flawed ?

An air France captain view and comments on Airbus systems following the AF 447 crash


PNC Contact, hôtesse de l'air, Steward, formations PNC et informations hôtesse de l'air. ? AF447 ? Le rapport de contre-expertise d?un pilote

The full version is in french have translated some exceprts below.

How can a stalled a/c enter a deep stall and then not recover ?

Looks like Airbus series have been designed as "Stall Proof" a/c, algorithms in their systems and flight controls will not be able to deal with stalls and their recoveries


Few minutes after the captain of AF 447 left the flight deck at 02:00 AM, alarms start to ring in the cockpit.
Autopilot disconnects; ATHR (autothrottle) disconnects as well.
These alarms ring continuously for 34''

A/C starts to be unstable due to switching to 'ALT LAW' mode as a consequence of erroneous speed readings due to clogged pitot tubes which at first glance the crew had no way of knowing.

First issue confusing the crew in their analysis of the situation is that the 'FWC' malfunction management system, able to warn the crew on erroneous altitude readings (NAV ALT discrepancy) is unable to do so for erroneous speed indications. the ECAM screen ( Displaying malfunctions and necessary actions to treat them) will be of little help.
Only a NAV IAS DISCREPANCY feature would have helped, but it is not available on the ECAM.

This particular issue provided it is positively identified, can only be adressed via a paper checklist, and as the BEA ( Bureau Enquetes Accidents) clearly states pp. 181 and 203 in the final report: ''ECAM does not provide any indication likely to enable the crew to spot a speed indication malfunction''
The BEA unlike Airbus raises this issue p. 182 and makes a series of recommendations.

Another point which will have a bearing as events unfold is that as a consequence of pitot tube clogging There is a loss of altitude, however marginal this loss ( 350 to 350 ft ) this point is crucial in the understanding of the sequence of events as it explains the crew first move is to pitch up in order to regain the lost altitude (BEA p.179)

Airbus features provide efficient aids through FD (Flight Director) and ''Speed Trends'' (an arrow indicating a speed prediction within the next 10 seconds), very useful during the deceleration and acceleration phases of flight.

FD's are bars helping the pilot to follow his trajectory.

In the time lapse between the onset of the trouble and the stall these aids have been confusing the crew, the Speed Trend indicating an acceleration tendency (!) after the stall warning bell went on for a mere 2'' before going silent again . (BEA p.100, p.186) At the same time these tendency bars went on and off erratically further impairing the pilot s judgement (BEA p.204)

All these events have been unfolding in a degraded environment, at night, in the vicinity of a storm, in a cockpit beset by alarms and bells...with multiple malfunctions to handle and an unstable manual pilotage in ALT Law mode, compounded with erroneous indications of overspeed and tendency bars ordering a pitch up.....(BEA p.183)

As the stall warning went on briefly (BEA p.195) for a mere 2'', whereby the ATHR bell rang continously for 34'', in such a saturated an stressful environment it could easily have gone unnoticed

45'' after the alarms rang, there have been alternate actions to pitch up and down by the crew
And here we need to bring up the peculiarity of Airbuses joysticks and TRIM.

Joysticks only function is to send inputs to the computer management system which either accepts them or rejects them (Hence the motto that airbuses cannot stall)

Whenever the computer acts on a control surface, the joysticks don't move and have no pressure feel, preventing the crew from receiving any sensorial feedback. Left and right joysticks are non interconnected preventing any feedback among the crew members themselves.

And the TRIM: On the Airbus series the trim is automatic, silent, and never used by the crew in normal operations.

The trim on the elevator works as follows: The pilot commands an action which moves the rear part of the elevator ( 1/3 of the surface) the computers then manage the 'trimming action' moving the PHR or ''Fixed part'' forward section of the elevator (2/3 of the surface)

The elevator of AF 447 within 45' from the triggering of the first alarms moves from straight and level to full
pitch up when the stall warning briefly goes on. (BEA pp.64. 66)

This elevator design would also prevent recovery from the stall as a pitch down push action on the yoke to reduce angle of attack would be acting on the rear portion of the elevator only (1/3 of its surface) , the remaining computer controlled 2/3 surface remaining unaffected unless the system senses the stall which may not be in the algorithms as by nature Airbus series are stall proof.

On p.193 of the accident report it is stated that it is not necessary to exert a pitch up action to compensate for a speed decay to maintain lift.

Therefore on the Airbus 330 with its uncoordinated joysticks with moving surfaces and auto trim! The crew have little or no sensory feedback to determine a speed decay.


One would wonder how a pilot would keep pulling on the stick while the stall warning is ringing.
This brings us to the peculiarity of the stall warning on the Airbus.

As noted above the stall warning rings for a mere 2'' in a stressful environment where the ATHR bell goes off consistently may have been overlooked by the crew, considering at this moment the Speed Trend bars show a speed increase tendency (!)

THE STALL WARNING ON THE AIRBUS DOES NOT COMPLY WITH CERTIFICATION REQUIREMENTS and prevented the crew from identifying the stall.

On pp. 143 however the BEA states that it '''complies with the certification requirements in in force at the time of type certificate approval'' though it is also stated (p.47) that if CAS speed values are below 60 Kts stall warning is inoperative (!!!!!!!)

This feature is destined to prevent triggering the warning while the a/c is on the ground.
Airbus considering that whenever the speed is below that value the a/c is on the ground.
Or is it ?

In contrast Boeing considers that an a/c is sitting on the ground when the front gear leg is compressed under its weight.

With reference to the ''Certification Specifications for Large Airplanes'' CS25
CS 25.207.c reads that the stall warning bell should be ringing continuously whenever pitch angle is above stall speed.

On AF447 during the deep stall speed was below 60 kts, with the Stall Warning inop per design.
What further aggravates matters is that it goes on again whenever speed increases to that value, ringing during a nose down action to reduce angle of attack.(BEA pp. 196, 197)

The PF made a corrective pitch down action (BEA p.25) when the a/c was in a deep stall at 40 deg pitch up fully stalled. He was confronted to a Stall Warning deactivating when he was pulling and activating again when he was pushing.

So here we have a situation where the ECAM does not say what is happening, pilot s aids are malfunctioning, the trim helping the a/c stall and a Stall Warning that works the other way around, how could any crew operate in such degraded environment ?

In this cockpit built by engineers, pilots did what they could.
Airbus a/c are by no means dangerous, they are ergonomically flawed, they are as easy to operate on a daily routine basis, as they are tricky and dangerous in critical situations.

There have been a series of stall incidents and accidents, and we can only hope that the Air Asia crash will not be a Rio bis
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Old 25th Jan 2015, 13:35
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An incompetent crew didn't follow the "unreliable airspeed" procedure and flew a perfectly flyable aircraft into a stall of their own making, and which they didn't even recognise.
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Old 25th Jan 2015, 13:42
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Thanks for a well written, concise, informative post.

It should also set the cat amongst the pigeons!
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Old 25th Jan 2015, 13:47
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An incompetent crew didn't follow the "unreliable airspeed" procedure and flew a perfectly flyable aircraft into a stall of their own making, and which they didn't even recognise.
whilst that is true I don´t see the same thing happening on an airplane with 2 interconnected sticks...when the bloody column hits your stomach, you get the message that the other dude is holding it back...
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Old 25th Jan 2015, 13:49
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It is worth noting that a Delta Crew encountered the same situation as the AF crew in a 330 on a Pacific flight. It was considered somewhat a non event. The crew flew the aircraft using pitch and power until they exited the weather and icing.
As far as the above post about control columns I don't see that as the problem. The issue is not back driving or interconnecting the controls be they sides sticks or yokes. I suspect if Airbus could revisit the issue the side sticks would be back driven. They won't do it now because of liability fears if they did not retrofit every aircraft.
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Old 25th Jan 2015, 13:49
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ALL aircraft are ergonomically flawed; man is a low-level, low-speed, 1g animal. It's just the degree of flaw that varies.
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Old 25th Jan 2015, 13:54
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Are Airbus a/c ergonomically flawed ?
No.

All of the BS contained in the original post has been already posted and busted repeatedly in 12 AF447 threads but attention seekers are just not giving up and removing of "As this is anonymous forum..." doesn't help.

Entertaining option would be merging this thread with 12th AF447 thread since OP says absolutely nothing about Airbus outside AF447 scope. Useful option would be letting it die.
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Old 25th Jan 2015, 13:55
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If Pprune had a 'post of the week' thread for the best and most concise comment ........ this should be this week's winner :

ALL aircraft are ergonomically flawed; man is a low-level, low-speed, 1g animal. It's just the degree of flaw that varies.
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Old 25th Jan 2015, 14:03
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As far as the above post about control columns I don't see that as the problem. The issue is not back driving the controls be they sides sticks or yokes.
If I understand the report correctly, the guy in the L/H seat never understood that Bonin held back the stick for quite some time if not all the time. If the other 2 guys would have understood that, they most likely would have taken the correct action, I believe.... maybe I´m wrong....
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