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AF 447 Thread No. 12

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AF 447 Thread No. 12

Old 14th Sep 2014, 18:14
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Owain Glyndwr,

system gain (roll acceleration/stick movement) was reduced as the airspeed fell, but against that you have to set worsening dutch roll damping and increased roll/sideslip ratio as AOA increases. And I don't think one can exclude Bonin learning how to fly it!
The slide slip and the yaw rates were not oscillatory before 2:12:15, the spiral mode is predominant. After that yes, at large AOA, the natural Dutch roll damping is slow, it requires a lot of effort and remarkable skills, to preserve stability...like standing on a ball, while holding a large umbrella in gusty wind. As far as I know, all the crashes due to control lost (stall) ended up in a spiral dive. Bonin kept the wing leveled, 38000ft stalled descent, i.e. see between 2:11:30 to 2:12:15, he used left rudder and sideslip, to increase the lift on right side of the wing, since roll command was insufficient, at left stop

Last edited by _Phoenix_; 14th Sep 2014 at 18:22. Reason: spelling
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Old 14th Sep 2014, 19:14
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Phoenix
Bonin kept the "wing leveled, 38000ft stalled descent, i.e. see between 2:11:30 to 2:12:15, he used left rudder and sideslip, to increase the lift on right side of the wing, since roll command was insufficient, at left stop
That is a hell of a confidence in a guy, who stalled a perfectly flying aircraft without noticing.
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Old 14th Sep 2014, 23:01
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Graydon Carter, editor of Vanity Fair, describes the article:

...Vanity Fair International Correspondent William Langewiesche’s “The Human Factor,” on page 256—a gripping minute-by-minute account of the loss of Air France 447, which inexplicably plunged into the Atlantic in 2009, claiming the lives of all 228 people on board. You probably remember the event: a routine flight from Rio to Paris, no calls of distress, and then the plane just goes missing. It was a disappearance that bedeviled the aviation business. As Langewiesche discovered, the pilots had become confused and then frantic after a momentary loss of airspeed indications—this in a fully automated, state-of-the-art airplane that was otherwise flying straight and level, and experiencing no mechanical problems. What happened? The explanation has to do with the enormous advances in aircraft automation and what this has done to pilots. In this case, when the autopilot disengaged, the pilots essentially forgot how to fly.
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Old 14th Sep 2014, 23:47
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RetiredF4,
That is a hell of a confidence in a guy, who stalled a perfectly flying aircraft without noticing.
How about hell of a confidence in that aircraft... capable of 2000ft/min climb with wing already stalled, see figure 28 of final report, 2:10:50 to 2:11:10.
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Old 15th Sep 2014, 07:50
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Originally Posted by Langewiesche, Carter, Vanity Fair 10,2014
fully automated
Are we in Tech log?
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Old 15th Sep 2014, 08:39
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Phoenix

We'll have to disagree on this.

As you say, there is very little sideslip in the early stages, but plenty of roll,which must say something about the roll/sidelip ratio
The yaw damper trace shows there was oscillatory behaviour in the yaw rate at a period that roughly equates to what one might expect for the dutch roll of an aircraft of this size in cruise, which contradicts your view that the motion was nonoscillatory
the spiral mode is never oscillatory so I don't see how you can claim that it dominated the initial rolling cyclic motion.

But anyway, all I was trying to say was that dutch roll characteristics get wprse as AOA increases, and your later remarks seem to confirm that.

Last edited by Owain Glyndwr; 15th Sep 2014 at 16:52. Reason: speling
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Old 15th Sep 2014, 10:57
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Originally Posted by jcjeant
Obviously when it will be talks of Airbus at the trial .. discussions on behavior and the system of THS going to be warm while it operated in accordance with the specifications
Autotrim is a damned if you do damned if you don't scenario. If its purpose and accepted effect is to reduce pilot workload in normal flight, then the corollary is that turning it off will _increase_ pilot workload, and turning it off in an emergency when pilot workload is already increased will...

I hope any discussion will include whether or not turning autotrim _off_ was the "right" thing for the system to do in incidents such as G-THOF and D-AXLA, where the pilots needed to trim manually and did not do so. It should also include other similar incidents e.g.:
"At the moment the stall warning activated, the horizontal stabilizer trim began to increase its pitch-up action in a progressive manner until it reached the pitch-up stop."
- Name that type, and why did the trim go up ?

It would be good if the discussion included not just the specifications but the certification requirements and the behaviour of other a/c / mfrs in similar circumstances if the requirements are not tightly specified.


The Boeing engineers' comments (sorry, I lost the link) regarding FLCH "trap" are also worth raising. They knew about and discussed the exact scenario that later planted a 777 onto a sea wall, and whether or not the system should override the pilot commands. In the end, Boeing chose to go with their central philosophy - that the pilots must have the final say in controlling the plane. After decades of censure from some quarters over its implementation of hard-protections overriding the pilot, would it not be ironic if AB were to be censured in this case for a mode where (due to bad sensor inputs) its plane acted like a Boeing and slavishly followed the pilot commands all the way to the sea / sea-wall ?

The STALL alarm system and behavior will certainly be put on the grill also
I would hope we find out what the "modifications" to the SWC that were alleged earlier in this thread actually are, and how they impact the false alarm probability and the possible impact on _perceived_ false alarm probability. Is it not a reasonable conjecture that 447 crew believed the stall alarm was false, especially given the BEA note that other UAS crews ignored stall alarms believing them to be false (no one seems to have investigated _why_ they thought they were false - or at least it isn't in the report) ?

If the certification standards don't specify SW behaviour in absence of AoA data, I would want to know what other types/mfrs do - alarm or not - before attaching blame to AB. Similarly on the air data unit (not, I think, by AB, and maybe used elsewhere) that invalidated AoA data in the first place. Also relevant is why AF rejected the BUSS option - on cost ?
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Old 15th Sep 2014, 16:14
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Originally Posted by _Phoenix_
See Figure 26 of final report. In only 2 seconds, after AP disconnect, the roll angle passes from 0 to 8.4 while roll input remained neutral. Next 15 seconds, PF continuously corrected the right tendency of the aircraft, 4 times, 2 times at left limit stop.
Right, so let's look at that a bit more dispassionately.

AP disconnects at the red line, seemingly while damping a left bank, presumably from turbulence. At AP disconnect, for 0.2 seconds the pink "Roll Attitude" graph indicates a cessation of roll (stable at about 0.8 deg right), followed by a subsequent right roll, beginning at 02:10:06.4 over 0.8 seconds, peaking at 8.4deg right around 02:10:07.2. This seems consistent with a "bump" caused by turbulence - we don't know how the situation would have progressed, because it is countered with a left-bank control input.

The fact that the input graph is rendered at 180 degrees (i.e. opposite) to the attitude graph makes it a bit of a 'mare to read, but Bonin moves the stick to the half-left position to correct what is presumably a turbulence-induced roll. This seems to be an instance of overcontrolling, because the aircraft then rolls 6 degrees to the left.

The controls over the next 15 seconds or so look very much like a case of opposite PIO in roll until he begins to get a feel for what he's doing, and the roll oscillations begin to stabilise.

For this reason I'm not sold on the "incipient spiral" theory. Furthermore, Bonin's completely unnecessary pitch inputs begin simultaneously with the roll inputs at 02:10:07. No reasonable time is given to assessing what is actually happening in terms of the aircraft's status, and from that point on, every input he makes is reactionary.

Originally Posted by infrequentflyer789
After decades of censure from some quarters over its implementation of hard-protections overriding the pilot
The hard protections don't "override" the pilot, they give the pilot what he or she is asking for up to the safe limit of the airframe, and will continually monitor the aircraft's status to comply with the commands given to the best of the aircraft's ability.

Last edited by DozyWannabe; 15th Sep 2014 at 16:30.
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Old 17th Sep 2014, 08:20
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Originally Posted by AdamFrisch
There's a pretty good in depth article about the accident and the culture of pilots in France in the latest issue of Vanity Fair. It's an interesting read.
I don't think the article was available online when you wrote this, but anyway, now it is:
Should Airplanes Be Flying Themselves? | Vanity Fair
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Old 17th Sep 2014, 11:52
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The "Hard protection" are they even a paradox
Let me explain ...
When the aircraft is in "normal" law (that is to say when things are going well .. the aircraft is in an optimum state and in the specification) all "hard protection" are active (recall that they are there for the pilot to fly the plane in an optimum way .. that is to say keep it from flying out of the flight domain)
When it starts to go wrong .. often the normal law no longer applies and some "hard protection" are no longer in force
Ironically it is during the time when there are problems and when the pilot would be glad to have all help he can (and the "hard protection" are flying aid) that "hard protection" choice to partially stop .. or totally

Vanity Fair:
Nice article .. nothing new ..

Last edited by jcjeant; 17th Sep 2014 at 13:27.
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Old 17th Sep 2014, 15:13
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Noske, AdamFrisch, Winnerhofer
Vanity Fair is not such a document we may quote as a reference document, in thesis, investigation, safety agencies reports, uni-works, serious exchanges.

Are the facts told there true or not, I have no possibility of verification. But in any case I already posted sometimes on PPRuNe, for semantic reasons, that I could not agree the CVR transcription and I rejected it.

If you maintain Vanity Fair as the truth your question must be now : "Why did the BEA modify facts and words ? Who from the regulators (DGAC including BEA) organised the lies, and why ? Why do the victims' families accept these lies ? And why do the specialists of human factor agree to sign the BEA's report(s) they know misexplaining the facts in that case.

I'm sure that Vanity Fair will not answer to these questions ! They will not be able to improve air safety by telling only some points of view to sell more paper without harming their concerned advisers.
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Old 17th Sep 2014, 15:16
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@jcj:
The vast majority of Airbus FBW pilots will never (on the line, at least) encounter a control law other than Normal (and briefly Flare) for the entirety of their careers. They'll likely never encounter the hard protections on the line either, but we know of at least a few where the hard protections did aid a positive outcome.

Last edited by DozyWannabe; 17th Sep 2014 at 15:59.
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Old 17th Sep 2014, 16:10
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As a low hours private pilot I found the Vanity Fair article to be a good account of events with reasonable inferrences drawn to cover the gaps in the official reports. As a Systems Analyst, I personally believe that the side stick control systems used by Airbus were the major contributory factor to this incident.

Side stick controllers are fine in single pilot military aircraft where weight, space constraints and the ability of the pilot to maintain control inputs under high G forces are paramount. They have no place in multi-pilot commercial aircraft unless physically linked so that each pilot is aware of conflicting inputs.

Fly by wire controls have improved aircraft safety, but by allowing aircraft to be flown using a sidestick control safety has been reduced by removing the physical and visual cues given by linked conventional control columns. Had this aircraft been fitted with such controls, it would have been immediately obvious that the control column was being held back and that exaggerated flight control inputs were being made. The situation would have been recognised at an earlier stage.

Clearly it is impossible for Airbus to modify their cockpit design to incorporate dual conventional control columns, but surely a means could be devised to ensure that each pilots control sidestick mirrors the position of the other. The force feedback mechanisms of PC joystick controllers could be a good starting point.

I personally dislike side stick controls to the extent that I have made it a point to never fly on a commercial aircraft that uses them. Fly by wire is fine, just so long as the cockpit looks "normal"!

The current practice of flying aircraft at the edge of the performance envelope in order to minimise fuel use and maximise profit can only lead to more of these upsets in the future, although most will be resolved with a less tragic outcome.
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Old 17th Sep 2014, 16:18
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@G0ULI:

If you check the original thread, you'll find that there are positive and negative aspects to both sides when it comes to connected versus non-connected controls. Remember also that even in a multi-crew airliner, only one of the crew is supposed to be using the controls to fly the aircraft at any one time.

The only reason they used to be physically connected was because prior to hydraulic controls being the norm, there were occasions where extra muscle power may be needed in the event of control failure - with hydraulic controls, that ceases to be an issue. Also note that there have been several similar accidents to this one in aircraft with connected controls.
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Old 17th Sep 2014, 16:31
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DW
You are absolutely correct and there were many other issues that complicated matters in this incident particularly CRM and the apparent lack of recent manual flying experience by all the crew. While there is no need for physically coupled controls with the advent of FBW systems, it does provide an additional level of safety by giving immediate visual and physical cues in stressful situations such as flight in abnormal attitudes. No need to interpret a display, the evidence is immediately in front of you.
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Old 17th Sep 2014, 16:51
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In theory yes. In practice it seems that whether the controls are connected or not, if the PNF lacks the confidence to assertively take control and keep hold of it then you're going to be in trouble.

Also, modern connected controls don't necessarily do what one might think - the NTSB found that in the EgyptAir 990 case, with one pilot pushing forward on the column and the other pulling back, the result was actually a split-elevator condition which seriously impacted the aircraft's stability.
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Old 17th Sep 2014, 19:07
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Winnerhofer
With a conventional linked control column, each pilot would feel resistance from the control inputs of the other. So positive physical feedback. The control positions are clearly visible to each pilot and to anyone else on the flightdeck. So positive visual feedback.

No need to check the instruments or press override buttons.

If it is a fight for the controls, then the stronger person will win, but that is more of a CRM issue.

It would seem obvious that the aircraft should respond to only a single control input and not try to interpret conflicting inputs from opposite sides of the cockpit by summing or subtracting the different control positions. The easiest way of accomplishing this is to have physically coupled controls in front of each pilot.
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Old 18th Sep 2014, 00:06
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Originally Posted by jcjeant
Vanity Fair:
Nice article .. nothing new ..
I don't know, this bit:
it was considered to be serious enough that Air France had decided to replace the probes with ones of an improved design and had sent out an advisory to warn pilots of the problem.
is a new one on me - I always thought it was Air BUS that advised that the probes should be replaced, and Air France that refused and then pushed back at the AB recommendation. See e.g. safetymatters: Shifting the Burden
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Old 18th Sep 2014, 01:36
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Owain,
...there is very little sideslip in the early stages, but plenty of roll,which must say something about the roll/sidelip ratio. The yaw damper trace shows there was oscillatory behaviour in the yaw rate
Are we looking to a Dutch roll mode or to an adverse yaw damping?
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Old 18th Sep 2014, 02:46
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DozyWannabe,
This seems to be an instance of overcontrolling, because the aircraft then rolls 6 degrees to the left.
...The controls over the next 15 seconds or so look very much like a case of opposite PIO in roll...
I just agree with your presumption of overcontrolling, that's normal startle affect at AP disconnection. Watch again the magenta graph after 2:10:10, the next 15 seconds. There is not a roll oscillation, equal quantity left-right. The roll attitude is always much larger on right side, therefore Bonin constantly corrected the right roll tendency. I think he was quite busy... keeping the aircraft leveled was only one of the tasks he had to do in the same time

...Bonin's completely unnecessary pitch inputs...
He trusted some instruments that look valid. He followed the FD bars.
see final report Figure 69: Evolution of FD crossbars
Why FD didn't step out, as AP disconnected, by design?
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