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AF 447 Search to resume

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AF 447 Search to resume

Old 23rd Jan 2011, 08:00
  #2641 (permalink)  
 
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tobias118118 be carefull, your CRM instructor appears to have spooky tendencies. Anything he tells you is most probably false.
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Old 23rd Jan 2011, 21:55
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Originally Posted by mmciau
Does anyone know if the Searchers are proposing to introduce additional audio/visual (evolutionary) equipment to the equipment they have used previously?
I gather that the same equipment will be used to search much of the area that was not covered during Phase 3.

Last edited by auv-ee; 24th Jan 2011 at 01:25.
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Old 27th Jan 2011, 08:47
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Seemingly Very Relevant to AF447......

Federal Aviation Administration
14 CFR Part 39
[Docket No. FAA-2011-0029; Directorate Identifier 2010-NM-279-AD; Amendment 39-16583; AD 2011-02-09]
Airworthiness Directives; Airbus Model A330-200 Series Airplanes; Model A330-300 Series Airplanes; Model A340-200 Series Airplanes; and Model A340-300 Series Airplanes
When there are significant differences between all airspeed sources, the flight controls of an Airbus A330 or A340 aeroplane will revert to alternate law, the autopilot (AP) and the auto-thrust (A/THR) automatically disconnect, and the Flight Directors (FD) bars are automatically removed.
It has been identified that, after such an event, if two airspeed sources become similar while still erroneous, the flight guidance computers will:
–Display FD bars again, and
–Enable autopilot and auto-thrust re-engagement
However, in some cases, the autopilot orders may be inappropriate, such as possible abrupt pitch command.
* * * * *
The unsafe condition is the potential for abrupt pitch command which may lead to unexpected maneuvers of the airplane and cause injuries of the crew and passengers, as well as reduced controllability of the airplane, and increased pilot workload. This AD requires actions that are intended to address the unsafe condition described in the MCAI.
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Old 27th Jan 2011, 21:20
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WeeWinkyWilly;

...and I note from the docket that the FAA considered it important enough to follow the EASA AD (with minor administrative changes), making it effective only 15 days from date of notification on 12 January 2011. Effective as of now.

Docket No. FAA-2011-0029 - a PDF file.

mm43

Last edited by mm43; 27th Jan 2011 at 23:14. Reason: Fixed broken link
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Old 27th Jan 2011, 22:31
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mm43

Unfortunatly, your link returns 404 error to me.
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Old 27th Jan 2011, 22:47
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Try this link - works for me

regards,
HN39
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Old 28th Jan 2011, 21:38
  #2647 (permalink)  
 
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HazelNuts39 & mm43

Now I've got it. Thank's a lot.

Best regards
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Old 29th Jan 2011, 06:03
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Consider this from page 86 of BEA accident report on A-320, D-AXLA
The Captain controlled a left roll movement, caused by the stall. The aeroplane’s high angle of attack and the roll movements generated asymmetry, and a speed variation between ADR 1 and 2 appeared. This increasing divergence caused a rejection of the three ADRs by the FAC then the ELAC. The flight control system then passed into Direct Law. It is likely that the crew did not notice this due to the emergency situation and the aural stall warning that covered the warning of a change of flight control laws............
Now assuming AF447 somehow departed controlled flight while in Normal Law while cruising on autopilot, does anyone see anything in the ACARS sequence that would preclude this type of scenario? (other than the vaunted Airbus protections). Loss of autopilot and autothrottle would be due to the ADR failures which would be caused by the departure from controlled flight.
The RTLU position equating to M 0.8/272 KT would then make sense.

Last edited by Machinbird; 29th Jan 2011 at 15:22. Reason: stray </font> removal, better wording
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Old 29th Jan 2011, 09:33
  #2649 (permalink)  
 
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Machinbird,

This is an interesting theorie !
@Bearfoil has also suggested this as a possible cause for the ADR disagree.
Technically that seems to be possible.

Looking more empirically at it, there have been multiple incidents with pitots freezing over, causing ADR disagree and subsequent disconnection of A/P and A/T.

However I'm not aware of any instance where an Airbus (or any other airliner for that matter) flying on A/P at cruise alt has ever all of a sudden developped such an attitude that would cause this assymetry.

Besides that also the RTLU setting would mean it had to stall/spin at M0,8.

The more likely connection to AF447 is that the pilots might have re-engaged the A/P to early and the A/P held the plane in a deep stall all the way down.
The intriguing part of this is that I was seriously wondering how you would deep stall a conventional tail airliner all the way down to FL 0.
You would probably have to pull the stick (or mis- trim it full nose-up) until the splash. When you are manually flying a fully mis- trimmed airplane it would feel/react strange. You should notice a very different aileron reaction at a stall AoA.
But if Otto's at the yoke and tells you everything is fine you might not notice it.
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Old 29th Jan 2011, 10:20
  #2650 (permalink)  
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Machin - I don't think your scenario will hold because it depends on a departure from controlled flight to initiate and we have no reason to expect that in a normal cruise scenario? Surely to arrive in that situation in a/p cruise would require a sensor malfunction first?

Re the AD - at last some common-sense emerging - don't implicitly trust the software. It is not that good. Fly the aeroplane.
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Old 29th Jan 2011, 19:49
  #2651 (permalink)  
 
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Hi Henra, BOAC.
I've been looking at what might have caused a dynamic (Oscillatory) departure from autopilot controlled flight and I think I have found two potential paths for that bad result:
  • The first is multi-pitot freezing causing erroneous airspeed input to the flight control system. This has the potential to foul up the flight control computer calculations and could lead to erroneous control loop gains being applied.
  • The second is an occurrence of hydraulic supply saturation causing control rate limiting of the flight control system.
The subject can get technical quickly, but for the simple explanation of the first condition look at the results from the X-31 pitot icing caused accident. My apologies for the over stated video presentation.
Aviation Video: X-31 Crash | Patrick's Aviation
A more factual report can be found here-X-31 Crash

The subject of hydraulic supply saturation and control rate limiting is a bit technical and is related to PIO causes (but cannot be called PIO since the automatics would be in control) however suppose AF447 flew into a vigorous Cb and the control surfaces began moving briskly to keep the aircraft upright. If total hydraulic usage exceeded pumping capacity + accumulator capacity, new non-linearities would be introduced into the control surface control loops. The control loops would develop phase lags and could even drive oscillations the wrong way! The triggering event could even be as simple as autothrottles commanding a power reduction.

To help you wrap your mind around this second concept, take a look at the following two links:

Fly-By-Wire A Primer for Aviation Accident Investigators
Pilot Induced Oscillation

Besides that also the RTLU setting would mean it had to stall/spin at M0,8.
So if this type of stall entry would break the airframe, we could pretty well conclude that that the two above scenarios couldn't happen since the aircraft appears to have impacted in an essentially intact condition.

Those with flight control design experience, test flying experience and related experience are particularly invited to join in.

Last edited by Machinbird; 29th Jan 2011 at 22:14. Reason: fixing funny mis-spelling
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Old 29th Jan 2011, 20:35
  #2652 (permalink)  
 
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Cool

Hi,

The more likely connection to AF447 is that the pilots might have re-engaged the A/P to early and the A/P held the plane in a deep stall all the way down.
Read the ACARS and post again ...
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Old 30th Jan 2011, 01:13
  #2653 (permalink)  
 
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Machinbird;
The control loops would develop phase lags and could even drive oscillations the wrong way!
I respect your knowledge on this subject, but my reasoning doesn't support the theory, simply because each actuator is either in pilot input or auto mode, and positional feedback from the control surfaces should avoid the hysteresis problem alluded to.

Or are you suggesting that the "hunting" results in effective loss of control as the hydraulics "stall"?
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Old 30th Jan 2011, 01:18
  #2654 (permalink)  
 
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Which ACARS Post?

jcjeant, which ACARS post? There are 2650 posts so far.
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Old 30th Jan 2011, 03:12
  #2655 (permalink)  
 
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Smilin Ed, he said "Read the ACARS and post again ..." Note the "and" in there. You must have missed it when reading it.

{^_-} (Sigh, getting old is not for the faint of heart or the terribly vain.)
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Old 30th Jan 2011, 07:29
  #2656 (permalink)  
 
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Smilin_Ed;

You might find the link to the ACARS list in Post #1611 useful.
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Old 30th Jan 2011, 08:44
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MM43
...my reasoning doesn't support the theory, simply because each actuator is either in pilot input or auto mode, and positional feedback from the control surfaces should avoid the hysteresis problem alluded to.
MM43, I will borrow from the subject of PIO since that situation is the closest approximation to what I am trying to say. I am not a flight control engineer, so the professionals can likely find some (hopefully) small problems with my analysis.

The case of the autopilot flying the aircraft is actually not as complex as that of a human flying the aircraft. A human can adopt any one of several thousand different control strategies. The autopilot can only adopt the strategy it is programmed for. As a result, from a design standpoint, it is much easier to identify control demands for autopilot flight than for direct human controlled flight. The possiblity of exceeding the hydraulic supply capability of the aircraft was undoubtedly addressed since the engineers could go through all their worst case situations and tradeoffs and have a reasonable assurance of proper control behavior.

For small control inputs at a low rate of change, a hydraulic actuator can keep up with the changing demand and position its control surface very accurately. This is linear behavior in that control response is directly proportional to control input and achieves the commanded position with minimal delay.

As the rate (frequency) and amplitude of control inputs increase, the control actuator has increasing difficulty in accurately achieving the commanded position and it takes longer for the actuator to achieve the new position until finally, it cannot since the signal has already reversed. This happens because at some point, the actuator is moving at its maximum possible speed as determined by its hydraulic control valve opening or by control rate limits built into the control system (to achieve predictable control performance).
Electrical engineers use a tool called a Bode Plot to graph out control system response to a constant amplitude sinusoidal input of varying frequency.

Below is a Bode Plot of a Low Pass electronic filter, an item with response characteristics similar to a hydraulic cylinder.
File:Bode Low-Pass.PNG - Wikipedia, the free encyclopedia
From the graph you can see that system response amplitude declines past some critical frequency (in this instance 100 Hz, but on an aircraft control probably more like 1 Hz).

At some higher frequency, there no useable control response available-the control just sits there and buzzes.

Meanwhile the phase lag of the response grows as frequency gets higher. If you consider the case of 95 degree phase lag, the response could be considered to consist of two components, one 90 degrees out of phase and one (albeit small) 180 degrees out of phase!

Flight control engineers can design adaptive features into their control designs for known features through clever use of integrators and differentiators and by filtering out critical undesirable frequencies from the feedback path, but they need a predictable basis for hydraulic cylinder performance. That basis is that normal hydraulic pressure is available. If hydraulic pressure surges up and down as various elements of the flight control system are actuated, the predictability of all the engineer’s control studies goes down the toilet. The controls will move in unpredictable non-linear ways and induce additional time lags. That is the thesis of the second pathway to loss of control.

This thesis cannot be proved with the presently available information. We will need the flight recorders to understand what happened. My intent is to merely point out another way that AF447 could have been lost.

It should be noted that control rate limiting is inherent in all mechanical actuators and can cause PIO under proper circumstances. Almost every Fly-By-Wire aircraft has had some PIO instances during its development and operational history.

Both pilots and autopilots do their job through closing the aircraft control feedback loop. Dynamic loss of control while on autopilot could be considered as a special case under the heading of PIO. Many of the principles are the same.
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Old 30th Jan 2011, 09:43
  #2658 (permalink)  
 
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Read the ACARS and post again ...
Good point !

I checked the sequence and indeed there seems to be no slot in the ACARS list where an A/P re- engagement could have taken place.
(Btw. I am assuming an A/P re- engagement would not appear in ACARS - am I right ?)

I thought maybe after 2:11 but at 2:13 was the PRIM1 / SEC1 fault.
That would disconnect it again and at least then the disconnection of A/P would appear in ACARS.

So I agree: you can skip that idea.
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Old 30th Jan 2011, 14:07
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Machinburd:
That basis is that normal hydraulic pressure is available. If hydraulic pressure surges up and down as various elements of the flight control system are actuated, the predictability of all the engineer’s control studies goes down the toilet. The controls will move in unpredictable non-linear ways and induce additional time lags. That is the thesis of the second pathway to loss of control
Seems to me that an "inner loop" driving the control cylinder rate and/or position to the pilot/autopilot requested values would have to exist.

This would remove the influence of instantenous hydraulic preasure variations. ("surges up or down"

This is similar to using feedback around an op-amp to precisely set gain even though the raw gain of the amp may vary 10:1 or more.

If the available raw "gain" (hydraulic pressure) is too low then the inner loop will fail but that would represent either a fundamental design flaw/or hydraulic system failure.

I am not familiar with real world details (work in electronics not hydraulics) so welcome insight from those who do know.
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Old 30th Jan 2011, 15:26
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Mea Culpa

JD-EE: Smilin Ed, he said "Read the ACARS and post again ..." Note the "and" in there. You must have missed it when reading it.
Mea culpa.


MM43, thanks for the link.

Ed
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