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AF 447 Thread No. 5

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AF 447 Thread No. 5

Old 26th Jul 2011, 16:15
  #721 (permalink)  
bearfoil
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Having a bit of a problem. As the a/c slows in a climb, AoA increases.

When the Nose dropped (it had to, yes?), the AoA immediately decreases (to +6). At this drop is when I propose the PF 'felt' the STALL, added TOGA, and 'relaxed' Back Pressure. He may not have relaxed it enough, and the a/c may have input NU instead of "minimizing altitude loss" (a very wrong thing to do, at this point!) PITCH is for drama? AoA is directly related to STALL, and what the PF "feels". In the dark, Pitch is "irrelevant"?
 
Old 26th Jul 2011, 16:33
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What is the effect of increasing thrust with underslung engines on the uncorrected pitch of an aircraft ?
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Old 26th Jul 2011, 17:05
  #723 (permalink)  
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Originally Posted by bear
When the Nose dropped (it had to, yes?), the AoA immediately decreases (to +6).
- where do you see this and why would the nose 'have to drop'?
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Old 26th Jul 2011, 17:14
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Originally Posted by Mr Optimistic
... the word 'stall' was not uttered, as the Chief Engineer stated some time back. I hope that the 60kt inhibit of the stall warning gets the attention it deserves.
I would go further than hope.... The Chief Engineer should be really worried and should drive the addressing of the problem that the word STALL was not BIG on the screen for everyone in the cockpit to see it clearly, after Stalling and for most of the STALL duration.
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Old 26th Jul 2011, 17:15
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Originally Posted by bearfoil
Having a bit of a problem. As the a/c slows in a climb, AoA increases.
When the Nose dropped (it had to, yes?), the AoA immediately decreases (to +6). At this drop is when I propose the PF 'felt' the STALL, added TOGA, and 'relaxed' Back Pressure. He may not have relaxed it enough, and the a/c may have input NU instead of "minimizing altitude loss" (a very wrong thing to do, at this point!) PITCH is for drama? AoA is directly related to STALL, and what the PF "feels". In the dark, Pitch is "irrelevant"?
Your literary efforts resemble those of 4th century 'philosophers'.
They're amusing.... which is why you're not on my "ignoramus" list yet, but they pollute the current discussions.
Some intelligent input from you would have been useful..... as I don't think you're just stupid.
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Old 26th Jul 2011, 17:39
  #726 (permalink)  
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BOAC

"had to drop". IF the PF had been trying to arrest the climb, (BEA have not located the NOSE Down inputs, time wise), He would have arrived at a ND a) eventually, b), in spite of elevator authority (due loss of energy), or c) STALL.

I think it extremely unlikely the a/c remained Nose up through the ordeal of the climb without it dropping @ "AoA decreased to 6, and STALL WARN." Yes?

ChristiaanJ sorry, I'll leave. Had a stroke last year. Memory and other things are a bit different.
 
Old 26th Jul 2011, 18:05
  #727 (permalink)  
 
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mach and stall

Thx, PJ, good review.

Gotta admit, that after a few years in straight wing planes I was then spoiled by sturdy subsonic wings and true supersonic wings. My straight wing jets "stalled" as you and the experts state. The shock waves over the wings caused by just a slight increase of AoA did more than cause the wing to lose lift, like the phenomena I described - aileron reversal, elevator reversal, etc. Scary, but I learned not to press the 'envelope" after some hairy moments.

THE RELEVANT POINT is if the crew tried to stop the "overspeed" warning by pulling up to slow down, then they could have entered a stall that they had not anticipated, way before speed was a factor. My training was to reduce power first, as pulling up got one closer to a stall and/or had other bad effects.

MY SECOND RELEVANT POINT ( IMHO) is that one can zoom at a comfortable AoA and gee and then run outta air molecules over the wing very quickly. By the time your AoA "protection" triggers, you are too slow for effective nose down control authority depending upon pitch moments determined by c.g. and basic aero characteristics of the jet. Back in 1979 we couldn't believe it would happen to our little rascal, but it did! It's why I joined the fray here when I saw more and more details of the crash. I simply wanted to point out that there are situations that the engineers never anticipated, whether "clever" maneuvers by the humans, or assumptions that the engineers made in the basic control laws.

Give me a 'bus and I believe I can duplicate the scenario easily. It is EXACTLY the scenario we discovered. Sure, pitch attitudes much lower, AoA much lower, but the identical scenario.

I pray that the users look at training, and remember the incidents of pitot-static failures.

And to beat a dead horse, I question the lack of design to allow for loss of the air data and still have a flyable jet. It ain't rocket science. Use last reliable data or use generic values depending upon gear up or gear down, etc. The Shuttle doesn't/didn't use air data until way slow. It was body rates, gee and such. The FBW systems use air data for "gains" - to command both rates and degree of control surface movement. Makes the jet "feel" like the old days, and it's a good thing. I would be the last pilot to recommend "direct" commands except as an absolute last-ditch maneuver. It's too easy to limit control surface movement according to "q". And body rates are easy to take into account without any air data at all. We only lost one jet I know of when the radome and pitot-static probes and AoA cones were ripped off. The guy flew for 10 minutes or more IFR (due to pelican blood over the canopy, heh heh). The body rates and 'standby gains" of the FLCS kept him flying on instruments until he gradually got into a PIO and had to bail.
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Old 26th Jul 2011, 19:01
  #728 (permalink)  
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My training was to reduce power first, as pulling up got one closer to a stall and/or had other bad effects.
Ours too - in mountain wave for example, the power comes off...sometimes to idle thrust, then picks up again as speed reduces and stabilizes. A manual pitch up in response to an "overspeed" is a very long way from a normal, trained-and-checked SOP response, (I don't believe it occurred here). The "Overspeed Law" will pitch the aircraft up at VMO + 6kts or at MMO + M0.010 to M0.015 depending upon initial pitch attitude. The response, IIRC, is about 0.05gee - very gentle, and the power reduces; (the Autopilot does disconnect in this circumstance).
By the time your AoA "protection" triggers, you are too slow for effective nose down control authority depending upon pitch moments determined by c.g. and basic aero characteristics of the jet.
IIRC, there have been comments that elevator effectiveness was not lost entirely, nor was it entirely lost throughout the descent. The discussion on whether elevator alone, without rolling the THS towards the ND position, and including the elevator's reduced effectiveness at its full-down position, could have recovered the aircraft has been had; we'll see on Friday whether this discussion was also had by the BEA.
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Old 26th Jul 2011, 19:13
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Originally Posted by Linktrained
What is the effect of increasing thrust with underslung engines on the uncorrected pitch of an aircraft ?
Pitch up of course, but what was the thrust setting before increase - 90+%? So the converse, throttling back the engines to flight-idle, which was tried later should have a more dramatic effect in what should have been the right direction.
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Old 26th Jul 2011, 19:22
  #730 (permalink)  
 
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@gums, PJ2

As far as I was aware there was no overspeed warning though - have I missed something?
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Old 26th Jul 2011, 19:43
  #731 (permalink)  
 
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@ Takata:

You are semantically right concerning internal/external faults but what matter is that those erroneous informations won't be used by the flight systems but still be displayed to the crew for information and troubleshooting. All relevant systems based on erroneous outputs would be declared inop during the fault isolation sequence (AP/FD, A/THR, PROT, RTLU, WINDSHEAR, SPD LIM, TCAS)

Concerning FMGCs and FCPCs monitoring, the effect is a rejection of the faulty sources (channel); in our case, all 3 ADRs are declared faulty by them and rejected. There is no cockpit circuit breaker at probe-pitot level and what could be displayed is a fault on ADR pannel; one may want to turn it off.
ADR Single and ADR Dual faults are detected by ADR itself.
A Triple ADR Failure message (ADR 1+2+3 FAULT) only exist if the BUSS option is installed, this is a level 3 (Red & Master Warning).
This Triple ADR monitoring was NOT installed on the A/C.
A local ADR fault without ECAM message doesn't seem logical to ECAM protocol.

I really can't find a statement that FCPCs would reject more than 1 ADR source.
If 1 ADR is rejected the FCPC(PRIM) will use the average of the other 2.
NAV ADR DISAGREE is triggered if these 2 values deviates a certain value.
or if all 3 sources deviates from each other.
This message and following ECAM crew action demands the crew to monitor and switch of the ADR in error, to prevent it being used by FCPC.

~o~

FCOM:

MAIN REASONS FOR ERRONEOUS AIRSPEED-ALTITUDE DATA

The most probable reason for erroneous airspeed and altitude information
is obstructed pitot tubes or static sources.
Depending on the level of obstruction, the symptoms visible to the flight
crew will be different.
However, in all cases, the data provided by the obstructed probe will be
false.
Since it is highly unlikely that the aircraft probes be obstructed at the
same time, by the same amount, and in the same way, the first indication
of erroneous airspeed-altitude data available to flight crews, will most
probably be a discrepancy between the various sources.



CONSEQUENCES OF OBSTRUCTED PITOT TUBES OR STATIC PORTS

All aircraft systems using anemometric data have built-in fault
accommodation logics.
The fault accommodation logics are not the same for the various systems;
but all rely on voting principles whereby when one source diverges from
the average value, it is automatically rejected and the system
continues to operate normally with the remaining two sources.
This principle applies to flight controls and flight guidance systems.

Normal situation
Each PRIM receives the speed information from all ADIRUs.
It compares the 3 values.
Pressure altitude information is not used by the PRIM.
Each FE (Flight Envelope computer) receives the speed and pressure
information from all ADIRUs.
For each of these two parameters, it compares the 3 values.

If one ADR output is erroneous and the two remaining ADRs are correct
The PRIM and the FE eliminate it without any cockpit effect (no caution;
normal operation is continued), except that one display is wrong and CAT III dual
can no longer be available on the FMA.

If two ADR outputs are erroneous, but different, and the remaining ADR is correct,
or if all three are erroneous, but different
The autopilot and the autothrust are disconnected by the FE (whichever
autopilot is engaged).
If the disagree lasts for more than 10 seconds, the PRIM triggers the ADR
DISAGREE ECAM caution.
It reverts to Alternate 2 law (without high and low speed protection).
On both PFD, “SPD LIM” flag is shown, no VLS and no VSW is displayed.
This situation is latched, until a PRIM reset is performed on the ground
without any hydraulic pressure.
However, if the anomaly was only transient, the autopilot and the
autothrust can be re-engaged when the disagree has disappeared.

If one ADR is correct but the other two ADRs provide the same erroneous output or
if all three ADRs provide consistent and erroneous data
The systems will reject the “good” ADR and will continue to operate using
the two “bad” ADRs.
This situation can be met when, for example, two or all three pitot tubes
are obstructed at the same time, by the same amount, and in the same way.
(Flight through cloud of volcanic ash, takeoff with two pitots obstructed by
foreign matter (mud, insects)).

Human beings (the pilot) tend to use the same type of “fault accommodation”
principles to detect an erroneous IAS/altitude indication.
Flight crews will tend to reject the outlier information, if the other
two outputs are consistent.
This choice is, in the great majority of cases, correct; but, all flight crews
should be aware of very extreme and unlikely situations where two (or
even three) speed/altitude indications can be consistent and wrong.

- BEWARE OF INSTINCTIVELY REJECTING AN OUTLIER ADR

The following chart provides a non-exhaustive list of the consequences of various cases of partially or
totally obstructed pitot tubes and static ports on airspeed and altitude indications. It should be noted that
the cases described below cover extreme situations (e.g. totally obsctructed or unobstructed drain holes)
and that there could be multiple intermediate configurations with similar, but not identical, consequences.


Based on the information given in the preceding chart, it is clear that no single rule can be given to
conclusively identify all possible cases of erroneous airspeed/altitude indications. However, any case of
erroneous speed/altitude indications will always be associated to one (or more) of the following cues :
a) Fluctuations of airspeed indications;.
b) Abnormal correlation of the basic flight parameters (IAS, pitch, attitude, thrust, climb rate) :
IAS increasing with large nose-up pitch attitude;
IAS decreasing with large nose down pitch attitude;
IAS decreasing with nose down pitch attitude and aircraft descending;

c) Abnormal AP/FD/ATHR behavior;
d) Undue stall warning or overspeed warnings;
e) Reduction of aerodynamic noise with increasing IAS;
f) Increase of aerodynamic noise with decreasing IAS.

RECOMMENDED PROCEDURES

The procedures described below are intended to provide flight crews with
general guidelines to be applied in case of suspected erroneous
airspeed/altitude indications.

FOLLOW ECAM ACTIONS
if failure undetected:
CROSSCHECK ALL IAS/ALTITUDE SOURCES:
ADR1,ADR2,ADR3 AND STANDBY INSTRUMENTS

If it is obvious that the outlier is wrong, select the corresponding ADR OFF
and reconfigure the PFD indications accordingly, by applying the ECAM drill

which will be automatically displayed.

Flight crews should, however, be aware that in very extreme

circumstances, it may happen that two, or even all three ADRs may

provide identical and erroneous data.
Therefore, the suspect ADR should only be switched OFF, if it is positively
confirmed that the two other ADR's are correct. If in doubt :




DISCONNECT AP,FD AND ATHR
FLY TARGET PITCH ATTITUDE AND THRUST SETTING

The initial pitch attitude and thrust values given in the QRH should be
considered as "Memory Items", since they allow "safe flight conditions" to
be rapidly established in all flight phases (takeoff, climb, cruise) and
aircraft configurations (Weight and slat/flaps).

Once the target pitch attitude and thrust values have been stabilized,

the expanded data of the QRH (Flight with Unreliable Speed Indication)
should be followed to determine the precise pitch attitude and power
setting required, as a function of the aircraft's weight, configuration and
desired speed.
After applying the QRH procedure, and when the aircraft is stable, the
flight crew should try to identify the faulty ADR (one or more).
Once the discrepant ADR has (or have) been positively identified, it
(they) should be switched OFF.
This will trigger the corresponding ECAM warnings and the associated
drills which should be followed to address all the consequences on the
various aircraft systems.

Last edited by A33Zab; 26th Jul 2011 at 20:05.
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Old 26th Jul 2011, 19:53
  #732 (permalink)  
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As far as I was aware there was no overspeed warning though - have I missed something?
The notion that the PF was responding to an "overspeed indication" on his PFD has been put forward a number of times as a reason for the pitch-up. There is no basis for the theory in any information currently available. The FWC does not send either the Stall or an Overspeed event to the ACARS, (which is, after all, a maintenance tool, not an investigative installation) and the BEA Update does not provide any information to support the notion.
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Old 26th Jul 2011, 20:43
  #733 (permalink)  
 
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Overspeed warning

Yeah, Doze, and PJ has corrected me.

Consider me "corrected", heh heh.

Looking for a plausible reason to command a pitch change, especially after the pilot stated he had assumed manual control and another comment about "alternate" laws.
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Old 26th Jul 2011, 21:36
  #734 (permalink)  
 
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Hopefully Friday we will know if an overspeed warning caused the pf to pitch up. I will be glad when that dispute is over. It is the simplest explanation of why they zoomed 3,000 ft. I flew sailplanes and towed them before the airline career began but like Sully, don't think it would have helped in either situation with a jet airliner. Good basic flying experience though. Something not encouraged much any more with automation unfortunately.
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Old 26th Jul 2011, 21:46
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Lame War Stories - No they're not

Gums
I'm sure I'm not alone in enjoying your insights into computer controlled flight based on your experiences in jet fighter aircraft and your associated 'war' stories. Please keep them coming. As to the possibility of flying the 330 into a similar engineer-unexpected, computer-defying, state as you suggest may have occurred, I hope that the designers of future passenger jets flight computers, systems, et-cetera, are informed in their approach to their task by the demise of AF447.
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Old 26th Jul 2011, 22:05
  #736 (permalink)  
 
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Dutch M, regarding the energy calculations you're asking questions that don't quite make sense in physicist terms.

If one wants to get VERY technical said airplane has an incredible amount of energy, and angular momentum, if calculated referenced to some as yet undetermined center of mass of the total observable universe. It's tiny speed changes won't change that energy more than a wiggle many decimal places out. It's not worth it to try to make that calculation simply because we don't observe it from that point in space time. We measure it typically from a reference on the Earth's surface. And the numbers in that sense work.

You mention stopping the Earth's rotation. For an Earth based observation that vastly changes the energy of the plane. Observing from the plane it makes no change, the energy is zero because the plane is not moving relative to the plane and is at zero height relative to the plane. Of course, this skirts the issue of what happens to the air mass over the Earth on the scale of the plane? I'm not sure it's fruitful to discuss this. By the time you appropriately account for the shifted frames of reference it all washes out anyway. It must or the physics that make your plane fly wouldn't work.

Referenced to a point on the Earth, conveniently directly below the plane, you can calculate an instantaneous "energy" value, mgh plus mV^2. (Note that squaring a velocity vector erases the direction part and leaves speed squared.) At 35000' and a given velocity it has one energy. At 37500' and a new velocity it has a new energy state. We also must consider the thust of the engines over time and the drag over time as two distinct energy inputs, initially balanced and more or less balanced once the plane is at the new altitude. What additional drag happens to subtract energy from the aircraft? We don't particularly know. And I certainly don't know how "more or less" my blithe "more or less balanced" is. Would it be accelerating once it levels out or not?

I certainly don't have the knowledge of the aircraft to handle these extra energy terms. BEA should. What I can do is work out the values and wave hands. It appears the altitude gain and speed loss were "sane", meaning there was no mother of all wind influences on the plane. It does not rule out a 100 mph level or smaller influence, at a semi scientific wild assed guess level of accuracy. The BEA phrasing doesn't tell us whether PF felt a downward movement and tried to counter it or not. It does imply a possibly unusual lag between a very serious climb and application of climb command on the stick.

What more can we say?

Edit: We can say that the ascent rate was about 84 miles per hour. So my 100 MPH wind level swag might be excessive.

Last edited by JD-EE; 26th Jul 2011 at 22:42.
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Old 26th Jul 2011, 22:11
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Gums
I'm sure I'm not alone in enjoying your insights into computer controlled flight based on your experiences in jet fighter aircraft and your associated 'war' stories. Please keep them coming.
You are most certainly not alone.
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Old 26th Jul 2011, 22:16
  #738 (permalink)  
 
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jcjeant, it's simplest to rule out more than one "failure" per incident. Look for a failure (in reliability speak terms) that could cause the given effect. There might be say 20 such failures. If nothing fits start looking for combinations of failures that could lead to the overall results. That's maybe 400 combinations to consider. Three such failures is 8000 combinations of those 20 items, alone, to consider. And once you consider one of the 20 plus something else you may discover something else might be 5 or it might be 50 different failures could lead to the results.

Considering "partly failed" becomes a nightmare.

And I suspect BEA is being just a whole lot more methodical working through the various trees and figuring out what external influences the plane experienced than we are.

So I figure it's not time to worry yet about conspiracies or pinning blame where it does not belong. And I note you're not working to go beyond "failure" to find "cause" which is required before you can assign "blame." You're before the cart the horse putting. (or something.) It does SEEM like you're very anxious to get to the blame part. OUR analysis here isn't anywhere near that, yet.

For people who lost a loved one it's quite natural to want to find a target at which they can lash out and vent their fury. Premature action leading to selection of the wrong victim for that fury can be most tragic. (This is often how feuds and wars begin.)
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Old 26th Jul 2011, 22:47
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Originally Posted by gums
Give me a 'bus and I believe I can duplicate the scenario easily.
Not so sure you could.

I think the scenario you describe explains why the “bus” FBW computers calculate a “phase advanced AoA” as mentioned in the A340 zoom climb incident report. Remember the bus has a pitch attitude protection (IIRC: 30 deg NU progressively reduced to 25 deg at low speed). With that limitation and factoring pitch inertial moment and tail effectiveness it must be relatively easy to anticipate and prevent the AoA to become greater than alpha prot. Of course, this works only in normal law.
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Old 26th Jul 2011, 22:48
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Originally Posted by HazelNuts39
As a student I piloted gliders. These routinely fly tight circles to stay in thermals. In a strong wind there is no airspeed loss turning downwind, nor gain turning upwind. I've never quite figured that out, may be as a physicist you can.
The same way a tacking sailboat can go faster than the wind. If you hit the wind at the right angle with an airfoil you can acquire energy from the wind. Of course, the sailboat has its keel to make this effect more pronounced. That allows its crumby airfoil called a sail to work. The sailplane has only its mass to provide the effect.
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