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AF 447 Thread No. 5

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AF 447 Thread No. 5

Old 19th Jul 2011, 21:49
  #501 (permalink)  
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I would like to add to the discussion on "Why the pitch up?" by quoting from and otherwise referencing from two sources - HtBJ by Davies, (1rst & 3rd ed, discussion on the "super-stall"), and from one of the papers to which I provided links a while back on this thread, entitled, "The Effect of High Altitude and Center of Gravity on the Handling Characteristics of Swept-wing Commercial Airplanes" published in the "Flight Operations" [FO] section in the April, 1998 issue of Boeing's AERO magazine.

The goal here is "continuing education" if you will, because the serious discussion on stalling heavy transport aircraft has become necessary as have some points regarding high altitude, high Mach number flight. There are some worthwhile papers which discuss this in great detail but are really tough sledding and the mathematics will be beyond most! I can't lead the discussion as HN39, gums and others might but I do wish to rejuvenate the discussion on the two items which continue to interest us all: Why the initial pitch up?, and the behaviour of a heavy transport aircraft in approach to, and in, the stall. Specifically, I am wondering if the low-damping forces of high altitude flight had anything to do with the eventual loss of control. - PJ2

First, from the Boeing document:

"Maneuvering Stability
"Maneuvering stability, like static stability, is influenced by CG location. However, when the CG is aft and near the neutral point, then altitude also has a significant effect. Since air density has a notable impact on the damping moment of the horizontal tail, higher pitch rates will result for the same elevator deflections as altitude increases. From the flight crew's perspective, as altitude increases, a pull force will result in a larger change in pitch angle, which translates into an increasing angle of attack and g. While a well-designed flight control system, either mechanical or electronic, will reduce the variation of stick force with CG and altitude, it is very difficult to completely eliminate the variation due to design limitations.

"For example, for the same control surface movement at constant airspeed, an airplane at 35,000 ft (10,670 m) experiences a higher pitch rate than an airplane at 5,000 ft (1,524 m) because there is less aerodynamic damping. The pitch rate is higher, but the resulting change in flight path is not. Therefore, the change in angle of attack is greater, creating more lift and more g. If the control system is designed to provide a fixed ratio of control column force to elevator deflection, it will take less column force to generate the same g as altitude increases.

"This principle is the essence of high-altitude handling characteristics for RSS airplanes. Unless an RSS airplane has an augmentation system to compensate its maneuvering stability, lighter column forces are required for maneuvering at altitude. Longitudinal maneuvering requires a pitch rate, and the atmosphere provides pitch rate damping. As air density decreases, the pitch rate damping decreases, resulting in decreased maneuvering stability (see figure 2 and "Maneuvering Stability" below).

"An additional effect is that for a given attitude change, the change in rate of climb is proportional to the true airspeed. Thus, for an attitude change for 500 ft per minute (fpm) at 290 knots indicated air speed (kias) at sea level, the same change in attitude at 290 kias (490 knots true air speed) at 35,000 ft would be almost 900 fpm. This characteristic is essentially true for small attitude changes, such as the kind used to hold altitude. It is also why smooth and small control inputs are required at high altitude, particularly when disconnecting the autopilot.

Summary
"The use of wing sweep and stability augmentation on modern commercial airplanes makes them more fuel efficient. However, flight crews must understand the effects of CG and altitude on performance and handling qualities. For example, operating at an aft CG improves cruise performance, but moving the CG aft reduces static longitudinal and maneuvering stability. Many modern commercial airplanes employ some form of stability augmentation to compensate for relaxed stability.

"However, as long as the CG is in the allowable range, the handling qualities will be adequate with or without augmentation. An understanding of static and maneuvering longitudinal stability is an essential element of flight crew training." (my bold/underlining).

. . . .

"Static Longitudinal Stability and Speed Stability
"STATIC LONGITUDINAL STABILITY
"The term "static longitudinal stability" is the name of the stability coefficient (Cm-alpha) for the pitching moment due to a change in angle of attack. In a stable, conventional airplane, the CG is forward of the neutral point of the airplane (wing plus tail). An increase in angle of attack from trim increases the amount of lift generated by the wing and results in an increasing pitch-down moment. This drives the airplane back toward its original angle of attack. If the CG is aft of the neutral point, increasing the angle of attack causes the airplane to pitch up, away from its original trimmed condition."



Next, from Davies; (NOTE 1: We have seen some of these illustrations from Davies before, posted by others during discussions on AoA, the stall and so on. I am providing pages 121 through 128. By referencing the so-called "deep stall", I am not implying that we have such here in AF 447 - I don't have the background to determine that. What I wish to provide is Davies' discussion on the broader elements of the deep stall, a discussion which I think is relevant to the behaviour of a heavy transport, regardless of kind of AFS installed. NOTE 2: "Cm" is referenced in the last paragraph of the Boeing document, above.

*Handling the Big Jets, D.P.Davies. 3rd ed. 1971. Civil Aviation Authority, London (OP)











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Old 19th Jul 2011, 22:05
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Hi,
Originally Posted by mm43
Numerous issues are raised, including simulator training and management, but nowhere in the article does the word "securitie" in the safety sense get a mention.
The word is not there, but the idea is: the "audit" about security has NOT been published (contrary to promises, imho). There were some comments (in French, and I am not sufficiently fluent in English to translate them):
- SÉCURITÉ - Ce rapport qui pointe les failles d'Air France - Le Point
- L'audit sur la scurit des vols d'Air France serait "trs critique" - LExpansion.com
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Old 19th Jul 2011, 22:36
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PJ2,
Thanks.
Can't find my own copy of Davies's HtBJ to cross-check. Is he discussing T-tails in that section you posted?
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Old 19th Jul 2011, 22:42
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jcjeant, mm43,
The 'La Dépèche' article seems to have been written by somebody who's either followed the PPRuNe threads on the subject..... or similar ones on French forums.
Nothing new there, and just the same speculations we've seen before.
I wouldn't pay too much attention to it.
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Old 19th Jul 2011, 22:48
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PJ2

Excellent post and reference , PJ2.

It´s basically what i learned in the aerodynamics-course in UPT 35 years ago, also in view of flying fighter jets. I didn´t learn anything in my ATPL course concerning this matter, but maybe i was not attentive enough.

I had some argument via PM with some posters here about how much the nose would have to be lowered to break the stall at an AOA of 60° and posted some concern to the effectiveness of the THS. This reference states that it is not enough to lower the nose somewhat, and that it would lead to high sink rates and a difficult pullout if altitude would be sufficient. Posters calculated altitude loss to some 5.000 feet. Please, go back and read this reference from PJ2 and reconsider your calculation.

I get the impression that this new generation of engineers and pilots are forgetting, that computers dont change aerodynamic fundamentals, they only can help in manage the equipment being used in a different way. But its still used in the same old environment from years ago, when your nice reference was written down, and even before, when the fathers of flying conquered the skies.

Aerodynamics is unchanged, any aircraft can stall, can be stalled, and any aircraft can end in an unrecoverable mode of stall or spin. Know before, when it could happen and under what conditions it can happen, and dont get near such an situation. If you fail and find yourself approaching a stall, try to get out of it as fast and efficient as possible. There might be only this one chance.
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Old 19th Jul 2011, 23:02
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Originally Posted by mm43
Protect the integrity of Airbus Industries and make Air France and its employees the "sacrificial lamb" is my read on this Irène Perrin article.
Why would you say that? Airbus acknowledged the pitot issue and released a service bulletin to the airlines months before the accident and simultaneously issued "workaround" instructions to pilots at the same time, while the issue was being fixed. What more could they reasonably have done?

I really don't get this "BEA protecting/covering up for Airbus/AF" meme that seems to have seeped in around the edges. The fact is (as I've said before) that not only has the BEA become a very different organisation than it was in the late '80s, but also that Airbus has been very quick to put its hand up to mistakes and fix them. Neither major manufacturer acquitted themselves particuarly well in the '80s and '90s - it took definitive proof that the 737 rudder hardovers had a technical cause before Boeing stopped pushing the angle that UA535 and USAir427 could have been pilot error, and it took the loss of Nick Warner to make Airbus look long and hard at their interface design. Both companies have since behaved considerably better in that regard.

We could be on the verge of a serious self-examination on the part of the airline industry as regards training, particularly with regard to making sure that pilots be properly-versed in the aspects of the aircraft they are expected to fly - which I gather from many posts on here people think is something long overdue, and yet it would appear that some people still think that finding the design of the aircraft (over and above the pitot issue) at fault in the face of the evidence would be a better outcome?

@CJ - yes, I'm pretty sure that sectoin of the book refers to deep stall in T-tail designs, but the thrust of the description is still very useful.
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Old 19th Jul 2011, 23:05
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Hello, CJ;

Yes, that's the section he's discussing: - T-tails and the "super-stall" and the section I have quoted comes right after that specific discussion, (pgs 115 - 120).

I understood his discussion on "the complete picture" at the beginning of page 121 to be relevant to all heavy transport types and not specific to the T-tail configuration, which is why I considered it pointing to what constitutes a "good education for pilots new to high altitude, high Mach number flight", if nothing else.

I'm no longer confident:
a) that this knowledge is taught either on the way to the ATPL or to new-hire pilots on the assumption that they already know it all and,
b) that the "assumption" is correct.

Edit: Thanks Retired F4 - confirms my sense of what's being taught and what is being assumed.

I get the strong sense that Davies' approach to matters of aviation and in particular heavy transport flight, is unfamiliar today, not because today's pilots are less keen and less interested in their profession but because bean-counting airline executives believe that "all today's airplanes are automatic" and so teaching things aeronautical as opposed to teaching the minimum "NTK", (need-to-know) to operate the airplane and pass the FAA/TC/JAR (haven't included Australia here, yet), rides has contributed to the present state of affairs and pilots, as a group, have let management get away with this brand of thinking and haven't taken the bull by the horns because many don't know what they don't know about flying airplanes. Just because one can manipulate the controls and land the thing doesn't mean one "knows stuff". The First Officer of the Colgan Q400 is sadly, tragically, one example. The poor soul was never taught about the business she was in or bread-and-butter aerodynamics, just the NTK about how to work the airplane.

Last edited by PJ2; 19th Jul 2011 at 23:15.
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Old 19th Jul 2011, 23:05
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Cool

Hi,

mid-'90s and the 777 has suffered only a single hull-loss, as has the A340 in service).
To remind that the loss of BA 777 (in England) was (as far know) due to a technical problem .. and the loss of AF A340 (in Canada) was (as far know) a pilot error.
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Old 19th Jul 2011, 23:07
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Cool

Hi,

jcjeant, mm43,
The 'La Dépèche' article seems to have been written by somebody who's either followed the PPRuNe threads on the subject..... or similar ones on French forums.
Nothing new there, and just the same speculations we've seen before.
I wouldn't pay too much attention to it.
Can we qualify the famous "audit report" as speculations ?SÉCURITÉ - Ce rapport qui pointe les failles d'Air France - Le Point

As far we know .. BEA not issued a grounding of all the A330 after all those days of investigations
We can conclude that at today the A330 is not technically implicated in any ways in the loss of the AF447
So with this knowledge .. we can conclude that the BEA final report will show that the actions of the pilots where not adequate for the situation the plane was.
It's maybe not name the responsibles .. but it's indicate a way to investigate more ....
Wonder what will be the final recommandations (to Airbus-AF and responsible bodies .. DGAC .. etc ...)

Last edited by jcjeant; 19th Jul 2011 at 23:27. Reason: Some additions
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Old 19th Jul 2011, 23:12
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If ice is the trigger for a fault in the function of static ports, what sort of evidence does that leave behind?

I can't recall any event where static icing was implicated and would be surprised to see icing problems at the static location if that be on the aircraft keel surface. As to evidence, like most icing, the evidence of a single event melts with the ice ...

The 737 proved that it wasn't as longitudinally stable as the 727 (unlike the 727 you couldn't rescue a fast approach by throwing the gear out early)

Saved more than a few hot/high approaches (don't we all love ATC track shortening ?) by reconfiguring early - if necessary back to final approach configuration well out to improve the descent angle to something better than a mile a thousand. Or have I missed your point here ?
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Old 19th Jul 2011, 23:23
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Originally Posted by jcjeant
To remind that the loss of BA 777 (in England) was (as far know) due to a technical problem .. and the loss of AF A340 (in Canada) was (as far know) a pilot error.
I wasn't going to make that distinction, as it's not important to the point I was making (early-life crashes that highlight airframe "quirks" are often written up as pilot error), but if you want to make that call, then go ahead.

Originally Posted by john_tullamarine
Saved more than a few hot/high approaches (don't we all love ATC track shortening ?) by reconfiguring early - if necessary back to final approach configuration well out to improve the descent angle to something better than a mile a thousand. Or have I missed your point here ?
I was referring to a document I read on 737 handling back when I had time to muck around with sims (the document referred to the real 737 however) - from memory it said something about some pilots trying some of the tricks they'd used on the 727 getting a nasty (albeit recoverable) surprise when they tried them on its little sister. I'm pretty sure "throwing out everything except an anchor" was one of those tricks...
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Old 19th Jul 2011, 23:26
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Originally Posted by jcjeant
Hi,
Can we qualify the famous "audit report" as speculations ?SÉCURITÉ - Ce rapport qui pointe les failles d'Air France - Le Point
Since your link dates from January 2011 and since we have seen no serious trace of that 'audit report' since... I don't quite understand what we are supposed to make of it.
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Old 20th Jul 2011, 00:43
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I'm pretty sure "throwing out everything except an anchor" was one of those tricks...

All my jet time is 727/737 and I can't say that I recall any problems in this arena save that the 737 required more planning due to its useless speedbrakes - the 727 was a much more readily versatile aeroplane ..
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Old 20th Jul 2011, 01:42
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Why would you say that? Airbus acknowledged the pitot issue and released a service bulletin to the airlines months before the accident and simultaneously issued "workaround" instructions to pilots at the same time, while the issue was being fixed. What more could they reasonably have done?
Did I say anything derogatory of Airbus, or imply a BEA cover-up? I can't see where I did. My comment was that the article in question had singled out Air France and the crew as being responsible before the BEA has released its final report - a long way off (time wise) methinks.

So don't get me wrong DW, I am not bashing Airbus and never have. My comments were reflecting the content of the article.

CJ - Agree.
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Old 20th Jul 2011, 03:53
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I had some argument via PM with some posters here about how much the nose would have to be lowered to break the stall at an AOA of 60° and posted some concern to the effectiveness of the THS. This reference states that it is not enough to lower the nose somewhat, and that it would lead to high sink rates and a difficult pullout if altitude would be sufficient. Posters calculated altitude loss to some 5.000 feet. Please, go back and read this reference from PJ2 and reconsider your calculation.
Retired F-4, as a fellow Phantom Pflyer, I think we both appreciate that it would likely take 10, 000 feet and more to recover AF447 from its deep stall. Until the AOA was reduced, it would not accelerate, and it would take a bit of time to run the trim down far enough to recover. If the recovery took more than 20,000 feet, it would be because the nose was left down too long after it began flying. Availability of airspeed in the recovery would be very beneficial (since there is no AOA gauge .)

PJ2, AF447 may have been a bit 'goosey' at altitude but to actually get and keep the nose well in the air as AF447 did implies more a lack of awareness (scan or instrument problem) than a control difficulty based on the limited information released. (There seems to be no indication of a dynamic departure from controlled flight for example.). There seems to have been adequate control authority to put the aircraft in a level attitude.

More like PF expecting the aircraft to take care of pitch attitude without the need to scan while attention is diverted elsewhere. Of course, this is just my personal opinion, and I await the BEA interim report to shed more light on what is presently poorly understood by those of us on the outside.
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Old 20th Jul 2011, 06:22
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A33Zab

In Flight:


The FMGC uses the weight and center of gravity from the FCMC (Fuel Computer) when available.
The GW and CG computed by the FE part are used:
as back-up in case of dual FCMC failure.
to trigger the aft CG caution and warning signals (independently of the FCMC).
FE Weight computation (back up)
When the aircraft is below 14625 feet and 255 knots :
GW = f(α, CAS, N1/EPR actual, CG from FE part, altitude)
When the aircraft is above 14625 feet or 255 knots :
GW = TOGW WFU
TOGW: takeoff gross weight
WFU: weight fuel used acquired from FADECs.
FE Center of gravity computation (back up/aft cg computation)
The CG is computed from the position of the horizontal stabilizer and is
function of the N1/EPR, Vc, ALT, MACH and GW from FE part.
thank you for detailed explanation A33Z

so in case of wrong CAS, MACH or ALT datas, what may be the result or the following aktion for the the FE Weight or Ceneter of gravity computation ??? in which direction will he put the wrong datas in his calculation? will he start pumping more fuel back or forward if the bird fly in level 350 with a (mayby) wrongly indicated CAS <255 ?
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Old 20th Jul 2011, 06:49
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Machinbird;

Thanks, I don't disagree frankly. I'm just trying to find something outside the box to figure out why someone would point an A330 15deg NU and keep it there at FL350+ and these papers got me thinking. I've flown the aircraft at altitude and I must say it was pretty responsive in pitch and roll in Normal Law. I've always doubted the notion that the airplane was slow and 'slushy' to respond to the stick at altitude and that a bit of PIO or inadvertent backstick while controlling roll resulted in an "accidental" climb that continued to 38,000ft but I'm outa ideas!
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Old 20th Jul 2011, 07:30
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...
....and that a bit of PIO or inadvertent backstick while controlling roll resulted in an "accidental" climb that continued to 38,000ft but I'm outa ideas!
Me too PJ2! I hope BEA is able to get to the crux of the problem. The CVR will be key in this.
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Old 20th Jul 2011, 08:17
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@Grity:

so in case of wrong CAS, MACH or ALT datas, what may be the result or the following aktion for the the FE Weight or Ceneter of gravity computation ??? in which direction will he put the wrong datas in his calculation? will he start pumping more fuel back or forward if the bird fly in level 350 with a (mayby) wrongly indicated CAS <255 ?

Transfer CG control is another question, pls read the first sentences:


In Flight:


The FMGC uses the weight and center of gravity from the FCMC (Fuel Computer) when available.
The GW and CG computed by the FE part are used:
as back-up in case of dual FCMC failure.



TRIM TRANSFER CONTROL

Before each flight, the crew inputs the A/C Zero Fuel Center of Gravity
(ZFCG) and Zero Fuel Weight (ZFW) data into the Flight Management
Guidance and Envelope Computers (FMGECs) via the MCDU.
The two FMGECs independently transmit this data to the FCMCs.
Each FCMC uses the ZFW from the FMGECs and the tanks fuel weight
to calculate the A/C Gross Weight (GW).
This GW is output to the ECAM FUEL page, and is used, in the FCMCs, to
obtain the target CG from the memorized CG versus percentage of Mean
Aerodynamic Chord (MAC) CG table.
In the same time, the FCMCs calculate the fuel weight CG using the FQI,
the pitch, roll and acceleration data from the Air Data Inertial Reference
Units (ADIRUs) 1 and 2, and the THS position from the Flight Control
Data Concentrators (FCDCs).
Then, with the ZFWCG transmitted by the FMGECs and the fuel weight
CG, the FCMCs calculate the Gross Weight Center of Gravity (GWCG).
This GWCG is output to the ECAM FUEL page.
The FCMCs, in their transfer logic part, compare the GWCG with
the target CG to determine if a FWD transfer or an aft transfer is needed.

CG control during automatic operation:
The master FCMC has full control of the A/C CG when the A/C is above
FL255, until the A/C descends below FL245.
The FMGEC independently monitors the CG of the A/C.
If it detects that the A/C CG is too far aft, it sends a signal ''CG target FWD'' to the FCMCs.
The master FCMC moves the target CG forward 2.0% MAC.
This is latched until the end of the flight.
If the FMGEC detects that the CG is still aft of the new target CG, it
sends a signal to the FCMCs.
The FCMCs then move the target CG forward 0.5% MAC for 10 minutes.
After 10 minutes the system goes back to normal operation.
This procedure can occur twice more until the target CG is first 4.0%
then 6.0% MAC forward of the initial position.
If the error is detected again, the FCMC stops the CG control and shows
an ECAM warning to the crew.
The crew then manually sets a forward transfer.

FCOM figure Posted earlier by Takata: LINK
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Old 20th Jul 2011, 11:24
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Originally Posted by mm43
So don't get me wrong DW, I am not bashing Airbus and never have. My comments were reflecting the content of the article.
My apologies - as I have said many times, the man or woman who invents a "tone-of-voice" reader for internet forums will make themselves a fortune...
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