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AF447 Thread No. 3

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AF447 Thread No. 3

Old 3rd Jun 2011, 05:21
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Effectiveness of Rudder?

If you go back to Tailspin Turtle's last graphic, it is easy to see that the airflow around the empennage and over the THS will create a turbulent mush over the rudder, making the rudder IMO useless.
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Old 3rd Jun 2011, 05:21
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gums, I figure your geometry is correct. And I suspect the AoA vane was at one of its stop positions. Given the phrasing it hits a stop not far beyond 35 degrees.
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Old 3rd Jun 2011, 05:29
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Garrison, if you're going to round the number it was 11,000'/min.
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Old 3rd Jun 2011, 05:35
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Graybeard
... so the plane was dropping at 10,912 fpm, or 107 knots. There must have been near zero horizontal velocity when the plane hit.
The BEA gave two components:-
... vertical speed of -10,912 ft/min, a ground speed of 107 kts
So, I don't understand how you have determined the near zero horizontal velocity.
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Old 3rd Jun 2011, 05:40
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Originally Posted by Machinbird
Bearfoil
Quote:
One cannot fly a brick, no matter the architecture. ....
Bear, I used to fly a brick-like object aboard ship routinely. The secret was maintaining control. Well, that and lots of thrust too.
(cough)(cough) With enough power you can fly a brick or perhaps more prosaically a lawnmower. Search for the flying lawnmower video on YouTube. It's hilarious. And it flies pretty well, too.
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Old 3rd Jun 2011, 05:48
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Originally Posted by Graybeard
There must have been near zero horizontal velocity when the plane hit.
I'm not so sure. Unless it was a fluke, the ship hit the water with 5° of bank, which indicates to me that the crew had roll control ie semi-effective ailerons. Had it been going straight down, no useable flow over the ailerons would have existed and it probably would have been falling like a leaf or tumbling.

100kt down and 107 G/S means it was "flying" at 140KIAS, albeit at 60° AoA.
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Old 3rd Jun 2011, 05:53
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MM43
If you go back to Tailspin Turtle's last graphic, it is easy to see that the airflow around the empennage and over the THS will create a turbulent mush over the rudder, making the rudder IMO useless.
It might not be quite that bad. Put 13 degrees of THS leading edge down on that THS and it doesn't block the rudder quite so badly. Also, even past the stall angle of attack, the inboard sections of a swept wing (or THS) are not as badly stalled as the sections further outboard. The VS & rudder being near the centerline should keep them in more energetic flow than it might initially seem.

Thinking about relative effectiveness of the wing and THS, the THS will operate closer to its intended operating point than the wing and thus its C/L should be less affected than the wing's. I.E. With a trimmed up THS (Leading edge down), the THS should become more effective in overpowering the wing pitching moment.
I don't do Aero , so any real Aero's on the thread can comment without hurting my feelings. If correct, that might explain why the stalled aircraft does not want to pitch down. That and the tendency of the center of lift to move forward on a stalled swept wing.
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Old 3rd Jun 2011, 05:54
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History

History
One of the more interesting (to me) on-line documents relating to the history of recommendations concerning the use (or non-use) of trim during recovery from upsets in transport aircraft is a talk given by Captain William Wainwright, Chief test pilot of Airbus.

The full text is available from at least two sources (one being the NTSB).
http://www.ntsb.gov/Events/2001/AA58...its/240005.pdf
http://www.skybrary.aero/bookshelf/books/435.pdf

Here are some extracts. The omission of text in these extracts is indicated by “………”.

--------- Start of quote extracts -----------
AIRPLANE UPSET RECOVERY TRAINING AID,
By Captain William Wainwright, Airbus Industrie.

INTRODUCTION
The idea for a joint industry working group to produce an Airplane Upset Recovery Training Aid was first proposed by ATA in June 1996. ........ The end result of 2 years work is a training package including a video and a CD-ROM, giving an airplane upset recovery training aid. This package is on free issue to all of you, to use or not to use as you wish. ………..The content of the package is not my subject today, but there are a few issues of general interest which I gained from my experience as a member of the working group which I would like to talk about.

2. THE BEGINNING
…… Right from the beginning there was a conflict between the technical advice given by the manufacturers' training pilots and that expressed by those of the principal airlines already practicing upset training. They naturally considered themselves to be the experts on this subject, based on the many .hours of training that they had already conducted on a large number of pilots in their simulators. At the beginning of 1997, the Flight Test Departments were asked to come in to support their training pilots. From then on, the chief test pilots of the 3 major manufacturers became members of the working group; XXX of Boeing, YYY of McDonnell-Douglas (now Boeing - Douglas Products), and myself. But the conflict over the different opinions on aircraft handling and recovery techniques continued for a long time until we finally achieved agreement at the last meeting in January 1998. The reasons for these differences of opinion are the subject of my talk today.

3. THE DIFFERENCES
The differences of opinion were mainly concentrated in the following areas:
(a) Procedures versus general advice.
(b) Ease of training versus failure cases.
(c) Stalling.
(d) Use of rudder.
(e) Use of simulators.

It is worth saying that there was never any difference of opinion between the three test pilots on the group. Although we come from different backgrounds and have worked in different organizations with different work cultures, we always agreed on our technical advice.

4. PROCEDURES VERSUS GENERAL ADVICE
The airlines wanted simplified procedures which were common to all aircraft in their fleets and which were easy to teach and easily reproducible. This is understandable because you are all interested in having a standard product at the end of your training programmes. And this is what they already had with the Airplane Upset Recovery Training that they were already doing. For the training managers from American Airlines, Delta, and United, the only thing necessary was to give an overall industry approval to their existing programmes; they already worked, because the many pilots that had undergone training all came out of it with the same standardized reactions to the standard upsets. For them, this was the necessary proof that their training programme worked.

Where we differed was in our conviction that there was no such thing as a standard upset and our reluctance to endorse simplified procedures for recovery from an upset. We wanted a general knowledge based approach, as opposed to a rule based one. For this, after proposing some initial actions, we talk about “additional techniqlies which may be tried”. This obviously is more difficult to teach. ...........

5. EASE OF TRAINING VERSUS FAILURE CASES
The training that was already being done considered upsets as being due to momentary inattention with a fully serviceable aircraft that was in trim when it was upset. We would like to consider other cases that involve failures of control systems or human errors leaving the aircraft with insufficient control authority for easy recovery.

This of course complicates the situation, because recovering an aircraft which is in trim, possessing full control authority and normal control forces, is not the same as rocovering an aircraft with limited control available or with unusual control forces. Thus, for us, an aircraft that is out-of-trim, for whatever reason, human or mechanical failure, should be re-trimmed. Whereas the airline instructors were against the use of trim because of concerns over the possibility of a pilot overtrimming and of trim runaways which are particularly likely on some older aircraft types which are still in their fleets. We spent a lot of time discussing the use of elevator trim, and we never reached agreement. All the major US airlines were adamant on their policy to recover first using “primary controls” which excluded any reference to trimming. Again, a compromise was necessary. What we have done is to talk about using trim if a sustained column force is required to obtain the desired response whilst mentioning that care must be used to avoid using too much trim. And, the use of trim is not mentioned in the simplified lists of actions to be taken.

6. STALLING
Another aspect that was 'being ignored in the existing training was the stall. By this I mean the difference between being fully stalled and the approach to the stall. In training, you do an approach to the stall with a recovery from stick shaker, which is often done by applying full thrust and maintaining existing pitch attitude in order to recover with minimum loss of height. Height cannot be maintained if an aircraft is actually stalled and should be of secondary importance. .......

There is a world of difference between being just before, or even just at, the stall, and going dynamically well into it. The training being given in the airlines at the time to recover from excessive nose-up pitch attitudes emphasised rolling rapidly towards 90" of bank. This is fun to do, and it was not surprising to find that most of the instructors doing the training were ex-fighter pilots who had spent a lot of time performing such manoeuvres in another life. The training was being done in the same way, with an aircraft starting in trim with a lot of energy and recovering while it still had some. However, the technique being taught only works if the aircraft is not stalled. ……..
If the airplane is stalled, it is imperative to first recover from the stalled condition before initiating the upset recovery technique. Do not confuse an approach to stall and a full stall. An approach to stall is controlled flight. An airplane that is stalled is out of control and must be recovered.
A stall is characterised by any, or a combination of the following:
--- Buffeting, which could be heavy at times.
--- A lack of pitch authority.
--- A lack of roll control.
--- Inability to arrest descent rate.
To recover from a stall, the angle of attack must be reduced below the stalling angle. Apply nose down pitch control and maintain it until stall recovery. Under certain conditions with under-wing mounted engines, it may be necessary to reduce thrust to prevent !he angle of attack from continuing to increase. Remember, in an upset situation. If the airplane is stalled, It is first necessary to recover from the stall before initiating upset recovery techniques. This is something that we are well aware of in testing, but it was either being totally ignored, or misunderstood. I consider the inclusion of this note to be one of our most important contributions.

8. USE OF SIMULATORS
We manufacturers were very concerned over the types of manoeuvres being flown in simulators and the conclusions that were being drawn from them. Simulators, like any computer system, are only as good as the data that goes into them. That means the data package that is given to the simulator manufacturer. And we test pilots do not deliberately lose control of our aircraft just to get data for the simulator. …………..

The complete data package includes a part that is drawn from actual flight tests, a part that uses wind tunnel data, and the rest which is pure extrapolation. If should be obvious that conclusions about aircraft behaviour can only be drawn from the parts of the flight envelope that are based on hard data. This in fact means being not far from the centre of the flight envelope; the part that is used in normal service. It does not cover the edges of the envelope. I should also add that most of the data actually collected in flight is from quasi-static manoeuvres. Thus, dynamic manoeuvring is not very well represented. …………

In other words, you have reasonable cover up to quite high sideslips and quite high AOAs, but not at the same time. Furthermore, the matching between aircraft stalling tests and the simulator concentrates mainly on the longitudinal axis. This means that the simulator model is able to correctly reproduce the stalling speeds and the pitching behaviour, but fidelity is not ensured for rolling efficiency (based on a sirrpfified model of wind tunnel data) or for possible asymmetric stalling of Ihe wings. ......

In fact, this is a perfectly adequate coverage to conduct all normal training needs. But it is insufficient to evaluate recovery techniques from loss of control incidents. Whereas, the training managers were all in the habit of demonstrating the handling characteristics beyond the stall; often telling their trainees that the rudder is far more effective than aileron and induces less drag and has no vices In short, they were developing handling techniques from simulators that were outside their guaranteed domain. Simulators can be used for upset training, but the training should be confined to the normal flight envelope; For example, training should stop at the stall warning. They are "virtual" aircraft and they should not be used to develop techniques at the edges of the flight envelope. This is work for test pilots and flight test engineers using their knowledge gained from flight testing the "real" aircraft.
--------- End of quote extracts -----------
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Old 3rd Jun 2011, 06:55
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(cough)(cough) With enough power you can fly a brick or perhaps more prosaically a lawnmower. Search for the flying lawnmower video on YouTube. It's hilarious. And it flies pretty well, too by ....... JD-EE
Here is the magic of lots of power and a good prop slipstream to help the flying controls to work aerodynamic wonders and lighten our day!

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Old 3rd Jun 2011, 07:19
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Originally Posted by mm43
The BEA gave two components:-
So, I don't understand how you have determined the near zero horizontal velocity.
10,912 fpm is, give or take 107kts, which would be some coincidence. Residual ground speed anyone?
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Old 3rd Jun 2011, 07:35
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Simple trigonometry: if the speed vector is at a 45 degrees angle then vertical speed is equal to horizontal speed.
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Old 3rd Jun 2011, 08:06
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Originally Posted by robertbartsch
Didn't the BEA permit AB to make a statement that they believe nothing needs to be modified in the A330 after the black box data was reviewed?
No, the AB statement was that no modifications (other than previously advised) were required yet - ie. the further investigation of the data may result in modification requirements

If that is true, BEA must believe it was a pilot only problem; right?
It isn't true, but that doesn't disprove that statement. I would hope however that until any other (than pitot) possible aircraft failures are eliminated, the conclusion remains open - and that will take a lot of work analysing the data.
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Old 3rd Jun 2011, 08:57
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Originally Posted by Capn Bloggs
...100kt down and 107 G/S means it was "flying" at 140KIAS, albeit at 60° AoA.
What about windspeed?

Pedantic maybe, but 140KTAS or 140KCAS. If the pitots only observe the normal component they may only report an "unbelievable" 70KIAS. With wind and sidelip it is easy to see the IAS from any/all unfrozen sensors moving across the 30-60kt range and responsible for initiating/blanking alarms.

Pitot tubes are designed to have little effect over +/-20 degree range, higher than this is possible with 'shroud' extensions

Last edited by sensor_validation; 3rd Jun 2011 at 09:29.
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Old 3rd Jun 2011, 09:07
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Originally Posted by Machinbird
I'm also thinking the first two "beeps" of stall warning on AF447 were quite possibly caused by the g incurred in the initial pull up! It will be interesting to see the pitch rates developed.
The stall warning AoA at M=0.8 gives you about 1.4 g. But then the BEA Update says 11 second later at 2:10:16 "The airplane's pitch attitude increased progressively beyond 10 degrees and the plane started to climb".
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Old 3rd Jun 2011, 09:32
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At 2.10.05 when the autopilot dropped out, due unreliable air speeds (showing about 60 kts), if the system had gone straight into Direct Law the aircraft would presumably have remained "speed stable" provided the elevator was not moved.

Would Direct Law be easier to handle with unreliable air speeds than Alternate Law?
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Old 3rd Jun 2011, 09:49
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if the system had gone straight into Direct Law the aircraft would presumably have remained "speed stable" provided the elevator was not moved.
- an assumption only, based on no idea what the a/c was actually doing at the time!
Would Direct Law be easier to handle with unreliable air speeds than Alternate Law?
- come on now, you claim to be a 320 pilot. Surely you know that that would run counter to all AB philosophy of 'protecting' the machine? The software know best, you, the pilot, do not need to know.
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Old 3rd Jun 2011, 10:05
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Hi BOAC,

Normal Law is brilliant. It wraps the aircraft in cotton wool.

Direct Law is just like a Boeing and I could cope with that.

Alternate Law is something in between.

During unreliable air speed whilst the crew held the wings level using constant left roll input, but with a mistaken light back pressure, it trimmed the aircraft fully nose up in response - without them realising. That's bad.
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Old 3rd Jun 2011, 10:18
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I quite agree but am pointing out that the philosophy change, in ceasing to 'protect' the aircraft from piloting, at AB, would be too dramatic
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Old 3rd Jun 2011, 10:18
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Originally Posted by JD-EE
gums, I figure your geometry is correct. And I suspect the AoA vane was at one of its stop positions. Given the phrasing it hits a stop not far beyond 35 degrees.
I can't find for sure what AoA probe was actually fitted to the AF447, but a likely candidate is a Thales C16291AA which has a catalog range of +/- 60 degrees - but this "Local Angle of Attack" gets corrected before use, so the limit is likely in the corrections/compensations.

Note this AoA probe has recently been subject to an AD to correct a manufacturing issue - excess oil which could impede motion at low temperatures. I don't know exactly what triggered this AD, the Perpignan A320 had Goodrich 0861ED probes, two of which probably froze at altitude due to water ingress during cleaning.
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Old 3rd Jun 2011, 10:51
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Originally Posted by susu42
Simple trigonometry: if the speed vector is at a 45 degrees angle then vertical speed is equal to horizontal speed.
I doubt that an object weighing hundreds of tonnes, moving at great speed under its own power and 10km in the air and designed to be in that environment, would end up in a simple trig equation, where vertical equaled horizontal. But it is one possibility I suppose.
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