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AF447 Thread No. 3

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AF447 Thread No. 3

Old 2nd Jun 2011, 22:47
  #1161 (permalink)  
 
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It appears to me -- with zero experience of flying this type of aircraft -- that recovery would not have required a great deal of height, once the wing was unstalled by trimming the THS and getting the pitch angle into the vicinity of the flight path angle, ie 20 degrees or so below the horizon.
Well, yes you could resume flying and generating lift, but you would be diving at a pretty severe angle and you have to generate enough G to turn the aircraft from that nose down path to horizontal. You have over 100 mph of vertical velocity and it's going to take some positive G to get rid of that. You aren't going to level out right away.
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Old 2nd Jun 2011, 22:59
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Vertical velocity is 170 ft/sec, and a 2G pullout would subtract 32 fps from that every second. Let's say 6 seconds, during which you would lose quite a bit less than 1,000 feet. Doesn't sound as if pulling out of a 25-degree glide at 250 knots would take that much room. At low altitude it should be possible to pull 2G without risk of a secondary stall. The point is, despite its high vertical velocity the airplane is not descending that steeply.
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Old 2nd Jun 2011, 23:14
  #1163 (permalink)  
 
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AoA versus velocity vector, etc

Sorry Garrison, but you must add the pitch attitude to the AoA to get the descent angle.

At impact, we know the BEA figures for 107 knots horizontal and vertical. Another contributor came up with same numbers I did, which was a basic 45 degree flight path with a 16 deg pitch attitude at impact. So AoA would have been around 61 degrees. When I use 107 knots vertical, I get about 10,800 feet per minute vertical vector, which seems to agree with BEA's numbers.

The last recorded values were a pitch attitude of 16.2 degrees nose-up, a roll angle of 5.3 degrees left and a vertical speed of -10,912 ft/min.
I point out the close agreement with my back-of-the-envelope calculations concerning vertical velocity

I can't find a BEA data point of 16 degrees pitch attitude and 35 - 40 degrees AoA in their report.


more on "trust your AoA" later for those debating its value when airspeed is FUBAR.
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Old 2nd Jun 2011, 23:18
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Lonewolf_50
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If your horizontal velocity is very low you have little lift (even if not stalled) and your downward component of velocity will increase so much that the AoA will go into the stall region.
Uh, not quite, I am not sure I understand you there. You can go slow (low horizontal velocity) and level if you are not stalled, and go merrily without falling, or stalling.

That said, once you begin to descend, the second part of your sentence begins to make sense.
I'm just looking for the cause of the two stall warnings right at the start, using the hypothesis that a stall warning is triggered not only by AoA but also when the (indicated) air speed falls into a region where a stall in level flight is imminent.

The further conjecture is that the PF acted on these (spurious) warnings according to the manual, and in doing so, perhaps in conjunction with an unfortunate updraft, got himself into a real stall.

So the invalid stall warnings helped get them into a real stall - and the lack of stall warnings (plus the resumption of warnings at times on correct pilot input) when they were in the real stall minimised the chance of them ever getting out of it.

Just a thought.
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Old 2nd Jun 2011, 23:23
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AoA indication

Excuse me for interjecting in a technical discussion as mere SLF (even if it is one who sits on an A340 across the pond rather often):

I am conscious of the various discussions about presence or absence of an AoA indication, and have read this entire thread from day 1 (sad?).
My personal background started in mathematics, moving into engineering, including the much maligned real-time software.

Anyway - a couple of comment on "vector maths", which most pilots should be familiar with from route planning?

If we assume that "pitch and power" were to be maintained, we would have an aircraft with pitch which was likely to be based on inertial reference. At this time, I believe this has not been questioned. A previous post then said that "pitch = airspeed, power = altitude". This makes sense, although I confess I have not had time to search back for the quote (credit to the author, never the less).

Then to the next stage - if you have pitch with reasonable reliability, the AoA is likely to be close to the vector sum of the forward and vertical velocities (not accounting for upward or downward air movement)

Hence - if you have reasonable, or at least some, indication of pitch and a large downward vertical velocity then do you not have a fair indication of a possible stall? The alternative is a very strong updraught - but how long is that likely to last?

Just a thought, I wonder if it is relevant?
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Old 2nd Jun 2011, 23:41
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Sorry Garrison, but you must add the pitch attitude to the AoA to get the descent angle.
Not on this planet. You have to add the pitch angle to the negative of the flight path angle to get the angle of attack (assuming that the flight path angle is negative for a descent).

With regard to the descent angle, the BEA report I am reading (27 May) says 15 to 16.2 degrees deck angle. At 35,000 feet, on the way back down, the angle of attack exceeds 40 degrees but during the 3+30 descent the angle of attack "remained greater than 35 degrees". Now, I suppose you could argue that "greater than 35" means 65, but the use of "exceeds" and "remained greater" strongly suggests that the AoA remained in that neighborhood -- that is, "greater than 35" means 36 or 37, not some much larger number.

I concede that the 107-knot groundspeed at impact is inconsistent with my calculation of 250 ktas, unless there is an improbably huge surface wind; but it is also inconsistent with the reported pitch angle and angle of attack. Maybe something changed in the final seconds; I don't have a good theory handy.
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Old 2nd Jun 2011, 23:47
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I haven't flown one in many years, but the Stall Horn on the 150 series I believe is nothing BUT an AoA indicator, albeit "aural". Zeroed to exclude angle of Incidence, (the from factory difference in chord line and fuselage longitudinal line) the horn starts to mouth off when air "leaves" the orifice. This I know for I used to test it by inhaling on the hole, and depending on effort, I could almost play it like a flute. Blowing into it accomplishes nothing. When the ANGLE of the airstream starts to impinge on the orifice at a suitable positive AoA, the annoying alert starts to pucker one's own orifice, more so as the AoA approaches the Stall Break.

Dozy. I am in earnest about evidence, something I define as facts, as do you.

Likewise, at Court, facts come with a guarantee, something lacking here.

You are not incorrect in your explanation of the Laws, I believe in you completely. Thus far, the bulk of trouble some have here is not in disallowing the Program, or its influences, but in the arena of suitability in crisis. This unsuitability is in discussion, and I warrant it has a basis in fact. something you seem unable to entertain.


I repeat. IF, upon selling a suite of hardware, one does not exhaustively train its usage to the client, one has sold a bill of goods.

one outsider. Please accept my offered apology for twaddle. I agree, though I think your Twaddle Launcher is pointed at the wrong target, just sayin'.

Garrison Absent any need for Pitch or AoA at impact, the Horizontal and Vertical velocities sum for a FLIGHT Path of 45 degrees descent Yes? One wishes for a mental picture of "almost" airborne, but BEA pitch at 16 and AoA at 35 + is discouraging.

ADD. I get an airspeed of about 135-145 knots (at impact) if BEA'S numbers for rate are accurate.

Last edited by bearfoil; 3rd Jun 2011 at 00:02.
 
Old 3rd Jun 2011, 00:01
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Abnormal Attitude Law

PVT
In the Flight Global article, they are making the distinction that the aircraft never went into the abnormal attitude law because the flight computer had rejected the information coming in from the ADRs before those parameters were met. The aircraft was already in alternate law from the loss of airspeed. The last underlined part is saying that the flight computers would have to see 'acceptable' data from the ADRs to trigger this law like the examples given.
If AOA is available for FDR it should be available for other systems too (benefits of ARINC429),
I've read in system schematics that there is a separate AOA backup signal bus to FCPC's.
On second thoughshould have been activated by AOA >30 @ 2h 11 min 40 ts that may be only on –BUSS- equipped A/C.
I'll come back to you if I got that in hand.

Besides that, if you combine the 3 BEA reports together, the Abnormal Attitude Law before NAV ADR DISAGREE warning msg @2h 12. (Received 2h 12 51)

@Bear:

Don't change the rules and call it something it ain't
.

Just trying to clear some operational 'misunderstanding' of FBW and laws from an E&M point of view. If not appreciated just say so.




@Dozy:

Agree with you.

Last edited by A33Zab; 3rd Jun 2011 at 00:16.
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Old 3rd Jun 2011, 00:01
  #1169 (permalink)  
 
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AoA Utility

As I have posted a couple of times before, AoA is highly useful at approach speeds (and in a stall situation) but not for maintaining a desired flight condition while in cruise. When encountering an Unreliable Airspeed condition, flying a specified AoA would not be very effective in cruise. At approach speeds, AoA varies relatively large amounts for relatively small changes in airspeed. At cruise speeds, the variation of AoA is relatively small for relatively large airspeed changes.

The U.S. Navy uses AoA for landing on aircraft carriers because it provides a precision reference for executing a precise approach. AoA indicators have not typically been used in transport category aircraft because that degree of precision is not required, but flying AoA does allow the pilot to fly the correct approach airspeed, regardless of weight and without reference to tables. The P-3, as initially introduced into the fleet, was equipped with AoA indicators but they were routinely ignored because "AoA is for jets". I don't know if those AoA indicators are ever used today.

IMHO, AoA is the single most useful instrument you can have to prevent a stall.
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Old 3rd Jun 2011, 00:09
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at impact, the Horizontal and Vertical velocities sum for a FLIGHT Path of 45 degrees descentYes?
Yes, and with 16 degrees nose up that gives an AoA of 61 degrees as asserted by gums. But the BEA numbers for most of the descent -- pitch 15, AoA 35-40 -- imply a much shallower descent. The airplane was almost uncannily stable during much of the descent -- steady AoA, no spin, and 40-degree wing rock correctable with ailerons is pretty nice behavior with AoA > 35 -- but that may have changed. It is not clear to me whether thrust went above idle again after 2:12:02; at low altitude thrust could affect angle of attack.
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Old 3rd Jun 2011, 01:20
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Garrison

Bear says:

Quote:
at impact, the Horizontal and Vertical velocities sum for a FLIGHT Path of 45 degrees descentYes?

Garrison says:

Yes, and with 16 degrees nose up that gives an AoA of 61 degrees as asserted by gums. But the BEA numbers for most of the descent -- pitch 15, AoA 35-40 -- imply a much shallower descent. The airplane was almost uncannily stable during much of the descent -- steady AoA, no spin, and 40-degree wing rock correctable with ailerons is pretty nice behavior with AoA > 35 -- but that may have changed. It is not clear to me whether thrust went above idle again after 2:12:02; at low altitude thrust could affect angle of attack.

Sorry, how is 55 degrees a much shallower descent than 61?

I am using BEA derived data re: velocities, basically H/V square. That gives a very simple Pythagorean solution of 45 degrees for hypotenuse, and velocities (integral) that compute to a summed velocity of 140 knots or thereabouts. I don't disagree with your assumption that these figures were or could be different at altitude, but so far no data with which to invest in an opinion?
 
Old 3rd Jun 2011, 01:21
  #1172 (permalink)  
 
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AF447 - Descent Profile at Impact

For those who need some help in envisaging what a high Angle of Attack looks like and how it will affect the pitot data, the following graphic should help.



It should also be noted that during the final stall, the AoA was never less than 35 degrees, and this was probably only achieved with N1 100%, with the AoA increasing once engines were at idle.

Last edited by mm43; 3rd Jun 2011 at 23:31. Reason: small change to graphic
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Old 3rd Jun 2011, 01:27
  #1173 (permalink)  
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"It should also be noted that during the final stall, the AoA was never less than 35 degrees, and this was probably only achieved with N1 100%, with the AoA increasing once engines were at idle."

Que?
 
Old 3rd Jun 2011, 01:27
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Can anyone clarify for me the operation of the AoA system? The real angle of attack must be obtained from a lookup table that translates the vane position into actual alpha. How far does the lookup table go? The interim report says that (my translation) "The angle of attack, when it is valid [or reliable], remains above 35 degrees." (Emphasis mine.) What limits the validity of the reading? Is the AoA recorded by the FDR and reported by the BEA valid/reliable for purposes of visualizing the attitude and flight path of the airplane?
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Old 3rd Jun 2011, 01:34
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Que?
If the horizontal component increased due to additional power and the RoD remained essentially the same, the AoA will have decreased. Do you want me to stretch the graphic to prove it?
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Old 3rd Jun 2011, 01:34
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bearfoil...
Sorry, how is 55 degrees a much shallower descent than 61?
I feel like Alice in Wonderland, except I'm a guy.

It's not 55, it's 25. Please please please draw a picture of an airplane, side view, nothing fancy, with its nose tilted upward 15 degrees. Then take a protractor and draw a line at a 40-degree angle downward from the airplane's longitudinal axis (which stands in, for purposes of discussion, for the mean aerodynamic chord). That is the flight path angle -- definition of angle of attack. It's 25 degrees.

Granted, the BEA numbers could be wrong -- garbage in, garbage out.

Maybe I should draw the picture. I'm not very good, but I'll try.
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Old 3rd Jun 2011, 01:38
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I think mm43's picture is cool, have you seen it? The long line is the Flight Path, from data via BEA. That qualifies as (Angle of) descent for me. Pitch and AoA are important how? Ride?

bear

mm43, hey two against one, no fair. With N1 100 %, doesn't the Pitch increase? and since increasing Pitch separates the AoA , it increases also? Lowering Thrust therefore closes the two (or...... This is a ballistic projectile. A brick with fins.

Last edited by bearfoil; 3rd Jun 2011 at 01:50.
 
Old 3rd Jun 2011, 01:47
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How do you post a picture? I drew one, it's a JPG, but I don't know how to insert it into a message. The "Insert image" icon asks for a URL.
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Old 3rd Jun 2011, 01:49
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MAC? maybe it's in Direct Law. You are entertaining a ballistic projectile, yes?

airfoilmod@sbcglobal. net. attachment?

I'm looking at 15 seconds before, and fifteen seconds after a/p disconnect. Here is where it happened.
 
Old 3rd Jun 2011, 01:56
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AoA and pitch attitude one mo' time

Sorry, all, I misplaced two of the vectors in my explanation. So I'll stick with my story, and only place I find a no-kidding AoA and pitch attitude and both velocity vectors was the one I quoted from the BEA report.

This is what I was trying to depict.



As shown here on my chalkboard, the AoA is the sum of the pitch attitude and the velocity vector ( assume airflow is opposite the velocity vector in the air).

Bear is correct about total velocity, 'cause if both horizontal and vertical velocities were identical, the total vector would be about 1.414 times either of them ( square root of two).

Need a real geometry wizard to make the call. Winner ( and referee) gets a free slug of Jeremiah Weed at my fishing cabin in Colorado in a few weeks.

I throw my hat in with the AoA mafia.

I don't understand why it isn't used more often, as it reflects the basic characteristics of the wing and its lift capability. In other words, weight of the jet is inherent, as the AoA must be greater to get the extra lift for the same speed. High lift devices, flaps, etc can change the stall AoA value, but that's an easy thing to depict on a display.

I call 'bird to the stand re: AoA indexer and gauges for the Navy jets.

Hate to hijack the thread from the "who was in what seat" discussion, but thot we could use a break.
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