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AF 447 Search to resume

Old 7th Apr 2011, 21:09
  #3161 (permalink)  

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There seems to be a slight misconception here about lowering the gear.

You don't normally lower the gear to get the speed down; but you (might) lower the gear to increase the rate of descent in Emerg Descent. I emphasise 'might'.

At low altitude (arriving for landing, say) you might lower the gear earlier than planned if you are a bit high on speed - there was a reference earlier to using gear as airbrakes.

Quite different situations.
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Old 7th Apr 2011, 21:14
  #3162 (permalink)  
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One might also consider lowering the gear to get the nose down.
 
Old 7th Apr 2011, 21:39
  #3163 (permalink)  
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Let's not get into pages of why we put the gear down, eh? We are merely trying to solve CliveL's dilemma about 'lowering the gear if you are to ditch', and I was simply hoping someone could identify from the images whether there were signs of a deliberate extension from jack positions etc. It really is not that important. I think it is fairly evident that ditching might not have been planned from the apparent state of the pax/cabin preparation, and no, CliveL, one would not normally lower the gear in a ditching scenario unless, perhaps a greater over-riding need arose.

To quote PJ2
"Again, when the MLG actually free-fell is immaterial - it did, and it, and its dressings have remained attached to the rear spar and support structure. I think it is more important to rule-in/rule-out the crew extending the gear. It is a debate until the recorders tell us."

Unless someone can tell from the pictures, we will indeed have to wait.
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Old 7th Apr 2011, 21:47
  #3164 (permalink)  
 
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dufc, it may depend on the capabilities of the retrieval ship, on whichever one the government selects. WHOI has several months remaining on its contracted period, so it is not as if they need to rush off to another task.

And WHOI may still be needed. When a Canadian Coast Guard cutter was pulling up a forward cargo door (considered a vital item) from Air India 182, the line snapped as it was about to be hoisted aboard, and the door sank back to the abyss, never to be found again.

I would expect that once retrieval starts, there will be a small flotilla of vessels on the scene, including the French navy, and possibly the Brazilian navy.
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Old 7th Apr 2011, 23:16
  #3165 (permalink)  
 
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Resolution of sonar images

Originally Posted by asc12
Can anyone enlighten us about the resolution of sidescan sonars? The BEA images show that the Remus AUV was using 120kHz. What sort of spatial resolution could one expect at this frequency? And how is the data displayed?
The speed of sound in water is approximately 1500 m/s (it varies in important ways with temperature, salinity and pressure, but that is not relevant to this question). The wavelength at 120kHz is thus: 1500m/s / 120kHz = 12.5mm. How many wavelengths across does an object need to be to detect it? That's hard to say, because it depends so much on the reflective and resonant properties of the target. I'm not an expert in this area (same math as radar, so maybe others will contribute), but I would say that 10-100 wavelengths would likely be detectable; so that is roughly 10cm to 1m. That's for detection: having a strong enough echo to detect above the noise and clutter..

Resolution is a different question. Along-track resolution is determined by the beam width of the transducer and the motion of the vehicle between pings. The second is easy: at 600m range, for conventional side-scan, the sound has to travel 1200m to leave the vehicle, reflect from a target and return. With some dead time between pings, that makes about 1 ping/second. In that time the vehicle has moved 2 m, so that will be the best possible along-track resolution (unless the sonar is chirped or uses some other way of getting more sound in the water). I don't know the beam width of the system they are using, but it is likely about 0.5degree; that gives about a 5m wide beam at 600m, so that is another limit. Remember that this is about resolution, not detectability.

The other resolution limit, not usually the controlling limit, is the across-track resolution; that is, in the direction between the target and the vehicle. This is limited by some transducer and wavelength considerations, and also by the number of samples, in time, made of the returning echos. Typically that is between 250 and 2000 (what the eye can see), and so this is easy to make better than the along-track resolution.

The data are commonly displayed as an image built up of scans where each scan is the echo intensity (normalized for range and absorption) as a function of range, or more accurately: time scaled and corrected to represent range. Each scan line is perpendicular to the motion of the vehicle (hence the term "side-scan", the sonar is looking to the port and starboard sides using two separate transducers and receivers).

In the released image, you can see that the vehicle proceeded in an "up" or "down" direction with respect to the image (nearly north or south, note the dark bands under the vehicle where there are no immediate reflections), and that the targets are smeared more in the along-track direction than in the across-track direction. I think that picture is a mosaic of views from both sides of the debris (there is more than one dark "nadir" under the vehicle).

In other words, what are we really looking at in this image? Are the redder parts colored to represent things that are physically located higher, or have a stronger return, or something else? What causes objects to have a stronger return with sonar? At 120kHz, how small an object could one expect to pick out?
The only thing the side-scan can measure is the time and amplitude (and phase, for some systems) of the returning echos. The colors chosen by the operator to represent the amplitude could be anything. In this case the darker colors represent weaker signals, and the brighter colors represent stronger signals.

Return strength from a target is a very complicated topic. It depends on the material, the internal cavities, the shape, the size the orientation toward the receiver, and many other factors. For this long range phase of the search, they were just looking for targets that did not look like the background everywhere else. While the resolution at 120kHz may not be great, higher frequencies are absorbed by the water and thus the maximum range gets shorter. A 55 gallon oil drum would be a reasonable target to detect using 120kHz at 600m range; it would not be possible to resolve the shape and thus identify this target at that frequency and range, using the side-scan presently installed on these vehicles.

After acquiring the debris field they will have surveyed it with higher ferquency side-scan, at intentionally shorter ranges (better along-track resolution). Of course, by now they will also have about 100,000 photos, most of which will show mud with no A/C parts, but the rest will be interesting.

Last edited by auv-ee; 8th Apr 2011 at 01:40. Reason: Clarify discussion of oil drum.
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Old 7th Apr 2011, 23:31
  #3166 (permalink)  
 
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deSitter:
...the neatly severed fin. There was much talk about the merits of composite materials and AA587...
Hardly a neat separation -- the forged lugs were ripped out from between their forged hoops!



A330 section 19:



Example from A400M:



The talk regarding composites and AA587 were/are irrelevant to this accident.

Last edited by Machaca; 7th Apr 2011 at 23:46.
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Old 7th Apr 2011, 23:48
  #3167 (permalink)  
 
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auv-ee,

Just to say thanks for that really excellent piece above - I understood all of it (I think)!

takata,

Did I see you and mm43 contributing to that "other place"? Dommage!
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Old 7th Apr 2011, 23:54
  #3168 (permalink)  
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Machaca - absolutely superb photos, thank you, - they explain/demonstrate a great deal.

PJ2
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Old 8th Apr 2011, 00:09
  #3169 (permalink)  
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Chris,

why do you say I'm wrong when you're emphasising what I said?

While you initially give a rate based on stick displacement, the airplane then holds the commanded attitude (with centered controls!). It hence does NOT behave like an airplane we all learnt to fly on, also the only airplanes we ever had any *real* upset recovery training in.

I rest my case, one should never implement systems which behave differently from what one learnt initially. We had a good example of that in Zurich not too long ago when a Saab 340 stuck it in near vertically shortly after takeoff, the Moldavian pilot had learnt to fly on AH's where the airplane symbol moves rather than the horizon. So he corrected the wrong way for an overbank condition.
 
Old 8th Apr 2011, 00:50
  #3170 (permalink)  
 
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Chris Scott,

Did I see (takata) and mm43 contributing to that "other place"? Dommage!
It was a "shameful" event! But shame on you for looking!

Sometimes a bit of "fact" injected into a nonsense thread will bring it to heel, and I suspect takata will agree.
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Old 8th Apr 2011, 00:55
  #3171 (permalink)  
 
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Machaca... Do you have any photos of the static dissipator tufts on the rudder after the vertical stab was recovered? It appeared in at least one photo during its recovery that several of the tufts were missing. After doing lightning analysis at a major avionics company, I think a possible root cause that should not be ruled out is high altitude lightning. A likely attach point forward could have been near the upper TCAS antenna. If the stroke traveled to the inside of the fuselage, a host of failures could have occurred. I know this is a stretch, but in the NASA F-106 studies that were done decades ago, the energies involved are very large. Recovery of electronic system components will certainly be valuable in the analysis.
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Old 8th Apr 2011, 01:21
  #3172 (permalink)  
 
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Lightning

Along that line, Lazerdog, nobody every gave a good explanation for the TCAS Fail in the ACARS message. Everybody just dismissed it as due to the pitot fails.

Again, a pitot fail will not affect the TCAS. A static fail will cause the transponder to revert to essentially Mode A function only, which will cause the TCAS to shut off, not Fail. Three things come to mind for a TCAS Fail:
Processor Fail
Lower TCAS antenna
Upper TCAS antenna

GB
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Old 8th Apr 2011, 01:29
  #3173 (permalink)  
 
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Takata
No-one can so far explain why switching at 0210 to ALT2, due to unreliable airspeed, would cause an immediate upset as none of the similar events ever caused one! After being switched to manual control, any manual imput could cause one in relation with other unknown factors (workload, weather, technical, etc.)
Perhaps this is stated backwards.
"Why would an upset cause unreliable airspeed indications??", might be a better question.
The answer is in the link below and subsequent discussion.
http://www.pprune.org/6210302-post2647.html

Note: With the discovery of the wreckage virtually under its 2010 position, the case for an immediate loss of control becomes stronger. The key question becomes how can it happen?

Last edited by Machinbird; 8th Apr 2011 at 02:00. Reason: Add note
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Old 8th Apr 2011, 02:19
  #3174 (permalink)  
 
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Originally posted by Graybeard ...
... nobody ever gave a good explanation for the TCAS Fail in the ACARS message. Everybody just dismissed it as due to the pitot fails.
I seem to remember that was dealt with by the BEA in Interim Report #2. From memory, the TCAS has an inbuilt function which checks that the static port pressure is valid and that the calculations that result fall within predetermined values. Should the TCAS detect erroneous values, it fails.

I need to check the report for accuracy, but the above is near to it.
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Old 8th Apr 2011, 02:30
  #3175 (permalink)  
 
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Chris, BOAC, let's take a wider view rather than a view immediately local to the MLG.

The belly slaps into the ocean. That provides a sharp dowards jolt to the gear. Perhaps tens of milliseconds later the wing becomes involved near the wing roots. There is a slap on the MLG doors mashing them upwards and likely shattering them. So far nothing has touched the MLG itself except a sharp deceleration. Perhaps it has already broken loose from its uplock and started to descend. At some point the MLG doors, whole or partly shattered touch the MLG. The doors are not nearly strong enough to contain the momentum of the MLG. It simply mashes through the doors or forces them (whole or not) out of the way as momentum pivots it to an extended position. Note that about the time the MLG hits the water the large wing surface is going to slap on the water providing a very sharp deceleration to the wing structure. (I'd not be surprised if the spars folded
up in the middle.) The MLG tries to keep going. The pivot point can't. But the truck and tires end can continue so it all pivots open.

I'd be surprised if the MLG was still neatly stowed. It would indicate a severe (too heavy) over design on the uplocks. I would expect to see a lot of torn and bent materials in this area due to these relative forces.

Mechanical Engineering is not my primary field. But visualizing the sequence simply requires a picture of the plane (Wikipoodle has some) and an ability to think in very small steps. Look at it and try to pretend you're seeing, in your mind's eye, that you are watching an extreme slow motion cutaway picture of the events.
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Old 8th Apr 2011, 03:05
  #3176 (permalink)  
 
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Cool

Hi,

When we have the exact location of the wreck .. it will be interesting to compare his position with the position of the vertical tail when discovered
Could there be from those data whether this vertical tail detached on impact or before?
Is the distance between the wreckage and the vertical tail is logic with reference to wind speeds and current in this area?
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Old 8th Apr 2011, 03:35
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Originally posted by JD-EE ...
The belly slaps into the ocean. That provides a sharp dowards jolt to the gear ....
I thought I had proposed a similar look at the relative forces involved in post #3106, but may be it got overlooked, because I believe that Chris Scott came up with a similar scheme a little later in post #3143.

Last edited by mm43; 8th Apr 2011 at 03:47. Reason: added Chris Scott post #.
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Old 8th Apr 2011, 03:49
  #3178 (permalink)  
 
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Oh nuts. That's a line I meant to edit and I posted it too soon. There is no real downward jolt to the wheel. There is a massive upwards jolt to the fuselage which would transfer to the wings as an initial upwards jerk on the hinge assembly and the uplock assembly.

That aside, your comment should not have been glossed over. It is a key to the landing gear down issue. It needs to be looked at step by step with the forces applied to the MLG assembly's various attachment points at least noted if not analyzed.

(Sometimes ya gotta mention things here several times to get them noticed.)
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Old 8th Apr 2011, 03:54
  #3179 (permalink)  
 
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JD-EE

I sometimes prefer to be invisible.

My vote is that the gear was never lowered, and flaps and slats were also not extended.
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Old 8th Apr 2011, 04:00
  #3180 (permalink)  
 
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And that raises an interesting naif style off topic question.

Presume you have something like an A330 falling like a leaf in some form of level stall with minimal forward velocity. What happens if you could extend flaps on ONE side only. Could that be guided into a new configuration from which escape from the stall would be possible?

I think it would tend to toss the wing with no flap extension downwards. Then the tail would point the nose downwards at a bloody scary angle. But that might allow enough airspeed pickup to allow the plane to get engines started and pull out to level flight.
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