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Atlas Air 767 down/Texas

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Atlas Air 767 down/Texas

Old 6th Mar 2019, 01:09
  #281 (permalink)  
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Originally Posted by filejw
Donít expect a comment on anything specific untill they do a full read with all teams in the investigation.
Agree, unless analysis of the data shows that there are reasons to communicate preventative safety actions on a timely basis.

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Old 6th Mar 2019, 01:25
  #282 (permalink)  
 
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Originally Posted by filejw


Donít expect a comment on anything specific untill they do a full read with all teams in the investigation.
agree, but I would have expected the team members to already be there and that no comment would have been given by the NTSB unless the team members agreed.

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Old 6th Mar 2019, 01:40
  #283 (permalink)  
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Originally Posted by lomapaseo
agree, but I would have expected the team members to already be there and that no comment would have been given by the NTSB unless the team members agreed.
Well the press release said the the team will assemble in the coming weeks so looks like it will be awhile.
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Old 6th Mar 2019, 02:12
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The exception would be if they quickly suspect there was some sort of failure that could exist on other 767s - in which case I'd expect something out very quickly to alert the rest of the fleet of a potential safety risk. Otherwise expect the sounds of silence for a while as they work through the voice recordings and data.
Satisfying public curiosity is not high on the NTSB agenda.
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Old 6th Mar 2019, 02:54
  #285 (permalink)  
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Originally Posted by tdracer
The exception would be if they quickly suspect there was some sort of failure that could exist on other 767s - in which case I'd expect something out very quickly to alert the rest of the fleet of a potential safety risk. Otherwise expect the sounds of silence for a while as they work through the voice recordings and data.
Satisfying public curiosity is not high on the NTSB agenda.
Precisely.
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Old 6th Mar 2019, 02:59
  #286 (permalink)  
 
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We should get a summary of the FDR data in the next few days according to the NTSB:

NTSB recorder investigators are currently verifying and validating the FDR data, and the NTSB plans to provide a summary in an investigative update in a few days.
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Old 6th Mar 2019, 03:42
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Originally Posted by Educated Airman
It would be interesting to know where the Atlas aircraft had been stored as the environment of storage to determine the probability and speed with which corrosion could occur
Goodyear and Marana for about 2 years. Did the freighter conversion in Singapore.
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Old 6th Mar 2019, 10:43
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Originally Posted by tdracer
Satisfying public curiosity is not high on the NTSB agenda.
Still the NTSB provides far more information, including detailed factual sequences, far faster and with far more transparency than other similar accident investigation bodies. Compare, for instance, the foot-dragging and secrecy endemic in Canada's TSB.
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Old 6th Mar 2019, 11:00
  #289 (permalink)  
 
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I did some calculation.

Starting at 6000 feet/2000 meter, the time in free fall to ground is 20,2 sec. This aircraft took about 18 sec from 6000 feet to ground. This means the G throughout the dive, from start to end, would have to be close to zero or slightly negative.

This seems to imply that the elevator/h.stabilizer must have been at average at approximately zero degrees angle of attack thus producing zero force.

I do wonder, if the jack screw for the stabilizer breaks loose, in which position will the aerodynamic forces put the stabilizer into? Would it "free float" into zero degrees AoA?
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Old 6th Mar 2019, 11:37
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Originally Posted by SteinarN
I do wonder, if the jack screw for the stabilizer breaks loose, in which position will the aerodynamic forces put the stabilizer into? Would it "free float" into zero degrees AoA?
While that might be true, more or less, for a control surface that's hinged at its leading edge (say an aileron), I don't think you can make that assumption for a THS where the pivot point certainly isn't at the L/E..
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Old 6th Mar 2019, 12:37
  #291 (permalink)  
 
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Originally Posted by SteinarN
I did some calculation.

Starting at 6000 feet/2000 meter, the time in free fall to ground is 20,2 sec. This aircraft took about 18 sec from 6000 feet to ground. This means the G throughout the dive, from start to end, would have to be close to zero or slightly negative.

This seems to imply that the elevator/h.stabilizer must have been at average at approximately zero degrees angle of attack thus producing zero force.

I do wonder, if the jack screw for the stabilizer breaks loose, in which position will the aerodynamic forces put the stabilizer into? Would it "free float" into zero degrees AoA?
I don’t think it would free float. It’s not a hinged device. Having said that if the aircraft was loaded with a forward CG and the stabilizer system suffered a catastrophic failure it would have thousands of lbs of force trying to move it to a neutral airload position like the Alaska MD80. That kind of force could drive it to that position and I doubt there is enough elevator force available to override it. To prevent that Boeing has both a main and secondary brake system on the stabilizer if I recall correctly. It would be interesting to know the CG of the aircraft as loaded that day.
Another possibility is that as the aircraft slowed the stabilizer was frozen and not moving. The autopilot would compensate with up elevator until at some point the autopilot desengaged which would produce a instant negative G nose down pitch mode.
It might be very difficult under negative G to apply much if any nose up elevator via the yoke depending on seat position and how tight the lap belts were plus shoulder harness usage. In fact I think it would be nearly impossible. The catch to this theory is the system has a stab out of trim caution to alert to this type of failure and the yoke itself would have been visually aft of its normally position.
If the aircraft had recent work in stabilizer area it would be interesting to know what was done.

Last edited by Sailvi767; 6th Mar 2019 at 12:53.
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Old 6th Mar 2019, 12:53
  #292 (permalink)  
 
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Originally Posted by DaveReidUK
While that might be true, more or less, for a control surface that's hinged at its leading edge (say an aileron), I don't think you can make that assumption for a THS where the pivot point certainly isn't at the L/E..
The question then would be if the pivot point is in front of, at, or behind the center of lift? If the pivot point is in front of the center of lift then the stabilizer should self adjust to zero degrees AoA. If the pivot point is very near or at the center of lift then the stabilizer position would be unpredictable. If the pivot point is behind the center of lift then the stabilizer would deflect to one or the other extreme position, either fully up or fully down.
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Old 6th Mar 2019, 12:57
  #293 (permalink)  
 
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Originally Posted by SteinarN
I did some calculation.

Starting at 6000 feet/2000 meter, the time in free fall to ground is 20,2 sec. This aircraft took about 18 sec from 6000 feet to ground. This means the G throughout the dive, from start to end, would have to be close to zero or slightly negative. Incorrect

This seems to imply that the elevator/h.stabilizer must have been at average at approximately zero degrees angle of attack thus producing zero force. Incorrect

I do wonder, if the jack screw for the stabilizer breaks loose, in which position will the aerodynamic forces put the stabilizer into? Would it "free float" into zero degrees AoA?

There is some flawed thinking here.
An object released to freefall will experience a 1G (gravitational) acceleration until atmospheric friction/drag will stabilize its speed at terminal velocity.
Depending on surface area and drag coefficient this terminal velocity may be significantly less then a dive while still under engine power.
https://www.grc.nasa.gov/www/k-12/airplane/termv.html

Recovery attempt from a steep dive can cause very high G-forces on the airframe



An example of this is here where they descended under power at appr. 12000fpm (!)
https://en.wikipedia.org/wiki/China_Airlines_Flight_006


The fact that the time of freefall more or less equals the time to descent is nothing but coincidence.

Last edited by B2N2; 6th Mar 2019 at 13:31.
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Old 6th Mar 2019, 13:01
  #294 (permalink)  
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On a less technical aspect, the jumpseat rider on flight 3591 Sean Archuleta had a class date with United Airlines.
The class was scheduled for Mar 12 a vacant chair will be in that particular class for Sean in his honor.
Sean's wife who flew up from Colombia on United was presented with Sean's wings and epaulets out of respect for him.
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Old 6th Mar 2019, 13:06
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SteinarN

Here is another example.
In this case an airshow crash of an F-18.
At 1:20 you see a high rate of descent/ high G combination

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Old 6th Mar 2019, 13:15
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Originally Posted by B2N2
There is some flawed thinking here.
An object released to freefall will experience a 1G acceleration until atmospheric friction/drag will stabilize its speed at terminal velocity.
Depending on surface area and drag coefficient this terminal velocity may be significantly less then a dive while still under engine power.
https://www.grc.nasa.gov/www/k-12/airplane/termv.html

Recovery attempt from a steep dive can cause very high G-forces on the airframe



An example of this is here where they descended under power at appr. 12000fpm (!)
https://en.wikipedia.org/wiki/China_Airlines_Flight_006


The fact that the time of freefall more or less equals the time to descent is nothing but coincidence.
No, you are not correct here.

As I am sitting here completely at stand still on my chair in front of my computer I am experiencing exactly 1G (in the vertical direction), the same 1G as the unfortunate pilots in this flight experienced while they briefly was semi stable at 6000 feet. If a 6000 feet hole suddenly appeared under my chair I would instantly start free falling at zero G. If I was to reach the bottom of the hole in 20 sec I would need to fall at an average zero (vertical) G. The acceleration you are describing does not count as a positive 1G. An object which is free falling is by definition experiencing zero G, this is according to laws set by Albert Einstein.
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Old 6th Mar 2019, 13:31
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[QUOTE There is some flawed thinking here.
An object released to freefall will experience a gravitational acceleration until atmospheric friction/drag will stabilize its speed at terminal velocity. ][/QUOTE]

Corrected for semantics.
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Old 6th Mar 2019, 13:32
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Actually, with Alaska 261, once the jackscrew cut loose from the acme nut, the THS was driven fully leading-edge up (nose down) by the aerodynamic forces. And beyond - it actually tilted leading-edge up so far and so hard that the leading edge ripped right through the motor fairing (the bulge on the tip of the MD vertical stabilizer), to 25į or more (normal limit was 2.1į). Resulting in unrecoverable nose-down aircraft pitch. In fact that aircraft performed a half-outside-loop (English Bunt) and was flying inverted on the reciprocal heading when it hit the water.

(There's a little NTSB video presentation on the failure progression here: https://en.wikipedia.org/wiki/Alaska...nes_Flight_261 )

767 may, of course, have different structural geometry - tdracer would probably know.

I'd rate a failed THS jackscrew mechanism in this incident as conceivable and possible, but, thus far, of unknown probability. Effects of a possible turbulence encounter on a weakened jackscrew unit, or switching from auto to manual control at that phase of flight, are suggestive, however.
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Old 6th Mar 2019, 13:38
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There is so much bollocks written on this forum as soon as any accident occurs.
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Old 6th Mar 2019, 13:49
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Originally Posted by B2N2
SteinarN

Here is another example.
In this case an airshow crash of an F-18.
At 1:20 you see a high rate of descent/ high G combination

https://www.youtube.com/watch?v=keCnK4kh65U
He actually did not have enough airspeed to generate a high G condition. He was however at 35 degrees AOA at impact.
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