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Atlas Air 767 down/Texas

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Atlas Air 767 down/Texas

Old 6th Mar 2019, 13:49
  #301 (permalink)  
 
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Originally Posted by B2N2 View Post
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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Old 6th Mar 2019, 13:50
  #302 (permalink)  
 
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Originally Posted by B2N2 View Post
[QUOTE There is some flawed thinking here.
An object released to freefall will experience a [i]gravitational acceleration until atmospheric friction/drag will stabilize its speed at terminal velocity. ]
Corrected for semantics.[/QUOTE]

I think we in reality agrees, but we might have a slightly different meaning regarding acceleration.

According to Einstein a sattelite in orbit is free falling around the earth in its orbit, experiencing zero G. Standing still on the face of the earth we are experiencing 1G.

What you was describing is what most people would count as an acceleration, the (vertical) speed starts at zero, then the speed increases as the object is free falling towards the earth. And thus the object must be experiencing a G of some magnitude since it is falling faster and faster. But this acceleration is only perceived and is dependent on the observer standing still and experiencing 1G. An observer free falling together with the object would see no acceleration of the object and would therefore conclude the object was not under any acceleration and thus experiencing zero G.
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Old 6th Mar 2019, 14:18
  #303 (permalink)  
 
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Yes thank you.
The rest of your assumptions remain flawed though.
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Old 6th Mar 2019, 14:23
  #304 (permalink)  
 
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Originally Posted by Contact Approach View Post
There is so much bollocks written on this forum as soon as any accident occurs.
Originally Posted by SteinarN View Post
According to Einstein a sattelite in orbit is free falling around the earth in its orbit, experiencing zero G.
Einstein said that? Who knew? I always thought it was Fig Newton...
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Old 6th Mar 2019, 15:24
  #305 (permalink)  
 
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Let us all wait for the NTSB to do the work.

I understand some of the PPRuNe group needs to vent all the ideas from the comfort of there arm chairs. But, all this discussion is not helping answer the question of "what really happened." Let us all wait for the professional to produce the result.

We lost three fellow airman, may they rest in peace.
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Old 6th Mar 2019, 15:32
  #306 (permalink)  
 
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mustanfsally, there's nothing wrong with venting ideas as long as it comes from those sufficiently qualified to make comment (though admittedly that's not always the case). That's what PPRuNe is all about didn't you know? If it upsets you, I guess it's best just to ignore this thread.
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Old 6th Mar 2019, 15:32
  #307 (permalink)  
 
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Originally Posted by Airbubba View Post
Einstein said that? Who knew? I always thought it was Fig Newton...
Well, they both did. You have Newtonian gravity, and you have Einsteinian gravity. In most cases the equations yeld practically identical results, however, when the values are extreme, as in an object in close orbit around another extremely dense object, like a white dwarf, a neutron star or a black hole, the equations yeld completely different answers.

Here on earth we need to resort to Einsteinian gravity and his theory of relativity in order to get the gps system to work, among other things. If we didnt apply Einsteins equations to the gps system we would see the position of a non moving gps receiver on the surface of the earth drift a couple km each hour. This is due to the fact that the time, and hence the clocks, on the gps satelites is running ever so slightly faster than the time and clocks on earth.
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Old 6th Mar 2019, 15:35
  #308 (permalink)  
 
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RE 767 ballscrew AD

https://www.federalregister.gov/docu...-767-airplanes
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Old 6th Mar 2019, 15:45
  #309 (permalink)  
 
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Originally Posted by B2N2 View Post
Yes thank you.
The rest of your assumptions remain flawed though.
Just a honest question.
What do you calculate the average vertical G to be in order to start at zero vertical speed at 6000 feet and then reach the ground in 18sec?
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Old 6th Mar 2019, 15:54
  #310 (permalink)  

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this is according to laws set by Albert Einstein.
No. The law was DISCOVERED by Einstein. It was always there.
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Old 6th Mar 2019, 16:06
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Response of the stabilizer to a failure of the jack screw or its nut and braking system would be greatly influenced by the position of the attached elevators. The horizontal tail normally carries as downward load (less so at aft CG, more so at forward CG). With the stabilizer hinge line forward of its center of pressure, the load on the jack screw is usually in the direction associated with rotating the stabilizer in the leading edge up (i.e., airplane nose down) direction. If the linkage controlling stabilizer position were to let loose I would expect the stabilizer to rotate in that direction causing the airplane to pitch nose down. In response (either by way of the autopilot or the crew) the elevator would be commanded in the airplane nose up direction (elevator surface trailing edge up) that would put further moment on the stabilizer to drive it in the airplane nose down direction.

We also need to remember that the lift generated by the tail is only a small portion of the total lift of the airplane. For the overall airplane to enter an essentially ballistic free-fall the wing angle of attack would have to decrease to that for zero lift. A horizontal tail mechanism failure that allows the stabilizer to float will result in nose down pitching moment that would tend to decrease wing AOA, but it is not correct to assume that a floating stabilizer leads to the wing experiencing AOA corresponding to zero lift. It is very possible that such a failure would result in sufficient nose down pitching moment to drive the wing to a significantly negative AOA to cause negative g.
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Old 6th Mar 2019, 16:07
  #312 (permalink)  
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CONSO, this indicates a "proposed" rule-making. Was the AD actually published and is it in force today? Tx.
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Old 6th Mar 2019, 16:22
  #313 (permalink)  
 
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Originally Posted by SteinarN View Post
Just a honest question.
What do you calculate the average vertical G to be in order to start at zero vertical speed at 6000 feet and then reach the ground in 18sec?
I donít know as this would depend on where terminal velocity is reached and this is dependent on a set of variables.

Unless Iím recalling my rusty theory incorrect G-forces are accelerating forces and indicate a rate of change rather then velocity. Same as a VSI shows rate of change.
Standing in an elevator I feel acceleration and deceleration forces.
If the vehicle that Iím in reaches a constant speed and Iím at the same speed I wonít feel ( measure) anything.
If the speed of the vehicle changes and Iím firmly attached to this vehicle I will sense (measure) a change.
Starting at altitude I can have an average rate of descent which is based on altitude change/time but it can be continuously accelerating or deceleration after initial acceleration.

This is where Vg diagrams come in and the definitions of the various speeds such as Va.
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Old 6th Mar 2019, 16:24
  #314 (permalink)  
 
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Intact airframes do not free fall in the atmosphere

Just a honest question.
What do you calculate the average vertical G to be in order to start at zero vertical speed at 6000 feet and then reach the ground in 18sec?
Drag, thrust and lift work in three dimensions. Only gravity works in a single dimension.

Etkin's Dynamics of Atmospheric Flight devotes 75 pages of serious math to General Equations of Unsteady Motion
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Old 6th Mar 2019, 16:26
  #315 (permalink)  
 
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Salute!

@ the wait-and-see folks ----- we should be fairly confident that our discussion of the MCAS on the 610 accident was worthwhikle and may have prevented another accident, ya think? And all that good info was before much of a formal report was released from the relevant safety agency. As tdracer , et al have stated, an obvious system or hardware failure that might exist, or be possible on other aircraft of this type would likely be announced early on by the NTSB.
If I were flying this type today I would be glommed onto this thread like flies on sierra.

Gums sends...





.
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Old 6th Mar 2019, 16:38
  #316 (permalink)  
 
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Originally Posted by PJ2 View Post
CONSO, this indicates a "proposed" rule-making. Was the AD actually published and is it in force today? Tx.
AD 2008-06-06. Still in force.
Attached Files
File Type: pdf
2008-06-06.pdf (29.0 KB, 45 views)
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Old 6th Mar 2019, 16:44
  #317 (permalink)  
 
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Originally Posted by B2N2 View Post
I donít know as this would depend on where terminal velocity is reached and this is dependent on a set of variables.
Correct me if I'm wrong, but average vertical acceleration would simply be a function of vertical distance travelled and time (if initial vertical velocity is zero).

6000' in 18 seconds gives a = 37 ft/s≤ if my maths is correct.

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Old 6th Mar 2019, 16:48
  #318 (permalink)  
 
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Originally Posted by FCeng84 View Post
Response of the stabilizer to a failure of the jack screw or its nut and braking system would be greatly influenced by the position of the attached elevators. The horizontal tail normally carries as downward load (less so at aft CG, more so at forward CG). With the stabilizer hinge line forward of its center of pressure, the load on the jack screw is usually in the direction associated with rotating the stabilizer in the leading edge up (i.e., airplane nose down) direction. If the linkage controlling stabilizer position were to let loose I would expect the stabilizer to rotate in that direction causing the airplane to pitch nose down. In response (either by way of the autopilot or the crew) the elevator would be commanded in the airplane nose up direction (elevator surface trailing edge up) that would put further moment on the stabilizer to drive it in the airplane nose down direction.

We also need to remember that the lift generated by the tail is only a small portion of the total lift of the airplane. For the overall airplane to enter an essentially ballistic free-fall the wing angle of attack would have to decrease to that for zero lift. A horizontal tail mechanism failure that allows the stabilizer to float will result in nose down pitching moment that would tend to decrease wing AOA, but it is not correct to assume that a floating stabilizer leads to the wing experiencing AOA corresponding to zero lift. It is very possible that such a failure would result in sufficient nose down pitching moment to drive the wing to a significantly negative AOA to cause negative g.
I was giving some thought to what the wings AoA would be after the stabilizer had reached zero AoA. As the aircraft should be more or less nose heavy the nose should drop down decreasing wings AoA. My thinking is that when the wings reach zero AoA then the complete aircraft is free falling (when talking about vertical movement) and the aircraft is no longer nose heavy, as there is zero G on every part of the aircraft. However there would still be some momentum left, causing the tipping down of the nose to continue past the zero AoA of the wings, and then the wings should start to produce negative lift as you speculate. When the wings produce negative lift and the nose is still heavy (as in center of mass beeing forward of wings center of lift) then the aircraft should go more nose up again. So there might be some ocillations. My thinking is then that as the vertical speed increases the aircraft must point more and more nose down in order to keep the wings at pluss/minus zero AoA.

So, I would speculate that a free floating HT where the hinge point is forward of the center of lift line will cause the aircraft to quickly reach approximately zero G and thereafter increase vertical speed at approximately 10m/s2 (1G) which would take an object from 6000 feet to the ground in 20 sec.
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Old 6th Mar 2019, 16:52
  #319 (permalink)  
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Originally Posted by FCeng84 View Post
. . . It is very possible that such a failure would result in sufficient nose down pitching moment to drive the wing to a significantly negative AOA to cause negative g.
Though the entire empennage is lost, the nose-down effect is the same - note the extreme downward bend of the wings just prior to impact. This came up briefly during the MCAS discussion: A26 video.
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Old 6th Mar 2019, 16:57
  #320 (permalink)  
 
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Originally Posted by DaveReidUK View Post
Correct me if I'm wrong, but average vertical acceleration would simply be a function of vertical distance travelled and time (if initial vertical velocity is zero).

6000' in 18 seconds gives a = 37 ft/s≤ if my maths is correct.
My idea with the "average" value is the same as yours, namely what value of constant G or acceleration would cause the object to travel the required vertical distance in the required time.

6000 feet in 20 sec gives zero G, free fall or approximately 10m/s2, 6000 feet in 18 sec gives your slightly larger value which is a slightly negative G.
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