Yes, though that doesn't seem to be relevant to what actually happened.

A "zero G" descent (or indeed a descent at any constant vertical acceleration value) follows a parabolic profile (ask any trainee astronaut). The actual descent profile, as far as it's possible to ascertain from FR24, was at an almost constant flightpath angle.

Well, I am not arguing this or that beeing the cause of the crash, I am only speculating as to the effect of an free floating HS.

With that said I really thought the vertical speed increased significantly all the way down to the ground?
If it was at an almost constant flight path angle, then the transition from more or less level flight about 20 sec prior to the aircraft hitting the ground, to the constant nose down angle must have happened extremely quickly and hence required a very high value of negative G for a few sec

Taking a step back it appears to me that something quite dramatic occurred to rapidly transition this airplane from relatively stable, level flight to a very steep dive that took it from 6000' to the water in less than half a minute. We can postulate all sorts of root causes - hopefully the combination of CVR, FDR, and examination of key portions of the wreckage will shed light on what actually went wrong. This event happened so quickly that I don't see how it could be associated with stabilizer motion at rates that the jack screw is capable of generating. It is possible that elevator hard over (both sides) could have led to this result. I would hope that the time histories of the tail surface positions, pitch attitude, roll attitude, and normal acceleration are made public sooner than later.

For now I think that it would be prudent for current 767 operators to make sure that they are up to date on all maintenance and inspections associated with horizontal stabilizer and elevator control. The 767 has a very extensive and solid history of safe operation so I would be very surprised to see design listed as a cause of this loss.

Sorry, but my expertise is in propulsion/engines - I'm not very knowledgeable about the structural aspects or details of how the flight controls work.

Pretty much all I can add in that respect is that I was on the Boeing Safety Review Board (SRB) for many years - in the aftermath of the Alaska MD-80 jackscrew crash Boeing did a study to determine if any of the Boeing models had a similar susceptibility and determined that the Boeing designs were still safe. But I don't recall any meaningful details.

BTW voyageur9 - I have worked with the NTSB on several occasions and have the highest respect for the people I worked with. Yes, they do tend to get information and reports out quickly - especially when things are reasonably clear cut. Just saying that the NTSB releasing information to the public is not a priority unless they believe there is some safety aspect that should be quickly addressed.

Take a look at post 97, some type of roll cloud or wind rotation could be the trigger to an aircraft structural failure or improper response by either the autoflight system or crew.

I don't think anyone will disagree with that. A screwjack/ballnut failure, on the other hand, could be implicated.

Quote:

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Originally Posted by mustangsally (Post 10408670)

We lost three fellow airman, may they rest in peace.

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Not in any way trying to detract from the technical discussion, here is a blog post about a moving tribute to the jumpseat rider.



https://thepilotwifelife.com/united-...c-seat-vacant/


https://cimg1.ibsrv.net/gimg/pprune....450b36590a.jpg

I'm sure those more familiar with proposed versus final AD, and how to trace can find or give a definitive answer- especially on this forum. I posted it for a link to related information on 767 horiz stab jackscrew and the mention of ballscrew versus MD " NUT" issues.

I have found a bit further related information at

https://www.federalregister.gov/docu...-767-airplanes

and I'll let the experts- and those most familiar with either maintainance records of a given airframe follow up with links if possible to factual data on that 767.
FWIW-around 1982 I had a small task to quickly make a manufacturing ' tool ' to drive the 767 horiz stabilizer during factory checkout before installation of any electrical or hydraulics. I was able to use a piece of 1/4 inch airhose, two screw band type clamps , one on hose over drive spline and one on hose over a 1/8 socket attached to a a low speed 1/4 hp air nutrunner. That kluge was used for a few months until a nicely fitting female match to the spline drive was made attached to a low speed nut runner to move the stabilizer thru its full motions after mounting and basic installation. Of course with no air-loads it was VERY easy and required very little torque. Sort of like your car steering wheel with front wheels off ground- which also uses a ball screw- jack type mechanism.

Correct me if I'm wrong, but we only know it was uncontrollably the last 18 seconds but not before that. Was there a problem already or not? So we can do the math with G forces or not, nobody knows at this time.
One thing I think we can scratch is if it was deliberate because than it would have been controlled.

Thank you, DaveReidUK & CONSO for info on the AD.

Hotel Tangop
 

HT, you miss read me. But, that is OK, my wife does the same. But frequently the comments posted come from someone with little knowledge of 767 systems and how they relate to this accident. I personally have over thirty five years as a crew member but have zero 767 experience. So I don't speculate on what happened or the cause and effect. If we could weed out the rift raft and get those with good understanding of the specific aircraft systems then I would agree with you. But that just does not happen in this pprune world. Just go back and reread the discussion on elevator trim and pivot points of such, jack screws and nuts et all. Maybe that is why I only look at this web sight for entertainment with a couple of pints.

Cheers

If by "two different things" you mean two different frames of reference, then yes. File under "interesting, but irrelevant".

Another video of the crash.

https://www.chron.com/news/houston-t...68603.php#next

In the video clip posted above by, Old Boeing Driver, in the zoomed in part of the video, 0:15s onwards, the rate/angle of descent moments before impact seems to decrease slightly, perhaps control was regained and a pull out of the dive was being attempted? Or maybe it's just an anomaly in the video footage?

-RP

In the second video it does look to me as well as if the plane begins to pull up a little. At that speed of descent such a manoeuvre would likely tear the wings off if the ground had not intervened first.

Without access to the Alert Service Bulletins mentioned in the AD it's difficult to determine how much of the content is applicable and the compliance times. The Bulletins would also indicate the means of terminating inspections and any other closing actions. Given that Wear Measurement and Lubrication are involved it's quite likely
that the repetitive actions have been included in the aircraft's Maintenance Program by now.

Two halves of the same thing:

A pull-out with careful attention to avoiding load limitations won't pull the wings off - it may very well also not be possible in the altitude/time available (BAD)
A pull-out with a hard enough pull to avoid the ground in the altitude/time available may very well exceed the load limitations (also BAD). Or produce an accelerated stall (see Hawker Hunter air show crash - also BAD)

Takes fine judgement and ingrained knowledge of an aircraft's capabilities to thread that needle - in the middle of a "startle" event.