I think the fact that the stick is also bent fwd (when it was originally straight) is a major clue.
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- Thread starter BillL
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I think the fact that the stick is also bent fwd (when it was originally straight) is a major clue.
I think it should be clarified that there was no connection between the service bulletins issued in early 2014 and this accident. When Service Bulletins 14-01-31 and 14-02-05 were issued by Vans in early 2014 (for potential horizontal stabilizer cracking near the spar and potential elevator spar cracking), there was early speculation that they were related to this September 2013 accident. The NTSB at the time quickly acknowledged that they were not.
See this information from Vans:
https://www.vansaircraft.com/images/open/NTSB-info.pdf
To my understanding, there were no design changes or Service Bulletins implemented as a result of this accident.
Noah
Well Known Member
Interesting. Sure looks like its bent aft to me. It looks like it's leaning forward because it's broken in two, but the bend appears to be aft to me.
Yes, but these service bulletins were the direct result of owner/builders inspecting their tail feathers in the aftermath of this crash.
As for the broken pilot-side control stick, please read the report instead of jumping to conclusions from pictures. The control surfaces would tear off long before the control stick would fail. The failure of the control stick is due to impact damage.
Clarification.....
Actually, only the one for elevator spar cracks was a result of owner discoveries.
Actually, only the one for elevator spar cracks was a result of owner discoveries.
Thank you for being on this board..
I lose count of the times Scott has to spoil opinions with facts here.
I appreciate it.
I lose count of the times Scott has to spoil opinions with facts here.
I appreciate it.
Back to G's and Failure Sequence
Based on the factual findings regarding both C-GNDY and N174BK, it looks like the forward VS spar failed first, just above the doubler that ties into the HS. As the VS moved aft it then broke at the aft spar just above where it ties into the bars that are used to mount both the VS and the HS. This aft movement then tears the rudder in half, followed by the lower rudder bearing being compressed and bent into failure. The lower rudder, now just attached by the control cables, flops back and forth with the cables sawing the fuselage.
As this sequence happens, ancillary damage occurs and varies depending upon many factors. Balance weight lost, rudder blown apart, and in the case of BK, left HS/Elev failure - due to a rolling moment.
The big questions I have center on the breakup speeds calculated as 234K and 220K, and the recorded/calculated maximum G loadings of 3.5 and 2.5.
So, was there a last pull that overloaded the structure that was unrecorded?
Could the speed alone start the failure sequence? The '7 rudder is offset to the left.
Flutter was discounted, but not eliminated as a factor in C-GNDY. Vans tested to 217K and the Canadians estimated that with the addition of 1 pound of filler and paint, the calculated flutter speed would be just above 250K.
It would be very reassuring if the probable cause showed a G loading in excess of 9 (or even 6). That is presumed, but not recorded or even directly addressed in either accident analysis.
What am I missing?
Best regards,
Merrill
Based on the factual findings regarding both C-GNDY and N174BK, it looks like the forward VS spar failed first, just above the doubler that ties into the HS. As the VS moved aft it then broke at the aft spar just above where it ties into the bars that are used to mount both the VS and the HS. This aft movement then tears the rudder in half, followed by the lower rudder bearing being compressed and bent into failure. The lower rudder, now just attached by the control cables, flops back and forth with the cables sawing the fuselage.
As this sequence happens, ancillary damage occurs and varies depending upon many factors. Balance weight lost, rudder blown apart, and in the case of BK, left HS/Elev failure - due to a rolling moment.
The big questions I have center on the breakup speeds calculated as 234K and 220K, and the recorded/calculated maximum G loadings of 3.5 and 2.5.
So, was there a last pull that overloaded the structure that was unrecorded?
Could the speed alone start the failure sequence? The '7 rudder is offset to the left.
Flutter was discounted, but not eliminated as a factor in C-GNDY. Vans tested to 217K and the Canadians estimated that with the addition of 1 pound of filler and paint, the calculated flutter speed would be just above 250K.
It would be very reassuring if the probable cause showed a G loading in excess of 9 (or even 6). That is presumed, but not recorded or even directly addressed in either accident analysis.
What am I missing?
Best regards,
Merrill
Well, I am not going to say you are missing anything, because without data showing what the flight dynamics where at the time of break up (how many G's were pulled, actual airspeed reached, etc, we will never be able to do much more than speculate.
I think that is why the NTSB didn't point a finger at a specific initial failure point.
An entirely different but fully plausible possibility is that the initial failure was the left stabilizer in the downward direction. This would align with the evidence that indicates a possible pull out from a high speed vertical decent (the loading would be downward on the stabilizer). This could then have started the sequence of the fwd vertical spar failure and all of the other parts that departed.
With the rudder bottom appearing to be the first part in the debris path it would have had to have been influenced aerodynamically in its fall, which I think is possible since this portion by it self would be rather light but have a lot of surface area. It appears to have landed ~500 ft from the majority of the other components that separated.
I admit, there is no evidence to indicate this scenario is any more likely than yours or any others, so my point is that I think the NTSB did the best the could with data was available, and that stating much more than a loss of control would be guessing on their part just like we are doing.
BillL
Well Known Member
Good question - -
Looking at both accident pictures and reports the sequence of failure suggested for the VS/rudder seems consistent with the pictures and comments in the structural NTSB report. Not sure when the counterweight might have separated, though. They did state (hint?) that "much" could have happened between radar return points. Looking at the plot of time vs G loading, it was not a rapidly increasing G loading. Unless the AC was not relatively stable roll wise. It seems a lack of g loading early in the maneuver allowed the speed to build up anyway. I don't understand the "roll" graph they had, and it could totally change the picture. It seems like the roll (presumably inverted) and pull to breakup was 6 seconds.
Bent stick. Either forward or backward, the loading case prior to impact would be normal force at the handle and a torque at the base. That kind of load yields a maximum bending stress at the pivot. So, if pilot forces were to bend it, it would not be nicely bent in the middle. Honestly, it looked like impact damage to me. Maybe someone would know if the stick was always bent that way.
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I used to fly a T-34A and was caught up in the 2004 wing spar failure, AD, and subsequent AMOC.
One of the many things I learned was that the 'damage' of one 6 G event equaled many, many 3 G events. So if you experience some number of high G events you can have unseen damage to the infrastructure, especially if the loads were asymetrical (...yanking and banking...).
And I'm not a metallurgist nor an aero engineer.
