Also this.
Had this recently. One of our guys could not figure out why his A-10C lacked elevator authority. Turns out an Xbox controller he had plugged in for other games got assigned the pitch axis. The Xbox mini-sticks are bad for sitting just off center to whatever side they were last pushed (needs a tiny nullzone) so it was commanding a minor pitch and fighting against his full command from the stick. The result was failure to takeoff.
Itās quite possible if the two devices are at opposite ends of their axis that sort of toggle up/down could happen.
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The tailplanes move all the time, even without stick input. Itās the FBW controlling the airplane. Of course this could be PIO, but it could be an error in the FBW as well. Bad tuning of the control circuit could easily cause such a thing. The question is just what are the exact circumstances that make it enter oscillation.
Surely with triple redundancy and cross checking of info that wouldnāt be the case here? Especially as it happened more than once (suggesting it is either a failure in the logic of the FBW occurring over more than one flight) or it has to be an external input. Which is why we need to know whatās happening with the stick or the position indication on the stabs.
The tailplanes move all the time, but this is an extreme input. Does setting altitude hold on the AP stop it? Or does it knock the AP off? If you hold a specific AOA does it continue? What about s specific throttle setting, can you power through it? Is it affected by speed brake operation or even Landing gear deployment. Anything that affects the airflow over the tail.
There has to be a reason for it and Iām convinced its physical hardware. Especially as it continues to happen when bank is increased and the nose starts to wander off sideways and the porpoises continue.
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DCS FBW doesnāt tend to show, at least not in the Hornet. The controls surfaces in the Hornet match the user controls pretty much all the time, within the limits allowed by FBW, but we donāt see any of the autonomous corrections like you see watching a real Hornet make a carrier approach.
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I wonder if someone with a viper could upload a video of it on a gentle power off decent and hands off stick and see what happens with the tail.
Maybe stick AP on altitude hold and cruise power too and see how far they deflect on its own. Just out of curiosity more than an answer to this problem.
All fine and dandy, but look at the video.
There you see the tailplanes going bang-bang from one limit to the other.
Thatās no FBW action.
Thatās something badly assigned in the control Axis.
DCS is an incredibly realistic product but hereās something that doesnāt make sense in that regard.
Sounds of hooves - most likely itās a horse and not a zebra.
I agree here, Iām prepared to be proven wrong but that is such a violent input on the stabs movement i cannot see it being anything caused by the plane itself.
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Au contraire, watch a Viper taxiing.
They arenāt going bang-bang, they are moving proportional to pitch rate. Problem is, the control circuit doesnāt apply enough damping.
See here, i wonāt convince you otherwise and you wonāt convince me. A track or video with activated controller input column will settle this in a heartbeat.
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Yep, totally agree here. All guesswork till more data comes in
Absolutely agree.
Also apologies, I didnāt mean to try and convince or procure bother. 
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I know you didnāt, neither did I. No apology necessary.
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Well, I tried a few ways of making it happen on purpose.
Pitot off, engine de-icing off, trim set poorly, two pitch axis with various forms of disagreement, INS shut off, etc. No luck, even intentionally. I can get it to buck, but it wonāt keep it up unless I time the inputs to amplify the problem - and even then it hardly dramatic.
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@Ragnar can you make sure you have the probe heat set to On?
I canāt test it myself until later but I suspect itās due to the pitot static system being iced up, which DCS has simulated for quite a while.
Your groundspeed is 700+ in the F2 view, as well as very low AOA, while your airspeed in the HUD is only around 250 and Mach .5.
Iām not super smart on the Viper systems but itās likely using the air data coming in from the pitot static system to program appropriate flight control inputs.
Thus, the aircraft believes its only at 250 knots and is programming way more control deflection than is needed at your actual speed.
Of course I could be completely wrong - looks like @Wes did some testing on this already since I started typing - and Iām making the assumption DCS uses air data instead of the raw speed from the sim. I also canāt see if thereās a super strong tailwind invalidating my speed comparison.
Iāll try to do some testing later as well. Just curious, what was the starting condition for the slot? Air start? Ramp?
Of course, itās another issue entirely as to whether or not youād actually ice up going that fastā¦
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After a bit of testing, I havenāt been able to recreate an icing condition - I canāt get any indications of a frozen pitot system - but I also canāt offer an explanation as to why the F2 menu speed and altitude differ so much from the HUD displayed values.
How did you get -6 for temperature? Lowest I can set mine is 0C.
There seems to be some confusion about the axes vs. stability of aircraft. I have spent many hours trying to explain this to ATPL studentsā¦
In this case, the pitching motion of an aicraft is along its longitudinal axis, but about its lateral axis.
Stability in pitch is characterized as longitudinal stability. It is the airplanes ability to dampen oscillations about the lateral axis.
I refer to Aerodynamics for Naval Aviators, pages 243 and 279. 
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There are NO controller conflicts, there is only the Saitek X52 Pro attached to the system.
As to pitot icing - possibly, but pitot heating is automatic (switch in centre position)
Also this problem is intermittent, it can happen above or below the cloud layer and self corrects after a while. I also noticed that the HUD speed was dramatically different to the ātrueā speed.
I have flown hundreds of sorties in the Viper in a variety of atmospheric conditions but have never had this experience before. I am sure it is related to the low temperature.
Solo sweep.miz (295.3 KB) )
In response to your initial reports, I discounted the possibility of icing, as I was under the impression DCS does not simulate such atmospheric conditions. I was wrong. It does. I think iced-over sensors sufficiently explain the phenomena. However, reproduction seems to be an issueā¦