I have a general question on APM 2.5 Throttle Failsafe, and then will follow up with tlog, log files and OSD video later...

I have been flying a reliable 3DR Robotics Y6 on release 3.1 for several weeks and yesterday had a bad crash from 400ft up.  (For equipment  Ardcopter 3.1 (APM 2.5), flying a 3D Robotics Y6, Frame 10, DragonLink, 5.8 ghz FPV etc.)

My question is,

  • If I had the throttle failsafe setup and tested (fires at loss from DragonLink set to 1026 low)
  • and show that Channel 3 raw did go to1026 for two seconds (some reason loss TX from DragonLink...  yet to be determined..  may be FPV CAM interference) ....
  • and if I see that the MP Tlogs show that the System stat went to "5 - Failsafe" two seconds after the TX loss ....
  • and it wasn't a BROWNOUT on the APM board...
  • and that the Battery FAILSE in not enabled ....
  • and was flying in Stabalize mode at 400 ft up ....
  • and I see in the TLOG (and MP telemetry log replay) that the APM board DISARMED immediately after the FAILSAFE event ...   INSTEAD of doing an RTL (the main reason I have Failsafe turned on!)


  • what kind of circumstances could have caused the APM to DISARM at the FAILSAFE event, when the copter wasn't sitting on the ground and/or with the throttle set to zero?


I have full OSD video of the flight, along with TLOG and downloaded LOG file.I will follow up with submitting these for help.

In the meantime, I would appreciate it if someone knows of any general conditions that could cause such an un-welcomed response from the APM Failsafe function....



I had this same failure on my Hexacopter on November 2013.  Randy helped me back then on 2.9, but we never found a root cause.  I subsequently thought it was a "flaky" GPS, but now on 3.1, a different APM board, GPS, another new Y6 model, new DragonLink TX (instead of 2.4), etc., I thought I had long left that issue behind.


I was fortunate that the fully loaded Y6 feel to its death from 400 feet upside down.  Had it hit feet first, my GoPro 3, Tarrot Gimbal, and 8000 mah bat would probably have been destroyed, if not in flames.

In any case, other than everything being sheered off the upper deck, including the APM 2.5 board, all parts seem to be functioning and just need to rebuild.

I just really need to find the root cause of this type of crash before risking again.  I know that there are others that have run into this issue and hope that someone might have put their finger on this issue.




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    • Sorry Randy.  I mis-spoke.  I'm still trying to determine whether or not this existing "bug" was the one that bit me twice.  In any case, I would like to apply some "repellent" until it is irradicated in the ppm encoder chip, just in case.

      Randy, do you (or any body else who has an opinion) think that because of the following setup, I might have experienced a "brownout" to my DragnonLink receiver.  My Y6 has a heavy payload and I was flying high taking on some wind, and also can see that the "amp pull" before the crash was getting spikes above 65 amps.

      I had the following power setup:

      The Y6 is totally run from an 8000 mah 4s LiPo.  There is an inverter off the 4s to supply the 12 volt devices (like tarot gimball, FPV Sony CAM, video switch between Sony CAM & GoPro 3 and mav-linked JD board / LED lights).  I had the 4.5 volt power to the DragonLink receiver being supplied from the input rail on the APM board.

       I'm in the process of rebuilding the Y6 now and plan on adding a 5am BEC to give dedicated 4.5 volt supply to the receiver, but unless the crash is determined to be caused by something other than the encoder bug, I'd sure like to feel that the BEC might have solved the problem for future flight.

      I'd appreciate any thoughts or facts on this issue. 

      Hopefully Rob might spot and/or confirm the "smoking" gun in the logs..




    • @ Mike Governale

      Perhaps a simple small (1300 mAH)  2S battery with a linear or (Switching / linear) BEC (I call that voltage regulator) would provide a "clean" power to your APM and receiver.

      Main Advantage: No ESC noise conducted or radiated (assuming you dont have long wires traveling near your "noisy" voltage lines..

      Bonus: Even if the main battery sags badly due to high power demand, your Rx / APM will always have a clean constant power.

      FYI: I did a simple test:

      Looked at my APM (Ver 3.1.2-RC1) altitude (on mission planner V1.2.95), Hexacopter on the ground was about 0.5 0.6 meter.. Good.

      Took the aircraft outside the garage (very very sunny and lots of UV).. Altitude jumped to 156 meters!

      Using a black foam which let the air circulating but stops the light on top of the MS5611-01BA03 cured the problem.

      How do you protect your barometric chip from light?



    • Hi Henri,

      "How do you protect your barometric chip from light?"

      I've seen lots of references like this and being intrigued/concerned, I have scoured the data sheets. I have not found any mention of light being a factor in measurement. What will make a difference is temperature (and voltage); exposing the unit to strong solar radiation will cause rapid increase in temperature. This will have a direct effect on the substrate of the device and it will cause a pressure rise within the APM case and the device case as the air expands. One could test this by using say a cree torch in a dark environment to illuminate the sensor while noting the altitude (pressure).

      The main advantage of adding foam will be to slow heating from solar radiation, and providing damping from wash when near the ground and noise from turbulence etc.

      One would assume that by the time you have set-up and the systems had settled, the vehicle should be at ambient and so temperature induced offsets would be constant(ish).

      I personally use a 20mm square of 5mm soft foam with a 10mm square removed from the centre, and another 20mm square that sits on top of the first. I've not experimented with type of foam etc.

      (Not intending to hi-jack your thread Mike)

    • Thanks Henri,

      My APM 2.5 is in a case.  I've previously checked the foam and added black electrical tape on the top of the case to avoid sun rays.

      In my rebuild, I'm adding a separate 3s 12volt Lipo for the FPV along with a separate 6 volt UBEC off the 4s flight battery dedicated just to the DragonLink receiver.

      Mike Governale

    • Back in the day of 5C Lipos we worried a lot about Rx brownouts when using BECs powered from the main batteries. I got in the habit of flying separate batteries entirely to power everything except the motor(s). It's more management but it sure eliminates any questions of the brownout sort. My T-Rex 600 even has dual redundant batteries and regulators (Gemini system). It feels nice knowing that the mains are not doing a thing except powering the motor. Presently I have a couple of little lipos on my Hex powering my Brand X nav lights and my Brand X brushless camera gimbal - that way if some bit of crappy hardware shorts out it doesn't take everything down with it.

    • Thanks Oliver.

      During my rebuild, I've removed the JD-board switch/ LED's and have gone back to providing power to the FPV CAM, Tarot Gimbal, an 5.8 ghz Video TX.  I will still hook the 8000 mah 4s to the APM 2.5's power model, but plan on stripping off a separate 4.5 volt supply to the DragonLink receiver with a 5amp UBEC.

      I hope to see a stable "hardware voltage" log on the APM 4.5 volt side after this, as others have indicated.

      In any case, would you then recommend this power arrangement?

      I don't know the wattage pull from the DragonLink (have a support question in now), but would think that it couldn't have been too much more than my original FRYSKY's receivers on other models.  Do you have input.

      In any case, I would hope that pulling an isolated 4.5 volts off of the main 4S flight battery with the 5a UBEC would provided added assurance that the DragonLink receiver would not experience a low-voltage brownout, but I could still be wrong if the 6 870kb motors are momentarily punched.

      The other alternative would be to supply the DragonLink 4.5 volts from the 12volt FPV system, but afraid that this might introduce an even more probably failure point, and expose the critical receiver to unknown power dirt.... 

      In any case, I would appreciate your (and any others) advice..


      Mike Governale

    • Mike, I changed my FrySky receiver for a Dragonlink, and noticed a couple of times, that when powering by USB, the Pixhawk would not always boot. Just a few times...

      And the Pixhawk supply is more solid than the APMs. I think the load to the USB is marginal, so not really an APM/PH issue.

      Anyways it seems that the Dragonlink pulls heavier than the original Rx's.

      To be on the safe side, I'm going to do a solid evaluation of the power distribution, although on battery, there doesn't seem to be any problem.

      I power the RX through the Pixhawk from the power module, my video and gimble off of a balance connector of one of the batteries.

    • Thanks David.

      Please let me know the results of your further investigations.

      Also,  if I get a response from DragonLink on their exact receiver power requirements, I'll let you know.


  • MR60

    Do you use a PPM encoder ? If yes I found this known bug on the google drone discuss group :

    By the way, we found what appears to be a bug in the current ppmencoder software today when using a ppmsum receiver and when power to the receiver is lost.

         The discussion is here:  http://www.diydrones.com/forum/topics/rtl-mode-cuts-motors-off-and-falls-to-the-ground

         the video is here:  https://www.youtube.com/watch?v=qDMRW04Ui2o

         The problem can be recreated by doing this:
              1. attach a ppmsum receiver to the APM2
              2. disconnect only the power wire to the receiver
              3. the throttle will drop temporarily to 1100 before dropping to 900

         If the user has the FS_THR_VALUE set to say 960 (like I do) this has the nasty effect of reducing the throttle to zero for a moment before the failsafe kicks in.  The bad news being that if you're in stabilize mode it causes the copter to disarm in the air instead of initiating the RTL!

         If this is fixed in the new version then I'm suddenly very interested in it being released.


    I have no idea if this corresponds to your context but it describes a situation where your copter could disarm in mid air instead of intiating a RTL as planned...

    • While awaiting Lefebvre to review my logs for this crash, I watched the video link and read the discussion.  This sure is the exact symptons of my two crashes but I don't know if it is the root cause yet.  In any case, I'm wondering since my DragonLink Receiver is pulling its power from the APM 2.5 board, whether or not it could be experiencing a temporary brownout (similiar to pulling the power wire, like you indicated would cause the two value response, and subsequent disarming).

      From the logs, when I could see the Amp draw shoot up as I was ascending (it was windy up there), and was thinking that maybe the secondary voltage dependencies (like the DragonLink receiver) might be suffering shortages.

      If so, would this cause the same symptons that you have referred to?


      Mike Governale

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