Plane crashed after lost telemetry signal


I had a crash today with my Skyhunter running on 3DR Pixhawk with latest firmware 3.1.1.

I don’t know why but after review of a tlog it seems I lost telemetry signal for a few seconds and then RTL mode switched over resulting in a crash because I was flying low and too close to the trees. But how is it possible that suddenly I had no telemetry signal when I was just 100-150m away ? I mean this is strange, my plane got destroyed badly and few months of work are gone apparently… If someone could look into my logs, I’d appreciate it.

I can upload onboard video if needed too…

Thanks a lot !

As for the telemetry loss: If you ran telemetry of a USB that does not provide full 5v , or anything else jammed you, it’s not that unlikely.
RTL should not be catastrophic anyway, and telemetry is not a critical feature.

1: I see you descended from about line 80k unlil RTL, was that controlled descend ? - it seems so,
Then RTL kicked in, and seems to maintain the rate of descent, reducing it barely.
It seems to me like it’s very likely to pitch down, like you struggle to fly level, and pull elevator a lot.
As I see it, you never pushed down, and barely a few times let the elevatr stick rest in center.

If you plot RCOUT.Ch2+RCIN,Ch2 (elevator) - it seems like higher PWM means elevator up , , you climb.
After RTL, AP increases RCOut.Ch2 - but the set PWM did not had the same effect as it did earlier in flight.

2: Were the PIDS tuned well ? - did it fly in AUTO modes/FBWA/RTL just fine before crash ?

Anyway - due to the deflection, and lack off response, I guess on a mechanical failure around elevator. Maybe the elevator itself, a servo, or it’s hinges become loose. That would also explain what I think looks like you struggled with a level flight.

BTW: if you switched to any other mode, you’d take over from RTL - for manual mode (where your flight mode selector already was), you needed to switch to any other mode, then back to manual.

Telemetry is critical for me because I don’t use any FPV gear. This plane has been designed to do long range photo mapping flights so FPV is not essential but telemetry in beyond VLOS is critical because it is the only information I have at that moment about the position and status of the plane.

To give you explanation about this particular flight… After my last crash few months ago I decided to put a carbon-fibre cloth with a lot of epoxy glue on top of the body. This resulted in very good protection of the body and thus main electronics are safe. The last crash actually proved that this was a good idea because body has no damage at all. The wings however are destroyed badly, I’m not sure I can fix them again. But the bad thing is that the weight of the body is now higher and this flight was the first to trim all the pitch/roll values on my RC during the flight. So that’s why you see I was trying to trim down the values and it took me almost 2 minutes until I was satisfied and took a controlled descent to do a low overhead flight. I’ve had GOPRO attached on top of the cabin so I wanted to have some closer shot of myself. But then something unexpected happened - uncontrolled crash into near trees. I had about 1 sec to realize what happened but I was too slow to realize that I could save the plane with a mode change using my RC switch. This was my most terrible crash with this plane so far… Very unfortunate circumstances…

What is a mystery for me is the loss of telemetry signal because how can you loose a signal from 67% to 0 during a close LOS flight ? I cannot believe my notebook has bad USB voltage so it’s all mystery. Next time I will use a better ground antenna for telemetry, those 2dbi whips are shit. Maybe it was positioned too low above the ground, I can only guess…

Anyway thanks for a look and response !

When I wrote" not critical" , that was meant as , not a reason to crash, that the autopilot would not be influenced by telemetry loss, in such way.
Of course telemetry is important to many missions and extra safety.

I am not sure how much carbon fibre you used, and how it was placed, but you do know that CF is conductive and can shield radio signals, right ?

CF is metal ? OK I didn’t know it’s conductive…

Anyway my special X-type telemetry antenna is attached to the body from outside. Well I don’t know how much signal can CF take away but I will do tests. I still think the problem was my ground setup, antenna has been attached to the back of my notebook that was sitting about 50cm above ground. Next time I try a mast with a better antenna.

Thanks anyway !

Carbon Fiber, is not metal, but it’s conductive, the carbon is. Just like a pencil.
If you have a big CF surface anywhere, it can easily shield the antenna at proper attitude. if it’s close to antenna, it can change the radiation pattern of the antenna, just like a metal object would do.

This is the Kevlar-CF cloth I have attached on the body and added multiple epoxy layers : … duct=10756
The top layer is this aluminum tape : … duct=43270
On top of that is the antenna in X configuration. Do you think I should remove something ? I mean I cannot remove the cloth anymore because I could destroy the body too. But do you think the alumunum tape has some conductivity too ?

post photo(s) of the result, antenna placement, and areac covered by CF/aluminumfoil (which is also a possible problem source)