Constant pitching down command when it lost RC signal

#1

Hello,

I flew seaplane type UAV last month and unfortunately crashed due to RC signal lost during the flight.

Here is the flight log. (Latter part of log after crash is trimmed)
https://logs.px4.io/plot_app?log=916e1a7b-a887-435d-bf82-a0a9cde214f5

When it loses RC signal, failsafe was activated, which was supposed to let aircraft go back to home point. However, it rather simply pitch down and dived into trees.

Here is pitch angle in the log.

Does anybody know what would lead this behavior?

Airspeed sensor was not working correctly in this flight, maybe because water went inside the pitot tube when cruising on water. And I suppose 0[m/s] reading in airspeed caused pitching down to gain speed and ended up with this behavior… What do you guys think?

#2

I’m sorry to hear about the crash.

What I can see from the log is that airspeed is always around 0:
https://logs.px4.io/plot_app?log=916e1a7b-a887-435d-bf82-a0a9cde214f5#Nav-Airspeed

Could it be that airspeed was not working correctly, or clogged? It would explain the pitching down because the estimator was thinking it was too slow and trying to prevent a stall.

I can see how we might be lacking a check for a working airspeed sensor which would warn the pilot as well as ignore it and fall back to the ground speed using GPS.

#3

Thank you for your comment!

Yes, there was something wrong with airspeed sensor at that time. I see, then that’s the cause.

Before this flight, I knew that airspeed reading is constant zero somehow. This happened only after cruising around water surface, thus I think water came inside the pitot tube and get clogged…

I still decided to fly because I was just flying in stabilized mode. RC signal lost is something that I didn’t expect, and it led auto mode then caused this pitching down.

Exactly, I would love to have that feature. I hope I or my team can contribute for this in the near future.

Do you think changing a parameter “FW_T_SPDWEIGHT” can prevent this?
It was default value 1.0. But if it was 0.0 then it does not control pitch to gain speed?

#4

I don’t know. Let me ping @dagar and @Paul_Riseborough, they might have thoughts on this.

#5

There is a pending PR to address airspeed sensor failure - see https://github.com/PX4/Firmware/pull/10733

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