Fly away dangerous racer 250 1.9.1 px4


Don’t know if this is the place but px4 just flew away with no response from input at all.Rejecting rtl and all other failsafes.The bad bit is the full throttle on 500w motors when on the ground it could have seriously hurt someone. I have flown with arducopter and betaflight type softwares many quads 15 ish for years and never had this happen. You need to find this and fix it . pronto(lol I was in panic mode). This software should be grounded immediately.


Hi @battlepenguin, thanks for reporting this and sorry to hear about the crash! We certainly would like to fix that. You could help us with that by providing the log file of the flight as well as detailed notes about the flight. So did you change anything? Was anything suspicious? What happened and how did you react? What were the error messages? Thanks.


Sorry , the cheap card that came with the pixhawk had a totally corrupt file system after. What concerned me at the time was that whilst flying away it refused to disarm and rtl (all reported to qgc at the time. It said rtl , posshold rejected ,disarm rejected due to being in wrong mode or something like that.No manual response in acro or stabilise. Now I realise that disarm is not a kill switch. Maybe that could be made a little clearer in the documentation.
The other thing was that after it hit the garage it fell then went full throttle whilst vertically pressed against a wall and a branch, until the battery ran out. Maybe some kind of sanity check is needed or perhaps the pixhawk had crashed completely by then.

I have since found that the external gps mag was in 100% fail but still. Other than that it is now the same as before the crash. I have successfully used arducopter with the 250mm emax rs2205 little bee fvt 30amp + gps frame for some years . The only different thing is the pixhawk is mounted firmly in soft urethane foam and runs px4. I guess it is yet another faulty pixhawk some how. It gives no errors or warnings from the hardware in qgc which was connected. It was poorly tuned but flew nicely in stabilise mode. I think I will go back to arducopter as it is very hard to tune nicely for acro and seems to have inconsistent flight characteristics. On that frame I use very low pids always. 0.09 0.09 0.0004 similar in arducopter.
You should make the logs available to users more easily also! and reduce requirements for the sd card. It all makes it safer.

If anything else turns up I will post it here.



Just remembered that the battery was not completely ran out so the pixhawk must have regained sanity at that point.


The radio link had low bandwidth so no data.
I am using a pixhawk standard power module only powering the tbs micro receiver and gps. It is possible that the crash caused/exacerbated a problem with the psu.

To sum up

Low flight controller voltage (4.8) warning before take off. Rectifies itself. No other warnings.
Faulty compass in m8n module but with 3d lock 12+sats but no automodes accepted.
Slow fly away no response to controls except switches ,actively and audibly qgc rejects disarm and other auto modes, rejects failsafe rtl,posh,land.
Hits wall , goes nuts flat against wall full throttle then slows motors after 2 min.

With new gps + compass (m8n) it flies fine with same hardware. Psu seems fine now!.

Very strange!. Prolly a bad gps/compass unit affecting psu and issue with automodes and external compass.

I am not putting that gps on again so I will never know.
But please make it clearer that disarm is not a kill switch .


Thanks for the feedback.

If it says POSHOLD rejected, than that likely means either strong mag interference or no GPS because the estimator does not have a confident position estimate anymore.

Yes, the kill switch should always work but disarm only works in manual mode or in other modes when land is detected.

It’s documented here:
but maybe a note about how normal stick disarming works (and when it doesn’t work would be good).
@hamishwillee FYI.

I agree, a crash detection would be nice. It’s always a bit tricky not to produce accidental false positives.
It’s on the list of feature requests but no one has taken it on yet unfortunately.

This is odd. So basically the mode switch was still working but not the controls?
I wonder if this is the same issue as others have been seeing with v1.9 where RC “sometimes” doesn’t work as it should: Without a log it’s difficult to say but it’s something to look into and hopefully fix anyway.
Could you share what receiver you are using? That might be a hint to see if it’s the same as reported in the issue above.


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It is a TBS micro receiver and a taranis x9d plus with a tbs module . I have not had any problems with the radio link other than that before or since.The issue is the rtl rejection and then refusal to change modes even though it can get the commands via the rc.

Seems the problems occur when I try to change into rtl . The wrong mode (loiter) getting selected and rtl rejected .

After it goes into Loiter and not RTL It goes crazy , cutting and starting motors , flying off , actively rejecting rc controls refusing to switch to other modes or just turns off the motors in the sky. Other times it will rtl perfectly. Have flown acro for over an hour to tune it and no problems at all.
The fall from the sky and the flyaway plus crazy part all happened after this strange going into loiter mode instead of rtl and cutting/restarting motors so I am reluctant to switch into rtl to test it.

Ok I have learned how to view the log and am using a good card . It is rejecting rtl and using loiter instead. There is not a reason given in the log for the rejection!.
Gps is good? High intermittent jamming? and the mag I don’t know for sure.There were pylons nearby but they have never caused a problem before.
In another flight with the same gps interference it has been ok apart from refusing to land.

Wrong mode selected for RTL and then erratic behaviour the log also shows strange oscillations in stabilise mode when acro is nearly fine.

ACRO tuning log , no problems since I didn’t change modes.

Phew that was a mouthful



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Wow, in loiter there are weird spikes in thrust, and I don’t understand yet where they are coming from.

@MaEtUgR does this make any sense to you? Could you have a look at the log just above? Thanks.


Also note lots of unusual yaw noise on that flight in the actuator controls fft. The extreme instability in stabilise mode ?.