Alt Hold uncontrolled rise on Pixhawk 2.1 setup

Switching flight controller 0n quad with kde 3520 400 kv motors.
Put Pixhawk 2.1 on and the latest beta 3.5 software for previous 2 days.
Had trouble tuning.
Saw 3.5 stable released so put it on yesterday. All seemed well.
Initial flight had P for roll and pitch set to channel 7 with .07 to .2 min/max
Took off with p at 1.5 (which is default.
Took off in Alt Hold mode and all seemed well. Previous squirrely flight Issues on the beta were gone as I wasn’t getting any pitch and roll rocking. After about 1 minute took it up to around 15-25 feet and turned dial up to what I’d guess was maybe .17 or .18 for the P on pitch and roll.
Quad start rising, and had no throttle response. (not sure but might have turned knob down for Pitch and Roll P during the rise). Also some yawing started but not extreme. Reluctantly switched to stabilize mode (which was terrible under the beta 3.5 on the previous 2 days) and with throttle very low it started falling straight down. I gave it some throttle and it reacted by hovering about 3-4 feet off ground. I then landed it very quickly. I think the yawing stopped when I switched to stabilize but can’t be sure. It this all happened in 5-10 seconds would be my guess. Afraid to fly again, till I figure out what was wrong. In meantime, I’m going to do another setup from scratch if I can figure out how to erase everything and start from scratch. I think this is the bin file from correct flight on Pixhawk 2.1.

Is there a tutorial on how to read the bin file for events?

You had a low-battery failsafe, hence the climb and yaw. (Failsafe is set to 22v).

There’s some good info on log analysis here:

Thank you. I will read the analyzing data logs. Maybe I can figure out how to read the logs. Giving Pixhawk another try after having mixed results with the first version 2 year ago.

Didn’t think to check my battery after landing. Wasn’t up that long and had plenty of battery when I took off. . I thought I had disabled low battery failsafe as I was just doing short test flights in my back yard. The battery is reading 23.4 right now. I have two large, but low c rating batteries for the quads. Using higher C ratings for my x8’s.

Does this mean that if I’m in alt hold or maybe flying in loiter that I will see it rise on low battery? But if I switch to stabilize I regain throttle control?

Since my battery is much higher now, I’m wondering if the battery failsafe was triggered momentarily during climb and then recovered a bit (back above the 3.6 failsafe) at the same time as I switched to stabilize mode. I didn’t think switching from one flight to another would eliminate the failsafe trigger. Just thinking out loud here as I am just learning and trying to catch up on arducopter.

You’re right that even after a failsafe is triggered, the pilot can always retake manual control by changing the flight mode. If the pilot does that, then he/she is back in control and the failsafe won’t trigger again. It’s really a safety feature, we wouldn’t want the autopilot to second guess what the pilot wants and try to repreatedly take control away from the pilot. If the pilot changes modes after the failsafe, we trust the pilot intends to do that and wants to resolve the failsafe himself/herself.

rmackay9, thanks. That makes perfect sense and is appreciated.

I have a switch set up to enable rtl. If failsafe is triggered by say low battery, then I change flight modes and regain control, then want to re-engage rtl, can I engage my switch and have rtl engaged?

If yes, I assume rtl would only work as long as I have a good gps signal?

Where can I find some help on reading logs. Want to understand vibrations, plus when I tune PID’s via a rc nob, what should I be looking at in the logs to show optimal settings. I’m not looking for a racing setup, but rather low and mid to large kv motors for filming.


Re the RTL, yes it will re-engage if you use a switch. Yes, RTL requires a good GPS signal.

Here’s some advice for large copter tuning:

  • For large copters (with props at least 13inch or 33cm diameter) set the Rate Roll and Pitch filters to 10hz (in Copter-3.3 these are RATE_RLL_FILT_HZ and RATE_PIT_FILT_HZ

This is from the AutoTune page:
The only other advice I have is in the tuning sections:

When tuning, the parameters to adjust are mostly the Rate Roll/Pitch P, I and D values.

I like running an autotune on a new vehicle, it often takes a few tries to get good numbers but it’s the best way to find the maximum accelerations for the frame, ATC_ACCEL_R_MAX, ATC_ACCEL_P_MAX

I will give autotune a shot. Just need to leave my back yard for that. Im on arducopter v 3.5 so assume I will set the Rate Roll and Pitch Filters to 10hz. Does auto tune start from where my setup is when I start flying as it’s not too bad right now.?

I’ve 3520 motors @ 400 with 14 and 15 " props depending on load.

I’d like to get it flying similar to my Inspire and mavic. Not sure if expectations are too high, but that’s what I’m aiming for.

Thanks again. Appreciate the help.