Warning #1: an issue has been found with Tower's Pause button which can cause the vehicle to fly to an old position if the vehicle has not sent a position update to Tower in some time.

Warning #2: Copter-3.3.2 fixes a bug found in Copter-3.3.1's desired climb rate initialisation which could lead to a sudden momentary drop when switching from Stabilize or Acro to AltHold, Loiter or PosHold.

Warning #3: Copter-3.3.2 fixes an issue found in Copter-3.3.1 which could lead to hard landings in RTL or AUTO if the WPNAV_SPEED_DN was set too high (i.e. >400 or 4m/s) and/or the WPNAV_ACCEL_Z was set too low (i.e. <100 or 1m/s/s).

Warning #4: a bug was found in Copter-3.3 which could cause a sudden crash if you abort a Take-off initiated from a ground station.  Video description is here.  The bug is fixed in Copter-3.3.1 so we recommend upgrading.

Note #1: AC3.3-rc8 corrected a long standing bug in the HDOP reporting.  HDOP values will appear about 40% lower than previously but this does not actually mean the GPS position is better than before.
Note #2: if upgrading from AC3.2.1 the vehicle's accelerometer calibration needs to be done again.
Note #3: set SERIAL2_PROTOCOL to "3" and reboot the board to enable FrSky telemetry like in previous versions.
Note #4: the wiki will be updated over the next few weeks to explain how to use the new features

Copter-3.3.1 is available through the mission planner.  The full list of changes vs AC3.2.1 can be see in the ReleaseNotes and below are the most recent changes since AC3.3.

Sadly this version (and all future versions) will not run on the APM2.x boards due to CPU speed, flash and RAM restrictions.

Changes from 3.3:

1) Bug fix to prevent potential crash if Follow-Me is used after an aborted takeoff

2) compiler upgraded to 4.9.3 (runs slightly faster than 4.7.2 which was used previously)

Changes from 3.3-rc11:

1) EKF recovers from pre-arm "Compass variance" failure if compasses are consistent

Changes from 3.3-rc10:

1) PreArm "Need 3D Fix" message replaced with detailed reason from EKF

Changes from 3.3-rc9
1) EKF improvements:
    a) simpler optical flow takeoff check
2) Bug Fixes/Minor enhancements:
    a) fix INS3_USE parameter eeprom location
    b) fix SToRM32 serial protocol driver to work with recent versions
    c) increase motor pwm->thrust conversion (aka MOT_THST_EXPO) to 0.65 (was 0.50)
    d) Firmware version sent to GCS in AUTOPILOT_VERSION message
3) Safety:
    a) pre-arm check of compass variance if arming in Loiter, PosHold, Guided
    b) always check GPS before arming in Loiter (previously could be disabled if ARMING_CHECK=0)
    c) sanity check locations received from GCS for follow-me, do-set-home, do-set-ROI
    d) fix optical flow failsafe (was not always triggering LAND when optical flow failed)
    e) failsafe RTL vs LAND decision based on hardcoded 5m from home check (previously used WPNAV_RADIUS parameter)

Thanks for your testing!

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Hi Dave,

So this has got me puzzled. We have Motor 1 going a little high for a moment during the full throttle burst but nothing too major. Maybe enough to suggest that the problem may have started there.

We see the negative yaw rate error starts we also see a small positive roll and negative pitch angle error appear and go away. This initial deviation suggests a loss of lift from motor 1.

This is where I would normally expect to see all other motors go low and the copter flip roll positive and pitch negative with a slight negative yaw. However this isn't what happens (obviously).

So the copter does something very strange at this point, it hardly loses roll and pitch control at all and instead starts to spin in yaw (negative) with both motor 1 and 2 high and 3 and 4 low. This is very unusual because motors 1 and 2 have very similar outputs and they stay that way all the way down.

Looking at the logs I see two possible causes of this combination of events.

1. We have partial failure of lift on both 1 and 2. This may be sync issues, loose props, interference on the esc signal line, ice build up on the props (thanks Martin)..... Any other ideas.....

2. One of the arms rotated in flight. Because motors 1 and 2 have the same output and we expect to see a lost of lift from a rotated arm, that would suggest motor 3 (being higher than 4). You have already said this doesn't look like the problem unless the cold allowed them to move. However, you should have been able to see this on the copter after it landed.

What arm broke?

This really has me stumped....

Looking at the video it looks like there is some serious vibration as you go full throttle and the initial yaw rotation looks almost violent and there appears to be a shock go through the frame just as it happens. Looking at the vibration logs you can clearly see this too. Vibrations steadily increase from the moment you go full throttle as the motors spool up (you can hear this on the video). Then BAM there is a massive amount of vibration just as you loose yaw control. You are getting clipping in the accelerometers meaning you are hitting 16g. Your climb rate reaches 3g so your copter is putting some power down during the climb.  So this is definitely pointing to a serious mechanical problem. So maybe the person suggesting your arm broke in flight rather than in the impact was correct. It may not have been a full break and instead twisted partially in place causing the yaw rotation.

Any more information you can think of??

Any update on getting terrain following in the next update?  Is this still on track?  Just curious after having to abort a mission yesterday in a remote location.  Flight started in a bit of a bowl and the intent was to fly the perimeter of a farm about 30 feet above the tree line.   But on the first leg with Iris about 300 yards away heading over rising terrain I didn't like the margins and engaged RTL.  Yes, I know I could sit down with a topo map to plan such a mission but how much simpler it would be if if the waypoint altitudes could be AGL and not relative to takeoff point.  Hoping to see this very useful feature...

I have got my Pixhawk fired up again and loaded with 3.3.2rc2 on my quad.  All seems ok and ready for a test flight except one thing has got me stumped.

When I push the safety button from solid red to put it in safe mode with red button blinking, after about 5 seconds there seems to be power going to the motors as if the ESCs are in "break" mode. I then push the button to get solid red (safety off and armed)  and the break mode goes off and I can arm it and motors all work fine.

When I raise the throttle and lower it quickly there is no breaking so I am 100% sure the ECS are not in "break" mode.

I did a full ESC recalibration and as well set them all to default setting just to be sure the ESCs  "break" mode is off. But even so it still goes into "break" when I turn the safety to off.

If I have it armed and no breaking is present and then just unplug the battery, the motors will spin freely as you would expect.

However if I disarm it and switch back to safety on, if I unplug then while it is in "break" mode, the motors stay in break mode, even with the power off.    They stay in this break condition for about 10-15min gradually fading away.

Anyone have any idea why this is happening?

what do you mean goes into breaking mode? Whenever you remove power from the ESCS/motors they will get "stiff" for a while until all the caps release their charge. This has been this way for well as long as brushless escs came out with caps on them.

Just fix a LIDAR to it.

Are the arms "pultruded" or woven carbon fiber? I've had pultruded lose torsional rigidity after a crash with no apparent loss of longitudinal strength. Longitudinal delamination of fibers allowed the motor to rotate very easily about the longitudinal axis of the tube...unflyable, yet no apparent damage... FWIW.

Hi Leonard,

My final post on this. I'm puzzled by the lack of response since it seems to me that my setup has revealed some problems with autotune.

I took my autotune of last week which was flying quite well and setup Roll_P on channel 6. I was then able to increase Roll_P by nearly 300% and the copter flew a lot better, much more locked-in - less overshoot on roll. This confirms my earlier manual tune where a high Roll_P gave good performance but could not be accompanied by a high Roll_I which led to massive instability.

Since this only happens on roll, my guess is that its something to do with the relatively high CG coupled with the active breaking of BLHeli. I attached a pic of my setup for reference.

(Incidentally the RCTimer SN16A ESCs are flying fine - no issues with temps, athough I have ordered a set of RG20's given all the hullabaloo).


So far, LIDAR sensors like LidarliteV2 don't work in auto flight mode.  I do a lot of mapping is areas with 50m trees lol, not so cool when your mission  is at 80m and your vertical data isn't spot on.  It would be nice if you could set a limited minimum distance between unit and ground independent of the AGL you set in Mission Planner.  If you could set say 20m as you safety margin.  Take off and fly the auto mission at say 80m AGL, but if something encroaches into that 20m the autopilot would ascend to whatever altitude that put 20m between it and the object.  After clearing the object it would return to the requested AGL (sensor pretty much useless beyond 40m).  Or just got get a Nvidia Jetson...  So want one of those lol!

What I was thinking is that if the GPS database includes elevation (maybe a bad assumption?) that when you load waypoints into Pixhawk you could specify AGL relative to each waypoint instead of relative to takeoff elevation.  Seems that would be far more useful and would streamline mission planning in hilly areas.... no need to break out the topo chart or guestimate.

Re-watching the video, I'm more convinced there's a distinct little 'snapping' sound at :43 seconds - like the sound of snapping your fingers.

Followed immediately by chaos.


What are the red motor mounts made of? It looks to have broken off at a narrow point. Consistent with hitting the ground, of course, but also where I might expect failure under sudden strain of full-throttle. Speculation.


Speaking of speculation - where was the black cat at the fateful moment?


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