Assistence explaining crash (loss of altitude despite throttle at max and switching modes)

Unfortunately I cannot provide any logs as my quadcopter is at the bottom of a lake. The flight controller was a cheap APM 3.1 Mini upgraded to 3.3. I have not yet performed AutoTune on it, but on the 10-15 flights I performed on it, it flew alright. The altitude hold was not perfect (after a sudden stop it would tend to gain altitude), but overall was ok.

I finally configured my transmitter to allow for more flight modes so I was able to try Position Hold for the first time. I ensured my GPS acquired satellites, thenI took off, flew around for a minute on a full battery, then went over a small lake. As I was returning I notice I was slowly losing altitude. I cranked up the throttle, but it did not seem to make a difference. I switched to Stabilize, but it kept losing altitude. It hit water before making it to shore.

It seemed as if it was in landing mode, as the decline in altitude was fairly smooth. However, I did not have Land as any of my failsafes, nor did I have the Land mode enabled as one of my available modes… so I don’t know why it would go into Land mode. The best explanation I could come up with is some software bug (I imagine with a hardware malfunction things would be more abrupt… but I might be wrong).

I did not go very far and my transmitter gave me no warnings of loss of signal. The battery was also full.

Probably what’s worse than losing the quad is not understanding why it happened. I have another APM mini board around, but I’m not sure I want to put it on my next build. Any suggestions would be greatly appreciated.

Here is a link to my apm parameters, in case it could be of any use:

Generally a slow lose of altitude is a weak battery or a battery that is going dead. We will never know.


There have been some developments. Firstly, it looks like my APM mini was running 3.2.1, so it looks like I posted in the wrong section. I apologize. Admin, if possible please move this thread to the correct section.

I went back today and managed to fish the quadcopter out of the pond. I managed to pull a video off the GOPRO’s SD card. I cleaned the APM with some rubbing alcohol, plugged it into the computer and it appears to be working. I was able to pull some data logs from the APM. However, I do not know how to interpret them. Any assistance with this would be greatly appreciated.

Here is the crash video:

Here are the data logs:

The log analysis had the following:

Log File C:\Users\user\AppData\Local\Temp\tmp354B.tmp.log
Size (kb) 601.2822265625
No of lines 7749
Duration -2 days, 20:16:18
Vehicletype ArduCopter
Firmware Version V3.2.1
Firmware Hash 36b405fb
Hardware Type
Free Mem 0
Skipped Lines 0

Test: Autotune = NA -
Test: Balance/Twist = NA -
Test: Brownout = FAIL - Truncated Log? Ends while armed at altitude 5.62m
Test: Compass = GOOD - No MAG data, unable to test mag_field interference

Test: Dupe Log Data = GOOD -
Test: Empty = GOOD -
Test: Event/Failsafe = FAIL - ERR found: FLT_MODE
Test: GPS = GOOD -
Test: IMU Mismatch = UNKNOWN - No IMU log data
Test: Parameters = GOOD -
Test: PM = NA -
Test: Pitch/Roll = NA -
Test: Thrust = NA -
Test: VCC = FAIL - VCC below minimum of 4.6v (4.487v)

The VCC error sounds like it could be a battery issue as Mike suggested. My battery is a 5200mah 10c and the quad had 4 800kv motors, vtx and a gimbal. Perhaps all those were drawing to much amps. I will need to do some math.


Congratulations on getting your vehicle back! That’s good news.

Your video states that you attempted stabilize mode. The log indicates you attempted AUTOTUNE mode (and that failed). AUTOTUNE can only be entered from ALT_HOLD or STABILIZE in the firmware you’re running.

It is unclear to me why CTUN.ThrOut wouldn’t be following RCIN.C3.

You didn’t mention an uncommanded climb out over the lake - there was a climb in the video, can you confirm it was uncommanded?

It looks like from the logs and the video that as it started to fly over the lake that is was going into the wind and this was causing an uncommanded climb as the logs show the desired altitude lagging behind the actual altitude.
Later the copter is turned around and now it is flying with the wind and the copter starts to fall. We can see from the logs that the desired altitude lagging behind the actual altitude again.
Since we don’t have the battery level or the RCOUT from the motors we can’t determine why the copter was falling.


Thanks for the insights, Peter and Mike.

The climb and fall were definitely uncommanded. I think the reason could be because I did not put black foam on my barometer. My APM Mini came in a plastic case, so I assumed I would not need the foam, but having done some reading since it sounds like I should have put some in there. I have been experiencing climbs when braking hard.

In a moment of panic, I did go into AUTOTUNE at one point by accident, but quickly switched to STABILIZE right after (just before the flight I added more flight modes using a combination of two 3-way switches). I added sounds for my modes to my transmitter, or otherwise I would not have realized I enabled AUTOTUNE. One thing I know is that I cranked up the throttle to the max once I noticed the quad started rapidly losing altitude… I’m guessing no matter mode I was in, that should have made it go up.

My theory is that this was battery related as Mike first suggested. I think I made a big newb mistake ignoring the C rating of my battery. The 5200mah 10c battery can only deliver a continuous current of 52 amps. The max continuous current drawn by my motors is 17 amps x 4 = 68 amps ( plus ~ 1 amp for the other electronics). It looks like it’s definitely possible to go over, as I did go full speed at one point on the lake.

Does this seem like the likely cause? Does the behavior in the video match what would have likely occurred if I exceeded the battery’s maximum output? Any other things you guys can suggest I can do to help analysis like this in the future (enable logging of given things? any hardware I could install?).