SOLVED: Very unusual loss of servo control, Nimbus 1800

I went to the field today to get some flying practice in. After a normal assembly and pre-flight check I put the bird at the launch site and armed the hardware via the GPS button. The bird sprang to life and was ready for takeoff as expected. After a few minutes I spooled the motors to about 5% throttle to see that I had good spin. About 3 seconds before I hit takeoff I heard a strange sound coming from the bird. I walked over and the left front servo was limp while the right front motor was behaving as expected. The noise was from the prop being stalled because the servo was limp but the motor was trying to spin the prop into the ground. The servo was acting like it either had no power or had no signal. Despite my efforts, I have no idea what’s going on with it except that it might be a software bug…

Airframe is Nimbus 1800
Firmware is 4.0.9
Pixhawk Cube Orange autopilot
This airframe has over a dozen flights on it, all were perfect
Via a voltage checker I am able to verify that the servo is getting 8.2V from the PixHawk Orange power rail (it’s a 2S servo).
Via a continuity checker I am able to verify that the servo has a continuous connection from the autopilot to the servo via the “signal” wire.
I can confirm that the servo itself is good because I connected it to a servo tester and it’s all great.
I have inspected every mechanical connection from the tip of the autopilot side connector to the servo and I can’t find any mechanical issues.
This behavior persists through all restarts and airframe assembling.
All other motors/ESCs/servos behave as expected.
Wire inspection shows no exposed cores or other damage.
No parameters were changed from last successful flight to this incident.
Airframe response to the hand held transmitter/controller behaves as normal.
No physical impact/mishandling occurred.
This happened either at the moment of arming via the hand held transmitter or very shortly after, the aircraft was LIVE and ARMED and ready for takeoff.
Servo is plugged into Aux Out 2.
No errors are being reported.

Log analyzer report:
Log File C:\Users\thear\AppData\Local\Temp\tmpAA6A.tmp.log
Size (kb) 31717.7705078125
No of lines 303800
Duration 0:02:18
Vehicletype ArduPlane
Firmware Version V4.0.9
Firmware Hash a632c813
Hardware Type
Free Mem 0
Skipped Lines 0
Test: Autotune = NA -
Test: Brownout = GOOD -
Test: Compass = WARN - WARN: Large compass offset params (X:-42.88, Y:-121.43, Z:286.79)
WARN: Large compass offset in MAG data (X:-42.00, Y:-121.00, Z:286.00)
mag_field interference within limits (4.43%)
Max mag field length (604.35) > recommended (550.00)

Test: Dupe Log Data = FAIL - Duplicate data chunks found in log (111339 and 117337)
Test: Empty = FAIL - Empty log? Throttle never above 20%
Test: Event/Failsafe = GOOD -
Test: GPS = GOOD -
Test: IMU Mismatch = GOOD - (Mismatch: 0.05, WARN: 0.75, FAIL: 1.50)
Test: Motor Balance = NA -
Test: NaNs = GOOD -
Test: OpticalFlow = FAIL - FAIL: no optical flow data

Test: Parameters = GOOD -
Test: PM = NA -
Test: Pitch/Roll = NA -
Test: Thrust = NA -
Test: VCC = UNKNOWN - No CURR log data

Parameter File: Nimbuss 1800 VTOL Params v2.7.param (21.8 KB)

Flight Log file is here.

Hi Chad,

Thanks for posting your .bin file. Your servo output functions from the log are below. Guess what function is missing? Function 75 for tiltMotorLeft. I’m not sure what happened here but it looks like your servo output was lost or written over.

Note that Function 75 for tiltMotorLeft is also missing from your attached Nimbuss 1800 VTOL Params v2.7.param file.


Changed servo 10 to a value of 75 and BINGO!

This happened because between my last real flight with it and this practice flight I was making a 1 axis gimbal mount for the payload, which meant that I was changing servo values. I must of accidentaly changed servo 10 in stead of servo 8 and didn’t realize it. The limpness of the servo went undetected as long as it did while sitting on the takeoff pad because as part of my pre flight list I manually move both servos to the 90deg “point to the sky” position in order to avoid having the props hit/dig into anything when the servos go live.

I do remember seeing the tilt of servo motors while walking back to the GCS as NOT being in the correct angle, but only slightly so so I let that detail go.