$100 Gift Card to whoever solves this: Airspeed Creep - Airspeed artificially increasing - Fixed Wing (ROUND TWO!)

This is another post on this problem, this will be the most thorough.  I am not the only one with this issue as it dates back to at least one year ago as far as I could tell from scanning these forums.  Here is a link to a previous post I did that is the same problem.  Disconnecting the airspeed sensor is the only solution, which is not acceptable on multiple levels.

DOWNLOAD LINK for tlog, rlog, waypoint files, parameter file, kmz f...


While sitting on the ground after awhile the airspeed will creep up to 20-31mph.  This happens within 10-30 minutes of bootup, sometimes sooner.  It only happens outdoors, today's temperature was 91deg F with no wind (1-4mph).

1.  A Preflight Calibration will put the airspeed down to 3-10mph with a mean of 8mph, BUT it will creep up again.

2.  Multiple calibrations does not solve this problem.  Update: Calibration hangs most of the time when attempting to calibrate.

3.  If the aircraft is sitting in my house with no wind and constant temperature the airspeed creep problem does not present itself within 9 hours of use. 

4.  The autopilot and the sensor have been replaced.  Wires from the sensor and the I2C board has been replaced.  No other wires have been replaced.

5.  I can not make this problem happen by physically manipulating any wire or component.

6.  If I blow into the airspeed sensor it reports back an increase in airspeed.

7.  Max operating temperature of the airspeed sensor is 221deg F, a temp reading of the sensor put it at 127deg F for the Goodluckbuy version, 2/3 of them, not trying a third.

8.  I have confirmed that the wires are connected to the proper pins for pixhawk, airspeed sensor and I2C board.

9.  This is a NEW pitot tube, it is not clogged or dirty in any way.

10. I had an electrical engineer deconstruct the circuit using documents provided by the manufacturer of the 4250DO component, it looks to be correctly interfaced and the tracers on the circuit look fine.

11. Wire length has been shortened to less than 7 inches, the problem persists.

12:  Disconnecting the Sonar does not solve the problem.

13: This very much seems to be a heat induced problem, either that or it is linked to spinning the motor (4 amp draw) somehow.  All of these tests are done on the ground for obvious reasons.

*****14. POST UPDATE:  So I built another bird (referred to as "second aircraft").  The photo of it is in the same link as the original posted link.  New aircraft, new transmitter, new pixhawk, new airspeed sensor SAME Problem. 


15.  POST UPDATE:  Putting the bird in auto calibrate for airspeed (and flying manual for a long time, 20 minutes) does change the airspeed ratio as it should but this does NOT fix the problem. Original value was default of 1.9733, new calibrated value fluctuated from 1.0023 to 4.756.

16.  POST UPDATE:  When in a throttle controlled mode (auto, RTL etc) the bird does cut the throttle to ZERO as it should because the airspeed is reading 47mph when it was actually traveling closer to 20mph.  It is trying to slow down to meet target airspeed (throttle stick was confirmed to be LESS than 50% so it was not interfering with the throttle nudge ability).  So I know for a fact that it is not the TECS energy management logic that is faulty it is for sure a bad sensor input.

17.  POST UPDATE:  Changing the ARSPD_TUBE_ORDER variable from 2 to 1 does make the airspeed more stable.  Two things, it causes Pixhawk to beep three rising tones (VIDEO OF THIS BEEPING LINKED HERE) when airspeed hits or appears to get close to zero (and does NOT report any errors) AND it does NOT solve airspeed creep

The weird stuff:

1.  I once disconnected the sonar to troubleshoot this.  After that, when I picked the bird up to see the affect on altitude THE AIRSPEED went to 15mph.  I was able to repeat this consistently 4 times, then stopped and cried.

2.  Sometimes with the airspeed sensor disconnected the sonar will report bad lidar health for no reason, indeed it once did not report anything at all (a zero for alt), a reboot or two solved the zero altitude problem but did not make the bad lidar health issue go away.  That still happens occasionally (rare).  The sonar checks out fine in all ways I can test it.  It's good.

3.  I armed the bird to let it sit so I could make a pure tlog file demonstrating this issue.  Upon getting the issue to appear I tried to re calibrate it, it told me the bird was flying when it was not and therefore it would not perform the calibration.


Goodluck Buy v1.1 airspeed sensor (2/3 melted), 2 other manufacturer types tried as well, JDrone and another no name brand.  All airspeed sensors tried thus far are digital.

V2.4.8 Pixhawk 1

Two 3DR gps/mag units

Castle Creastions Pro BEC

RFD900+ Telemetry Radio

I2C board

7S Power Module (not from 3DR, they don't make one)

Castle Creations ESC

Maxbotic I2C XL 1242 Sonar

4 Digital Hitec servos with some wire extensions to reach the autopilot

buzzer/switch/usb extension

Futaba receiver with a PWM module to connect to pixhawk


1.  Goodluckbuy has picked a manufacturer that is making these sensors in a different way than 3DR endorsed airspeed sensors were, thus causing this problem - I had two out of three of this brand fail by melting down at 127deg F (see pics).  Not trying the third I just threw it away instead.

2.  This is an airspeed algorithm issue - making the most sense at this point

3.  Temperature is causing this issue somehow - No longer think this is an option due to multiple platforms being used and in much cooler temperatures (72deg F ambient to 103deg F ambient)

4.  Jupiter has aligned with Mars and the age of Aquarius is upon us - probably the best cause in my humble opinion

5.  Noise from the servos due to long (12inch) extensions is somehow playing into this (except for when the bird is indoors?) - No longer thing this as other platforms have long servo cables and work, also the signal inputs from servos are isolated from the inputs from the I2C components

6.  GPS inaccuracies are feeding sensor data to an algorithm that over a short time reports back that the only way to make all that jumping around make sense is to say the bird is flying and flying at 20-31mph. - Not likely but I can not rule this out because I have not seen or understand the code itself

7.  Could the Sonar be causing this somehow? - No longer think this because the NEW platform does not have a sonar and it too has the airspeed creep issue.

8.  POST UPDATE:  I now firmly believe that this is a software issue that started somewhere after 3.0.

The question:

What is causing the airspeed to creep up artificially and thus causing the aircraft to stall?

Others with the same problem:




I am offering $100 Gift Card to whoever can help me solve this.  I flew for YEARS without this issue and now all of a sudden it's a problem.  Will mail the gift card to anywhere in the world.


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I think this is an issue with airspeed sensors with extended cable lengths. Most of the clone airspeed sensors are missing the necessary level-shifter chip to bring up the voltage on the I2C bus to 5v. That's not a problem for the short cables, but when you extend them more than about 10 inches it cause dropouts and errors. 

One solution is to just mount the airspeed sensor closer to the flight controller so you can use a shorter cable. 

Cable length is 7.75 inches.  Maybe that length plus some Texas heat causes this.  Is there a way to tell if the airspeed sensor has that level shifter?  I have a magnifying scope I can see components with.  If not, can you point me towards an airspeed sensor that you are sure has this ability?  I'll buy it ASAP, I'm dying over here :)


I'm trying a sensor from JDrones at this point.

So here are some updates. 

I bought three new airspeed sensors from different manufacturers.  One is running right now and I get the same problem.  This new sensor is running on a shortened cable, its less than 7 inches long.  The ASS (Airspeed Sensor :) used in the logs provided in this thread got so hot one day that it BURNT MY FINGER TO TOUCH IT.  I've retired it.

I examined that hot sensor under a stereo microscope and saw nothing to say that this ASS is faulty save for the slight browning of the 4250DO component.  I have not taken the 4250DO sensor apart.

I had an electrical engineer deconstruct the circuit using documents provided by the manufacturer of the 4250DO component, it looks to be correctly interfaced and the tracers on the circuit look fine.

I studied up on I2C.  Line noise could still be a culprit here but adding an amplifying circuit to all of my builds is not an option.  I do not have a means to measure line noise.

$100 USD Gift card to whoever can help me solve this issue.  Will mail to anywhere in the world.

No one?

No solution but perhaps vaguely similar. Analog 3DR sensor, cable length 135mm. Multiple reboots, calibrations etc. Airspeed refuses to be zero when it is blatantly zero (indoor and sensor covered), see attached pics (units set to mph).

try a three wire shielded cable. I use a shielded cable from old computers which was used in old type of CD drives. also cover the pitot holes when you powerup the APM if there is wind around. Not needed if the breeze is calm.

So, it sounds like you have spent lots of time diagnosing and debugging the airspeed sensor. My guess is that it isn't the airspeed sensor directly.

Going back to basics, I would first ask if you made any changes in either software or hardware in the months or weeks before this started happening. Sometimes I make a change that creates a bug, but I won't catch it until weeks later during certain kinds of testing.

It also sounds like you have noticed this relates to heat. I know you're in Texas, but is it possible to fly this in cool weather. Maybe you can fly it really early in the morning before it gets hot. If not, perhaps an indoor, mock wind tunnel just using some box fans might help you to simulate it.

That would at least allow you to know if it is a temperature problem.

Thanks guys!  The analog sensor not hitting zero is correct.  It has to do with the  equation that drives the output.  When you get up to speed, say more than 10-15 mph its accuracy gets very good.  Mine has always bounced from 3-8, I have built more than 40 of these things (different types and configurations) and they all did that with airspeed sensor.  TY for your reply Andreas!

@Hushel  I think you are right in that its not the airspeed sensor directly.  I'm thinking it has to be code somehow, specifically somewhere in the way GPS inaccuracies getting fed back into the airspeed estimation but I do not code so I can not say.  I can't think of anything I've done differently here but I'll keep looking.  I'm going to disable use, but it in auto cal mode and throw her in the air to do donuts in the sky and see what happens.  TY!

Thank you for clarifying this. I have been noticing that, when flying, the AS values are broadly believable. On the other hand, the non zero values I see on the ground are generally small so I 'd been wondering if the vehicle simply flies with a small constant error.

Anyway, best of luck solving your problem.

Have you a photo of the aircraft and the airspeed sensor on it?

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