Question Why does my Garmin G3X and G5 show 38 mph when not in motion?

[Neal]

Went to my maintenance shop today for a few inspections including the 24 month transponder cert. The guy asked why my airspeed was reading what it was when not in motion and I never really paid attention to it as it seems to work fine for flight, etc. What are others seeing on theirs and if you have any suggestions what may be going on with mine let me know. The tech alluded to it may be a setting somewhere.

 

[Cactus Charlie]

My IAS on ground and not in motion is as shown below. I see see non zero IAS during run up which is due to poor static port position but I have come to accept that.

 

[Neal]

I reached out to g3xpert, got a quick response which is nice and they instructed me to recalibrate the ADAHRS which I'll do next time out to the plane.

 

[Cactus Charlie]

I find it very surprising that the G5 and GDU both display the same airspeed error. The G5 has its own internal sensors and does not depend on the GSU 25 data. It is improbable that the G5 sensors have drifted by exactly the same amount as the GSU 25 sensors.

Except for G3X reversion modes the only thing the G5 and GDU airspeed displays have in common is the aircraft pitot static system.

When did this IAS error first appear? (data log review?)

I would open the pitot line to eliminate any possibility of trapped pressure before I attempted any calibration.

 

[Neal]

Yes, I thought the G5 was separate so I'm not sure how both are doing this. Good idea on the data log. I went back to initial, delivery, and various and it shows around 10 kts for IAS. It seems to only have gone up to around 14 to July which is my latest data I have on my computer.

The one thing that may be of interest is it was raining on the day of this observation so I wonder if there was water over the pitot static port or on the pitot tube? I'll see how things look next time at the airplane and also run the pitot heat a bit in case I have a guest inside even though I always keep the pitot tube cover on when hangared.

 

[Neal]

I find it very surprising that the G5 and GDU both display the same airspeed error. The G5 has its own internal sensors and does not depend on the GSU 25 data. It is improbable that the G5 sensors have drifted by exactly the same amount as the GSU 25 sensors.

Except for G3X reversion modes the only thing the G5 and GDU airspeed displays have in common is the aircraft pitot static system.

When did this IAS error first appear? (data log review?)

I would open the pitot line to eliminate any possibility of trapped pressure before I attempted any calibration.

How would you suggest I do this?

 

[Cactus Charlie]

I installed my G5 so it may not be the same as a factory installation.

On my aircraft the easiest way to open the pitot line would be to pull the G5 and disconnect the pitot line from from it. I have enough slack in the harness and tubing to do that. If you don't you may need to pull the GDU.

Other places to open the line would be at the left wing root (in line fitting) and near the GSU 25C (T fitting). Pulling the G5 allows a comfortable seated working position and only requires one captive screw to be touched.

I looked at my own data logs for a very early CubCrafters engine run and for the time at which I made the screen shot I gave earlier. The results were interesting. I expected the IAS to be zero or "no computed data" but both were about 13 kt. I assumed there must be a "minimum displayable airspeed" configuration setting but I have not seen it.

A search for - G3X touch "minimum displayable airspeed" gave some interesting AI generated information and links to discussion at SuperCub.org and EAA. It seems likely the minimum displayable IAS is not configurable and is 20. That agrees with the end of tape value in my screen shot.

Note that Garmin cautions that incorrect usage of an air data test set can damage the differential pressure sensors. Since the G5 and GSU would be subject to the same damaging input they could both show errors. Can you correlate the increase in IAS error with anyone working on your pitot static system?

 

[Neal]

No one has touched my plane since taking delivery other than yesterday for compression check, borescope, and transponder cert. I have not touched anything pitot static related to date, everything has been as expected there, AOA, etc.

 

[Neal]

Went to the plane today and turned things on and this is what it showed. Guess maybe it was rain water over the port?

 

[Cactus Charlie]

What is the static IAS in the flight log after this miraculous recovery?

You seem to have about 30 ft difference in altitude readouts. Has that been constant or has it been increasing with time?

My altitude difference increased until it became intolerable and I had to run a G5 altimeter calibration.

Garmin's claim that their sensors don't drift is somewhat removed from reality.

 

[Neal]

My last plane didn’t have the G5 calibrated so CC paid to have that done under warranty. The new plane under a new avionics person had it nicely calibrated to around 10ft difference and it’s drifted since. I’m curious of the DA affect and need to see if it changes between summer and winter.