Sometimes the G3X sticks at 20 plus knots when stopped. Sometimes it decays to zero over time.G5 generally agrees with the G3x except it never indicates speed when stationary.
The Wind velocity and direction indication on the G3 X is wildly out usual high windspeed, direction as much as 180 degrees out. I believe that the G5 AHARS is independent of the G3X
The fact that the avionics work to some algorithm make this harder to diagnose than with round dials. Seems that partially blocked pitot is part of the mix but could this combine with a hardware issue in the G3X. Incorrect airspeed messes up the wind calculation but could that drive very large errors in wind direction.
I am waiting for the engineers to blow through the pitot lines but wonder if this is only part of the problem. Opinions welcome
I had a similar issue (with mechanical gauges) and it turned out to be a drop of water in a bend in the static line, so it wouldn’t reproduce on the ground. It would cause more problems when descending for landing (although in my draughty old Auster, you can gauge the airspeed quite well just by how the airflow sounds, so it was a bit easier to land without excess airspeed).
This could be anything.
The box will have baro sensors connected to the pitot/static pipework. These sensors tend to be cheap car-type stuff.
Amazed @rate2 you don’t have a normal ASI as a backup. I would never fly a plane like that. And I do embedded hardware/software as a job 
Do the alt and VSI work properly?
Firstly, ignore the wind indications. These are based on comparing the air data/aircraft heading with ground speed and GPS track. Inaccurate air data is going to result in a faulty wind indicator.
G5 and G3X have their own separate ADAHRS and if they disagree you will get a miscompare message. So this is probably just a blocked pitot, no different to diagnose than with a round gauge. Go through your plumbing first as this isn’t going to be an identical hardware fault with both the GSU25 and the G5.
I am not clear that a round dial ASI would help that much when partial pitot blockage is the issue. As far as I can see the altimeter and the VSI readings are sensible. I have little doubt there is something wrong with the pitot system. I am too old and inflexible to get under the panel to blow back through the whole system but gentle prodding with thin wire appeared to show the pitot head and its drain hole to be clear. Unfortunately there is no break point in the lines accessible inside the wing. Thinking about this again I was wrong about a possible the G3x error. Say simplistically I am tracking and heading 270 with nil actual wind the G3X expects to see ground speed equal IAS but if ground speed is 30 knots faster then it will assume there is a 30 knot tail wind potentially a 180 degree error.
That’s good news. How big are the differences in airspeed between G3X and G5, and is there any difference in indicated altitude?
IO390 wrote:
How big are the differences in airspeed between G3X and G5, and is there any difference in indicated altitude?
We have an aircraft with a G3X and a G5 as backup. There’s a 2 knot IAS difference and a 40 feet altitude difference. This does baffle me, as I would have thought both units were calibrated during installation.
Yes mine shows a very small difference too, although it’s closer to 0.5kt and 10-15ft. I have not looked into calibration – the G3X does not support this but I think the G5 does.
