What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

GSU 25 non-zero IAS when stationary

Draker

Well Known Member
Well, I'm working through yet another G3X issue, and I'm wondering if anyone else has seen/corrected this: Examining my data_logs, I noticed that my G3X is logging Indicated Airspeed (IAS) of around 15-20 knots while on the ground, stationary, even with the engine off. I know I'm not experiencing a 20 knot headwind inside my hangar with the doors closed. What might cause this? Airspeed equipment: GSU 25 ADAHRS + GAP 26 Pitot+AoA

I had a local shop perform a pitotstatic test (per 14 CFR § 91.411) about a year ago, and the shop performing the test reported no issue. Looking back at data_logs collected on my CF card, this erroneous IAS has been happening from the very beginning. If I can't trust the number on the ground, how can I trust it in the air?
 
GSU-25 LED

The GSU-25 has a LED that indicates the status of it.
See G3X installation manual Rev: AQ. pg 891.
Then you know if its a hardware fault or if it's a communication problem.
Also pg 692. Enter set up mode for the G3X, scroll to ADAHRS, there should
be green check marks for GSU25, Air data and GPS data.
Suggest a new calibration if no faults are found.
Also check for condensed water in pitot lines.

Good luck
 
Not a problem, this is why the display doesn't show you a number at very low airspeeds. Carry on.
 
Sounds like you have some water trapped in the system. Temp and outside pressure inducing pressure in the system, causing the reading when stationary.
 
I think this is a question for the Garmin guys, but is there a possibility that it always shows some random small number in the tapes until the system comes alive at a real airspeed? It's not like it's showing that on the tape right?
 
I had this issue and it required a replacement GSU25. If you call support and describe it, they know exactly what the issue is and its hardware related. Major bummer...
 
Thanks for the pointers. This is hopefully purely a data logging issue.

The actual display UI seems correct. When stopped on the ground, the tape says "--" and when you switch to round gauge mode, the needle points to zero. I do have a backup steam airspeed indicator which correctly displays zero while stopped, and a G5 standby display which also displays "--", so it seems there is no unexpected pressure in the pitot line. Next time I fly, I will closely compare the airspeed readouts from all three sources and verify they agree.

I've also tried two different GSU AHRS units and found no water in the pitot line when (recently) replacing it. Data logging behavior is the same regardless of the GSU unit.

I have also examined other G3X logs from the system of another VAF pilot, and his data logs correctly show zero while stopped on the ground.

I've posed this question to g3xpert over E-mail, but was hoping someone here also had experience with this. Will continue with Garmin.

EDIT: Since a picture(plot) is worth a thousand words, here's a plot of GS vs. IAS. Something is majorly wrong on the ground, but in the air, it seems correct:

 
Last edited:
The fact that the display intentionally does not show very low airspeed values should be your clue that the data is meaningless at very low airspeeds. This isn't something worth worrying about and there's nothing wrong with your equipment.
 
Thanks for the pointers. This is hopefully purely a data logging issue.

The actual display UI seems correct. When stopped on the ground, the tape says "--" and when you switch to round gauge mode, the needle points to zero. I do have a backup steam airspeed indicator which correctly displays zero while stopped, and a G5 standby display which also displays "--", so it seems there is no unexpected pressure in the pitot line. Next time I fly, I will closely compare the airspeed readouts from all three sources and verify they agree.

I've also tried two different GSU AHRS units and found no water in the pitot line when (recently) replacing it. Data logging behavior is the same regardless of the GSU unit.

I have also examined other G3X logs from the system of another VAF pilot, and his data logs correctly show zero while stopped on the ground.

I've posed this question to g3xpert over E-mail, but was hoping someone here also had experience with this. Will continue with Garmin.

EDIT: Since a picture(plot) is worth a thousand words, here's a plot of GS vs. IAS. Something is majorly wrong on the ground, but in the air, it seems correct:


The fact that the display intentionally does not show very low airspeed values should be your clue that the data is meaningless at very low airspeeds. This isn't something worth worrying about and there's nothing wrong with your equipment.


Ryan,

Listen to Matt, he knows a few things about the inner workings of Garmin equipment. :p
 
The fact that the display intentionally does not show very low airspeed values should be your clue that the data is meaningless at very low airspeeds. This isn't something worth worrying about and there's nothing wrong with your equipment.

Matt,

Just for our edification - what's the deadband/minimum for IAS?
I have a similar setup and I see IAS in the bouncing around 1 - 5kts in the data, but no indication on the UI.

B
 
GSU 25

Hey Ryan,

There is a non-linear relationship of pitot pressure to airspeed. At normal flight airspeeds, slight changes in pitot pressure translate to incremental changes in airspeed. At the low end (stationary), the pressure readings are smaller than the pressure sensors ability to distinguish from noise, this is why we do not display airspeed below a certain threshold. As pitot pressure increases, accuracy drastically increases.

Thanks,

Justin
 
Hey Ryan,

There is a non-linear relationship of pitot pressure to airspeed. At normal flight airspeeds, slight changes in pitot pressure translate to incremental changes in airspeed. At the low end (stationary), the pressure readings are smaller than the pressure sensors ability to distinguish from noise, this is why we do not display airspeed below a certain threshold. As pitot pressure increases, accuracy drastically increases.

Thanks,

Justin

Thank you, Justin. I understand that with steam gauges, airspeed only becomes "alive" around 20-30 knots, and I'll accept that the electronic device has a similar limitation. I'm worried that there could be significant delta between IAS and GS at normal flight speeds (>Vs0) that cannot be explained by wind. Since I'm early in phase I, and have not performed an inflight airspeed accuracy test, perhaps I should do that exercise first, and examine the results of that. There may be significant calibration error.
 
Since I'm early in phase I, and have not performed an inflight airspeed accuracy test, perhaps I should do that exercise first, and examine the results of that.

If you're worried about your ADAHRS, the thing to do would be to use a calibrated pitot/static test set to verify it on the ground, not try to work backwards from unknown data collected in-flight.
 
Thanks, guys. I suppose I am just fishing for a more satisfying answer than "live with it". The pitotstatic test I had done last year wasn't that time consuming or expensive. Sounds like re-doing it for peace of mind might be the best bet.
 
Impact Pressure Readings

Thanks, guys. I suppose I am just fishing for a more satisfying answer than "live with it". The pitotstatic test I had done last year wasn't that time consuming or expensive. Sounds like re-doing it for peace of mind might be the best bet.

A pitot-static test would work just fine, it will be the most accurate way to gather the data you are looking for. Its less of a "live with it" answer, and more "as designed". There is a lot of information available out there on the relationship between impact pressure and Indicated airspeed if you are interested in digging into it any further.

Let us know if anything else comes up!

Thanks,

Justin
 
Back
Top