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-   -   Altitude Discrepency between G3X and G5? (https://vansairforce.net/community/showthread.php?t=170415)

Bavafa 04-09-2019 10:03 AM

Quote:

Originally Posted by Brantel (Post 1338106)
I will have to disagree with you there. The static pressure definitely plays a part in how the altimeter works/reads on the ground or in the air.

I would love to learn... please help me understand how it can effect it on the ground when the pressure in the cockpit and outside of the plane is the same as well as no pressure in the pitot tube.
My assumption is that the plane is in the hanger, garage or just on the ramp and not moving.

g3xpert 04-09-2019 10:42 AM

Both the GSU 25 and G5 have the ability to calibrate the altitude pressure sensors within current software. However, this should only be done by an avionics shop with a test set that is used as a truth source. All Garmin air data computers are calibrated out of the factory and typically do not need any calibration. Some small difference is not unusual as no sensor has zero error. Please do not just adjust this yourself.


Thanks,
Levi Self

Brantel 04-09-2019 11:31 AM

Quote:

Originally Posted by Bavafa (Post 1338117)
I would love to learn... please help me understand how it can effect it on the ground when the pressure in the cockpit and outside of the plane is the same as well as no pressure in the pitot tube.
My assumption is that the plane is in the hanger, garage or just on the ramp and not moving.

The pitot tube has nothing to do with the altimeter other than the warning I mentioned when doing a check or recalibration as noted by Garmin with their AD-AHRS units.

Regardless of whether the airplane is moving or not, there is still a "pressure" on the static pressure port. This pressure changes relative to altitude. The altimeter compares this static pressure to the pressure of a standard atmosphere at sea level when the baro setting is set to 29.92inHg. This is basically how an altimeter works. On a standard day at sea level, the altimeter should indicate ZERO when the baro is set to 29.92 inHg. Even then there is a "pressure" on the static port. Guess what it is? Hint....29.92 inHg.

Bavafa 04-09-2019 11:50 AM

Quote:

Originally Posted by Brantel (Post 1338152)
The pitot tube has nothing to do with the altimeter other than the warning I mentioned when doing a check or recalibration as noted by Garmin with their AD-AHRS units.

Regardless of whether the airplane is moving or not, there is still a "pressure" on the static pressure port. This pressure changes relative to altitude. The altimeter compares this static pressure to the pressure of a standard atmosphere at sea level when the baro setting is set to 29.92inHg. This is basically how an altimeter works. On a standard day at sea level, the altimeter should indicate ZERO when the baro is set to 29.92 inHg. Even then there is a "pressure" on the static port. Guess what it is? Hint....29.92 inHg.


I believe I understand how the altimeter works which is as you have described here.

In the context of this conversation, which I set the assumption that the units are on the ground and not moving, you indicated that the static pressure still plays a part.

Considering the static pressure will be the same in the cabin or outside of the cabin AND will be the same for both units, how would this effect one device differently than the other.

Brantel 04-09-2019 12:29 PM

Quote:

Originally Posted by Bavafa (Post 1338159)
I believe I understand how the altimeter works which is as you have described here.

In the context of this conversation, which I set the assumption that the units are on the ground and not moving, you indicated that the static pressure still plays a part.

Considering the static pressure will be the same in the cabin or outside of the cabin AND will be the same for both units, how would this effect one device differently than the other.

I did not say it would have a different effect on one vs the other (different effect no but what they read yes...more about that below). What I said was static pressure does have an effect. You said it had nothing to do with it which it does.

This conversation has nothing to do with static position error or the effect of leaks in the system or the difference between the pressure outside the plane vs the inside while in flight.

Since static pressure is half of the equation in how an altimeter determines altitude, I say static pressure is very much involved. I do agree that on the ground it will be involved the same on 1 or 10 altimeters (what they read is a different story...see below).

The calibration simply determines what this static pressure is referenced to and in some cases how linear it is across the span. The Garmin AD-AHRS units have the ability to calibrate to the zero and a minimum of 2 and optionally 3 other points. This improves the accuracy across the entire span.

That being said, since there can be error in the calibration at one point where there is none at another point and this could be different on different altimeters, the static pressure at different field elevations could definitely cause a different "reading" on different altimeters attached to the same static system. The same airplane may not show any difference at sea level on a standard day but might be off by some amount at 5000ft or any other point in the span. Or they could be dead on at 1300ft but off at sea level on a standard day. Or dead on from sea level to 10,000 but start to drift apart at higher altitudes.

Example: My airplane may have zero difference between the altimeters while on the ground at my home base which is 1313ft of elevation. It may have 20ft of difference at El Paso which is 3958ft of elevation with the exact same atmospheric conditions. The only thing that changed is the static pressure because of the increase in field elevation.

I also agree that the only way to know which one is off is to use a calibrated test set with enough resolution to act as a certified reference. Like Levi warns above, don't mess with it if the difference is within tolerance... You could make it worse!

Carl Froehlich 04-09-2019 03:09 PM

Back to the OP issue.

Do the IFR Pitot/Static check and have the shop issue the paper that your primary altimeter (G3X) meets specs. You may or may not need to do the simple offset into the G3X during the test. Do not bother to have the shop do the G5. Most shops charge extra for each instrument and once the G3X is done you have what you need for IFR.

If you can have this done before first flight, great. We have a shop that comes to the airpark with the big dollar test boxes when we get five or more planes done the same day. Cost is about $300 for the IFR version.

Now at your leisure you can do the needed offset for the G5 so it matches the G3X. Do this on the ground. The G5 is a backup so this does not impact your IFR system sign off. On my RV-10 the SkyView was dead on up to 25K?. The analog altimeter was 100? off. As the SkyView was my primary altimeter the plane passed the check. I adjusted the analog altimeter while sitting in the cockpit as he did the system check.

Before you do the test build a simple manometer to check for leaks. This will save you a lot of time and money.

Carl

Brantel 04-09-2019 03:25 PM

There is no simple offset in either the G5 or the G3X systems. Changes to the altimeter require a full blown calibration.

airguy 04-09-2019 03:36 PM

Quote:

Originally Posted by Brantel (Post 1338225)
There is no simple offset in either the G5 or the G3X systems. Changes to the altimeter require a full blown calibration.

Confirmed through careful reading of the install docs using procedure C. I was looking specifically for an altitude offset adjustment, which does not exist. I can live with that, I'll run that procedure during my next pitot/static cert check.

Carl Froehlich 04-09-2019 03:58 PM

Quote:

Originally Posted by airguy (Post 1338230)
Confirmed through careful reading of the install docs using procedure C. I was looking specifically for an altitude offset adjustment, which does not exist. I can live with that, I'll run that procedure during my next pitot/static cert check.

Sorry - not a Garmin guy and just assumed they would make this simple.

The main point of my post stands. Do the pitot/static check with the G3X as the altimeter of record. Do whatever you want on the G5 - leave it be or whatever.

Carl

RONSIM 04-09-2019 05:15 PM

Just curious --- what about the encoder?
 
I assume either EFIS/PFD can be used at the encoder to the transponder ( I use my Dynon D10A but could use my AFS5600T) ---- do these calibration issues have an impact on the encoder?

Ron


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