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Altitude Discrepency between G3X and G5?

dmat

Well Known Member
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Hi all,

I recently fired up my new glass panel and noticed that the G3x (non-touch) screen was 40ft different from the G5. Both had a pressure altitude of 29.91.

Has anyone had this issue before? Easy fix?

Thanks,
D
 
I've got the exact issue with my G5, versus the Dynon altimeter. The pitot-static check on the Dynon tells me it's right - but the G5 does not have an altitude offset adjustment to correct it's reading. I believe it should have one added to the software.
 
It looks like one or possibly both of them the need to be calibrated. If you are getting a different altitude for the same baro settings, it means that is one of them is out of calibration.

For accuracy, one will need to have a good and certified source for calibration. If you are planning on IFR test, this will be done when the person test for leak and accuracy.

Hope this helps.
 
Sounds pretty good to me!

For comparison, the Airbus I fly at work has 3 Air Data Reference systems, any one of which can serve as a primary source of altitude data. In addition, we have an independent standby gauge that’s pretty much the airliner equivalent to the G5 (plus about $100k).

On the ground, the primary sources can be 20 feet off and the standby can be 100’ off. At 5,000’, we’re allowed 50’ and 100’.

In the old days of analog altimeters, I think 1 needle’s width was good for almost 100 feet! ‘Course most GA aircraft didn’t have 2 altitude sources so nobody knew any different.
 
Yeah, I am planning on getting the system IFR certified so I can start IFR training.

If the two units are in a loop (pitot - static plumbing) I wouldn't see leaks as being the culprit.

Recalibration does sound like the logical course of action. Hopefully (fingers crossed) calibrations and possibly taxi-testing can be done this weekend... 7+months in the making and a blown budget is starting to shape up. But by the time the this bird is done, she will be basically new.


@ G3xpert: would this be a possible software update?

D
 
One needs to understand that it may not be easy or practical to get the two altimeters reading EXACTLY the same across the entire altitude range. Most of our modern EFIS equipment will get the two altimeters very close with proper calibration and some of them will be dead on across most of the range of altitudes that we commonly use.
 
Yeah, I am planning on getting the system IFR certified so I can start IFR training.

If the two units are in a loop (pitot - static plumbing) I wouldn't see leaks as being the culprit.

Recalibration does sound like the logical course of action. Hopefully (fingers crossed) calibrations and possibly taxi-testing can be done this weekend... 7+months in the making and a blown budget is starting to shape up. But by the time the this bird is done, she will be basically new.


@ G3xpert: would this be a possible software update?

D

Keep in mind that the G5's altitude display normally comes from its built in air data sensors. The G3X's altitude display normally comes from the air data sensors in whatever AD-AHRS is providing data to that screen. So in other words, it is two different air data sensors or similar to multiple stand alone altimeters.

In my setup I have 3 altimeters. The Primary G3X PFD connected to a GSU 73, the G3X MFD which is configured to connect to the GSU 25, and the G5 with its own internal AD-AHRS.

While my entire setup passes the xponer/static/alt testing requirements of 91.411 and 91.413, they are likely never going to agree 100% on all three at all times and the testing requirements allow a tolerance for this fact.

Garmin warns that the G5 and the G3X AD-AHRS's are low drift sensors and that they rarely if ever will need re-calibration. (if you screw up the calibration procedure, it could render the G5 or the G3X worthless without either doing it again correctly or sending it into Garmin)

Garmin also warns that their air data sensors can be damaged by an improper test where the pitot and the static line are not both connected to the test set while performing the test and or any re-calibration. Apparently they use a differential pressure sensor that can be damaged by too much differential pressure which can happen if both the pitot and static lines are not cross connected during these procedures.
 
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From the original post, the reported discrepancies sounded to be on the ground and not in the air and if that assumption is still correct the Pitot/static does not play any role in this.

The calibration of one or possibly both will set it correct as you don?t know exactly which one is correct, otherwise you could take care of it yourself. You are correct that if there was a leak, it would affect both systems equally.
 
From the original post, the reported discrepancies sounded to be on the ground and not in the air and if that assumption is still correct the Pitot/static does not play any role in this.

The calibration of one or possibly both will set it correct as you don’t know exactly which one is correct, otherwise you could take care of it yourself. You are correct that if there was a leak, it would affect both systems equally.

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

The EFIS as an encoder always outputs whatever altitude that is displayed at 29.92inHg. So if the altimeters is out of calibration, the encoder output will be wrong as well by the same amount. Keep in mind that the resolution of the encoder protocols can be as little as 100ft so most tiny errors are not usually a problem.
 
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

If I am not mistaken FAR 91.411 requires that “each” altimeter in the airplane be tested before use in IFR. Don’t think it matters for VFR.
 
A couple of related points on meeting IFR requirements.

If the , say, G5 is new, the rules give you 24 months from date of manufacture before the part 43 altimeter test is required.

The static system can be done by any airframe mechanic if he has a little equipment. This might save someone a few bucks.

r
 
A couple of related points on meeting IFR requirements.

If the , say, G5 is new, the rules give you 24 months from date of manufacture before the part 43 altimeter test is required.

The static system can be done by any airframe mechanic if he has a little equipment. This might save someone a few bucks.

r

I don't believe the G5 is TSO'd and I doubt most mechanics have a certified P-S tester to perform the leak test.

(c) Altimeter and altitude reporting equipment approved under Technical Standard Orders are considered to be tested and inspected as of the date of their manufacture.
 
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discrepancy

Hi,
I am late to this discussion but I had some offset between my AFS 5600 and G5. At my IFR check, the technician at Higginsville Avionics took them both and put them through the full calibration. Both passed but had 20-40 ft. difference. He told me that was within limits. He also explained that a guy with one altimeter always knows his altitude but a guy with two altimeters always wonders which is correct. That struck me as really funny but I got the point. My discrepancy was due to my OCD not the instruments being out of limits.

John
 
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