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04-09-2019, 06:40 AM
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Join Date: Oct 2010
Location: Union County, NJ
Posts: 933
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Altitude Discrepency between G3X and G5?
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
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RV6A: N6389-->N456RJ - Avionics Upgrade & Restoration COMPLETE & Flying
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04-09-2019, 08:29 AM
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Join Date: May 2011
Location: Roseville
Posts: 386
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That's what I see in my set up as well- G3x and G5. Never could figure it out.
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RV-9A N435KR, Vans calendar March 2018
LIO 320 B1A, Dual LS Plasma III, AFP injection
G3x touch
Roseville, Ca
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04-09-2019, 08:35 AM
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Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,145
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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.
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Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2020 dues paid 
N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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04-09-2019, 08:42 AM
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Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,351
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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.
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Mehrdad
N825SM RV7A - IO360M1B - SOLD
N825MS RV14A - IO390 - Flying
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04-09-2019, 08:42 AM
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Join Date: Jan 2005
Location: Brighton, Colorado
Posts: 393
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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.
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Brighton, Colorado (CO12)
RV-3 N87CT (Thanks Chuck!)
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04-09-2019, 09:01 AM
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Join Date: Oct 2010
Location: Union County, NJ
Posts: 933
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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
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RV6A: N6389-->N456RJ - Avionics Upgrade & Restoration COMPLETE & Flying
WTB RV4/8?- Looking for a 4 or 8 that needs some love and TLC. PM me if you have one!
Donations Paid- Thank you VAF!
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04-09-2019, 09:08 AM
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Join Date: Aug 2007
Location: Ontario, Canada
Posts: 2,291
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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.
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04-09-2019, 09:20 AM
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Join Date: Mar 2006
Location: Newport, TN
Posts: 7,496
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Quote:
Originally Posted by dmat
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
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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.
Last edited by Brantel : 04-09-2019 at 09:22 AM.
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04-09-2019, 09:33 AM
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Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,351
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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.
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Mehrdad
N825SM RV7A - IO360M1B - SOLD
N825MS RV14A - IO390 - Flying
Dues paid
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04-09-2019, 09:36 AM
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Join Date: Mar 2006
Location: Newport, TN
Posts: 7,496
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Quote:
Originally Posted by Bavafa
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.
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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.
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