G5 Airspeed Indicator issue

[Greg Ellis]

So, for all of you far more knowledgeable than me, what would be some causes of the Airspeed indicator strip on the G5 being about 10knots slower than the indicated airspeed on the analog gauge?  I will preface this with, I do understand that the strip on the G5 is not primary.  I also am leaning towards trusting my analog gauge because if I follow the analog gauge the airplane stalls at the correct indications on the gauge where it is a lot lower on the G5, when I landed according to the airspeed on the analog the airplane landed fine, if I land with the G5 at the correct landing speed, the analog is way high and I float half way down the runway.  So I am leaning towards trusting my analog airspeed indicator but I am curious as to what could be the cause of the discrepancy.  My avionics shop is on notice but they are extremely busy and trying to fit me back in but I thought I would see what the collective has to say and if anything could point me in the right direction.  Thanks.

[EricJ]

There's no calibration procedure for the airspeed in the G5 installation manual (at least the one I have), just a normal leak check.    It's possible that your airspeed indicator may be a little off, but the only way for sure to tell whether one or both is off significantly is with a calibrated pitot-static ramp tester.   It's the same device that you get your normal pitot-static and transponder certs with every couple of years, so if you're close to due for that you might just schedule a check.

 

[PT20J]

24 minutes ago, EricJ said:

There's no calibration procedure for the airspeed in the G5 installation manual (at least the one I have), just a normal leak check.    It's possible that your airspeed indicator may be a little off, but the only way for sure to tell whether one or both is off significantly is with a calibrated pitot-static ramp tester.   It's the same device that you get your normal pitot-static and transponder certs with every couple of years, so if you're close to due for that you might just schedule a check.

 

Airspeed calibration is not part of the required pitot-static and transponder tests, so it is not routinely done. A couple of years ago while trying to get my Aspen AoA to calibrate properly (I was never able to, but that's another story) I wanted to have my airspeed indicator checked and took it to the repair station that does my pitot-static checks. That's when I learned that their tester didn't have that capability. Evidently, that's an option for the tester and not every shop has one since it's rarely used and an extra calibration expense to maintain. Check with your shop, but that would be the way to do it unless you want to go the trouble to build a water manometer.

Skip

[Greg Ellis]

9 minutes ago, EricJ said:

There's no calibration procedure for the airspeed in the G5 installation manual (at least the one I have), just a normal leak check.    It's possible that your airspeed indicator may be a little off, but the only way for sure to tell whether one or both is off significantly is with a calibrated pitot-static ramp tester.   It's the same device that you get your normal pitot-static and transponder certs with every couple of years, so if you're close to due for that you might just schedule a check.

 

So, they did one and attached are the results.  According to the results, I should be following the G5 since it seems way more accurate.  However, in real life flying for instance, when stalling with gear up and flaps up my stall speed should be 60 knots.  The G5 showed 57 and the ASI showed 63.  When stalling with gear down and flaps down my stall speed should be 56 knots.  The G5 showed 47 and the ASI showed 55.  I tried two different landings.  One where I used the speeds on the ASI only and ignored the G5.  The plane landed perfectly and I was off the runway in short order.  The second landing I followed the speed on the G5 ignoring the ASI and I floated and floated and floated.

At cruise speed for instance, the ASI will be at 130knots which is the bottom of the yellow arc but the G5 only shows 121 knots.

Another concern is that the gear speed on a C model is around 104 knots.  If I follow the G5 then I am below 104 knots well before the ASI gets to 104 knots and am I putting the gear down at too high an airspeed??

And if you look at the image that is attached, it shows that the G5 has an indicated airspeed of 79 knots where the airspeed indicator shows 89-90 knots.

Thanks for posting your advice.  I appreciate it.

 

G5 vs ASI test.pdf

[47U]

This is probably a dumb question, but does your second G5 have pitot/static connections?  Can you configure it to show backup attitude with airspeed?

Second dumb question, does the altitude match up (or consistent error) between the round dial altimeter and the G5(s)?

Third dumb question, how old is your pitot/static plumbing?  Was it all replaced with your (awesome) panel upgrade?

(Sorry, not a Garmin expert.) 

[EricJ]

4 hours ago, Greg Ellis said:

So, they did one and attached are the results.  According to the results, I should be following the G5 since it seems way more accurate.  However, in real life flying for instance, when stalling with gear up and flaps up my stall speed should be 60 knots.  The G5 showed 57 and the ASI showed 63.  When stalling with gear down and flaps down my stall speed should be 56 knots.  The G5 showed 47 and the ASI showed 55.  I tried two different landings.  One where I used the speeds on the ASI only and ignored the G5.  The plane landed perfectly and I was off the runway in short order.  The second landing I followed the speed on the G5 ignoring the ASI and I floated and floated and floated.

At cruise speed for instance, the ASI will be at 130knots which is the bottom of the yellow arc but the G5 only shows 121 knots.

Another concern is that the gear speed on a C model is around 104 knots.  If I follow the G5 then I am below 104 knots well before the ASI gets to 104 knots and am I putting the gear down at too high an airspeed??

And if you look at the image that is attached, it shows that the G5 has an indicated airspeed of 79 knots where the airspeed indicator shows 89-90 knots.

Thanks for posting your advice.  I appreciate it.

 

G5 vs ASI test.pdf 77.47 kB · 3 downloads

When I bought my airplane I figured out just from seat-of-the-pants on approaches that the ASI was reading probably 10 kts fast or so, so I was subtracting about that much and was getting decent results.   When I had the ASI overhauled they said it was reading 15 kts fast, so I was doing approaches consistently 5 kts slower than I thought.    I don't think my landings have ever been as good as they were when it was off. 

Anyway, it might be worth just getting your ASI overhauled or getting a replacement.    Another advantage of an overhaul is that it'll look like new, plus it'll probaby give you more confidence in it and likely match the G5 better.

Edited May 31, 2022 by EricJ

[EricJ]

22 minutes ago, 47U said:

This is probably a dumb question, but does your second G5 have pitot/static connections?  Can you configure it to show backup attitude with airspeed?

G5s have both pitot and static inputs, since they can display airspeed and altitude and rate of climb.    They can easily be switched between HSI and PFD (AI) modes.    Mine always read the same when both are in PFD/AI mode, which suggests that they share cal/display info or something, but I don't know what the strategy is.    The G5 integration software seems to work pretty well when you have two installed.   They talk to each other in ways that seem to make sense.

22 minutes ago, 47U said:

Second dumb question, does the altitude match up (or consistent error) between the round dial altimeter and the G5(s)?

Unlike airspeed, there is a cal procedure for altitude on the G5.   My G5s and the air altimeter disagree somewhat, but since the G5s aren't primary, I don't worry about it.   I don't think it's easy to get them calibrated to read identically the same with the altimeter.   I have a pitot-static tester and have been thinking of redoing the cal (I didn't do it initially) to see I if I can get them closer, but there's some risk to the air instruments every time you hook one of those things up, so I've not been in a rush to do it.

Edited May 31, 2022 by EricJ

[Greg Ellis]

1 hour ago, 47U said:

This is probably a dumb question, but does your second G5 have pitot/static connections?  Can you configure it to show backup attitude with airspeed?

Second dumb question, does the altitude match up (or consistent error) between the round dial altimeter and the G5(s)?

Third dumb question, how old is your pitot/static plumbing?  Was it all replaced with your (awesome) panel upgrade?

(Sorry, not a Garmin expert.) 

The second G5 is setup to be an HSI.  When you switch it to the Attitude page I cannot recall if it displays airspeed or not but good idea and I will check.

The altitude matches spot on with the round dial altimeter.

I am not sure about the pitot static plumbing but I will ask.

Thanks and none of these are dumb questions.  Thanks for helping out.

[Ragsf15e]

6 hours ago, Greg Ellis said:

So, they did one and attached are the results.  According to the results, I should be following the G5 since it seems way more accurate.  However, in real life flying for instance, when stalling with gear up and flaps up my stall speed should be 60 knots.  The G5 showed 57 and the ASI showed 63.  When stalling with gear down and flaps down my stall speed should be 56 knots.  The G5 showed 47 and the ASI showed 55.  I tried two different landings.  One where I used the speeds on the ASI only and ignored the G5.  The plane landed perfectly and I was off the runway in short order.  The second landing I followed the speed on the G5 ignoring the ASI and I floated and floated and floated.

At cruise speed for instance, the ASI will be at 130knots which is the bottom of the yellow arc but the G5 only shows 121 knots.

Another concern is that the gear speed on a C model is around 104 knots.  If I follow the G5 then I am below 104 knots well before the ASI gets to 104 knots and am I putting the gear down at too high an airspeed??

And if you look at the image that is attached, it shows that the G5 has an indicated airspeed of 79 knots where the airspeed indicator shows 89-90 knots.

Thanks for posting your advice.  I appreciate it.

 

G5 vs ASI test.pdf 77.47 kB · 5 downloads

I’d want them to be pretty close (mine are very close).  That being said, the stall speeds you mentioned seem legit if you’re light.  The published ones should be mgw, but it’s also a little hard to nail that down as you do a stall.  3 to 4 knots below published due to weight being less seems ok.

If a ground tester isn’t available, you can do this with your gps… so, go out and do a very precise 3 way gps groundspeed run (smooth air and calm wind helps).  Take notes of exact indicated airspeeds, ground speeds, temp, altitude, pressure, etc.    Enter the groundspeeds into one of the online true airspeed calculators.  Usually you need 3 legs at least 60 degrees off from each other.  You’ll get True, then you can use the other info to go back to equivalent, calibrated, and finally indicated.  It sounds bad, but it’s not terrible.  There’s a correction in your poh for indicated to calibrated.  So that’s the “I-C-E-T-G” of airspeeds but in reverse.  You will then know which one was right (or at least closer).

[EricJ]

It's not compliant to the installation manual if it doesn't have the pitot and static lines connected.   That's required at installation, even for HSI.

 

[KB4]

Installer programs the speeds for your model which produces the floating Icons on the tape.  Verify speeds for your Model. My guess, they got it right and the Analog is way off. 

[FlySafe]

And verify units kts vs mph… i know silly but it has happened;)

[201Steve]

My G5 is dead on with my round dial. Fwiw

The altitude was calibrated together, they are within 10’

[Greg Ellis]

Thanks to all for your responses.  I will check some of this out this weekend when I get to the airport.  I will also mention a few of these things to my avionics shop.  I am very grateful to have a resource such as Mooneyspace to provide advice and direction.

[dbier]

I'm having a G5 installed and they indicated my AI has a static leak (pitot system also had monor leaks which they resolved) and supposedly is under-reporting my airspeed. So, while doing some research I found this thrad and of coure am now curious how everything turnd out for you (Greg Ellis). Any update? 

[Greg Ellis]

36 minutes ago, dbier said:

I'm having a G5 installed and they indicated my AI has a static leak (pitot system also had monor leaks which they resolved) and supposedly is under-reporting my airspeed. So, while doing some research I found this thrad and of coure am now curious how everything turnd out for you (Greg Ellis). Any update? 

I had a leak in my airspeed indicator which they believe may be the issue.  I purchased another airspeed indicator (used) which also had a leak (even though I was told it was in perfect working order).  One of them is currently being overhauled to fix the leak and will be put in.  I can report back if that fixes the issue.  The current issue is the shop that is doing the overhaul is woefully behind and it is taking a very long time to get it back.  Fortunately I still have the one installed in the airplane to keep flying while I wait.

And, just to add, the shop took out the G5 and tested it and it was dead on accurate with their testing equipment and the analog airspeed indicator was off.

[M20E for me]

Who is rebuilding your ASI?  I might need to get mine rebuilt as well.

[Greg Ellis]

14 hours ago, M20E for me said:

Who is rebuilding your ASI?  I might need to get mine rebuilt as well.

It is currently at Air Dallas (airdallas.com)  They have been taking a very long time to do the work.  My avionics shop also uses rudyaircraftinstruments.com, and fieldtech (ftav.com)

Field Tech is at Meacham (KFTW).  They don't use them as often because they said that even if they can't fix it they charge you quite a bit.  Rudy Instruments is in Arkansas and they are a small Mom and Pop shop.  Air Dallas is big, in Lewisville, Texas and currently where my Airspeed Indicator is.  They told me they sent one in there in November and just got it back.  But they are apparently very good and that is why they use them.

[M20E for me]

Thanks!  I took the Mooney to a shop, Instrument Overhaul Services, at Gillespie Field in El Cajon Ca where they discovered two problems, one the airspeed indicator is way out of calibration and the Dukes airspeed switch leaks.  He thinks he can fix the switch and they pulled the indicator to see if it can be serviced.  They have a good reputation here in the San Diego area.  Hopefully it can be fixed.

[kortopates]

Thanks!  I took the Mooney to a shop, Instrument Overhaul Services, at Gillespie Field in El Cajon Ca where they discovered two problems, one the airspeed indicator is way out of calibration and the Dukes airspeed switch leaks.  He thinks he can fix the switch and they pulled the indicator to see if it can be serviced.  They have a good reputation here in the San Diego area.  Hopefully it can be fixed.

They do great work and there is very little they can’t do!


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[wombat]

I have a similar problem in that the steam gauge ASI and the digital ones disagree by about 8 knots when I'm on final.   At cruise speed they seem similar.