G5& Steam Altimeter

[Dr_Jim]

Hey folks, I currently had a GFC500 installed in my 20C airplane.  During the installation I asked them to do a pitot static check thinking that that would true up both the G5 and steam altimeter which did not have that result.  They are still both off by 200 feet.  This just does not seem correct to me.  The G5 is showing the correct altitude on the ground and steam gauge is not.  Thoughts?

[philiplane]

Oddly enough, you can have each one off by the max allowable limit, in opposite directions, and they will be legal and you will have a split of no more than 40 feet. The limit for each altimeter is 20 feet at sea level. But 200 feet off means the shop tested them but did not adjust them, meaning that they should not have issued a certification because they are well out of limits.

Some shops will certify one as the primary altimeter, and not even test the other. You should expect to pay extra to have both tested, adjusted if needed, and you will have two altimeter cards in your log entry. If you only have one, they only did one.

Edited September 24, 2021 by philiplane

[Vance Harral]

Bear in mind that the G5 is not certified as a primary altimeter.  The information it provides is advisory only, from a legal perspective.

So the way this is "supposed" to work is that when you ask for a static system check, the shop should first test/adjust/certify the primary altimeter, i.e. the classic steam gauge.  In the OP's case, sounds like this was done incorrectly, or not at all.

Once that's done, the G5 altimeter can also be tested, and adjustments made to match it up with the reference standard, so the two altimeters are close.  Different shops have different attitudes about this (ask me how I know). Some shops adjust the G5 as a matter of course, while others ignore it, saying, "that thing is only advisory".  In the latter case, you can explicitly ask them to adjust it - for an extra fee, of course.

[N201MKTurbo]

I have a G5 that I use as an HSI, but a few button pushes and it is an ADI. At my last static inspection the steam altimeter and the G5 matched within 5 feet except at 15200 it was off by 50 feet and then went back to matching. there must be a burr on one of the wheels. I think it does that because I had Howard tune up my altimeter. It is a WWII Square D altimeter. Howard says most techs don't know how to adjust them. He says they have adjustable pivots. He says that it allows you to set them up very precisely, unless you don't know how to do that, in which case you can make them very bad. He said Mooney bought thousands of these surplus altimeters and every plane up till the mid 80 got one.

[Mooney Dog]

1 hour ago, Vance Harral said:

Bear in mind that the G5 is not certified as a primary altimeter.

Does this still hold true? I thought the STCs changed that if you had 2 of them installed but im probably mistaken. 

 

2 hours ago, Dr_Jim said:

The G5 is showing the correct altitude on the ground and steam gauge is not.  Thoughts?

Well they can be +/- 75 feet of field elevation, so 200 might be a bit much, but i would always expect a little error. 

[Vance Harral]

28 minutes ago, Mooney Dog said:

Does this still hold true? I thought the STCs changed that if you had 2 of them installed but im probably mistaken. 

You raise a good point.  Garmin revises the G5 STC w/ AML installation manual frequently.

Best as I can tell, the latest release of this document is revision 26, dated May, 2021: https://static.garmin.com/pumac/190-01112-10_26.pdf

The verbiage about the altimeter in that document appears unchanged to me from earlier revisions.  Section 1.6.1.5 makes reference to G5 air data functions that include secondary airspeed, altitude, and vertical speed.  Later in the document, it says, "The installation of the G5 requires the retention of the mechanical airspeed indicator, altimeter, and vertical speed indicator."  I don't find any verbiage in the document that removes this restriction when two G5s are installed.

In real life, I know a lot of people are flying around referencing only their single or dual G5s, and not looking at all at the mechanical instruments that surround them.  As an instructor, my stance is this: for VFR, I have no issue with it.  For IFR, I have no issue with it in the enroute environment, especially when using the altitude hold function of an attached GFC500 autopilot.  But when it comes to instrument approaches, missed approaches, and departure procedures, I expect pilots to use the steam gauge altimeter as the reference for minimum altitudes.

My rationale for this stance is not just pedantry about the STC.  It's also because, for whatever reason, every single G5 I've flown with (about half a dozen) indicates higher than the mechanical altimeter.  In cases where the G5 was installed and the altimeter not adjusted by the shop, indicated altitude on the G5 runs almost 100' higher than the mechanical altimeter.  In cases where the G5 has been adjusted, it still runs 30-40' higher.  In these airplanes, using the G5 as a reference puts you 30-100' below the DA/MDA as indicated by the certified instrument.  That's a big deal for 200' AGL minimums on an ILS or LPV approach.  I don't know if there is something systematic about the G5 that causes this, or all the data points in my small sample size just happen to be on the high side.  But I've watched from the right seat as an instrument student with less-than-perfect skills descended 100' past minimums on an ILS using an uncalibrated G5 as a reference.  It's eye opening, and I decided at that moment I was not cool with just calling the uncertified G5 altimeter good enough.

[N201MKTurbo]

You really need to be there when they are doing the static check and see how the 4 altimeters are tracking. The 4 are the master altimeter in the test box, the mechanical altimeter, the G5 and your altitude encoder. 

They should all track within spec. But you should have a good feeling about how they are all tracking.

[EricJ]

I just replaced my vacuum attitude indicator with a second G5, as I already had one in the HSI spot.   I haven't compared my HSI (in PFD mode) to the mechanical altimeter, but the new G5 is very close to the mechanical on the ground, within 0-20 ft, but as I climb it increases error up to about 60-80 feet higher than the mechanical at about 10000 feet.    It also seems to not be 100% consistent in the error at the higher altitudes.  I did a leak check but wasn't planning to do the calibration until my next cert is due, since I'm among the crowd that thinks the mechanical is primary.   I couldn't get clarity either way from reading the docs.

I did, however, discover that, at least with the latest software revision, the HSI will automatically revert to PFD if the AI/PFD unit loses power.    I shut the PFD down a couple of times to do configuration while everything else was running, and whenever I did the HSI switched to PFD on its own.   It was still possible to easily switch it back to an HSI if one wanted to.    I like that, so that's a bonus.   The G5s seem a lot more solid in this area than the 275s, which is one reason I was very comfortable just putting in a 2nd G5.

[dzeleski]

My G5s are within 30 feet of my primary analog altitude indicator. I would be asking for everything to be re calibrated together, 200 feet is far too much.

[Ragsf15e]

6 hours ago, Vance Harral said:

Bear in mind that the G5 is not certified as a primary altimeter.  The information it provides is advisory only, from a legal perspective.

So the way this is "supposed" to work is that when you ask for a static system check, the shop should first test/adjust/certify the primary altimeter, i.e. the classic steam gauge.  In the OP's case, sounds like this was done incorrectly, or not at all.

Once that's done, the G5 altimeter can also be tested, and adjustments made to match it up with the reference standard, so the two altimeters are close.  Different shops have different attitudes about this (ask me how I know). Some shops adjust the G5 as a matter of course, while others ignore it, saying, "that thing is only advisory".  In the latter case, you can explicitly ask them to adjust it - for an extra fee, of course.

One weird thing after first acknowledging that you’re correct the altitude on the G5 isn’t primary… the G5 is allowed to be your altitude encoder.  So how the heck is my non-primary g5 altitude feeding my encoded altitude to ATC while i fly off my “primary” mechanical altimeter?!  Luckily mine are within about 50’, but still, atc is seeing exactly what my g5 is showing.

[Vance Harral]

15 minutes ago, Ragsf15e said:

atc is seeing exactly what my g5 is showing.

Well, not exactly, which is probably why this isn't a big deal in practice, even though the legalities are a bit questionable.

Your transponder doesn't transmit indicated altitude, it transmits pressure altitude, and then only to the nearest 100'.  What ATC sees on their display is that 100' increment pressure altitude, adjusted by whatever altimeter setting the ATC display itself applies - call that their "Kollsman" setting.  I don't know how ATC's pressure setting actually works - I'm sure it's more sophisticated than a single dial that applies to everything on the display.  Perhaps we have a TRACON or ARTCC'er here that can elaborate.  But whatever the mechanism, one assumes the "indicated" altitude they see for you would vary from your indicated altitude by a bit for that reason alone, even setting aside all the other inaccuracies in the system.  If you report you're at 7000, and their display shows 7100, my understanding is they don't care.

The combination of all this error, plus some wiggle room for human scan rates, means ATC separate standards are large, e.g. IFR traffic is separated by a minimum of 1000' vertically.  And they're not going to question a deviation from assigned altitude or below MVA by you, until it gets to about 300' off, which is the point at which the discrepancy can't be explained away by the 100' resolution of the system, Kollsman adjustments, the accuracy variation in all the various altimeters out there etc.

So... if your G5 was off by 300' or so, it might affect the system.  Inaccuracies below that might be interesting to individual pilots, but not to ATC.

It's entirely possible the recent update to the STC which allows the G5 to be used as an altitude encoder hasn't been fully thought through.  But my guess is it's not going to be noticed in practice, until/unless some huge portion of the fleet is using the G5 as an altitude encoder.

[Ragsf15e]

50 minutes ago, Vance Harral said:

Well, not exactly, which is probably why this isn't a big deal in practice, even though the legalities are a bit questionable.

Your transponder doesn't transmit indicated altitude, it transmits pressure altitude, and then only to the nearest 100'.  What ATC sees on their display is that 100' increment pressure altitude, adjusted by whatever altimeter setting the ATC display itself applies - call that their "Kollsman" setting.  I don't know how ATC's pressure setting actually works - I'm sure it's more sophisticated than a single dial that applies to everything on the display.  Perhaps we have a TRACON or ARTCC'er here that can elaborate.  But whatever the mechanism, one assumes the "indicated" altitude they see for you would vary from your indicated altitude by a bit for that reason alone, even setting aside all the other inaccuracies in the system.  If you report you're at 7000, and their display shows 7100, my understanding is they don't care.

The combination of all this error, plus some wiggle room for human scan rates, means ATC separate standards are large, e.g. IFR traffic is separated by a minimum of 1000' vertically.  And they're not going to question a deviation from assigned altitude or below MVA by you, until it gets to about 300' off, which is the point at which the discrepancy can't be explained away by the 100' resolution of the system, Kollsman adjustments, the accuracy variation in all the various altimeters out there etc.

So... if your G5 was off by 300' or so, it might affect the system.  Inaccuracies below that might be interesting to individual pilots, but not to ATC.

It's entirely possible the recent update to the STC which allows the G5 to be used as an altitude encoder hasn't been fully thought through.  But my guess is it's not going to be noticed in practice, until/unless some huge portion of the fleet is using the G5 as an altitude encoder.

Yeah I agree with that.  In practice though I like it better because now I can set my altitude by g5 and the corresponding pressure altitude from the g5 goes to the transponder.  Before there was an encoder between the altimeter and transponder.  I feel like there’s less error now than before.  Everything is digital and it’s all in one system.

[Mooney Dog]

On 9/24/2021 at 12:51 PM, Vance Harral said:

You raise a good point.  Garmin revises the G5 STC w/ AML installation manual frequently.

Best as I can tell, the latest release of this document is revision 26, dated May, 2021: https://static.garmin.com/pumac/190-01112-10_26.pdf

The verbiage about the altimeter in that document appears unchanged to me from earlier revisions.  Section 1.6.1.5 makes reference to G5 air data functions that include secondary airspeed, altitude, and vertical speed.  Later in the document, it says, "The installation of the G5 requires the retention of the mechanical airspeed indicator, altimeter, and vertical speed indicator."  I don't find any verbiage in the document that removes this restriction when two G5s are installed.

In real life...

I generally see more error on the steam gauge in my airplane, but it does vary from plane to plane. We have 3 G5 planes here so ill take a look at those as well. Defiantly something im going to be keeping my eyes on for a good while now, double so in my own plane. 

[carusoam]

Welcome aboard Dr.J.

You got plenty of responses above.

My question is… what ancient altimeter do you have in your M20C?

When was the last time it has seen service?

They can get pretty dirty over the years…

Either way… they should all have IFR precision if used for IFR flight…

Got a pic of your altimeter?

Pp thoughts only…

Best regards,

-a-

[bradp]

So on an ILS at minimums will you be 200 feet too high or 200 feet too low

I had my G5 calibrated by the shop- they are spot on now, less than 10 feet. 
 

Interestingly the GFC 500 altitude selector and autopilot computer both use the G5 altimeter.  So it’s primary in that particular scenario.  Autopilot is certified to 800AGL on the way up, 200 AGL on the way down .  

[Dr_Jim]

I had another shop take a look and redo the pitot static and no they are within 10 feet of each other.  Thanks for all the replies.

 

Jim

[Vance Harral]

On 9/27/2021 at 3:41 AM, bradp said:

Autopilot is certified to 800AGL on the way up, 200 AGL on the way down .

Can you provide a reference for this?  Not doubting you, I'd just like to see exactly how that clause of the STC is written.

Yes, the autopilot gets altitude information from the G5, so if you tell it to hold "X" feet, it's going to target X as displayed on the G5.  But my guess is that any information about altitudes at which the autopilot may be engaged/must be disengaged make reference to the primary altimeter in the airplane, not the secondary altitude displayed on a G5.

[bradp]

This is not my Mooney/AFMS (for a piper, found on the internet), but the altitude limitations are with respect to AGL, not MSL, and are the same for all GFC-500 installations 

 

 

[Mooney Dog]

1 hour ago, Vance Harral said:

Can you provide a reference for this?  Not doubting you, I'd just like to see exactly how that clause of the STC is written.

Yes, the autopilot gets altitude information from the G5, so if you tell it to hold "X" feet, it's going to target X as displayed on the G5.  But my guess is that any information about altitudes at which the autopilot may be engaged/must be disengaged make reference to the primary altimeter in the airplane, not the secondary altitude displayed on a G5.

So i went over this with my AP today. The G5 is the primary alt for my plane and has since been approved for use airplanes. He didnt get into the specifics, but just that all my alt information comes from my G5 now. 

[EricJ]

1 hour ago, Mooney Dog said:

So i went over this with my AP today. The G5 is the primary alt for my plane and has since been approved for use airplanes. He didnt get into the specifics, but just that all my alt information comes from my G5 now. 

I just put in my G5 attitude indicator, after having had a G5 HSI for several years.   The latest installation manual (May, 2021, rev 26) says G5 air data, including altitude, is secondary (Sec. 1.6.1, 1.6.1.5).

[Mooney Dog]

45 minutes ago, EricJ said:

I just put in my G5 attitude indicator, after having had a G5 HSI for several years.   The latest installation manual (May, 2021, rev 26) says G5 air data, including altitude, is secondary (Sec. 1.6.1, 1.6.1.5).

No you're absolutely correct they do. That contradicts what my AP was telling me today about how my panel is wired up. Ill have to have a look at it tomorrow.

[Ragsf15e]

5 hours ago, Mooney Dog said:

No you're absolutely correct they do. That contradicts what my AP was telling me today about how my panel is wired up. Ill have to have a look at it tomorrow.

Well I definitely believe your panel is all wired up so that the G5 provides altitude to whatever needs it (gfc500, gps, transponder, etc), but the “primary altitude” is still the mechanical altitude indicator.  I’m not exactly sure how you’re supposed to reconcile that, but I think that’s where we are.

[Dr_Jim]

My G5 is the reference for the autopilot, but I have also been told that it is NOT the primary altimeter in the eyes of the FAA.  I am not sure which avionics shop made the comment but I was told that if the G5 was able to be the primary the steam altimeter would no longer be required like the G5 taking the place of the turn bank indicator.