Garmin G5 HSI

[Feassarian]

Let's talk altitude for a minute. Pre-taxi check includes getting current altimeter setting and entering it into the altimeter. Let's use 1K1 as an example - field elevation is 1340 - when I enter the altimeter setting into the altimeter, I should read the same field elevation of 1340 on the altimeter.

However, the altimeter setting needed in the Garmin G5 to get the same field elevation is often off by 0.05 Hg or less and during pre-taxi the adjustment is made to the G5 to match the altimeter's reading.

Why? Why is the G5 and the altimeter not matching? Is it possible the G5 is factoring in non-standard temperature and presenting an altitude adjusted for both Hg and temp or is there some other reason for the variation?

Thanks!

 

[murphey]

Using the Rule of Thumb that 1.0 Hg is 1000 ft, the 0.05 Hg is 50 ft, which is completely acceptable until you're IMC attempting to land.

 

[Feassarian]

Appreciate that rule - and actually the issue has been more pronounced now that I am working through instrument
Issue is far more noticeable when flying and approach or even enroute with the the aid of the autopilot - the AP is coupled to the altitude from the G5, yet the altimeter is considered primary so it is what gets updated most frequently - a good habit to begin updating the G5 every time the altimeter is updated is being integrated into my scans - but I still was curious why the two do not stay in sync and to be in sync requires differing altimeter settings

 

[nauga]

Is it possible the G5 is factoring in non-standard temperature and presenting an altitude adjusted for both Hg and temp or is there some other reason for the variation?

Anything's possible, but the basis for indicated altitude is pressure and pressure corrected to indicate height able MSL, so there is no temperature correction assuming Garmin is doing it right - which is probably a safe assumption. I tend to distrust the provided altimeter settings as much as I distrust my own altimeter(s).

Nauga,
under pressure

 

[RussR]

In any airplane I've flown with more than one altimeter, the altimeters never quite match. This includes factory installations like G1000 and G3000, and various FMS systems.

The plumbing is different, the sensors are slightly different, the components within the systems are slightly different, etc. Very small changes in component values (but within tolerances) can add up.

It's the "man who has two watches never knows what time it is" scenario (neglecting GPS-set watches, of course).

 

[mondtster]

Have you had a static system and altimeter check since the G5 was installed? How much error was present during the testing?

Bear in mind that a digital readout doesn't imply precision or accuracy. There are a lot of places where measurement or calibration errors could creep in and cause an indication to be slightly off. I've also had the same experience as Russ; two G1000s and an Aspen in the same airplane all will indicate slightly differently.

 

[TCABM]

… Let's use 1K1 as an example - field elevation is 1340 - when I enter the altimeter setting into the altimeter, I should read the same field elevation of 1340 on the altimeter.…

There is an ADC baro calibration procedure in the install manual that should have been completed during install.

In all the G5 equipped planes I’ve flown, it’s been coincidental for the primary altimeter and the G5 to have identical readings, but the only time they’ve been off enough to make a difference is when the pressure settings were different.

Also keep in mind the G5 is secondary for altitude.

 

[RussR]

Let's use 1K1 as an example - field elevation is 1340 - when I enter the altimeter setting into the altimeter, I should read the same field elevation of 1340 on the altimeter.

I did want to comment on this, although it's not related to the difference you're seeing. What you state is a common misunderstanding.

From the Pilot Controller Glossary: AIRPORT ELEVATION- The highest point of an airport's usable runways measured in feet from mean sea level.

It's the highest point on any of the _runways_. That's important, because usually we do the altimeter check at some point other than the highest point on the runway. We may do it at the end of the runway in the runup area (which may or may not be the highest point), or we may do it outside the hangar or on the parking ramp, neither of which are even considered in Airport Elevation.

Some airports have pretty significant differences in elevation around the airport. At one of my primary airports, as you taxi on the parallel down to runway 17, you can see the altimeter slowly decrease - the approach end of 17 is about 20 feet lower than the midfield parking area - and that's in Oklahoma, not considered particularly hilly!

The nearby Class D has 30 feet variance between runway elevations.

You use 1K1 as an example, an airport I'm quite familiar with. That has a published 0.3% slope up to the north. The runway is 5106 ft long, which means the south end of the runway is about 15 ft lower than the north end. Pretty flat in this case, but again, this doesn't include taxiways and parking areas.

The bigger the airport (meaning more runways across more acres) and the hillier the area, the more variation you'll have, even when runway slope isn't a factor worth considering. And the published Airport Elevation may be way out at the far end of the most remote runway. It's not uncommon to see differences of over 100 feet.

 

[RussR]

Let's use 1K1 as an example - field elevation is 1340 - when I enter the altimeter setting into the altimeter, I should read the same field elevation of 1340 on the altimeter.

Also, the airport elevation I see for 1K1 is 1364 from multiple sources.

 

[CT583]

I have a funny preflight procedure concerning the G5.
I’ll set it to airport altimeter setting.
It is habitually 40 feet higher than airport elevation.
what I also do is set the steam altimeter to field elevation. I like having both to reference. only when I level off at cruise will I set the steam altimeter to match the G5.

Now when I’m shooting an approach, I add about 50 feet to any altitude I’m flying along the approach past the FAF. It’s interesting that is almost perfectly on glideslope when on an LPV.