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My school airplane has a Garmin G5 Electronic Flight Instrument and round gauge altimeter, but they have almost 50ft differential. The G5 shows 50 feet higher on the same altimeter setting.

So in IFR, when I shoot the approach or cruise flight, my primary altimeter is the G5 so I follow my G5 altimeter.

But my instructor said "you hit the minimum, because standby (round gauge) showing below (-50ft) the minimum..."

In this case should I follow my round gauge altimeter? Even though the G5 is alive?

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    $\begingroup$ "my primary altimeter is the G5" Why? $\endgroup$
    – Mast
    May 1, 2023 at 14:56
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    $\begingroup$ As a non-aviator layperson, I would always chose whichever shows the lowest altitude - I'd much rather find that there's 50ft of unexpected descent left before touchdown, than the opposite! $\endgroup$
    – noughtnaut
    May 2, 2023 at 10:39
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    $\begingroup$ @Noughtnaut Satellite (GPS) and barometric (normal) altimeters are never used to determine touchdown height, or used at all below the MDA or DH ("decision height"). In that regime only radio (radar) altimeters are useful, and still are not a primary landing aid except for autoland. $\endgroup$
    – TypeIA
    May 2, 2023 at 15:17
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    $\begingroup$ The old "man with two watches never knows what time it is" conundrum. $\endgroup$
    – Criggie
    May 2, 2023 at 18:13
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    $\begingroup$ @noughtnaut, as a non-aviator layperson, hopefully you (and 15 others...) learned something from Chris' answer! I get your point though. ;) $\endgroup$ May 3, 2023 at 19:41

2 Answers 2

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The G5 is not certified as a primary altimeter- its information is advisory only. From a legal standpoint, your primary altimeter is always the certified one- in your case, the steam gauge one.

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Garmin itself says:

With the Auto Calibration feature turned on, the altimeter accuracy can range from +/- 50 to 125 feet (same as GPS elevation).

You can draw your own conclusions from this!

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