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Old 12-06-2017, 08:56 PM   #1
2013grandsport
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Default Garmin G5 Airspeed accuracy

So my new G5 is indicating an airspeed lower than my mechanical airspeed indicator... Which is to be trusted more as they both use pitot/static?


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Old 12-06-2017, 09:20 PM   #2
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So my new G5 is indicating an airspeed lower than my mechanical airspeed indicator... Which is to be trusted more as they both use pitot/static?
I'm guessing that the G5 is more accurate, unless there's a problem with the hoses leading to its sensor. Both should be measuring indicated airspeed using the difference between dynamic and static pressure, and both should suffer from identical errors vs calibrated airspeed (e.g. when one of the ports is partially masked at certain angles of attack or yaw).

A mechanical ASI occasionally needs to be adjusted/calibrated: mine got seriously out a few years ago, but only at lower speeds. When's the last time yours was checked in an instrument shop?


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Old 12-06-2017, 11:32 PM   #3
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hasn't been checked every since my ownership, and the needle sitting still isn't centered. its pointing more towards 40 mph sitting still... so its probably wacky.
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Old 12-06-2017, 11:46 PM   #4
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Sounds like a pitot static check is in order. Chances are your new instrument is accurate.
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Old 12-07-2017, 12:35 AM   #5
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So my new G5 is indicating an airspeed lower than my mechanical airspeed indicator... Which is to be trusted more as they both use pitot/static?
What are you seeing on each for 50, 100, and top KIAS?

I wonder if there is a calibration setting for the G5?
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Old 12-07-2017, 12:42 AM   #6
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Is the G5 in the HSI position? How do you like the PFD display?


Garmin G5 Part 23 AML STC Electronic Flight Instrument Pilot's guide para 3.1 “The G5 has two main pages, the PFD page and the HSI page. When installed as an ADI, the G5 displays the PFD Page only. When installed as an HSI, the G5 displays the HSI Page primarily but can display the PFD page as a backup to the ADI.”
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Old 12-07-2017, 12:55 AM   #7
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I have both G5's and its AWESOME. Im betting the G5 is accurate, and my mechanical AI is out of calibration.
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Old 12-07-2017, 01:02 PM   #8
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hasn't been checked every since my ownership, and the needle sitting still isn't centered. its pointing more towards 40 mph sitting still... so its probably wacky.
I'd say that a/s instrument is off. Have your pitot-static system checked. We had this problem years ago and had to replace the a/s indicator as it was 'stuck' at 40 when sitting still.

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Old 12-07-2017, 08:17 PM   #9
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The best way to check accuracy of the ASI (including the pitot system) is to take its reading, convert it to true airspeed (taking into account altitude and OAT, and compare it to TAS derived using a three-course GPS calculation. Fly the calibration courses one right after another to minimize errors from changes in wind speed, and best to do the flight in turbulence-free air.
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Old 12-07-2017, 10:36 PM   #10
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The best way to check accuracy of the ASI (including the pitot system) is to take its reading, convert it to true airspeed (taking into account altitude and OAT, and compare it to TAS derived using a three-course GPS calculation. Fly the calibration courses one right after another to minimize errors from changes in wind speed, and best to do the flight in turbulence-free air.
That might land you within a few knots, but it would be good to try for reasonably low winds as well as calm air, because you don't get your wind-correction angle loss back on the different headings (only the direct headwind/tailwind components). That means that the three-course GPS calculation will always give you a TAS that's too low—by only a little in light winds, or by a lot of the winds are particularly strong.

You'd also have to run the test in different speed regimes -- when my ASI was miscalibrated, it was still fairly accurate at cruise speeds, but way off down near the stall. Down there, the IAS/CAS deviation can be big, too, so you have to convert IAS => CAS => TAS (you should do that at cruise speed, too, but the IAS/CAS deviation is usually negligible there).

tl;dr this probably isn't the easiest way to test ASI calibration


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