Garmin 300

HAPPYDAN · Post by **HAPPYDAN** » Thu Jul 02, 2015 9:30 am

I have just a quick question about the Heading Indicator on the Garmin 300 panel in the Cessna Skycatcher. It seems to be very stable and accurate as compared to, say, a magnetic compass (we enjoy a 18-22 degree east variation here!). Is it gyro-stabilized like on the 152, 172, etc.? Or is it linked to GPS, and therefore, aligned to true north? My best guess is GPS. Also, as a back up, maybe adding a mag compass would be a good idea?

drseti · Post by **drseti** » Thu Jul 02, 2015 9:51 am

Should be a Flux gate compass. Way more stable than a whiskey compass, though still prone to magnetic dip errors in turns, climbs, descents, acceleration, and deceleration.

FastEddieB · Post by **FastEddieB** » Thu Jul 02, 2015 11:17 am

HAPPYDAN wrote:Is it gyro-stabilized like on the 152, 172, etc.? Or is it linked to GPS, and therefore, aligned to true north? My best guess is GPS. Also, as a back up, maybe adding a mag compass would be a good idea?

I have not heard of a heading indicator that used a GPS for heading input.

Even the fanciest Garmin and Avidyne equipment needs a remote compass somewhere.

In general, a GPS can give location and track, but not heading - it does not know which way the nose of the plane is pointed.

Even if it is utilizing GPS data somehow, it would absolutely have to display magnetic heading, since...

91.205 (Equipment requirements for VFR)

...(3) Magnetic direction indicator...

MrMorden · Post by **MrMorden** » Thu Jul 02, 2015 11:50 am

FastEddieB wrote:
HAPPYDAN wrote:Is it gyro-stabilized like on the 152, 172, etc.? Or is it linked to GPS, and therefore, aligned to true north? My best guess is GPS. Also, as a back up, maybe adding a mag compass would be a good idea?
I have not heard of a heading indicator that used a GPS for heading input.

Yeah, I thought the whole idea behind the compass requirement was to have *some* form of "this shall not fail" direction finding if/when everything else goes Tango Uniform.

drseti · Post by **drseti** » Thu Jul 02, 2015 12:01 pm

As Andy suggests, whiskey compasses are failure proof (unless the pilot drinks their contents!) That's why even planes with a fancy EFIS still contain them.

SportPilot · Post by **SportPilot** » Thu Jul 02, 2015 12:20 pm

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FastEddieB · Post by **FastEddieB** » Thu Jul 02, 2015 12:35 pm

SportPilot wrote:If you're moving at all, the GPS heading information is going to be pretty accurate.

Does it actually compute and show magnetic heading without some sort of remote or self-contained magnetic compass?

drseti · Post by **drseti** » Thu Jul 02, 2015 12:47 pm

All the GPS can show is ground track, which is different from heading (unless you have zero winds, or a direct headwind or tailwind). I can't speak for the G300, but every EFIS I've ever flown used a flux gate magnetic compass for heading information.

SportPilot · Post by **SportPilot** » Thu Jul 02, 2015 12:55 pm

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rsteele · Post by **rsteele** » Thu Jul 02, 2015 1:03 pm

I wonder if some the newer equipment uses solid state MEMS compass, that are now showing up in cell phones. These can be quite accurate, and if combined with other MEMS sensors can compensate for a lot of movement.
MEMS = MicroElectoroMechanicalSensor which means the mechanical bits are etched on a chip. If you google you can find micrographs of these things which I think are pretty cool.

Ron

drseti · Post by **drseti** » Thu Jul 02, 2015 1:05 pm

SportPilot wrote: I don't think you really need a wet compass in a SkyCatcher.

According to FAR 91.205, you need a direct magnetic direction indicator in every aircraft. It can be a wet compass, a vertical card compass, a MEMS compass, or a flux gate compass. GPS alone is not a legal substitute.

FastEddieB · Post by **FastEddieB** » Thu Jul 02, 2015 1:11 pm

SportPilot wrote:My point is, I don't think you really need a wet compass in a SkyCatcher.

Well, to be fair...

1) By law, you need a magnetic direction indicator of some sort.

2) You also need the ability to fly an assigned heading.

Before I calibrated my vertical card compass, it was off so much as to make it virtually unusable. The one in my Dynon was even worse. So, if Knoxville Approach, let's say, told me to fly a heading of 050, I'd pretty much have to fudge it based on my understanding of the approximate winds aloft at my altitude.

Never got called on it, and I've got it "swung" pretty well now, but that's why you do need to be able to fly an assigned heading, not a specified ground track - which with high winds can be radically different things.

SportPilot · Post by **SportPilot** » Thu Jul 02, 2015 1:11 pm

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SportPilot · Post by **SportPilot** » Thu Jul 02, 2015 1:13 pm

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FastEddieB · Post by **FastEddieB** » Thu Jul 02, 2015 1:13 pm

I've sensed zero animosity nor argumentativeness to this point in this thread.

But maybe I'm calibrated differently!

Back on point, you DO need a magnetic compass of some sort, both for legal and practical reasons - and pointing that out is as much for the benefit of newcomers, fledgling pilots and lurkers as it is for you.

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