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enewton57(at)cableone.net
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 Posted: Wed Dec 26, 2007 5:50 pm Post subject: Compass Requirements in Experimental Aircraft (updated) |
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Hi All,
Well its been kind of slow so I thought I would give a little update regarding my research into whether or not a "wet" compass is required if you have a EFIS with a remote external compass feeding a heading to it. After much research and searching archives of all of the various airplane groups that I belong to, I finally decided to just mount an airpath compass on my glareshield and be done with it. I said to myself - "no sense getting into a pissing match with the DAR over a $160 instrument, just put it in and forget it".
So I purchased the Airpath C2400 lighted compass. It arrived sometime last week and over Christmas vacation, I finally got out to the airport to see how it would work with my Bearhawk. I had heard that some people were having as much as 30 degrees of installation errors in a Bearhawk with a glareshield mounted compass.
So there I was standing beside my airplane which was pointed in a generally northward direction. I held the compass out in front of me and it read 358 degrees. I then reached inside and sat it on the glareshield centered and as far back from the "V" tubes as I could. It read "N" 360 degrees. PERFECT, only about 2 degrees off right??? WRONG!!!
I then grabbed the tail and swung it around to point the nose in a generally south direction. Poked my head inside for a look at the compass and . . . What's this???? It still says "N" 360 degrees!!! I grabbed the compass out of the airplane and once again stood next to the airplane and sure enough the compass now read 178 degrees. Put back on the glare shield and . . . "N" 360 degrees.
So there I stood, scratching my head, thinking about what I had done to cause what Erbman would call a "Gross Buffoonery". As I was standing looking at things, I saw the problem. My lightening fast brain finally figured out what I had done. When I mounted my XM satellite radio, I didn't want to run the external antenna all the way outside so I had simply mounted it up near the windshield right between the "V" braces. The antenna has a strong magnet built in to allow it to stick to the roof of your car. Of course it didn't stick to my aluminum boot cowl so I had just double stick velcroed it in place.
The wire running from the antenna to the Xm radio on the right side of the instrument panel was included in the main wiring harness and is well wrapped within the bundle of wires running from the comm and intercom system. Removing and replacing the XM satellite antenna would take a long time laying on my back under the instrument panel cutting every tie wrap I made.
So I decided to do what I should have done in the first place. I called my DAR to ask him his opinion on the magnetic direction indicator in the Dynon. I described the Dynon D100 EFIS with its remote magnetic compass feeding heading information to the Dynon D100. He asked me again what I was building. I said a Bearhawk. He said "That's Experimental right?" I said "yes it is". He said "Well then, it falls under FAR part 91.205 which specifically says "Magnetic direction indicator" . He said as long as the Dynon receives, and displays, a heading based on the Earth's magnetic field, then it qualifies as a "magnetic heading indicator" as required in Part 91.205 and he doesn't see why I would need an old whiskey compass littering up my glareshield.
We talked a bit about redundency and when I told him about my battery backup of the Dynon as well as a GPS with battery backup as well as a handheld radio with a VOR indicator, he was more than satisfied.
So problem solved. I called Aircraft Spruce for an RMA number and as of this morning, the compass is on its way back to Corona, CA and I'll have a credit for it. Guess I could have saved some shipping $ by simply calling the DAR first.
Lesson learned and shared.
Eric Newton - Long Beach, MS
BH #682- Mississippi Mudbug
BEARHAWK BUILDER'S MANUALS
http://mybearhawk.com
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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Thu Dec 27, 2007 11:19 am Post subject: Re: Compass Requirements in Experimental Aircraft (updated) |
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Let's not forget the charming Gordon Baxter (Bax Seat) tale about getting into an airplane that had seen better days, taking off northbound, flying north by the the compass, landing on a north-pointing runway. and finding out later that the compass hadn't worked at all.
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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
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bobf(at)feldtman.com
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| Posted: Fri Dec 28, 2007 8:38 am Post subject: Compass Requirements in Experimental Aircraft (updated) |
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NExt time you start your airplane, watch the compass as you crank. You will be surprised at the way it swings in reaction to the high current loads from the battery to the starter. And that can magnetize local ferrous structures - thus the need to "swing" the compass on a recurrent basis to catch induced "new" errors.
Most importantly - use the compass to make sure you are on the right runway as final before take off checklist item.............. a crash a couple years ago due to wrong runway!
Secondly - most of us use our GPS to set our DG anyway - not the compass.
I've enjoyed the back and forth about the need for a compass in experimentals - My glastar has one, so I am covered.
bobf
On 12/28/07, Tim Olson <Tim(at)myrv10.com (Tim(at)myrv10.com)> wrote: [quote]--> AeroElectric-List message posted by: Tim Olson <Tim(at)MyRV10.com (Tim(at)MyRV10.com) >
Another builder asked me to forward this because he's on
vacation, but wanted to join in the thread:
"But another very important issue regarding magnetism is that the steel
parts in the plane can pick up residual magnetism from permanent magnets
and it can be a real problem to sniff out and get rid of with a
magnaflux (I think he means degaussing -Tim) tool. I believe a I made
a complete posting regarding this in the RV10-list archive. But even
the helical steel coil in scat tubing is capable of causing totally
eronous compass readings. I had this occur when we re-did the
instrument panel in our glastar. I even avoid using magnetic tip tools
now that I've seen how difficult it can be to solve the residual
magnetism problem.
-bob newman
Tim Olson wrote:
| Quote: | --> AeroElectric-List message posted by: Tim Olson <Tim(at)MyRV10.com (Tim(at)MyRV10.com)>
So one of the morals to the story is that magnetic base antennas
have no place in an airplane. They won't just screw with your
nearby compass but can screw with your much more sensitive EFIS
magnetometer as well. Any antenna with a magnet should be cut
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[b]
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