cheap and unreliable
I had one like it on my car and it was marginal. The steel in the car may have had something to do with it. It does have a calibration mode to account for compass corrections, but after I played with it for weeks and drove with it for a few months it was kind of a joke. It may work better in an aluminum plane. This was a few years ago and the electronic compass may be better now?
I would recommend a traditional wet compass. It never needs a battery and is consistent and a known quantity. Now if you find with your flight test the el-cheapo (some not so cheap) electronic mag compass works well what the heck why not. For me I am going with the Dynon magnetometer remote mounted only, no additional compass. There is no controversy about this and I have double checked with the FAA. There are rumors of a few DAR not signing off without a wet compass but that is their mistake, your FSDO mileage may very. My mission is day night VFR by the way so I find the Dynon EFIS mag heading is plenty accurate and there is no need for redundancy.
You must have as required equipment a compass for day VFR and above. Some say you need to have a non-electronic compass, but that is NOT true. Part 91 is clear and it mentions a compass but makes no distinctions as to being electric or non-electric. However Part 23 does require the compass to NOT be electrical, but that does NOT apply to us, part 91 is good enough. In the jets I fly we still have a wet compass. Guess where it is. In the center above the wind screen and folds out of the way, only visible when folded out. Guess how much we use it? never. The point is you legally could use your auto/electronic mag compass, but should you?
Frankly anyone who has flown for a few hours with a GPS knows we could care less about heading most of the time since we now have full time position, track and ground speed. Of course when ATC says fly heading XYZ they intend that to be magnetic, but this is not a big issue for a VFR guy.
HOWEVER regardless of VFR or IFR I would advise on having something reasonably accurate for a mag direction indication (independent of GPS track), if for no other reason than if you have to have an instrument in your plane, it should be accurate. It is required anyway. If I did not have a Dynon mag heading and relied on the wet compass, I would likely go with a traditional wet compass, but the newer designs vs. the WWII stuff (but airpath is is still fine). The vertical card compass I don't think are so great from my memory.
Back in the day with out GPS and Vacuum gyros, we where updating the DG all the time and the wet compass played a more important roll. Now with GPS we just drive the track we want and the heading is the heading. Also most have done away with the Vac DG and have EFIS with the magnetometer built in anyway.
Other wise I don't use the mag compass much. George
PS: The big use of mag heading in my mind is calculating wind and direction enroute and making sure I am taking off the correct runway, although knowing just ground speed is good enough. Who cares what the direction is enroute except to determine that it matches the forecast? In the pattern I don't look at the wet compass to figure wind direction, ATIS is good enough.
