The International Cessna^® 170 Association

I'd like to swing my compass using the GPS on a calm morning. Has anyone here done this ? I believe the GPS bearing and tracking data is magnetic (compensated for variation). Or is it relative to true north ?
Rudy

My circa 1998 Garmin GPS uses magnetic north.

There are already so many errors in magnetic compasses, I don't know why you'd want to introduce yet another variable.
I'd suggest you find an airport with a compass rose (which is also/already compensated for magnetic variation) and swing your compass according to the aviation standard.

I'd suggest that if you had a compass rose and a couple buddies it'd still be a lot harder, and possibly less accurate, way to swing a compass than flying w/ a gps on a calm day.

The "right" way w/ a rose is to simulate flying conditions: tail about 5' in the air, engine running. Unless your compass rose is built on a turntable it takes a lot of manuevering to center on every one of the 8 different headings. Do you lower the tail each time? Turn off the engine? No matter how much help and time you've got it's a royal pain.

I can turn to 8 different gps headings pretty easily when the plane's flying, and with the engine at cruise and the wings level it simulates flying conditions pretty well too <g>.

Spiro's message precisely reflects my thoughts. Raising the tail, engine running on a compass rose- it's a lot of trouble. Flying on a calm mornng is much easier and probably more accurate. Also more pleasant.
Now, can someone tell me- does the GPS read magnetic or true ?
Eric's old unit read true-
I use a Garmin 295- I suppose I should ask Garmin, but does anyone here know? George ?
Rudy

Whoops- sorry Eric, I misread your message- if your '98 Garmin reads Magnetic, then my 295 does also.
Rudy

Rudy, if nothing else, look at the GPS display-- as I recall, mine sez "mag" or something like that up in the corner of the bearing & track datablocks. There might be a menu option to set those to either magnetic or true. I would look in the Garmin owner's manual to be sure.

Eric

I Did my compass in the air from my 295 and I belive its closer now than it ever was....

Compass accuracy is pretty overrated anyway if you are flying with a gps. No matter what your compass says, you're going to correct your heading left/right x-number of degrees to satisfy your gps readout.
A compass is important when dead reckoning and using pilotage. (Most pilotage these days are actually gps-assisted...but that's not what I'm talking about.)
If real accuracy is desired, then swinging the compass with a rose is the way to do it. (But if your compass accuracy is only needed to be "approximate" then the gps method is as good as any....I just wanted to point out it's not as accurate as might be assumed.)
Using a rose: Tail in the air is an easy matter with a wheeled dolly. Two guys at the dolly to wheel you around makes it simple. In any case, tail up or down on the ground, there's less likely hood of error (in my opinion) to using a compass rose than a gps. Despite what you might think, winds aloft still exist even when it's calm on the ground.

Last Saturday I flew some visiting friends to Everglades City for a grouper sandwich (outstanding by the way- I highly recommend it when you're in Florida) and Horrors ! left my briefcase with GPS at home.

Had to actually navigate using the compass and VOR ! It was really quite fun- we get so spoiled with GPS !

George is right about the winds aloft not necessarily being calm. I think the best time to swing the compass with GPS would be when the surface wind is calm and fly low, maybe 500 ft.
Rudy

All the Garmins allow you to set up whether the compass shows true, automag correction (based on your location), or manual mag correction in the set up menu.
Doesn't anyone read the instruction manual anymore, or a least play with the settings in their GPS so they at least know how to change the time zone when the fly far from home!!!!!!!

On my 196 it is in the setup menu under the "location" tab under the "heading" choice.

Read the manual ? Only as a last resort, when all else fails-
Rudy

Very few compass roses are currently certified, If you use a non certified compass rose and one of the FAA nit pickers wants to be a jerk he she or it could cause some grief for you. FAA does not keep up with the movement of the magnetic pole in most locations. A rose that was painted in 19XX may be off by 5 or 6 degrees. Have you noticed that some runway numbers have changed in the last few years, ( rounded to the nearest 10 degrees magnetic ) but the runway is the same as it was before. As a practical matter a compass rose wll get you in the ball park but so will the GPS or a VOR. The pole wanders around so much that over time your compass will become off even if it could be adjusted precisely on a given day. Your location in relation to the magnetic pole will make a difference also. Around here the current deviation is about 24 degrees E, In the lower states somewhere between the E coast and mid continent the deviation is 0. The direction of movement of the magnetic pole in relation to where you currently are will have a bearing on how far off your compass may become over time. If you are in the 0 degree area and the magnetic pole moves S ( or N ) 5 degrees it won't affect you but if you are W ( or E )of the magnetic pole you will be off 5 degrees. The magnetic pole is in an area of northern Canada quite a ways from the true N pole.

George mentioned winds aloft, but here's the point: GPS does not tell you your aircraft heading, it tells you the track over the ground. It doesn't actually know where you've got your nose pointed! To swing a compass, you have to compare the compass with something else that's telling you the aircraft's heading, not ground track.

I live in an area where magnetic variation is very small, and I don't have to go far to find a point where it's zero. I have swung my compass in flight by lining up with an east-west road, then setting my DG (I have a fairly new DG that is rock-solid). Then I swung the compass with reference to the DG. I'm sure this is not an approved method, but for these airplanes (slow, limited range) I think it's plenty good enough.

Best Regards,

John

It's true that even with no wind, the track over the ground is not necessarily the same as the heading. But the track is what you're really interested in. For example, if an airplane is way out of rig and flies through the air in a skid, you'd fly the compass in the direction you want to go, the track, and not the heading.

BTW, my 295 reads Magnetic. I learned this not by reading the manual, but because there's a tiny letter "m" next to the tracking and bearing numbers. Whew ! Almost had to read the manual !
Rudy