Calibrating a compass using a GPS, in flight
 

There are many problems with the traditional ground based compass swinging:

- should be done at cruise power but cruise RPM often can't be achieved on the ground

- hard to line up the plane accurately

- a hassle to do in bad weather, or at grass locations when the grass is soft

- compasses can be heavily affected by electrics e.g. alternator load, which lights are on, etc. Wingtip-mounted fluxgate magnetometers tend to be a lot better but all these things are affected by e.g. a steel hangar 50-100m away.

As a result most compasses are way off, notwithstanding the deviation card carrying impressive figures like 1 degree max error.

It is intuitively obvious one could use the highly accurate GPS track for this. Suprisingly, I did not find much on google... there is the usual "TAS from GPS" stuff like this, which is commonly used for calibrating one's speedo.

In zero wind, track=heading. But how can you be sure there is no wind aloft? Fly two reciprocal headings, and if this yields reciprocal tracks, then either wind=0 or you are flying in line with the wind, and in both cases track=heading.

If there is wind, it will usually be unknown, but if one flies reciprocal headings and increases them (say 15 degree increments) until one is seeing reciprocal tracks, then you have nailed the above case.

Unfortunately, a compass has one screw for N-S adjustment and another screw for E-W adjustment, so one really wants a method which yields more or less direct N-S and E-W headings.

It is hard to explain without a diagram, but it is fairly obvious that if you fly N-S reciprocal headings, and (assuming there is wind) you get non-reciprocal tracks, then your compass error will be determined from the assymetry between the non-reciprocity of the tracks and the headings being flown, and you can set this error directly on the compass. Then repeat this for E-W.

Example:

Fly 360 and 180.
If the track is 360 and 180, you have no (relevant) wind so set the compass to 360/180 (the easy case).
If the track is 010 and 170 (obviously a westerly wind from 270) then the non-reciprocity is symmetrical about the heading, so set the compass to 360/180 as before.
If the track is 008 and 168 (obviously a slight southerly component in the above wind) then your actual heading is 358.

I hope I got that right, but it seems a useful method.

The above works to a first order accuracy i.e. the wind effect is small relative to your TAS. If you fly at 50kt and the wind is 50kt, the geometry becomes quite complex, and possibly needs and iterative solution.

I'll check my copy tomorrow if I can find it, but I think that there's a method in this book.

G

Does it matter other than as an excercise in OCD?

I can't see a case for the level of accuracy you are trying to achieve. Can you explain more the need?

"As a result most compasses are way off, notwithstanding the deviation card carrying impressive figures like 1 degree max error".

Is this really the case? The effect from electrics in the aircraft itself aren't going to change whether you're airborne or on the ground. Provided you're using a properly calibrated site then good results are easily achieveable. A proper base should be accurate to +/- 0.2 degrees, and since I can only read my sighting compass to 1 degree and the Airpath and H.S.I in the aircraft have 5 degree increments, where are the big inaccuracies coming from - must be the pewrson doing the adjustments!

I dutifully align my DI to the compass and then cross check with GPS, but GPS gives track and compass/HI give heading so surely you can only reference compass to GPS in nil wind?

In practice I always follow the GPS track so what th compass and HI read is largely irrelevant:E

In some cases, wiring. I can "swing" my vertical card compass by 10 degrees just by revving up the engine, or operating the taxi/landing lights.

The engine prob99 needs to be running, whereas most compass swinging I see done doesn't do that.

One could indeed argue that an accurate heading is not needed if using a GPS, and that is true, but I quite like getting things right :)

No; see my original post.

At the end of the day does it really matter if the compass is 2° out? If you are flying VFR you should be looking outside and if you are flying IFR you are tracking based off a VOR, NDB, ILS or GPS so heading in either case doesn't require heading to be accurate to more than ±5°

The only times I find myself looking at the magnetic compass is when lining up on the runway to ensure it is reading correctly and at TOC to check it matches the RIC slaved HSI.

If you don't have a RIC then chances are you are VFR and should be looking outside anyway.

We almost bought a new DI because of a faulty compass! The compass seemed to have developed a massive error E-W but nothing N-S, and we didn't know it.

We'd be flying and setting the DI then turn through S and the DI would be 60 degrees off, it was like this for quite a while as we thought the DI was on its way out and drifting.

We then started flying on the GPS track and ignoring the compass and we then figured out from the GPS that it was in fact the Compass that was way off and so we borrowed some swinging compasses from our maintenance place. Sure enough, 60 degrees of error through E-W, 0 degrees through N-S. I hate to think what would have happened had we had to dead reckon somewhere!

Can you not set the GPS to give magnetic heading, and fly in zero wind and use that to calibrate the compass?

How can you calibrate a magnet device with a totally non magnetic device?

Of course; and an IFR GPS gives you MH anyway. But you need zero wind.

My long post described a way of doing this when the wind is not zero.

I don't think the method would work as stated. Generally, if you measure two things (the two tracks in this case), you can estimate two things, everything else must be assumed. Here we want to know Crosswind, Headwind, and compass error, which are three things.

If there is only cross wind the method would work, in principle. The key image is an isosceles triangle which is rotated by the compass error. But a head wind would also 'rotate' the triangle. You could not distinguish between the two effects.

Fortunately, as you are not correcting for the crosswind, you can simply measure the headwind (and calculate TAS while you are at it) by timing the two legs. In principle you now have all you need.

Assuming sufficient accuracy, known (local) magnetic variation, etc, etc!

PS For the angles, for better accuracy, I would suggest plotting the GPS tracks on a half-million chart and measuring the true tracks that way. If you can access the time-stamps for the plotted points, you would get excellent timing information too. Obviously you would not get the results in flight, but the measurements would have been made under flight conditions.

Different method?
 

My be there is a different method.

Let's suppose we know the magnetic (or true) course between two points on the earth clearly visible from the air, lets say two towers, couple of miles apart. We could position our aircraft's axis during flight to the extended line connecting this two towers. A short glace at the compass in the right moment when everything is in row will give us the reading of the compass with known course.

If we can do this for more headings, we will have at least the readings of the compass for known heading, even if not for the right 360/90/180/270 headings. Unless we find the right towers of course :) The rest is a little interpolation.

Miroc

I think my method will work just fine.

The key to it is that second order effects remain so, which in GA means the TAS is a lot bigger than the wind.

Obviously if you find a calm day then you can knock it off in a flash.

With the reference to ground objects the wind is no more an issue.

You can reduce the task of calibrating the compass during flight conditions to determine the exact physical heading of the plane and the reading of the compass in the same moment. If you want to get the physical heading from the GPS, you will be affected by the movement of the air (unknown).

Miroc

Do you not need to be a licenced engineer or hold an ATPL to adjust a compass on an aeroplane, or do the FAA have different requirements?

I would get it done by an A&P/IA and signed off appropriately.

There are A&P/IAs who are pilots also :)

No idea what the minimum requirements for compass adjustments are, for private aircraft. I would sincerely hope at least 20 JAA exams and 10,000 hours. Nobody can understand how a compass works with any lower qualifications.

IO540, my apologies, the headwind effect is indeed second order, as you said all along.

Specifically, about 60hx degrees where 'h' and 'x' are the head and cross winds expressed as fractions of the TAS. So for 225/30 wind the error is about 2.5 degrees, which is comparable to the accuracy of flying a heading manually.

Eliminating the error would involve something like this:

1) Take off, then set the DI by visual reference to something outside. This does not have to be very accurate. Plus or minus 10 degrees should be fine.

2) Fly a 'one minute square', ie one minute E, then S, then W, then N, using the DI for direction. Analysing the GPS tracks afterwards will give you headwind, cross wind, and TAS.

3) Fly one leg, North by the compass, long enough to get a good True Track estimate from the GPS track afterwards.

4) Solve the wind triangle problem to get the True Heading, then apply the best estimate of the local magnetic variation (Whopity's point, I think) to calculate the actual magnetic heading.

A lot more complicated, and probably pointless, but more accurate with respect to the wind!

And you are going to do all this while flying and trying to adjust a compass that will be bouncing around with the aircraft....... :ugh::ugh:

Is it a slow news week or something?

No, as I explained earlier, the GPS records the track in-flight for post-flight analysis. Setting the DI that way is a little unusual, but otherwise all you have to do in flight is hold some headings for some times. Covered in most PPL courses, I think!

We are talking about calibrating a compass in flight......

Perhaps I am being daft but i really can't understand what the purpose of doing so is?

Perhaps someone could enlighten me as to the purpose of the exercise above and beyond a standard compass swing that has served us well enough for a century?

I think IO540 meant "under flight conditions".

I'll leave him to explain the benefits over the compass swing.:)

Can't you just compare the HSI with the compass ;)

You could, but unless I am mistaken the HSI remote compass also requires swinging so it could be the blind leading the blind......

Ahh, I see....We actually took one of the "swinging" compasses into the aeroplane with us when we were checking ours. I can borrow one if you need to use one....

I did this for real for 40 Hrs of long VFR xcountry in a plane (not my own) last year where the compass went AWOL, setting the DI every 10 min off the GPS allowing for percieved drift. After a while I forgot the compass even existed.

I think the typical GA compass provides an ideal platform for velcroing a standby GPS onto.