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New How to mount and swing a compass



There is no mystery to adjusting a magnetic compass. The only things needed are a non magnetic screwdriver and maybe some masking tape. No compass rose, no pelorus, no special equipment. Just follow the instructions below. These adjustments should be made away from any hangar buildings or other possible sources of magnetism. They should also be made with the engine running at enough RPM to assure that the voltages are at cruising levels, and with all radios and normal electrical equipment turned on. If the airplane has a canopy, it should be closed. It is important in the following steps that the 180 degree turn be done as precisely as possible. If you have a gyro, use it. If not, mark left and right wing shadows with tape on the ground, and make the turns using the shadow. If this is the case, be aware that the sun moves 1/4 degree each minute.

  1. Go north (or south) by the magnetic compass.
  2. Zero the gyro (un-slaved) or put masking tape on ground.
  3. Do a 180 degree turn by the gyro or shadow.
  4. Halve the compass error using the N-S adjustment screw (non-magnetic screwdriver).
  5. Repeat steps 1 through 4 until there is no error.
  6. Go east (or west) by the compass.
  7. Zero the gyro - or use tape.
  8. Do a 180 degree turn by the gyro or tape.
  9. Halve the error using the E-W adjustment screw.
  10. Repeat steps 6-9 until there is no error.
Now go north by the compass and zero the gyro if necessary. Make turns of 30 degrees by the gyro and note any errors on the magnetic compass. These errors should be recorded for the compass correction card. The compass is now adjusted as accurately as it can be without changing external factors. Never change the adjustments except when on a N-S or E-W heading, and then only the proper screw. It is not possible to adjust headings other than the cardinal ones without upsetting the entire adjustment of the compass. If these adjustments won't correct the compass on N-S and/or E-W headings, then something in the airplane is amiss. You will have to research whether the problem is with the airframe, under the panel, or elsewhere. There is a strong magnetic field lurking in there someplace. Don't even try to adjust your compass using an airport compass rose because of the difficulty of aligning the airplane accurately. The method outlined above will produce better results in a fraction of the time! Now, why does this method work? It's easy! Look at Figure 1. The magnetic disturbance in the airplane is to the right of the compass so the compass has a clockwise error. Figure 2 shows that in doing a 180 degree turn, the disturbance is now on the left of the compass and it now has a counter-clockwise error. This explanation is for the N-S errors, but applies equally well to the E-W ones.

If the compass is adjusted so that when you do an exact 180 degree turn by the gyro or shadow, and the magnetic compass also makes a 180 degree turn, then all of the magnetic forces or disturbances inside the airplane must be balanced on both sides of the compass. The adjusting of the N-S screw made the compass think that there was an equal and opposite force on the other side of the airplane. See Figure 3. The magnetic compass is now acted upon only by forces outside the airplane - and that force is the earth's magnetic field.

Now you can go and check the compass rose at your airport. If it doesn't agree with your compass, then the compass rose is probably wrong!

Your Compass - Functional or Decorative?

By Ned McIntosh

How many of you have a compass in your trike cockpit? Lots of raised hands, that’s excellent! Compasses are simple devices which always point towards the North magnetic pole. By using the “lubber-line” we can see what direction we are pointing, which may not necessarily be the direction we are flying! Magnetic compasses, as you all perhaps know, require some corrections, the principal ones of which are Variation and Deviation.

We all know about magnetic variation. The WAC or VNC for your area will tell you what the magnetic variation is, since both have the lines of magnetic variation shown (look for them!). The VTC has the variation displayed near the legend.

We also know that Variation has to be subtracted from (if Easterly), or added to (if Westerly), the True headings from a chart to convert them to the magnetic heading to fly. (“Variation East, Magnetic is Least” is the old rhyme sailors learn). For example, if the magnetic Variation is 12 degrees East, then your compass will read Zero Degrees Magnetic when the True Bearing is 12 degrees. Therefore, on a True heading of 013 degrees, your magnetic compass will read 001 degrees (13 - 12 = 1 degree...Variation East so subtract).

The second correction, Deviation, is a little different. Deviation arises due to magnetic fields in your trike near the compass. Steel tubing is one source, steel brackets or clips used to mount instruments are another. Yet another source of deviation are wires – any wires – carrying an electric current. Whenever an electric current flows, a magnetic field exists at right angles to the direction of current flow.  Deviation can – and will – affect your compass, possibly by several degrees or more. Unlike Variation, Deviation is constant because it is from local magnetic fields in your aircraft... although adding extra instruments or wiring will probably alter it, as will flying over a large iron-ore body, but we will ignore such circumstances for the remainder of this discussion.

I’m sorry to be the bearer of bad news, but a compass that hasn’t had its deviation determined is unreliable, even positively dangerous, as an indicator of direction. It is decorative, rather than functional. Deviation is determined by “swinging the compass”.

Now, how many of you have actually swung the compass in your trikes? Oh dear... I’m not seeing as many hands raised now, am I?      

“But I use GPS!” I hear you say. Well, that’s fine, but GPS is not a primary navigational instrument. A compass is. If you have a compass in your trike, you need to know it’s deviation, and you also need a record of it in the cockpit. Determining deviation means you have to “swing the compass”.

There is a practical and simple method of doing this. It involves setting the trike base up on a series of accurately determined magnetic headings and recording what the cockpit compass reads against the known magnetic bearings. A table of Magnetic versus Compass is then made, usually at 30 degree intervals all the way around the compass rose. The difference between known magnetic and what the trike compass indicates is the deviation. Once these amounts are determined, a card is drawn up showing the deviation (see sample at the end of this article). Mount the deviation card somewhere prominent in the cockpit where you can see it clearly – preferably with non-magnetic fasteners – and the job is done!

This reduces the compass swing procedure to that of accurately setting the trike base to point along known magnetic directions. Back in the old days (when Pontius was a pilot!) many airfields had a compass-swinging area set aside (away from hangars, etc) with directions accurately surveyed and painted on the concrete. This made life very easy, but few airfields have a compass-swinging area now. How else can we determine accurately the real magnetic bearings? 

Time to meet what mariners call a Hand Bearing Compass. This is an accurate handheld compass with a scale subdivided down to half-degree increments, a sighting-prism and a handle enabling it to be held up at eye-level (and arm’s length!) so you can sight precisely on a distant object. Somehow, you need to beg, borrow or steal a hand bearing compass – and someone who knows how to use it.

You’ll also need to keep a record of your work, so rule up a worksheet with three columns headed “Magnetic”, “Compass” and “Deviation”. Under the Magnetic heading write twelve numbers, starting at 000 degrees and proceeding around the compass in 30 degree increments, thus 000, 030, 060, 090, 120, etc... all the way to 330 degrees (see the sample worksheet at the end of this article).

The procedure is simple enough. Set your trike base up somewhere away from metal fences, hangars, etc. The middle of your local airfield is good, if it can be arranged (hint: choose a day when the weather is totally overcast at about 500 feet so no-one is flying!). If you’re really keen you can put the wing on, but there is so little ferrous material and wiring inside a trike wing it is not significant and you can leave it in its bag... unless you plan on flying after you’ve finished.

Switch on the ignition, install your radio and switch it on, plug your headset in and leave it on the seat. Leave the engine off for the swing; you can turn it on afterwards and see if it affects the compass, but remember to set the revs so the base isn’t subject to a lot of vibration so the compass can settle! You want the trike base set up as closely to its in-flight configuration as possible. Empty your pockets of any ferrous objects (Leatherman tools, bottle-openers, etc), key-holders and so on. Keys themselves are usually brass with a thin nickel plating. Keep any Boy Scouts away... they are infamous for carrying little magnets in their pockets and we don’t want that!

Align the base with the nose pointing where your trike compass indicates North is. Then, walk to a position about ten metres behind the trike... directly behind it so you are exactly on its centre-line. (This distance will ensure the hand bearing compass isn’t affected by the trike’s magnetic fields!)

Now, sight with the hand bearing compass straight through the centre-line of the base and see if it really does face North. If it doesn’t, lift the nose of the trike around and repeat the process until it is facing exactly magnetic North according to the hand bearing compass.

Walk back to the trike and note the reading on the compass in the cockpit. Chances are it will not be North, but a few degrees one way or the other. Make a note of it in the “Compass” column next to Magnetic 000.

Now, move the trike base around in exact 30 degrees (clockwise) steps, according to the hand bearing compass and again read the heading on the trike cockpit compass. Tabulate them until you’ve gone around the full circle.

Finally, get into the base, note the compass heading, start the engine, set the revs so the base doesn’t vibrate, let the compass settle and see if it is different to the engine-off condition (mine wasn’t). Whatever error you find will be constant for every heading, so add or subtract it from your recorded trike compass readings accordingly.

Engine-running is the usual condition under which you will be using your compass.

That’s the hard physical stuff done, now for the easy bit! What you have is a table of actual Magnetic headings versus what your trike compass reads. If you’re really keen, calculate the difference between them and write this figure in the third column (Deviation). Use positive and negative signs according to the difference.

With your table of Magnetic and Compass values you now have all you need to draw up the Deviation Card. Try the format set out below, it works quite well.

You don’t need to go any further unless you’re really curious. If you are, plot Deviation (horizontal axis) against Magnetic (vertical axis). You should get something like a sine-curve. Not perfect, but passable (the zero deviation points will probably not be at 000 and 180 degrees). That’s also normal because aircraft rarely have equal amounts of local magnetic field, nor is it necessarily symmetrical either side of the centreline.

Well, that’s it. Now you’ve swung your compass and know what it’s really telling you. For the curious, below is a sample worksheet and a Deviation Card from an actual swing. Remember, Deviation is a fixed correction, but if you add extra wiring or instruments, you should do a new swing.

            Magnetic        Compass         Deviation


            000                  004                  +4                    Note: Varying value and sign

            030                  031                  +1                    form approximate Sine-Curve.

            060                  058                  -2

            090                  088                  -2

            120                  114                  -6

            150                  144                  -6

            180                  172                  -8

            210                  203                  -7

            240                  238                  -2

            270                  269                  -1

            300                  302                  +2

            330                  334                  +4


                        Figure 1: - Sample Compass Swing Worksheet




                                                Compass Deviation Card


            FOR:         000   030  060  090  120  150  180  210  240  270  300  330

            STEER:    004   031  058  088  114  144  172  203  238  269  302  334



                        Figure 2: - Sample Deviation Card from above worksheet

How to mount and swing a compass

Compass deviatdion card

Copyright 2003, SkySports International


The process of mounting and adjusting ("swinging") an aircraft compass seems like a simple task, and with attention to some details, it can be. Still, locating and adjusting a compass to insure an accurate reading can be frustrating to the point of madness, since slight variations in positioning of the instrument in the cockpit can have a major effect on the adjustment process. Take it slow, read these instructions carefully from start to finish, and the task is very managable.



A magnetic compass is a simple instrument. The most common are fluid filled chambers containing a rotating disk mounted on a pivot. A small magnet is mounted on the disk, and the outer rim of the disk is marked from 0 to 360 degrees. The outer rim of the disk is visible through a window in the chamber, and the center of the window is marked with a thin indicator line (the "lubber line"). The disk tends to want to stay aligned with the earth's magnetic field, so as the chamber is rotated the magnetic heading appears in the window under the lubber line. The fluid in the compass is usually a highly refined petroleum distillate which won't freeze no matter what, and remains clear for decades. The fluid dampens the tilting and rotation of the disk, so turbulance doesn't render the compass useless.

Compasses are equipped with compensators, which are mechanically adjustable ferrous metal pieces which can be moved in relation to the magnet to make the disk align with the earth's magnetic field. Using these compensators, the compass can be set to read 0º when the aircraft is pointing north, 90º for east, 180º for south and 270º for west, and be accurate at all intermediate points as well.

A note of caution though: you''ll never get the reading to track the magnetic directions throughout the entire range of headings. The earth's own magnetic idiosyncracies, as well as those of your airplane, cause compass errors that just can't be adjusted away. Our goal is to minimize these errors.



Compasses can be mounted in the instrument panel, on top of the instrument panel, on the windshield or windshield support. Charles Lindbergh had to mount his compass over his head (his panel was full) and read it in a mirror. The critical factor is magnetic interference from other instruments and radios. Many electrical instruments have small magnets inside, creating enough magnetic force to affect a nearby compass. If you want to mount the compass in the panel, you'll need to carefully place the compass near the various intruments in the panel to see which ones cause the compass disk to swing, and to be sure you can mount the compass as far away from those instruments as possible. Test the same way near radios and radio indicators with the power applied. Try transmitting on your comm radios too; often a comm radio presents significant magnetic interference only when transmitting. If you can't find a place in the panel which is not affected by interference, you'll need to move the compass to the instrument panel glare shield or to the top of the windscreen.

It's usually not possible to "shield" a compass from magnetic interference, as any effective shield would have to be ferrous (i.e., iron based). The shield itself would cause the compass magnet to turn.



If you live south of the equator, make sure and buy a compass compensated for the southern hemisphere. Otherwise, the disk will be tilted in the compass case and may be impossible to read



You'll need a small, brass, non-magnetic screwdriver and an airport with a compass rose. A compass rose is a pattern painted on the ground which accurately depicts magnetic headings. If you can't find an airport that has one, you can sometimes make your own using the known magnetic orientation of the runways at an airport. Be careful, just because a runway is marked "27" doen't mean it is oriented at 270º. It might be 274º or 268º. Check with your state aeronautics department to get the precise orientation.

Position the aircarft at a known north heading using the compass rose or runway reference. WARNING: most compass compensators turn only one-half turn (180º). Turning more than that can ruin the instrument. Compensators are usually capable of together producing a +/-20º correction, which should be more than enough.

1. With the aircraft oriented North, the engine running, and all radios switched on, adjust the N-S adjusting screw until the compass reads due North, or 0º.

2. Rotate the aircraft to a known East heading, and use the E-W adjusting screw to make the compass read due East, or 90º

3. Rotate the aircraft to a known South heading. Note how many degrees off South the compass reads. Turn the N-S adjusting screw to remove one-half of the error.

4. Rotate the aircraft to a known West heading. Note how many degrees off West the compass reads. Adjust the E-W adjusting screw to remove one-half of the error.

5. Rotate the aircraft through the N,S,E,W headings again, confirming that the errors for North and South are the same, and the errors for East and West are the same. You may want to repeat steps 1-4 to fine tune the correction.

6. Now rotate the aircraft from North, stopping at each 30º point on the compass rose (e.g., N, NNE, NE, ENE, E, etc.) Make a note of the actual compass reading at each point, and complete the compass calibration card, which you should then mount near the compass for reference.


If you cannot get a usable set of readings, you need to locate sources of magnetic interference. Look for steel screws, washers or other components near the compass. You should also experiment with aircraft electrical systems (lights, for example), to see whether activation of that equipment causes the compass to misbehave.






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