THE FASTEST, EASIEST AND MOST ACCURATE WAY TO ADJUST A COMPASS

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