ArcticTern
Topic Author
Posts: 89
Joined: Sat Dec 03, 2005 11:55 pm

### Compass Swing?

Hi,

Could someone please let me know the exact procedure of a compass swing?

How often do can one perform a compass swing.

Thanks,
Sushant
I wanted to fly even before I knew how to pronounce the word 'Pilot'

Tristarsteve
Posts: 3406
Joined: Tue Nov 22, 2005 11:04 pm

### RE: Compass Swing?

On modern airliners with INS, compass swings are not required. A form is left in the cockpit and the crew fill in the details of heading from INS, GPS, and standby compass. After enough readings are accumulated, the engineer calculates the adjustments to the standby compass.

OzLAME
Posts: 336
Joined: Thu Feb 17, 2005 8:40 am

### RE: Compass Swing?

Sushant, there are a number of different techniques that may be used, but probably the simplest is as follows:

First of all, set the compass compensators to neutral and align the aircraft at or near to magnetic North, East, South and West on an approved compass swing pad, recording the actual headings as shown by an external compass.
Now, suppose that at North the aircraft compass shows 003 degrees with the external compass showing 001 degrees; at East the a/c compass shows 094 degrees and the external shows 092 degrees; at South the a/c compass shows 180 degrees and the external shows 178 degrees; and at West the a/c compass shows 272 degrees and the external shows 270 degrees. The result is that there is an error of two degrees at all four cardinal headings; because the error is the same at each heading this is an error of alignment of the compass in the aircraft, perhaps the compass mounting bracket is not straight. This is called coefficient A Error and is corrected by adjusting the fore-aft alignment of the compass in the aircraft.
Once you have got rid of the Coefficient A Error, align the aircraft at or close to magnetic North again. Determine the actual heading of the a/c and adjust the N-S Compensator in the compass so that the aircraft compass reads the same as the external compass. Next align the a/c at or near magnetic East, determine the aircraft heading and adjust the E-W Compensator so that the a/c compass reads the same as the external compass. Then position the aircraft at or near South.
Now for the sake of argument, say that the internal compass shows a heading of 176 degrees while the external compass shows 180 degrees, giving a difference of four degrees. You then adjust the N-S Compensator again to remove half of the error, so that the aircraft compass now reads 178 degrees. You then position the aircraft on a heading at or near West; suppose the a/c compass shows 273 and the external shows 269. You then adjust the E-W Compensator so the a/c compass reads 271 degrees. After you do this you then position the aircraft at various headings (in Australia at each thirty degrees, i.e 300, 330, 000, 030, 060 etc.) and record the differences between the headings indicated by the aircraft compass and the external compass. You then calculate and enter the corrected headings on the Compass Correction Card.
Simple really, once you have done a couple.
Monty Python's Flying Circus has nothing to do with aviation, except perhaps for Management personnel.

EMBQA
Posts: 7798
Joined: Sat Oct 25, 2003 3:52 am

### RE: Compass Swing?

 Quoting ArcticTern (Thread starter):Could someone please let me know the exact procedure of a compass swing?

Compass swings are done most often to calibrate the on-board stand by compass. Most airports have a painted 'compass rose' that is calibrated and shows the primary 8 headings. You taxi the aircraft and line up with each 'rose' heading. You match your heading to the calibrated rose heading and that will give you your off set. Often you will also have a mechanic outside shooting a heading back toward the plane with a hand held calibrated compass to act as a third reference point. He will line up his compass with the painted compass and the center line of the plane to give a much more accurate reading. As to what you need to do inside the plane... each aircraft is a little bit different. Always refer to the manuals on how to configure the aircraft. Often you will be required to have the generators off and other system configured before you take your readings.
"It's not the size of the dog in the fight, but the size of the fight in the dog"

Dougloid
Posts: 7248
Joined: Tue Jul 05, 2005 2:44 am

### RE: Compass Swing?

SOP at Douglas was to haul them out to the compass rose and swing the compass with everything up and running.
If you believe in coincidence, you haven't looked close enough-Joe Leaphorn

fr8mech
Posts: 6835
Joined: Mon Sep 26, 2005 9:00 am

### RE: Compass Swing?

Haven't seen a compass rose in years. We use the INS and tow the aircraft to the cardinal points and the intermidiate points. Plug and chug a formula (don't have it handy) and come up with the deviation.
When seconds count...the police are minutes away.

OzLAME
Posts: 336
Joined: Thu Feb 17, 2005 8:40 am

### RE: Compass Swing?

I took so long typing out my previous post that I forgot that you asked when a compass swing is performed.

A compass swing used to be done on Australian aircraft at least once every three years, but the regulations were changed a couple of years ago, so now there is no set time laid down in the Australian Regs. However, I would do a compass swing every time the compass is removed and also every time an avionics modification is done to an aircraft. I have done some very extensive mods in my time, installing wiring looms as thick as a man's arm in some aircraft, and sometimes the magnetic field set up by the new wires will cause the compass to read five degrees differently or occasionally even more.
Monty Python's Flying Circus has nothing to do with aviation, except perhaps for Management personnel.

HAWK21M
Posts: 30002
Joined: Fri Jan 05, 2001 10:05 pm

### RE: Compass Swing?

A good Article on Compass Swing.
regds
MEL
I may not win often, but I damn well never lose!!!

saintsman
Posts: 2037
Joined: Fri Mar 15, 2002 12:34 am

### RE: Compass Swing?

On the VC-10s we used to do a 12 point swing with a Watts Datum Compass on the outside and adjust the two main aircraft compass systems to an accuracy of 0.2 of a degree. That was on a mechanical system too.

Whiskeyflyer
Posts: 223
Joined: Mon May 13, 2002 3:07 pm

### RE: Compass Swing?

we do a compass swing every year AND after an engine change, avionics mod, and when we position the aircraft at a new base (we lease aircraft, so they move hemispheres)

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