Magnetic Compass

Understanding magnetic compasses 1) is somewhat difficult and 2) seems outdated.  However, since every aircraft has one and they technically never fail, it's important to know how they work and how they'll react in turns.  So here it goes...

Compass turns, standard rate... 

• Time to turn:  degrees to turn/3 = time; so 60° / 3 = 20”
• Bank:  airspeed/10 + 5 = bank;  90 kts --> 9+5 = 14°
• Rollout - 1/2 your bank angle in degrees prior to desired heading

“ANDS”

• ANDS is an acronym for Accelerate North Decelerate South. This is one type of magnetic compass error. This error primarily occurs if you are on an approximate heading of east or west, and most of this type of error occurs between 060 degrees and 120 degrees if the heading is easterly, or 240 degrees through 300 degrees if the heading is westerly.
• The magnetic compass will indicate a turn to the north if the aircraft is on one of these headings, and will indicate a turn to the south if the aircraft is decelerated while in this heading range. This error occurs due to magnetic dip. The error is eliminated when the airspeed stabilized.

“UNOS”

• UNOS is an acronym for Undershoot North Overshoot South. A false error occurs on the magnetic compass when the aircraft is turned. This phenomenon of the magnetic compass is called northerly turning error. This error is zero on east or west headings, but is pronounced on turns to either north or south. When turning from east/west to north, the magnetic compass card actually lags the heading of the aircraft, so we undershoot, or turn what appears to be early, when rolling out to a northerly heading. The opposite is true when turning to a southerly heading and we must turn what appears to be past the south heading.
• How do we compensate for this error?
• We must factor in latitude at which we are operating, plus one-half of our bank angle. We will use a latitude of 30 degrees for this session. With a bank angle of 16 degrees, we would take half of that (8 degrees) and calculate that into our bearing and roll out on a predetermined magnetic heading. The following is the initial part of the equation at 30 degrees latitude. This data reflects the amount of error when turning to various magnetic headings: If you are turning to:
• Amount of Error on the Compass 
• 360 Degrees 30 degrees of error
• 030 and 330 degrees 20 degrees of error
• 060 and 300 degrees 10 degrees of 
• 090 and 270 degrees 0 degrees of error
• 120 and 240 degrees 10 degrees of error
• 150 and 210 degrees 20 degrees of error
• 180 degrees 30 degrees of error
• Now we must calculate the lead by adding or subtracting ½ of the bank angle and applying this to the compass card readings at different points on the turn. Let’s say we are turning right to a heading of north from a heading of west. There is no error when we are on the initial heading of 270 degrees. We now roll into a 16-degree bank to the right. The compass card is now indicating a turn to the right, but the rate is faster than the turn. Using the information on the last page, the adjusted roll out point is 330 degrees as we are going to UNDERSHOOT NORTH. We deduct ½ of the bank angle (8 degrees) and start our roll out when the compass card is 322 degrees. Here are some other examples, but remember that we OVERSHOOT SOUTH. We will assume a bank angle of 16 degrees for these examples.
• Direction/New Heading Roll Out Heading Bank Angle Correction Roll Out
• Left to 360 Degrees 030 degrees 8 degrees 038 degrees
• Right to 180 Degrees 210 degrees 8 degrees 202 degrees
• Left to 180 degrees 150 degrees 8 degrees 158 degrees
• Right to 300 degrees 290 degrees 8 degrees 282 degrees
• Left to 300 degrees 310 degrees 8 degrees 318 degrees
• Right to 150 degrees 170 degrees 8 degrees 162 degrees.
• Remember that the magnetic compass is the primary navigation reference in an aircraft. It is important that you have a general working knowledge of this instrument. These calculations are only used to offset the compass error while you are in a turn. The compass will stabilize when you roll to wings level. A smaller amount of roll out lead is required with smaller bank angles. So if you are at 30 degrees latitude and you are turning left to a heading of 180 degrees with a 5- degree bank angle, you would overshoot the desired heading by 30 degrees and be within 2 ½ degrees when you roll out. I have heard down south they teach “the south leads and the north lags.” Seems like this may be one way to remember what correction to apply.

- FIG -