Aircraft compass turns

Aircraft compass turns

In aviation, aircraft compass turns are turns made in an aircraft using only a magnetic compass for guidance.


A magnetic compass aboard an aircraft displays the current magnetic heading of the aircraft, "i.e.," the aircraft's direction of travel relative to the geomagnetic field, which has a roughly north-south orientation. The compass can be used in turns to verify that the aircraft is travelling in the desired direction at the conclusion of a turn. The nature of the instrument, and the alignment of the magnetic pole of the earth, cause the magnetic compass to have several severe limitations when used for navigation. A pilot aware of those limitations can use the compass as an effective instrument which is not liable to electrical failure, or icing effects.

Compass turns are the exception to the rule in most aircraft today, and are generally only performed in an aircraft when other directional instruments, such as the directional gyro, have failed. Although a magnetic compass is theoretically very easy to use to guide a turn, in practice various characteristics of real-world magnetic compasses complicate their use for this purpose, and pilots must learn to compensate for errors introduced by these characteristics. While turning to a heading of North, undershoot by 30 degrees.While going south, conversely the turn radius is only 20 degrees but overshoot.

Most of the errors inherent in the heading indications of a magnetic compass are related to the compass' construction. The compass in an aircraft can be thought of as an upside-down bowl balanced on a pin and enclosed in a case filled with non-acidic kerosene. A magnetized bar attached to the inside of the compass tends to align itself with the local geomagnetic field and in so doing turns the visible bowl of the compass to indicate a magnetic heading. As the aircraft turns, the bowl tries to remain stationary due to this alignment, hence the aircraft is free to turn around the stationary bowl.

The standard practice when flying with a gyro-stabilized compass (or heading indicator) is to read the magnetic compass only while in straight and level flight. This reading is then used to set the gyro-stabilized compass. The gyro compass will read correctly in a turn, whereas the magnetic compass can't be read properly while turning. Thus the pilot will always ignore the magnetic compass while turning, but periodically check it in straight and level flight.

Errors inherent to compass turns

Several types of errors will affect the heading indication provided by a magnetic compass in a turn. Thus the magnetic compass cannot be used reliably when turning.

Pitch limits

A limitation imposed by a compass' construction is that the balancing bowl's pin, which is connected to a pivot point, only allows for, in most compasses, a pitch or bank of 18 degrees before the compass will touch the side of the casing. When the compass touches the side of the casing the freedom of the plane to rotate around the compass is lost and the compass becomes unreliable.

Magnetic dip

A second limitation is "magnetic dip." When the aircraft is at any altitude above the earth's surface, the compass dial will tend to align itself with the geomagnetic field and dip toward the northern magnetic pole when in the northern hemisphere, or toward the southern magnetic pole when in the southern hemisphere. At the equator this error is negligible. As an aircraft flies closer to either pole the dipping error becomes more prevalent to the point that the compass can become unreliable because its pivot point has surpassed its 18 degrees of tilt.

When in straight and level flight the effect of magnetic dip is of no concern. However when the aircraft is turned to a new heading the following two rules apply for the northern hemisphere:

First, when on an easterly or westerly heading and the aircraft accelerates, the compass will show a false turn towards the north if in the northern hemisphere or vice versa a false turn towards the south if in the southern hemisphere. Also if the aircraft is decelerated the compass will show a false turn towards the south in the northern hemisphere and false turn towards the north in the southern hemisphere. This can further be explained by imagining the compass needle's attraction towards the pole not wanting to move. However, the aircraft's acceleration generates a force greater than that of the magnetic force. The force is neutralized when the aircraft has reached its velocity and the magnetic compass will then read the proper heading. Pilots in the northern hemisphere remember this by the acronym "ANDS"; accelerate north, decelerate south. The opposite occurs when you decelerate. This error is eliminated while accelerating or decelerating on heading of exactly North or exactly South.

Second, when on a northerly heading and a turn towards the east or west is made the compass will lag behind the actual heading the aircraft is flying through. This lag will slowly diminish as the aircraft approaches either east or west and will be approximately correct when on an east or west heading. When the aircraft turns further towards South, the magnetic compass needle will tend to lead the actual heading of the aircraft. When a turn is made from south to an east or west heading the compass will lead the actual heading the aircraft is flying through, it will diminish as the aircraft approaches either east or west, and it will lag as the aircraft turns further towards North. The magnitude of the lead/lag may be about 20 to 30 degrees when making standard rate turn. The pilots community uses acronym UNOS (undershoot North overshoot South) to memorize this rule. Some other acronyms which pilots find easier to remember is NOSE (North Opposite, South Exceeds), OSUN (Overshoot South, Undershoot North), and South Leads, North Lags [opposite in the southern hemisphere]

tandard-rate compass turns

Standard rate turn is a standardized rate at which the aircraft will make a 360 degree turn in two minutes (120 seconds). Standard rate turn is indicated on turn coordinator or turn-slip indicator.

All turns during flights under instrument rules shall be made at standard turn rate, but no more than 30 degrees of bank. In case of vacuum-driven instruments failure (i.e. directional gyro, attitude indicator) the rollout to new heading is timed: let's say the aircraft is flying 060 degrees heading and it needs to fly new heading 360. The turn will be 60 degrees. Since the standard rate turn is 360 degrees in 120 seconds, the plane will need 20-second standard rate turn to the left.

In case of electrical instrument failure, which include turn coordinator or turn-slip indicator, the following formula will help to determine turn bank at which the turn will be made at standard rate: In order to calculate bank angle for a standard rate turn knowledge of air speed must be known. The rule of thumb using air speed requires that the last digit of the air speed be dropped then add five. For example if the air speed is 90 knots drop the zero and add five. The bank angle in this example would be (9+5=14) 14 degrees. Or if the air speed is 122 knots drop the two and add five. The bank angle in this example would be (12+5=17) 17 degrees. The line of latitude is the maximum lead or lag a compass will have.

The following explanations are for the northern hemisphere.

For example an aircraft flying at 45°N latitude making a turn to north from east or west maintaining a standard rate turn a pilot would need to roll out of the turn when the compass was 45 degrees plus one half of the bank angle before north. (From east to north at 90 knots 0+45+7=52) A pilot would begin to roll out to straight flight and on a heading of north when 52 degrees was read from the compass. (From west to north at 90 knots (360-45-7=308). A pilot would begin to roll the aircraft out of the bank at 308 degrees read from the compass to fly on a north heading. Making a turn towards south from west the pilot would have to roll the aircraft out of the turn when the compass was 45 degrees minus half the bank angle (from west to south at 90 knots 180-45+7=142, from east to south 180+45-7=218).

From the examples we see that when turning to north from east or west the bank angle used to calculate the time to roll the plane out of the turn must begin at the greatest amount of degrees or further away from north. Conversely for turns to south from east or west the bank angle is calculated to decrease the number of degrees to lead the roll out or closer to south.

Generally pilots will practice making these turns using half standard rate turns. This will decrease the bank angle so that it is half of the calculated bank angle. When turns are made at half standard rate the line of latitude will only cause the compass to have an error of half as much. So our new calculation using a half standard rate turn is as follows: (From east to north at 90 knots 0+22.5+3.5=26) the lead roll out heading read from the compass would be 26 degrees to fly on a north heading. (From west to north 360-22.5-3.5=334) The lead roll out heading read off the compass would be 334 degrees.

Turns made for other directions should be interpolated. For example a left turn made from a heading of west to south east (SE). The compass would initially show a heading that is correct as the turn gets closer to south the compass would indicate a lead heading of the greatest error, as the aircraft passes through south the error would decrease and show less of a lead. As the aircraft approaches south east the error would only lead half as much as it did when the aircraft was rolling through south. So if the turn was made using a half standard rate at 90 knots and the SE heading required to fly was 135 degrees the roll out heading would be 135-11.25+3.5=127 degrees. Hence a roll out heading read from the compass of 127 degrees would be used to actually fly the heading of 135 degrees.


*cite book
last =Gleim
first =Irvin N.
title =Private Pilot FAA Written Exam
publisher =Gleim Publishing
date =January 1, 2001
pages =440
isbn =1560276185

*cite book
last =Federal Aviation Administration
title =Pilot's Handbook of Aeronautical Knowledge: FAA-H-8083-25 December 2003
publisher =Aviation Supplies & Academics, Inc.
date=September 28, 2004
pages =352
isbn =1560275405

*cite book
last =Kershner
first =William K.
title =The Student Pilot's Flight Manual
publisher =Aviation Supplies & Academics, Inc.
date =November 1, 2000
pages =440
isbn =1581941285

*cite book
last =Kershner
first =William K.
title =The Instrument Flight Manual: The Instrument Rating and Beyond
publisher =Aviation Supplies & Academics, Inc.
date =January 1, 2002
pages =320
isbn =1560276193

*cite book
last =Machado
first =Rod
title =Private Pilot Handbook: The Ultimate Private Pilot Book
publisher =Aviation Speakers Bureau
date =March 1996
pages =572
isbn =0963122991

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