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Originally Posted by Wiki
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.
[edit] 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.
[edit] 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. Magnetic dip is caused by the downward pull of the magnetic poles and is greatest near the poles themselves. A weight is often placed on the compass of the aircraft on the equatorial side to help negate this effect. Compass navigation near the polar regions, however, is nearly impossible due to the errors caused by this effect.
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 inertia effect of the weight on the compass in the magnet causes the compass to 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. 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 flying in the southern hemisphere. 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 inertia effect of the weight on the compass in the magnet causes the compass to 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 will be approximately equal to the aircraft's latitude. (An aircraft at 30° north latitude will need to undershoot 30° while turning directly north, and overshoot 30° while turning directly south) 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] This guideline is based not on a standard-rate turn, but on a bank angle of 15°-18°, which would equal a standard rate turn at the airspeeds typical of light aircraft.
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