occurs when flying in the southern hemisphere. This error is eliminated while accelerating or decelerating on a heading of exactly North or exactly South. Second, when on a northerly heading and a turn towards the east or west is made, 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. This happens in a coordinated turn because of the bank of the aircraft and resulting bank of the compass. The North-seeking pole of the magnet is pulled towards the earth's magnetic field in the turn. This results in an angular displacement of the compass. 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). 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. The pilot community uses the mnemonic UNOS (undershoot North overshoot South) to memorize this rule in the Northern hemisphere. In the Southern hemisphere the mnemonic ONUS is used. Other mnemonics used in the northern hemisphere are NOSE (North Opposite, South Exaggerates), OSUN (Overshoot South, Undershoot North), and South Leads, North Lags. These are reversed in the Southern hemisphere. Standard-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