Instrument Flying Handbook Flight Instruments Basic Aviation Magnetic Compass

Instrument Flying
Handbook

Preface

 Figure 3-19 A compass correction card shows the deviation correction for any heading. The corrections for variation and deviation must be applied in the correct sequence and is shown below starting from the true course desired. Step 1: Determine the Magnetic Course True Course (180°) ± Variation (+10°) = Magnetic Course (190°) The Magnetic Course (190°) is steered if there is no deviation error to be applied.. The compass card must now he considered for the compass course of 190°. Step 2: Determine the Compass Course Magnetic Course (190°, from step 1) ± Deviation (-2°, from correction card) Compass Course (188°) NOTE: Intermediate magnetic courses between those listed on the compass card need to he interpreted. Therefore, to steer a true course of 180°, the pilot would follow a compass course of 188°. course is known: Compass Course ± Deviation Magnetic Course ± Variation = True Course Dip Errors The lines of magnetic flux are considered to leave the Earth at the magnetic north pole and enter at the magnetic South Pole. At both locations the lines are perpendicular to the Earth's surface. At the magnetic equator, which is halfway between the poles, the lines are parallel with the surface. The magnets in a compass align with this field and near the poles they dip, or tilt, the float and card. The float is balanced with a small dip compensating weight, so it stays relatively level when operating in the middle latitudes of the northern hemisphere. This dip along with this weight causes two very noticeable errors: northerly turning error and acceleration error. The pull of the vertical component of the Earth's magnetic field causes northerly turning error, which is apparent on a heading of north or south. When an aircraft flying on a heading of north makes a turn toward east, the aircraft banks to the right, and the compass card tilts to the right. The vertical component of the Earth's magnetic field pulls the north-seeking end of the magnet to the fight. and the float rotates, causing the card to rotate toward west, the direction opposite the direction the turn is being made. [Figure 3-20] If the turn is made from north to west, the aircraft banks to the left and the compass card tilts down on the left side. The magnetic field pulls on the end of the magnet that causes the card to rotate toward east. This indication is again opposite to the direction the turn is being made. The rule for this error is: when starting a turn from a northerly heading, the compass indication lags behind the turn.
 Figure 3-20. Northerly Turning Error.

3-13