IFH 5-11

Airspeed for best single-engine rate-of-climb at gross weight and Sea Level 80 60 I20 I00 I60 I40 I20 I00 200 I80 Figure 3-15. Color codes for an airspeed indicator. 240 220 200 I80 I60 I40 I20 I00 80 60 40 KNOTS Blue radial line Green arc White arc Yellow arc Red radial line Figure 5-15. Color codes for an ASI. Figure 5-16. A magnetic compass. The vertical line is called the lubber line. N-S E-W Figure 3-16. A Magnetic compass. field of another, the unlike poles attract each other and like poles repel. An aircraft magnetic compass, such as the one in Figure 5-16, has two small magnets attached to a metal float sealed inside a bowl of clear compass fluid similar to kerosene. A graduated scale, called a card, is wrapped around the float and viewed through a glass window with a lubber line across it. The card is marked with letters representing the cardinal directions, north, east, south, and west, and a number for each 30° between these letters. The final “0” is omitted from these directions; for example, 3 = 30°, 6 = 60°, and 33 = 330°. There are long and short graduation marks between the letters and numbers, with each long mark representing 10° and each short mark representing 5°. Magnetic Compass Construction The float and card assembly has a hardened steel pivot in its center that rides inside a special, spring-loaded, hard-glass jewel cup. The buoyancy of the float takes most of the weight off the pivot, and the fluid damps the oscillation of the float and card. This jewel-and-pivot type mounting allows the float freedom to rotate and tilt up to approximately 18° angle of bank. At steeper bank angles, the compass indications are erratic and unpredictable. The compass housing is entirely full of compass fluid. To prevent damage or leakage when the fluid expands and contracts with temperature changes, the rear of the compass case is sealed with a flexible diaphragm, or with a metal bellows in some compasses. Magnetic Compass Theory of Operations The magnets align with the Earth’s magnetic field and the pilot reads the direction on the scale opposite the lubber line. Note that in Figure 5-16, the pilot sees the compass card from its backside. When the pilot is flying north as the compass shows, east is to the pilot’s right, but on the card “33”, which represents 330° (west of north), is to the right of north. The reason for this apparent backward graduation is that the card remains stationary, and the compass housing and the pilot turn around it, always viewing the card from its backside. Magnetic fields caused by aircraft electronics and wiring can effect the accuracy of the magnetic compass. This induced error is called compass deviation. Compensator assemblies mounted on the compass allow aviation