Flight Instruments

Figure 8-31. Angle of attack indicators. state of an airplane is the balance between airspeed, altitude, There are long and short graduation marks between the letters drag, and thrust and represents how efficiently the airfoil is and numbers, each long mark representing 10° and each short operating. mark representing 5°. Compass Systems The float and card assembly has a hardened steel pivot in its center that rides inside a special, spring-loaded, hard glass The Earth is a huge magnet, spinning in space, surrounded jewel cup. The buoyancy of the float takes most of the weight by a magnetic field made up of invisible lines of flux. These off of the pivot, and the fluid damps the oscillation of the lines leave the surface at the magnetic North Pole and reenter float and card. This jewel-and-pivot type mounting allows the at the magnetic South Pole. float freedom to rotate and tilt up to approximately 18° angle of bank. At steeper bank angles, the compass indications are Lines of magnetic flux have two important characteristics: erratic and unpredictable. any magnet that is free to rotate will align with them, and an electrical current is induced into any conductor that cuts The compass housing is entirely full of compass fluid. To across them. Most direction indicators installed in aircraft prevent damage or leakage when the fluid expands and make use of one of these two characteristics. contracts with temperature changes, the rear of the compass case is sealed with a flexible diaphragm, or with a metal Magnetic Compass bellows in some compasses. One of the oldest and simplest instruments for indicating direction is the magnetic compass. It is also one of the basic The magnets align with the Earth’s magnetic field and the instruments required by Title 14 of the Code of Federal pilot reads the direction on the scale opposite the lubber Regulations (14 CFR) part 91 for both VFR and IFR flight. line. Note that in Figure 8-32, the pilot views the compass A magnet is a piece of material, usually a metal containing iron, that attracts and holds lines of magnetic flux. Regardless of size, every magnet has two poles: north and south. When one magnet is placed in the field of another, the unlike poles attract each other, and like poles repel. An aircraft magnetic compass, such as the one in Figure 8-32, 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, N-S E-W north, east, south, and west, and a number for each 30° between these letters. The final “0” is omitted from these Figure 8-32. A magnetic compass. The vertical line is called the directions. For example, 3 = 30°, 6 = 60°, and 33 = 330°. lubber line. 8-23