Flight Instruments

Plane of Precession Figure 8-19. Precession of a gyroscope resulting from an applied deflective force. 8-16 P al n e o f F o r c e Plane of Rotation F O R C E to move to the left. There is a need to turn the handlebars at of pressure in the system (vacuum is measured in inches of low speeds because of the instability of the slowly turning mercury less than ambient pressure). gyros and also to increase the rate of turn. As shown in Figure 8-20, air is drawn into the vacuum Precession can also create some minor errors in some system by the engine-driven vacuum pump. It first goes instruments. [Figure 8-19] Precession can cause a freely through a filter, which prevents foreign matter from entering spinning gyro to become displaced from its intended plane the vacuum or pressure system. The air then moves through of rotation through bearing friction, etc. Certain instruments the attitude and heading indicators where it causes the gyros may require corrective realignment during flight, such as the to spin. A relief valve prevents the vacuum pressure, or heading indicator. suction, from exceeding prescribed limits. After that, the air is expelled overboard or used in other systems, such as for Sources of Power inflating pneumatic deicing boots. In some aircraft, all the gyros are vacuum, pressure, or electrically operated. In other aircraft, vacuum or pressure It is important to monitor vacuum pressure during flight, systems provide the power for the heading and attitude because the attitude and heading indicators may not provide indicators, while the electrical system provides the power for reliable information when suction pressure is low. The the turn coordinator. Most aircraft have at least two sources vacuum, or suction, gauge is generally marked to indicate of power to ensure at least one source of bank information is the normal range. Some aircraft are equipped with a warning available if one power source fails. The vacuum or pressure light that illuminates when the vacuum pressure drops below system spins the gyro by drawing a stream of air against the the acceptable level. rotor vanes to spin the rotor at high speed, much like the operation of a waterwheel or turbine. The amount of vacuum When the vacuum pressure drops below the normal operating or pressure required for instrument operation varies, but is range, the gyroscopic instruments may become unstable and usually between 4.5 "Hg and 5.5 "Hg. inaccurate. Cross-checking the instruments routinely is a good habit to develop. One source of vacuum for the gyros is a vane-type engine- driven pump that is mounted on the accessory case of the Turn Indicators engine. Pump capacity varies in different aircraft, depending Aircraft use two types of turn indicators: turn-and-slip on the number of gyros. indicators and turn coordinators. Because of the way the gyro is mounted, the turn-and-slip indicator shows only the rate of A typical vacuum system consists of an engine-driven turn in degrees per second. The turn coordinator is mounted vacuum pump, relief valve, air filter, gauge, and tubing at an angle, or canted, so it can initially show roll rate. When necessary to complete the connections. The gauge is mounted the roll stabilizes, it indicates rate of turn. Both instruments in the aircraft’s instrument panel and indicates the amount indicate turn direction and quality (coordination), and also serve as a backup source of bank information in the event an attitude indicator fails. Coordination is achieved by referring to the inclinometer, which consists of a liquid-filled curved tube with a ball inside. [Figure 8-21] Turn-and-Slip Indicator The gyro in the turn-and-slip indicator rotates in the vertical plane corresponding to the aircraft’s longitudinal axis. A single gimbal limits the planes in which the gyro can tilt, and a spring works to maintain a center position. Because of precession, a yawing force causes the gyro to tilt left or right, as viewed from the pilot seat. The turn-and-slip indicator uses a pointer, called the turn needle, to show the direction and rate of turn. The turn-and-slip indicator is incapable of “tumbling” off its rotational axis because of the restraining springs. When extreme forces are applied to a gyro, the gyro is displaced from its normal plane of rotation, rendering its indications invalid. Certain instruments have specific pitch and bank limits that induce a tumble of the gyro.