Flight Instruments

a signal relayed to the heading indicator unit, which is also or “spills” and no longer gives the correct indication until remotely mounted. This signal operates a torque motor in reset. After spilling, it may be reset with the caging knob. the heading indicator unit that processes the gyro unit until Many of the modern instruments used are designed in such it is aligned with the transmitter signal. The magnetic slaving a manner so that they do not tumble. transmitter is connected electrically to the HSI. An additional precession error may occur due to a gyro not There are a number of designs of the remote indicating spinning fast enough to maintain its alignment. When the compass; therefore, only the basic features of the system are vacuum system stops producing adequate suction to maintain covered here. Instrument pilots must become familiar with the gyro speed, the heading indicator and the attitude indicator the characteristics of the equipment in their aircraft. gyros begin to slow down. As they slow, they become more susceptible to deflection from the plane of rotation. Some As instrument panels become more crowded and the pilot’s aircraft have warning lights to indicate that a low vacuum available scan time is reduced by a heavier flight deck situation has occurred. Other aircraft may have only a vacuum workload, instrument manufacturers have worked toward gauge that indicates the suction. combining instruments. One good example of this is the RMI in Figure 8-30. The compass card is driven by signals Instrument Check from the flux valve, and the two pointers are driven by an As the gyro spools up, make sure there are no abnormal automatic direction finder (ADF) and a very high frequency sounds. While taxiing, the instrument should indicate turns in (VHF) omni-directional radio range (VOR). the correct direction, and precession should be normal. At idle power settings, the gyroscopic instruments using the vacuum Heading indicators that do not have this automatic system might not be up to operating speeds and precession northseeking capability are called “free” gyros and require might occur more rapidly than during flight. periodic adjustment. It is important to check the indications frequently (approximately every 15 minutes) and reset the Angle of Attack Indicators heading indicator to align it with the magnetic compass The purpose of an AOA indicator is to give the pilot better when required. Adjust the heading indicator to the magnetic situational awareness pertaining to the aerodynamic health compass heading when the aircraft is straight and level at a of the airfoil. This can also be referred to as stall margin constant speed to avoid compass errors. awareness. More simply explained, it is the margin that exists between the current AOA that the airfoil is operating at, and The bank and pitch limits of the heading indicator vary the AOA at which the airfoil will stall (critical AOA). with the particular design and make of instrument. On some heading indicators found in light aircraft, the limits are Speed by itself is not a reliable parameter to avoid a stall. approximately 55° of pitch and 55° of bank. When either of An airplane can stall at any speed. Angle of attack is a better these attitude limits is exceeded, the instrument “tumbles” parameter to use to avoid a stall. For a given configuration, the airplane always stalls at the same AOA, referred to as the critical AOA. This critical AOA does not change with: • Weight • Bank Angle • Temperature • Density Altitude • Center of Gravity An AOA indicator can have several benefits when installed in General Aviation aircraft, not the least of which is increased situational awareness. Without an AOA indicator, the AOA is “invisible” to pilots. These devices measure several parameters simultaneously and determine the current AOA providing a visual image to the pilot of the current AOA along with representations of the proximity to the critical AOA. Figure 8-30. Driven by signals from a flux valve, the compass card [Figure 8-31] These devices can give a visual representation in this RMI indicates the heading of the aircraft opposite the upper of the energy management state of the airplane. The energy center index mark. The green pointer is driven by the ADF. 8-22