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PRESSURE ALTIMETER ERRORS

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1,000-foot counter and then add the 100-foot pointer indication. CAUTION It is possible to misinterpret the counter- pointer altimeter by 1,000 feet immediately before or after the 1,000-foot counter moves. This error is possible because the 1,000-foot counter changes when the foot-pointer is between the 900- and 1,000-foot position. Counter-Drum-Pointer Altimeter.— The only real difference between this altimeter and the counter-pointer altimeter is the addition of a 100-foot drum (fig. 2-9). This drum follows the 100-foot pointer, and it is this drum that actuates the 1,000-foot counter. In this way it prevents the reading error when the 1,000-foot counter switches. There are two methods of reading the indicated pressure altitude. One way is to read the counter-drum window without referring to the 100-foot pointer. This will give a direct readout of both thousands and hundreds of feet. The second way is to read the counter window and then add the 100-foot pointer indication. The pointer serves as a precise readout of values less than 100 feet. This sample altimeter has a servoed mode and a pressure mode of operation. The mode of operation is controlled by a spring-loaded, self-centering mode switch, placarded RESET and STBY. In the servoed mode, the altimeter displays altitude, corrected for position error, from the synchro output of the air data computer. In the standby mode, the altimeter operates as a standard altimeter. In this mode, it uses static Figure 2-9.-Counter-drum-pointer altimeter. pressure from the static system that is uncorrected for position error. The servoed mode is selected by placing the mode switch to RESET for 3 seconds. The ac power must be on. During standby operation, a red STBY flag appears on the dial face. The altimeter automatically switches to standby operation during an electrical power loss or when the altimeter or altitude computer fails. The standby operation is selected by placing the mode switch to STBY. An ac-powered internal vibrator automatically energizes in the standby mode to lessen friction in the display mechanism. PRESSURE ALTIMETER ERRORS.— There are five categories of errors relating to pressure altimeters. They are the mechanical error, the scale error, installation/position error, reversal error, and hysteresis error. Mechanical Error.— Mechanical error is caused by misalignments in gears and levers that transmit the aneroid cell expansion and contraction to the pointers of the altimeter. This error is not constant, and it must be checked before each flight by the setting procedure. Scale Error.— Scale error is caused by irregular expansion of the aneroid cells. It is recorded on a scale correction card maintained for each altimeter in the instrument maintenance shop. Installation/Position Error.— Installation/ position error is caused by the airflow around the static ports. This error varies with the type of aircraft, airspeed, and altitude. The magnitude and direction of this error can be determined by referring to the performance data section in the aircraft NATOPS manual. An altimeter correction card is installed in some aircraft that combines the installation/position and the scale errors. This card shows the amount of correction needed at different altitudes and airspeeds. Reversal Error.— Reversal error is caused by inducing false static pressure into the system. This normally occurs during abrupt or huge pitch changes. This error appears on the altimeter as a momentary indication in the opposite direction. Hysteresis Error.— Hysteresis error is a lag in altitude indication due to the elastic properties of the material within the altimeter. This occurs after an aircraft has maintained a constant altitude for an extended period of time and then makes a large, rapid altitude change. After a rapid descent, altimeter 2-9



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