Aviation Electronics Technician 1 - Historic Naval Ships Association

1,000-foot counter and then add the 100-foot pointerindication.CAUTIONIt is possible to misinterpret the counterpointeraltimeter by 1,000 feet immediatelybefore or after the 1,000-foot countermoves. This error is possible because the1,000-foot counter changes when thefoot-pointer is between the 900- and1,000-foot position.Counter-Drum-Pointer Altimeter.— The onlyreal difference between this altimeter and thecounter-pointer altimeter is the addition of a 100-footdrum (fig. 2-9). This drum follows the 100-footpointer, and it is this drum that actuates the1,000-foot counter. In this way it prevents thereading error when the 1,000-foot counter switches.There are two methods of reading the indicatedpressure altitude. One way is to read thecounter-drum window without referring to the100-foot pointer. This will give a direct readout ofboth thousands and hundreds of feet. The second wayis to read the counter window and then add the100-foot pointer indication. The pointer serves as aprecise readout of values less than 100 feet.This sample altimeter has a servoed mode and apressure mode of operation. The mode of operation iscontrolled by a spring-loaded, self-centering modeswitch, placarded RESET and STBY. In the servoedmode, the altimeter displays altitude, corrected forposition error, from the synchro output of the air datacomputer. In the standby mode, the altimeter operatesas a standard altimeter. In this mode, it uses staticpressure from the static system that is uncorrected forposition error.The servoed mode is selected by placing the modeswitch to RESET for 3 seconds. The ac power mustbe on. During standby operation, a red STBY flagappears on the dial face. The altimeter automaticallyswitches to standby operation during an electricalpower loss or when the altimeter or altitude computerfails. The standby operation is selected by placing themode switch to STBY. An ac-powered internalvibrator automatically energizes in the standby modeto lessen friction in the display mechanism.PRESSURE ALTIMETER ERRORS.— Thereare five categories of errors relating to pressurealtimeters. They are the mechanical error, the scaleerror, installation/position error, reversal error, andhysteresis error.Mechanical Error.— Mechanical error is causedby misalignments in gears and levers that transmit theaneroid cell expansion and contraction to the pointersof the altimeter. This error is not constant, and it mustbe checked before each flight by the settingprocedure.Scale Error.— Scale error is caused by irregularexpansion of the aneroid cells. It is recorded on ascale correction card maintained for each altimeter inthe instrument maintenance shop.Installation/Position Error.— Installation/position error is caused by the airflow around thestatic ports. This error varies with the type of aircraft,airspeed, and altitude. The magnitude and directionof this error can be determined by referring to theperformance data section in the aircraft NATOPSmanual.An altimeter correction card is installed in someaircraft that combines the installation/position and thescale errors. This card shows the amount of correctionneeded at different altitudes and airspeeds.Reversal Error.— Reversal error is caused byinducing false static pressure into the system. Thisnormally occurs during abrupt or huge pitch changes.This error appears on the altimeter as a momentaryindication in the opposite direction.Figure 2-9.-Counter-drum-pointer altimeter.Hysteresis Error.— Hysteresis error is a lag inaltitude indication due to the elastic properties of thematerial within the altimeter. This occurs after anaircraft has maintained a constant altitude for anextended period of time and then makes a large, rapidaltitude change. After a rapid descent, altimeter2-9