AEROGRAPHER'S MATE 3 & 2 



Maintenance 



The exterior of the case should be dusted 

 whenever required, and the dial window should 

 be wiped with a clean damp cloth if necessary. 

 Inspect the general physical appearance of the 

 instrument. A cracked dial window, dents, bends, 

 and other external physical damage probably 

 indicate a need for overhaul of the Instrument. 

 Sufficient impact to cause external damage is 

 usually sufficient to render the instrument in- 

 operative or of suspect accuracy. 



No repair, parts replacement, or lubrication 

 are to be attempted at the observer's maintenance 

 level. 



In mounting the aneroid barometer, keep It 

 away from areas where it might be exposed to 

 sudden shocks or rapid thermal changes, but 

 place it in an area easily reached by the ob- 

 server. Before each reading, tap the case slightly 

 to remove the drag effects of linkage friction. 

 To minimize the effects of vibration on shore 

 stations, and to minimize the effects of pitch, 

 roll, and vibration in ships, the precision aneroid 

 barometer should be shock-mounted. 



ALTIMETERS 



The Aerographer should not underrate the 

 importance of the altimeter. Particularly im- 

 portant is the altimeter setting which ensures 

 that the pilot always has a correct reading 

 available to him. Many aircraft accidents may 

 have been caused by a faulty altimeter setting. 

 Today, three types of altimeters are in general 

 use— the pressure altimeter, radio altimeter, and 

 the radar altimeter, all of which are briefly 

 discussed in the following paragraphs. 



PRESSURE ALTIMETER 



The pressure altimeter is primarily an aner- 

 oid barometer calibrated to indicate altitude in 

 feet instead of units of pressure. The pressure 

 altimeter reads accurately only in a standard 

 atmosphere and when the correct altimeter setting 

 is used. Since standard conditions seldom (if 

 ever) exist, the altimeter reading usually requires 

 correction. It will indicate 10,000 feet when the 

 pressure is 697 millibars, whether or not the 

 altitude is actually 10,000 feet. 



The altimeter is generally corrected to read 

 zero at sea level. A procedure used in aircraft 

 on the ground is to set the altimeter reading to 

 the elevation of the airfield. The altimeter then 



reads the altitude above sea level and the Kollsman 

 window indicates the current altimeter setting. 

 (See fig. 2-8.) 



Altimeter Errors Due to 

 Change in Surface Pressure 



The atmospheric pressure frequently differs 

 at the point of landing from that of takeoff; 

 therefore, an altimeter correctly set at takeoff 

 may be considerably in error at the time of 

 landing. Altimeter settings are obtained in flight 

 by radio from navigational aids with voice 

 facilities. Otherwise, the expected altimeter set- 

 ting for landing should be obtained by the pilot 

 before takeoff. 



To illustrate this point, figure 2-9 shows the 

 pattern of Isobars in a cross section of the 

 atmosphere from New Orleans, Louisiana, to 

 Miami, Florida. The pressure at Miami is 

 1,019 millibars and the pressure at New Orleans 

 is 1,009 millibars, a difference of 10 millibars. 

 Assume that an aircraft takes off from Miami 

 to fly to New Orleans at an altitude of 500 feet. 

 A decrease in the mean sea level pressure of 

 10 millibars from Miami to New Orleans would 



219.85 



Figure 2-8.— Pressure altimeter. 



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