Chapter 2 — PRESSURE 



NEW ORLEANS 

 1009 MB 



MIAMI 

 1019 MB 



Figure 2-9.— Altimeter errors due to change in surface pressure. 



cause the aircraft to gradually lose altitude; 

 and although the altimeter indicates 500 feet, 

 the aircraft would be actually flying at approx- 

 imately 200 feet over New Orleans. The correct 

 altitude can be determined by obtaining the correct 

 altimeter setting from New Orleans, resetting 

 the altimeter to agree with the destination 

 adjustment. NOTE: The following relationships 

 generally hold true up to approximately 15,000 

 feet: 34 millibars = 1 inch (Hg) = 1,000 feet 

 elevation. Since 1 millibar is equal to about 

 30 feet below 10,000 feet altitude, a change of 

 10 millibars would result in an approximate 

 error of 300 feet. 



Altimeter Errors Due to 

 Variation From Standard 

 Temperature 



Another type of altimeter error is due to 

 nonstandard temperatures. Even though the alti- 

 meter is properly set for surface conditions, 

 it will often be incorrect at higher levels. If 

 the air is warmer than the standard for the 

 flight altitude, the aircraft will be higher than 

 the altimeter indicates; if the air is colder than 

 standard for flight altitude, the aircraft will be 

 lower than the altimeter indicates. (See fig. 

 2-10.) 



RADIO ALTIMETERS 



This type of altimeter transmits a signal 

 that varies in frequency at an extremely rapid 

 and constant rate. The receiver picks up the 

 reflected signal in a frequency measuring device, 

 which measures the difference in frequency 

 between the transmitted and reflected signal at 

 any instant. This difference generates a voltage 

 which is linked to an indicator calibrated in 

 feet. Radio altimeters work better over water 

 than land because the water reflects the radio 

 signals better. 



Radio altimeters are very helpful in deter- 

 mining clearance from the surface. As such, 

 they are extremely helpful in avoiding collision 

 with the terrain. They are also useful under all 

 instrument conditions and are essential for hur- 

 ricane penetration to determine pressure levels 

 and altimeter settings for the pressure alti- 

 meter. They are, however, not generally used for 

 pressure-pattern or jet stream flight because 

 their operational altitude is lower than that 

 flown by most long-range transports. 



RADAR ALTIMETERS 



This type of altimeter works similar to the 

 basic radar set in that it transmits ultra-high 

 or super-high frequency pulses that are reflected 



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