Table 65 269 



ALTIMETER SETTING COMPUTATION FACTORS 1 



The altimeter setting is a pressure used for setting a pressure-scale type of sensitive 

 altimeter in an aircraft so that upon landing of the aircraft at an airport the instrument 

 will indicate an altitude reading equal to or very close to the field elevation above sea 

 level, provided that the instrument is functioning properly. The altimeter setting furnished 

 to a pilot for setting the sensitive altimeter just prior to landing should be determined 

 near the time and place of landing at a station equipped with suitable instruments for 

 computing the altimeter setting. 



The altimeter setting in millibars, Amb., is given by: 



/4» b . = (£»b. - 0.3)F (1) 



and in inches of mercury, Am., by: 



^i». = (*i».-0.01)F (2) 



where : 



pmb. = station pressure in millibars, 

 Pm. = station pressure in inches of mercury, 

 F = altimeter setting computation factor. 



F is a dimensionless factor given by equation (3), valid only below the height of the 

 standard atmosphere tropopause. It is derivable from the relations given in Table 63 

 and depends only on the station pressure p and the station elevation Hb. 



M I+ (^)f]" (3) 



where : 



po = standard sea level pressure (1013.25 mb. or 29.921 in. Hg.), 

 px= Omb. — 0.3) when p = 1013.25 mb. 



= On. -0.01) when />» = 29.921 in. Hg., 

 a = lapse rate in NACA standard atmosphere below the isothermal layer 



(0.0065 °C. m." 1 ), 

 F = standard sea-level temperature (288 °A.), 



Hb = station elevation in meters (elevation for which station pressures are given), 

 n = aR = 0.190284, where R is the gas constant for dry air. 



The correction to p of —0.3 mb. (or —0.01 in. Hg.) arises from the fact that the 

 altimeter of an airplane is usually about 10 feet above the landing gear and it desirable to 

 indicate the height of the landing gear rather than of the cockpit. 



Example. — Given, station elevation Hb = 1236 m. 



when p — 910.0 mb., F = 1.15901, A = 1054.4 mb. 

 p — 909.9 mb., F = 1.15902, ,4 = 1054.2 mb. 

 p = 909.8 mb., F = 1.15902, A — 1054.1 mb., etc. 



For data in English units the appropriate values of F may be found by converting to 

 metric units, then apply equation (2) to obtain A. 



Given Hb = 4964 ft. (= 1513 m.) 

 when p = 26.00 in. Hg. (=880.5 mb.), F = 1.19862, ,4=31.15 in. Hg. 



p = 25.99 in. Hg. (=880.1 mb.), F = 1.19865, ,4 = 31.14 in. Hg., etc. 



1 An alternative method for computing altimeter settings is given in Table 66. 



{continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



