| Standard Atmosphere
In order to provide a common denominator for comparison of the
performance of various aircraft, a standard atmosphere has been adopted. The set
of standard conditions presently used in the U.S. is known as the International
Standard Atmosphere (ISA) and has been adopted by most of the nations and
airlines of the world - the International Civil Aviation Organization.
The standard atmosphere actually represents the mean or average
properties of the atmosphere that is, it represents the year round average of
the pressure/height temperature soundings observed over a period of years.
In the standard atmosphere, sea level pressure is 29.92" Hg and
15 degrees C. (59 degrees F.); the standard lapse rate (decrease) for pressure
are approximately 1" Hg per 1,000 feet increase in altitude and 2 degrees C.
(3.5 degrees F.) per 1,000 feet increase (up to the tropopause). Fig. 17-62
{should be Fig. 17-69 - Ed.} illustrates the variation of the most important
properties of the air throughout the standard atmosphere.
Since all airplane performance is compared and evaluated in the
environment of the standard atmosphere, all of the airplane performance
instrumentation is calibrated for the standard atmosphere. Thus, certain
corrections must apply to the instrumentation, as well as the airplane
performance, if the actual operating conditions do not fit the standard
atmosphere. In order to account properly for the nonstandard atmosphere certain
related terms must be defined.
STANDARD ATMOSPHERE
Figure 17-69
---------------------------------------------------------
Altitude Pressure
Temp. Temp. Density -
(ft) (in.
Hg) (C.)
(F.) slugs / ft^3
0
29.92 15.0
59.0 0.002378
1,000
28.86 13.0
55.4 0.002309
2,000
27.82 11.0
51.9 0.002242
3,000
26.82
9.1 48.3 0.002176
4,000
25.84
7.1 44.7 0.002112
5,000
24.89
5.1 41.2 0.002049
6,000
23.98
3.1 37.6 0.001988
7,000
23.09
1.1 34.0 0.001928
8,000
22.22 -0.9
30.5 0.001869
9,000
21.38 -2.8
26.9 0.001812
10,000
20.57 -4.8
23.3 0.001756
11,000
19.79 -6.8
19.8 0.001701
12,000
19.02 -8.8
16.2 0.001648
13,000
18.29 -10.8
12.6 0.001596
14,000
17.57 -12.7
9.1 0.001545
15,000
16.88 -14.7
5.5 0.001496
16,000
16.21 -16.7
1.9 0.001448
17,000
15.56 -18.7
-1.6 0.001401
18,000
14.94 -20.7
-5.2 0.001355
19,000
14.33 -22.6
-8.8 0.001310
20,000
13.74 -24.6
-12.3 0.001267
---------------------------------------------------------
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