Pressure altitude is the height above the standard datum plane (SDP). The aircraft altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere. If the altimeter is set for 29.92 “Hg SDP, the altitude indicated is the pressure altitude—the altitude in the standard atmosphere corresponding to the sensed pressure.
The SDP is a theoretical level at which the pressure of the atmosphere is 29.92 “Hg and the weight of air is 14.7 psi. As atmospheric pressure changes, the SDP may be below, at, or above sea level. Pressure altitude is important as a basis for determining aircraft performance, as well as for assigning flight levels to aircraft operating at above 18,000 feet.
The pressure altitude can be determined by any of the three following methods:
- By setting the barometric scale of the altimeter to 29.92 “Hg and reading the indicated altitude,
- By applying a correction factor to the indicated altitude according to the reported “altimeter setting,” see figure below.
- By using a CX-3 Flight Computer.
Let’s try a sample problem using the above chart.
1. Determine the pressure altitude at an airport that is 1,386 feet MSL with an altimeter setting of 29.55.
A—1,631 feet MSL.
B—1,731 feet MSL.
C—1,778 feet MSL.
Looking at the above chart you will see that 29.65 is not actually shown so we will need to interpolate between 29.50 and 29.60. Subtract 298 from 392 to get 94 and then divide by 2 since 29.55 is directly in the middle. You get 47 which you can now add to 298 to come up with 345 altitude correction. Add 345 onto you airport elevation to find the pressure altitude, 1,386 + 345 = 1,731 PAlt.
The correct answer is B, 1,731 feet MSL.
Here is another problem which can be solved by using your CX-3 Flight Computer.
2. Determine the pressure altitude at an airport that is 3,563 feet MSL with an altimeter setting of 29.96.
A—3,527 feet MSL.
B—3,556 feet MSL.
C—3,639 feet MSL.
Using you CX-3 open the FLT menu and select Altitude from the list. Enter your IAlt (indicated altitude) or airport elevation of 3,563 feet. Scroll down to the Baro field and enter your altimeter setting of 29.96. The CX-3 will then give you a PAlt of 3,527 feet MSL. The correct answer is A.
