Since barometric pressure effects air density, most airplane performance charts ask for pressure altitude. Pressure altitude is the altitude at which the pressure at your altitude exists in the standard atmosphere. Using pressure altitude in performance charts is a simple method of factoring in non-standard pressure.
Pressure altitude is the altitude shown on the altimeter when it is set to 29.92. Normally, field elevation and the altimeter setting will be used to determine pressure altitude for use with airplane performance charts.
Example Pressure Altitude Calculation One
Altimeter Setting: 30.02
Field Elevation: 1,000 feet
To find pressure altitude, determine the pressure deviation from standard. The standard pressure is 29.92 inches. In this case, the pressure is 0.10 inches higher than normal. This pressure difference will have the effect of increasing air density, slightly improving performance.
Every inch of pressure difference is roughly equal to 1,000 feet of altitude. Multiplying 0.10 times 1,000 gives us a 100 foot altitude correction. Now it must be determined whether to add or subtract this 100 foot correction to the field elevation.
Air density decreases with higher altitude and increases with lower altitude. Since 30.02 is higher than standard pressure, the air is made more dense by atmospheric pressure. Higher density is found at lower altitudes. So, we subtract the 100 feet to determine the pressure altitude to be 900 feet.
If the pressure is higher than standard, subtract the correction. If the pressure is lower than standard, add the correction.
Example Pressure Altitude Calculation Two
Altimeter Setting: 29.87
Field Elevation: 206 feet
In this case, the pressure is lower than standard. This will have the effect of lowering air density. The correction will be added to the field elevation to find the pressure altitude.
Subtracting 29.87 from 29.92 shows the deviation from standard pressure to be 0.05 inches. Multiplying 1000 times 0.05 gives a 50 foot altitude correction, which is added to the field elevation. In this case, the pressure altitude is 256 feet.
Pressure Altitude Correction References
| Pressure Deviation | Estimated Altitude Correction |
| 0.01 inches | 10 feet |
| 0.05 inches | 50 feet |
| 0.10 inches | 100 feet |
| 0.50 inches | 500 feet |
| 1.00 inches | 1,000 feet |
Alternatively, a table of altitude corrections may be used to determine pressure altitude.
| Altimeter Setting | Altitude Correction |
| 29.60 | 298 |
| 29.65 | 252 |
| 29.70 | 205 |
| 29.75 | 158 |
| 29.80 | 112 |
| 29.85 | 67 |
| 29.90 | 20 |
| 29.92 | 0 |
| 30.00 | -73 |
| 30.05 | -119 |
| 30.10 | -165 |
| 30.15 | -211 |
| 30.20 | -257 |
| 30.25 | -303 |
| 30.30 | -384 |
Altimeter Settings
Local altimeter settings correct the altimeter for the difference between standard pressure (29.92 inches) and existing atmospheric pressure.
Before flight, the pilot should set the airplane's altimeter to the local altimeter setting. If a local altimeter setting is not available, then the altimeter should be adjusted to the elevation of the departure field. Doing this will result in the altimeter displaying true altitude at ground level, regardless of whether existing atmospheric pressure is higher or lower than standard.
During flight, pilots must keep the altimeter set to that of a reporting station within 100 miles of the airplane's location. Prior to landing, the destination airport's altimeter setting should be set into the altimeter. Pilot's who fly above 18,000 feet MSL, in the flight levels, set their altimeters to 2992 when climbing through 18,000 feet.
When a pilot flies from an area of high atmospheric pressure to an area of lower pressure, the altimeter senses this lowering pressure as an increase in airplane altitude. Since this change is very gradual, the pilot will not notice it. Maintaining the same indicated altitude, the pilot will unknowingly descend to a lower altitude at a very low rate.
If a pilot were to fly from high temperature air into colder air, the air density would be increasing, since cold air is more dense. As the pilot maintains the same indicates altitude, the airplane is descended to a lower altitude. In the colder air, that same air density that resulted in a particular altitude being displayed is achieved when the airplane is actually at a lower altitude.
The opposite of both of these statements are also true, resulting in the pilot actually climbing the airplane, while maintaining the same indicated altitude, when moving from cold to hot air or from low to high pressure areas.
The easiest way to apply this knowledge is to remember this: High to low, hot to cold, look out below.
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