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Nov 1, 2010 --- The basis for predicting aircraft performance is pressure altitude. By definition it is indicated altitude corrected for non-standard pressure. As a pilot you are responsible for calculating the effect that weather conditions will have on your takeoff, climb, cruise and landing performance. You will have to compute the pressure altitude for the airports being used, review your aircrafts performance charts and determine the numbers for the expected conditions.

By internationally accepted standards, a standard day has a barometric pressure of 29.92" Hg and a temperature of 15 degrees Celsius with the aircraft at sea level. All of your aircrafts airspeeds, rates of climb, fuel burns and takeoff and landing distances are determined by the aircraft manufacturer using these standard conditions. Lower pressure, higher altitude and higher temperature will have a negative effect on aircraft performance. Higher pressures and lower temperatures can improve aircraft performance. It's important to note that high humidity can also have a negative effect on performance, but few performance charts take this into account.

There are different ways to compute pressure altitude:

1. Determine the airports field elevation. Then get the altimeter setting from an airports hourly weather observation report. Use a manual or electronic E6B flight computer following the instructions and examples that came with the computer.
2. Sit in the airplane, putting 29.92"Hg in the Kollsman window of the altimeter. The altimeter will then indicate pressure altitude.
3. Determine the airports field elevation. Then get the altimeter setting from an airports hourly weather observation report. Determine the difference between 29.92" Hg and the reported altimeter setting and apply the difference to the field elevation of the airport as in this example:
   
   Altimeter Setting = 30.22 - 29.92 = .30
   .30 x 1000 = 300 feet
   Field Elevation = 575 feet - 300 = 275 feet pressure altitude

Once you have determined the pressure altitude you can then refer to the aircrafts performance charts and compute the required distances, speeds and fuel consumptions.