By J. Mac McClellan
Ask a pilot, particularly those who fly jets, about the temperature aloft and you will almost never hear an actual temperature, like minus 54 degrees C. Instead we all talk about it being plus or minus some number of degrees. This measurement of plus or minus a standard is just one more example of unique pilot speak, but air temperature and the standard are absolutely critical to airplane performance. That's why FltPlan.com shows temperature aloft relative to standard, rather than an specific number of degrees.
With the wind aloft forecast on your FltPlan.com Navigation Log page, you will see temperature shown as a plus or minus. That is the number of degrees forecast to be above or below standard, not the actual air temperature.
The standard is, of course, the International Standard Atmosphere (ISA) which was agreed on decades ago by engineers and government regulators. It is essential to have a standard so that airplane performance can be compared to published performance data. Without the standard an airplane would climb higher and, faster one day than the next as the temperature aloft varied, and pilots would have no way to know what performance to expect.
ISA does not really represent an "average" day, and the exact conditions of ISA may never actually exist on any one day. But atmospheric conditions do exist that straddle the ISA so we can predict how an airplane will perform when we compare back to the standard.
ISA assumes the atmosphere is a perfectly uniform dry gas, which it is not. But the moisture content and some other variables do not change airplane performance much. The really important considerations are atmospheric pressure and air temperature. These are not important in themselves, but they are measures of air density, and density is what matters. Airplane engines and wings crave dense air, and the denser the air the better they perform. Lower the density and engines lose thrust and wings lose lift.
Naturally enough, ISA begins at the surface with an air temperature of 15 degrees C, and barometric pressure of 29.92 inches of mercury. ISA says the air temperature drops steadily until just over 36,000 feet where, at minus 56.5 degrees C the air stops cooling with altitude until you climb beyond 65,000 feet. The level where the air temperature stops decreasing with altitude is called the tropopause, and it is the boundary between the troposphere were we live and the stratosphere.
At every altitude in the flight a wide range of air temperatures is possible. It is counterintuitive, but generally speaking the high altitude temperatures are coldest over the equator and warmest over the poles. But ISA treats all parts of the globe as though the atmosphere was the same, so we are always flying in air that is above or below-or very occasionally-right on standard.
Imagine if airplane manufacturers and their test pilots had to collect performance data for every possible temperature at all usable altitudes. It would be a nightmare for them and for we pilots trying to read such a chart.
But by using ISA as a baseline, airplane performance data can be extrapolated to whatever the actual air temperature may be above or below standard. Test pilots collect many data points under conditions that are both warmer and colder than standard. With those data, charts can be created that account for the entire range of operating temperatures.
The goal at FltPlan.com is to make flight planning as easy, and as accurate, as possible for any pilot. By showing predicted air temperatures as either above or below standard, FltPlan.com makes it easy to look in performance manuals to see what to expect from your airplane on the flight.
If, for some reason, you want to know what the actual air temperature is forecast to be at your cruise altitude, there is an easy rule of thumb-multiply your cruise flight level by two and subtract 15 for altitudes up to 36,000 feet. So, let's say you plan to cruise at 35,000 feet. Multiply 35 by two and get 70. Subtract 15 and you have 55 degrees which is very close to ISA for that level. For all altitudes at 36,000 feet and above ISA will be minus 56.5 degrees. For those allergic to math, just look at the top of the Winds box on your NavLog, (or on the Winds Aloft Matrix) and ISA is given for each of your different cruise altitudes.