Imaginary yardstick

First, ISA is not “standard” in the sense that it’s normal, or average, or expected. There’s no sea level airport in the world where the average temperature is 15 degrees Celsius all day every day and the barometric pressure is always 29.92 inches Hg. But that’s what the ISA airport is. Instead, ISA is an imaginary yardstick used to predict how an airplane will perform under the infinitely variable conditions of the atmosphere we fly in. Because turbine airplanes operate over a vastly larger parcel of the atmosphere and are larger and heavier, they are affected by changes in that atmosphere to a much greater extent than piston-powered airplanes. That’s why ISA, and the atmospheric conditions it depicts, governs how fast, how far, how high, and how much runway our turbine airplanes can deliver on any given day.

The concept of standardizing the air, and thus airplane performance, dates to at least the 1920s. Very early in the era of powered flight pilots and engineers saw that an airplane performed differently on a hot day than a cold one. And that airports at higher elevations, and thus lower atmospheric pressure, robbed performance. What was needed is a metric to measure how much variations in the atmosphere impact airplane behavior.

The ISA we use now was formulated in the 1950s and adopted by the International Civil Aviation Organization. ISA has been tinkered with a little since then, but those changes are not significant to our flying. 

Bottom line, ISA is a model of air density. Because it is air density that provides lift to a wing, and air that powers the combustion in our engines, any change in air density impacts performance. 

The big factors in air density change are air pressure and air temperature. Humidity and even dust in the air can create small changes in density, but they are so slight ISA assumes the atmosphere is a dry and clean gas. So, the two issues that rule our flying are pressure and temperature. 

As pilots of turbine aircraft, we care a lot about air temperature at cruise altitude. Because we fly at a flight level, we are holding a constant air pressure. Changes in the pressure relative to ISA will place our constant flight level higher or lower relative to the ground, but not really do anything to performance.

But air temperature deviations from standard at turbine cruise altitudes can have a huge impact on all aspects of performance. Warmer air is less dense so at the same flight level—constant air pressure—our wings and engines have less air to work with. With less lift and power, we won’t be able to climb as high at a given weight, and with less power available cruise speed will be slower.

On the ground air temperature still matters a lot for runway performance, but the air pressure also assumes a major role. Air pressure goes up and down with the movement of weather systems, and we certainly notice that when setting the altimeter above or below the 29.92 in Hg standard.

But really big air pressure changes come with airport elevation. At Teterboro, with its near-sea-level elevation, your engines and wings have about 14.7 pounds of atmospheric pressure to work with. At Aspen, which is almost 8,000 feet above sea level, there is just a little over 10.5 pounds of air pressure. That’s at standard air temperature, which is 15 degrees Celsius at Teterboro but below freezing at Aspen. Raise the Aspen temperature to a summer day and you can see how little air density is available. That’s why airport elevation, thus lower available air pressure, is so devastating to takeoff performance.

The FAA and many safety advocates have been on a crusade to remind pilots to “check density altitude” before takeoff. Many automated weather reporting systems will include the density altitude in their broadcasts. That’s essential advice, but not something we need to do in turbine airplanes because the FAA-approved airplane flight manual (AFM) has already done that for us. And so do the flight management systems; nearly all can calculate performance. 

So, in a jet I really don’t know what the density altitude is, and there’s no place in the AFM to look for it or apply that information. Instead, we consult a chart or FMS that considers airport elevation—pressure altitude, more specifically—and air temperature and tells us how much runway is required. We are, in fact, considering all the ramifications of density altitude without ever knowing or applying that number. 

It’s really ISA, or deviations above or below ISA temperature, that affect our flying. That’s why quality flight planning services provide a cruise altitude temperature as plus or minus ISA. And the digital electronic air data computers nearly all of us fly with show air temperature as plus or minus ISA. 

In the bad old days, the only outside air temperature we had in the cockpit was ram air temperature (RAT) which is warmer than the actual temperature depending on speed and altitude. Charts in the AFM converted RAT into static air temperature (SAT) or what was often called TAT for true air temperature, and then another chart that compared that value to ISA. Or you could perform the calculation with one of those little circular whiz wheels. Or you could look at a cruise chart that showed performance for a given RAT and not consider ISA. Crude.

Now, with air data computers, we are constantly aware of ISA and the actual conditions compared to it. We also see how jumbled the real atmosphere is. Instead of decreasing at a constant rate with altitude as ISA models, the air temperature can jump all over the place. That’s because ISA doesn’t consider wind or convection, the two big factors that stir up the atmosphere and are the source of turbulence. 

Also, there is the height of the tropopause, the level where the atmosphere changes from the troposphere to the stratosphere. ISA places the “trop” level at about 36,000 feet. Above that level ISA temperature remains constant at minus-56.5 degrees C until climbing to about 70,000 feet where the air temperature begins to decline again. 

But the height of the “trop” varies with latitude and season, and movement of air masses. The trop is typically highest at the equator and lowest over the poles. The trop is lower in winter and higher in summer. At typical business jet altitudes, it’s not uncommon to see the air temperature decrease during the early segment of descent because we were cruising above the trop level. And a low altitude trop can put air temp 10 degrees C or more above ISA at flight level 400 and above, which is a real killer on cruise performance. 

Lower levels of the atmosphere often mimic what’s going on at the surface. We usually see colder than standard temperatures for the first several thousand feet during winter, or warmer during summer. But at jet cruise levels those often reverse making it impossible to generalize about temperature at cruise levels based on surface conditions. Way below standard conditions on the runway can give you terrific performance margins for takeoff, but in the higher flight levels above-standard temperatures can rob speed and range. 

For all these reasons and more, ISA governs how we plan trips and fly. It also sets the standards for aircraft design and certification and is key to comparing airplane capabilities. Without ISA we’d be wondering how our airplanes were going to perform in real-life conditions. It’s the “standard” in ISA that gives us the information we need to adjust to the atmosphere as we find it.

J. Mac McClellan is a corporate pilot with more than 12,000 hours, and a retired aviation magazine editor living in Grand Haven, Michigan.