But how well these three do their interconnected jobs of making the airplane fly depends on how thick or thin the air is. As air thins, wings generate less lift, propellers deliver less thrust, and normally aspirated engines lose horsepower—all adding up to a thin-air performance penalty.

If you want to see this performance penalty for the airplane you train in, simply go to your airplane’s pilot’s operating handbook and look at the takeoff charts or graphs. If you compare the sea level takeoff roll to the takeoff roll for the maximum elevation published in the handbook, you’ll probably see that the distance more than doubles from one end of the chart to the other because air thins with increasing altitude.

But don’t be surprised if you find similar conditions at lower altitudes, even near sea level.

On hot days, air expands, creating high-altitude thin air at lower elevations. This effect is called density altitude—essentially the thinness of the air expressed in altitude units—and it has a bigger impact than many pilots realize, simply because most flight training takes place where it really doesn’t much matter. Let’s take Florida, for instance, which just happens to be one of the states with plenty of flight schools conducting flight training.

In Florida, on a warm summer day, an airplane’s wings, engine, and prop can all easily “behave” as if the field were a good 2,000 feet higher than its measured elevation, due to expanding/thinning warm air. Given that the average field elevation in Florida is 100 feet msl, this is hardly an issue. Yeah, it adds to the takeoff run, but not by all that much—something like 15 percent more, and the airplane performs just fine. So, many pilots get lazy about checking density altitude during preflight.

But airplanes go places.

What if you were in the Mile High City? If you “raise” Denver's elevation by 2,000 feet, you are nearly off the performance charts; your takeoff run doubles, and the airplane will climb like a dog. Or, if heavily loaded, and the day is truly hot—not just warm—the airplane might not climb at all.

So even if you are training in the “flatlands,” where it doesn’t much matter, create good habits around always checking—and respecting—density altitude.

These good habits will keep you safe as you spread your wings and go places.