True airspeed indicator
True airspeed indicator

How fast is it?

If you’re talking about an airplane, this short, simple question can produce a long, complicated answer.

My friend Hugh has a highly developed scientific mind, and when he sees something about aviation that piques his interest, he often calls me for details. One question—“How fast is it?”—seems as if it ought to be pretty simple, but my answers often leave him frustrated. How can speed be so complicated?

With a car, it’s not. If the speedometer in Hugh’s Camry reads 60 mph, it takes one minute to go one mile.

But with airplanes, the air they move through is much more of a factor. And air is not only an amorphous gas, it’s almost always moving, itself—sometimes pretty fast. In the Boeing 767 I was riding in recently, the headwind measured 117 mph at one point. 

So that’s why “how fast you’re going” must be calculated in groundspeed—the measure of how fast the airplane is moving over the Earth—and it depends on wind direction and speed. 

“OK,” Hugh says, “then forget about the wind and just tell me how fast the [expletive deleted] airplane is moving through the air.” Good question. And airspeed is an excellent way of measuring one airplane against its competitors. With prop-driven models that tend to fly at relatively low altitudes, “high-speed cruise” is one of the specs you’ll find in the brochures. But the “how fast is it?” waters muddy pretty quickly unless you understand what the numbers mean. 

For example, the “never exceed” airspeed (also in the brochure) is the limit for safe operation—to keep the wings and other parts attached—and it has little to do with how quickly you’ll get from A to B. “Never exceed” speed usually assumes the airplane is in a dive. Any airplane will go much faster in that situation, but does that count when Hugh’s asking, “How fast can it go?”

It’s probably more reasonable to use the “high-speed cruise” number to describe an airplane’s ability to perform its stated mission. But most flights aren’t made at high-speed cruise. “Long-range cruise” is slower, but is the speed at which the airplane flies most efficiently.The faster you go, the greater the drag (and fuel burn), so the sweet spot on the airspeed indicator for long-distance travel is always slower than the high-speed-cruise mark. A few miles per hour faster doesn’t help if it means you’ve got to make another fuel stop to get to Grandma’s house.

And speaking of the airspeed indicator, there’s another wrinkle. “Indicated airspeed” is what shows up on the dial in the cockpit, but it seldom equals the actual speed the airplane is moving through the air. That’s “true airspeed,” and the only time it’s the same as what you see on the dial is at sea level and standard atmospheric conditions of temperature, pressure and humidity. Climb a few thousand feet on a hot, humid day with unusually high or low pressure and you’ll have to whip out a calculator to know your “true” airspeed. Generally, the higher you fly, the greater your true airspeed, because the air is thinner and causes less resistance.

When Hugh asks how fast a jet is, my answer is usually its maximum Mach number—the percentage of the speed of sound. That’s because at the altitudes where jets fly, indicated and true-airspeed numbers are so skewed by the extreme conditions that it’s more useful to measure the airplane’s speed in comparison with the speed of sound. Both are affected equally by the ambient conditions, so it’s a way to standardize performance from one aircraft model to another. The fastest business jet, the Cessna Citation X+, can fly at Mach 0.935, while military jets can often exceed Mach 2.5—two-and-a-half times the speed of sound.

Hugh complains that my answers are the equivalent of telling him the history of Rolex when all he asked was, “What time is it?” And I get that. In fact, I see that I’m already close to exceeding my allotted word count for this column, and I haven’t even gotten to the difference between miles per hour and knots. 

But maybe, like Hugh, you’ve already had enough of this conversation.


Mark Phelps is a private pilot and a managing editor at BJT sister publication Aviation International News.

 
 
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