Our two nearly identical airplanes were being jostled by turbulence at 4,500 feet, so I suggested climbing above the fair-weather cumulus clouds on this summer afternoon for a smoother ride. I guessed 8,500 feet ought to do it.
“Not worth the effort,” my fellow flier replied. “We’d burn so much more avgas in the climb that it’s better to just stay down here and tolerate the bumps for the next 90 minutes.”
I’m notoriously bad at math, but my cohort’s statement got me curious. When does climbing higher pay off in terms of fuel efficiency? How long do you have to be at a higher altitude to make up for the fuel penalty of the high-power climb required to get there?
The short answer for normally aspirated piston engines (and even moreso for turbocharged or turbine models) is that the break-even point comes astonishingly fast. For a 4,000-foot climb, you break even after just six minutes at cruise, and you get further and further ahead in terms of fuel consumption after that.
This doesn’t take into account variables such as wind speed and direction, nor greater true airspeeds at higher altitudes. It’s a coarse, imperfect, ballpark estimate—but here goes. The next time you have a choice between bumpy air down low and smooth air up high, go high—as long as you’ll be there more than a few minutes before coming down.
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A second opinion
Payoffs start around 60 nm
Not trusting my own arithmetic, I sought out friend and colleague David Kenny, an accomplished pilot and professional statistician. He pointed out that 90 percent of the extra fuel spent during the higher climb gets refunded during a shortened cruise phase and a longer descent.
He also came up with this nugget: Assuming both airplanes maintain an 80-knot groundspeed during the climb and 120 knots in both cruise and descent, Kenny put the break-even distance for the higher climb at 57.3 nautical miles, at which point both airplanes will have burned 4.3 gallons.
“The lower one will arrive at that point in 30 minutes, and the high one in 32, due to additional time in the climb,” he said. “It’s an interesting mathematical exercise—with a really surprising result.”
The same time estimates hold for bigger, thirstier piston engines. They obviously burn more gallons per hour, but the payoff comes at about the same rate in cruise and descent.—DH
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