Come on up and try it
My discussion last month of climbing after takeoff-the effects of ground effect immediately after liftoff, the transition to initial climb, and the factors that go into choosing best angle, best rate, or a cruise-climb profile (see "Continuing Ed: Time to Climb," April 2009 AOPA Flight Training)-ended by saying that "once you have the climb part figured out, it's time to concentrate on the descent." Well, that's true if you'll be staying in the pattern to practice bump-and-runs. Otherwise, there's a phase of flight that bridges climb and descent. It's called cruise.
What are those factors? The first is the airplane you'll be flying, with performance being the primary consideration. Obviously, a four-place fixed-gear single with a normally aspirated engine of 180 horsepower or less, turning a fixed-pitch propeller, typically cruises at a much lower altitude than a turbocharged airplane simply because it doesn't have the muscle to climb high and cruise fast.
That's not to say you can't go high in simple light aircraft. I bought my first airplane, a Cessna 172M with a 150-horsepower engine, in California and flew it across the country to my home in Maryland. East of the Front Range of the Rockies over Colorado, the air was so rough that I laboriously climbed to 13,500 feet mean sea level (msl) hoping to top it for a half-hour, the maximum time in which the pilot of an unpressurized airplane is allowed to remain at that altitude unless breathing supplemental oxygen. At 13,500 feet I was still getting tossed around, however, so my stay up there was all too brief. I descended to 11,500 feet msl and wished I had bought a turbocharged airplane instead.
Turbocharging enables a piston-powered aircraft to climb quicker and fly higher and faster than a normally aspirated airplane can because a turbocharged engine produces much more power in the thin air at high altitudes. However, not all pilots of turbocharged aircraft use these capabilities wisely.
The owner of a high-performance turbocharged single at the airport I fly at once told me he doesn't fly higher than about 5,000 feet. I was astonished. The elevation at our airport is less than 20 feet msl. It gets hot in the summer, but density altitude at the airport hardly ever rises above about 2,500 feet, and the surrounding terrain is as flat as it gets. There is no need for turbocharging to compensate for high-density-altitude takeoffs or clearing terrain.
The only practical justification for operating a turbocharged airplane in my part of the country is when the primary mission is long trips, where flying as high and as fast as possible makes the most sense. The disadvantage to flying high in a turbocharged, unpressurized airplane-and it is a significant disadvantage-is that pilot and passengers have to breath supplemental oxygen.
I didn't get a chance to ask the owner of that turbocharged single why he flies so low, but I suspect the answer is that he has never received effective instruction on the special considerations of flying above 10,000 feet. He may not even understand the mechanics of turbocharging, and therefore is reluctant to use the capability.
Most of us fly lower-performance aircraft with normally aspirated engines, so our choice of cruise altitude is based on performance limitations-relatively modest climb rate, much-reduced power above about 5,000 feet msl because of the lower-density air at altitude, and no supplemental oxygen.
Those limitations dictate relatively low cruise altitudes, but not too low. There's probably some psychological component at work on pilots who routinely cruise at 2,500 to 4,500 feet msl. Flying low, where you can see a lot of detail on the ground and the airplane's progress over the surface is obvious, is comforting. Flying relatively high, on the other hand-7,500 to 9,500 feet, for example-usually makes most sense on longer trips, but if we've not been exposed to that profile and are not familiar with all of the factors involved, we may not be comfortable going that high.
Why high? It yields the most efficient combination of true airspeed (think of true airspeed as your no-wind groundspeed) and fuel consumption. Take a look at the performance charts for the airplane you fly and you'll see that for a given power setting, say 65 percent, you go faster if you fly higher. For example, the performance charts for my airplane show that at the same intermediate power setting, the airplane goes 13 mph faster-7 percent-at 6,000 feet than it does at 2,000 feet. Given the choice, I'm climbing to a cruise altitude above 6,000 feet.
The higher you fly the thinner the air, which means the engine takes in less air and thus produces less power. The good news is that fuel consumption decreases significantly, provided you lean the mixture properly, while true airspeed remains relatively high.
I have an owner's manual for a 180-horsepower retractable single with constant-speed propeller, and the performance charts show that at 70-percent power at 2,500 feet msl, the airplane should cruise at about 134 knots true airspeed and burn 9.8 gallons per hour. At 10,000 feet and 69 percent power the fuel consumption is the same, but true airspeed is 149 knots-an 11-percent increase.
Not only are you going faster when you fly high, you can fly farther because the airplane is more efficient-more speed from the same amount of fuel, or the same speed on less fuel compared with flying low.
The ideal power setting to achieve the optimum ratio of speed and fuel efficiency is about 55 percent, but for most of us that means going too slow in a machine intended to get places fast. So when flying high we're typically at full throttle, which equates to a power setting of 75 percent or less depending on altitude. (The engine manufacturer may not recommend leaning above 75-percent power-check the pilot's operating handbook or airplane flight manual for your airplane to be sure.) The higher we fly, the less horsepower the engine is capable of producing, but that power is more productive at generating higher true airspeeds.
There are other significant advantages to flying high. Visibility is almost always better, and the air is smoother because you're above the turbulence associated with thermals or terrain. The higher you fly, the less traffic you'll encounter.
It's cooler and therefore more comfortable up high in the summer, although the payback comes in the winter when it can be very cold-too cold for comfort, even-up high.
Up high you're above a lot of controlled-access airspace including Class C and D terminal areas, reception from ground-based navaids and communications antennas is better, and you can overfly all but the tallest mountains. Flying high also gives you more time to deal with a problem should one come up, especially if you find you have to descend. You simply have more options when you fly high.
Flying low is fun, but flying high is efficient. Given all of the advantages, how do I pick the optimum cruise altitude? That's a good subject for another column.
Mark Twombly is a writer and editor who has been flying since 1968. He is a commercial pilot with instrument and multiengine ratings and flies a Piper Aztec.
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