You'd think that after making the round trip a few dozen times I'd have figured out the best cruise altitude. And I have-to a point. Yesterday was a good example of being half-right, half-wrong.
We departed in the dark at 6 a.m. I had filed IFR because a thin but solid cloud deck hung over the land at about 4,300 feet msl. We were southeast-bound, and my cruise altitude of choice was 7,000 feet msl.
The flight-planning program I use computes time and fuel consumption for the route at four different altitudes, in this case 3,000; 5,000; 7,000; and 9,000 feet msl. The computations are based on forecast winds for my planned departure time, and aircraft performance figures that I had to enter when setting up the program initially. My flight plan yesterday morning said the trip would take 40 minutes at 7,000 feet and 41 minutes at the other three altitudes. OK, there's a minute saved.
It also predicted that the airplane, a piston twin, would use 20.7 gallons if I flew the trip at 7,000 feet, and 20.2 gallons at 9,000 feet. Hmmm, that's one-half gallon less fuel if I flew 2,000 feet higher. (Trip fuel consumption at 5,000 feet and 3,000 feet were projected to be 21.2 gallons and 21.4 gallons, respectively. More fuel, and a minute more of flight time. Not an option.)
I'd probably rather save a bit of fuel than one minute of flight time, so why didn't I flight plan for 9,000 feet? Because the computer-generated flight plan can't anticipate the effects of air traffic control (ATC) clearances. I've learned from experience that, on this particular route, ATC usually lets me climb unimpeded to whatever cruise altitude I've filed for, but I don't remain there very long before Miami Air Route Traffic Control Center instructs me to descend so I will be below heavy jet traffic inbound to Miami International (KMIA). So, it would be inefficient to climb to 9,000 only to have to descend almost immediately. I would use more fuel in the climb than if I stopped at 7,000 feet and spent a few minutes cruising there at a properly leaned, reduced fuel flow.
The morning flight worked out according to plan. I was briefly held at 5,000 feet, then cleared to 7,000 where I remained for about 15 minutes before getting the descent clearance. Flight time was right on the money, and fuel consumption was even a bit less than the flight plan called for. That was the half-right part of the day. The late-afternoon VFR-direct flight back did not go quite as smoothly-literally.
The flight-planning program said the quickest, most fuel-efficient altitude for the return flight would be 2,000 feet msl because headwinds were significantly higher at the higher altitudes. Fine, especially since on departure the Miami controllers always hold me at 2,000 feet for 30 miles to keep me below the base of Miami Class Bravo airspace and those KMIA arrivals. Once clear of the Bravo airspace I wanted to climb because it was uncomfortably bumpy down low. So I climbed to 4,500 feet msl.
I didn't want to go higher because I knew I'd have little time at altitude before beginning my descent. Unfortunately, it was just as bumpy at 4,500 feet as it was at 2,000. After a few minutes I could see that I was riding near the top of the haze layer, and that it was clearer and probably much smoother just above that. But it was too late to climb, so I gritted my teeth and bumped along for a few more minutes until beginning the washboard-rough descent to home base.
Yesterday's flight underscored something I learned a long time ago: Selecting a cruise altitude for a cross-country flight is based on science but leavened with art and, you hope, a little bit of luck.
The science lies in evaluating the long and complex roster of factors that come into play when selecting a cruise altitude: clouds, precipitation, ambient temperature, winds aloft, reported turbulence, aircraft weight and performance, fuel consumption, obstacles, terrain and bodies of water, airspace, trip distance, communications and navigation (VOR) radio reception, and pilot and passenger comfort. The art comes in applying experience, judgment, and common sense to weigh the various factors.
The atmosphere is dynamic, ever changing, and a cruise altitude that is ideal one day may not be the best choice the next time you fly the same route. My 4,500-foot choice on the return flight would have been ideal if the air had been smooth. As it turned out, 6,500 feet would have been better.
My simple rule of thumb is to cruise as high as possible, and sensible, to take advantage of the combination of lower fuel consumption and higher true airspeed. Other benefits of flying high include less traffic, better radio reception, and less controlled-access airspace (Class C, D, and B) with which to contend. Once I determine the ideal altitude for the trip length, I decide whether I'll have to settle for lower because of winds, weather, reported turbulence, or some other factor.
Working through some complete trip planning exercises using the performance charts in the pilot's operating handbook or airplane flight manual invites understanding and insight of the relationships between ambient conditions (winds and temperatures aloft), power settings, true airspeed, altitude, fuel consumption, and range and endurance for the airplane you fly. At some point, however, you'll probably turn over much of that work to the software in a computer-based flight-planning program.
Most programs do a good job of crunching the numbers to determine a good cruising altitude, but the programs are only as good as the information fed to them. Assuming that the program takes into account forecast winds for the planned route and time, your task is to make sure you've armed it with real-world information about your aircraft's performance including climb, cruise, and descent speeds and fuel consumption. It simply doesn't work to tell the computer that the airplane cruises at 120 knots true airspeed, regardless of altitude. It may well do that at a best-power setting at, say, 5,500 feet msl in standard conditions, but cruise speed will vary according to density altitude and power setting. Plus, airplanes generally climb slower and descend faster than they cruise.
As good as flight-planning software is these days, it can't anticipate the influence of such factors as clouds, turbulence, and air traffic control on your choice of cruise altitude. It's up to you to apply your experience and knowledge to correct whatever the software recommends to come up with a final, real-world flight plan and cruise altitude.
You've done your best to plan the ideal cruise altitude for the trip, but we all know what can happen to the best-laid plans. From the moment of takeoff, be prepared to make adjustments to account for whatever the atmosphere, the airspace, and air traffic control may throw your way.
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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