Frugal Flyer: Finding a tailwind

In the world of compromises known as aviation, it’s rare for all indications to point in one direction. But while planning a recent flight from the Madison, Wisconsin, area home to Frederick, Maryland, in AOPA’s 2009 Sweepstakes Cirrus SR22, all signs pointed the same way to an economical trip: Higher!

Sure, aircraft engines consume less fuel (and make less power) as they breathe thinner air at high altitudes. But changes in the speed or direction of winds aloft, clouds, turbulence, freezing levels, traffic flows, and other factors usually offer diminishing returns for climbing beyond a certain point. Most nonturbocharged piston aircraft achieve their best true airspeeds between 6,000 and 9,000 feet, and the SR22 is no exception.

But the airplane’s service ceiling is all the way up at 17,000 feet. And on an eastbound trip in the fall, the ability to climb high and catch a tailwind can dramatically improve overall flight efficiency, significantly improving range and reducing total fuel consumption.

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The route of flight on this trip went straight across Lake Michigan, so a higher altitude meant less time beyond gliding range from shore. A broken cloud layer topped out at 7,000 feet with temperatures warm enough that icing wasn’t a concern, and there were clear skies above. The wind direction was out of the northwest at all altitudes with speeds increasing steadily at higher altitudes. At 15,000 feet, winds ranged from about 30 knots over Wisconsin to 70 knots over West Virginia.

The only factors arguing against flying as much of the trip as high as possible were an airmet for turbulence at 17,000 feet and above, and the aerodynamic penalty of high induced drag that all aircraft pay when flying near their service ceilings.

With a flight plan filed for 15,000 feet and a full oxygen bottle, Madison Departure soon cleared the SR22 to 13,000 feet, then 15,000 feet approaching Lake Michigan’s western shore at Milwaukee.

The tailwind showed up right away in the form of higher-than-normal groundspeeds, and the SR22 climbed as rapidly as practical (120 KIAS) to get maximum advantage from more time in the increasingly powerful tailwinds above. Crossing Lake Michigan, the tailwind exceeded 30 knots and groundspeeds approached 200 knots with a full-throttle fuel flow just more than 10 gph.

In Ohio, the barometric pressure steadily dropped approaching a tight low-pressure system and the SR22’s groundspeed increased to more than 200 knots. The S-TEC System Fifty-Five X autopilot was following a preprogrammed airway route, but ATC offered a shortcut all the way to Martinsburg, West Virginia, and the relatively straight line shaved more than 40 miles from the 620-nm leg.

In cruise flight the airplane’s indicated airspeed hovered around 125 knots, but the true airspeed was about 40 knots faster (165 KTAS) and the groundspeed topped 230 knots. Most important from an efficiency standpoint, the SR22 was getting better fuel mileage (23 nm per gallon) than my Chrysler Sebring gets on the road—and moving a whole lot faster, with a far better view.

The airplane’s 310-horsepower engine was only producing about 50 percent power (17 inches of manifold pressure) at 15,000 feet. And the propeller was turning 2,700 rpm. (The SR22 has a single power lever that controls engine power and propeller rpm, and sadly from a noise and efficiency standpoint, the propeller is permanently set to fine pitch/high rpm at full throttle.)

The tailwind topped out at 72 knots near Martinsburg, and I snapped a few photos of the airplane’s PFD during the long descent to record groundspeeds above 250 knots—something I’d never seen before in the SR22, or any other single-engine piston airplane (other than a warbird).

The price for the booming tailwind was jolting turbulence below 9,000 feet as the strong surface winds collided with the jagged Appalachians and swirled on the lee side of the mountains. But the bumps were a minor inconvenience compared to the impressive savings from cutting a full hour off what normally would have been a four-hour trip, and crossing an entire time zone on 38 gallons of avgas (less than a half tank in a Cirrus SR22).

Free and subscription weather forecasting tools give increasingly accurate pictures of the conditions we can expect aloft. The free AOPA’s Internet Flight Planner tells you the current and forecast winds aloft and calculates how long it will take to fly particular trips at any number of altitudes. Fltplan.com offers a free, highly streamlined printout that compares time and total fuel usage at a variety of altitudes.

And Seattle Avionics with its subscription Voyager software packages ($79 or $149) will flat out tell you the most efficient altitude and route to fly based on your airplane’s performance and forecast winds. Seattle Avionics offers a free version of its flight planning software, but only the subscription versions have the ability to show “best time” and “best economy” altitudes and routes.

With such potent weather tools, and aircraft versatile enough to take advantage of them, we should all be able to find tailwinds—or at least lesser headwinds—more often.

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