Tuesday, March 22, 2011
There’s No Such Thing As Tailwinds
Why is it that we always only remember headwinds?
If you fly many trips in the mountains, the issue becomes more confused. As mentioned above, any irregularity on the Earth’s surface would seem to maximize friction between sky and ground, but mountains make their own rules. Wind speed tends to increase because of Venturi effect, the same phenomenon that produces lift on a wing. Funnel wind thru a narrow opening, and it inevitably speeds up.
In fact, a combination of geographic features can sometimes produce spectacularly dangerous winds in the mountains. Little 6,300-foot Mount Washington in New Hampshire, barely a hill by the standards of the 14,000-foot Rockies and Sierra Nevada, has experienced winds of more than 200 knots. The winds are so consistently ferocious that the government weather station at the top regularly records wind chills below -100 degrees F, not that dissimilar to conditions at the South Pole.
Turbulence is another factor that universally kills lift and causes airplanes to fly slower. While updrafts may impart a temporary increase in airspeed (assuming you maintain altitude), a downdraft can cause a dramatic drop in speed. I flew a Caravan to Seoul, Korea, last year, and as I left the Sea of Japan and crossed the Korean coast at Pusan, I watched airspeed slide back and forth from 140 to 75 knots for the next 200 miles because of spectacular up and down drafts that plague the area in winter.
In the Midwest and other regions where the wind really does come whipping down the plain, the sky can deliver surprising surface gusts that may tempt a pilot to fly low to take advantage of the free speed. Not a good idea, as those gusts can become violent and ruin more than your day.
Sadly, even when mid-altitude winds are glycerine-smooth and consistent, they most often won’t help you on a two-way flight. You’ll always lose more time in headwinds than you’ll gain in tailwinds, simply because headwinds act on the aircraft longer than tailwinds. Crunch some numbers on your flight computer, and you’ll see this to be the case. If you fly an airplane with a 200-knot true airspeed on a 400 nm out-and-back trip, and there are 20-knot winds directly on your course line, you’ll need 2+13 in the headwind direction and 1+49 for the return flight. That’s two minutes longer than if you flew both legs in no-wind conditions. The situation gets even worse as the trip gets longer.
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