Tuesday, June 8, 2010
Dealing With Convective Weather
Some pilots simply lock their airplanes in the hangar when convective weather is about. Others learn to cope.
According to Barry Schiff in his excellent book The Proficient Pilot II, one simple rule for guessing a more realistic Va is to subtract two knots for every hundred pounds under gross. If you’re flying 400 pounds below gross, for instance, you should use a Va that’s eight knots slower than the book number.
When turbulence passes the moderate level, another technique is to try to maintain constant power and constant wings-level attitude, but not a constant altitude. If you’re filed IFR, you’ll need to advise ATC that you’re unable to maintain altitude (assuming there are any other idiots penetrating the same horrible weather), though you may find the vertical winds will cancel one another, and you’ll wind up hovering somewhere near your original height. We’ve all heard stories of spectacular up and down drafts inside cumulus clouds, but the reality usually is far less dramatic.
If you’re over mountains or other high terrain, you may have no choice but to fight the downdrafts and hold altitude as best you can. Just remember that if you’re jockeying throttle and yoke in an attempt to maintain a precise altitude, you may be subjecting the airplane to considerably more stress than if you were simply riding out the turbulence.
But let’s say you’re flying in turbulence and you get caught in a downdraft that seems determined to take you to the ground. This may have been what brought down the late Steve Fossett in California’s Sierra Nevada mountains back in September 2007. Fossett was flying a friend’s Super Decathlon and crashed in an area of known convective activity.
We’ll probably never know exactly what brought down Steve Fossett, but if it was a strong downdraft, there’s a technique that may save your life, provided you have the nerve to use it. Everyone knows if you’re flying in strong downdrafts, the standard technique is to pitch to Vy and apply full power. Right?
Not necessarily. Sailplane pilots (who don’t have the privilege of power) are taught something that seems almost counterintuitive to aviators of powered machines. The best course of action may be to lower the nose to a cruise attitude, and fly through the downdraft as quickly as possible. This isn’t as suicidal as it sounds, provided you have the altitude to make it work. Increased forward speed has the effect of actually reducing the angle of descent. Similarly, the faster you fly through the downdraft, the quicker you’ll reach the updraft that’s probably waiting on the other side.
Conversely, if you’re caught in a strong updraft, resist the temptation to lower the nose and maintain altitude. You’ll add airspeed in a hurry, often pushing the airplane close to its Vne in a few seconds. Ride with the updraft and reduce power to help control altitude. Updrafts can be nearly as dangerous as downdrafts in that even idle power may not stop the climb. In California’s Owens Valley near Bishop back in the last century, I was once forced to ride a Globe Swift to 17,000 feet before the updraft finally released me. Had the elevator ride continued, I would have been in trouble, as I had no supplemental oxygen on board.
No one in their right (or left) mind flies through the center of a thunderstorm, but simple cumulus clouds may be manageable. If you’re willing to make some accommodations, you can learn to live with convective weather.
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Labels: Flight Hazards, In-Flight Emergencies, Pilot Skills, Weather Flying, Weather Skills, Pilot Safety