Plane & Pilot
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.

Once, on the 2,000 nm leg from Honolulu to Majuro, Marshall Islands, in an A36 Bonanza, I dropped from 14,000 feet to 100 feet above the water, and drove the last 700 miles on into Majuro at that height. The turbulence abated as I dropped lower and was easily tolerable at the reduced altitude.

Forecasting Is A Precise Science
The good news is that weather forecasting and real-time product uploads have become so accurate, we rarely encounter anything we don’t know about well in advance. My buddy Jeff Kopps of the National Weather Service in Monterey, Calif., usually can pinpoint areas of cumulus buildup and possible CB activity over the Pacific, and he’s almost never wrong. If we know there’s nastiness waiting for us on the ocean, we usually don’t leave in the first place. It’s the oldest advice in aviation, carried to the ultimate extreme on a long leg: “When in doubt, don’t.”

Another benefit some pilots have today is reasonably accurate weather-depiction avionics such as Stormscope, Strike Finder and radar. The state of the art in graphic weather presentation is exponentially better than it was even a decade ago. XM weather can allow a real-time display of the atmospherics ahead, easily as accurate as onboard detection devices.

The Long View
Regardless of what electronic help I have on the panel, I’ll often ask Jeff Kopps to look two to four days ahead and temper my go/no-go decision based on his long-range forecast. If I’m headed for Indonesia, Japan or the Philippines through Honolulu, Majuro and Guam, and there’s a typhoon threatening the Western Pacific, even though it’s still 5,000 miles away, I may delay the entire trip for a while. I’ve waited in Honolulu as long as an extra five days for the ITCZ that borders the equator to calm down. (Hey, it’s a tough job, but…)

The Weight Consideration
Of course, at the weights we fly, we have an extra incentive not to penetrate major weather systems. Our airplanes typically are loaded 15% to 25% over gross with fuel, and that reduces the allowable G-tolerance considerably. On my recent Caravan trip to Korea, for example, I was operating with 430 gallons of ferry fuel on board at a max takeoff weight of 10,915 pounds, compared to the Grand Caravan’s normal max gross of 8,750 pounds. That means I was flying the airplane 2,165 pounds over gross (on a ferry permit), roughly 25% above the normal limit.
It’s the oldest advice in aviation, carried to the ultimate extreme on a long leg: when in doubt, don’t.

The math doesn’t demand a physicist. Normal category allows a max load of 3.8 times the approved gross weight of 8,750 pounds, a total load of 33,250 pounds (3.8 x 8,750 pounds). Operating at 10,915 pounds, my max G-load is therefore reduced to 3 G’s (33,250/10,915). While it’s unlikely I’d ever see that kind of stress in convective weather, I avoid turbulence whenever possible.

“What To Do If…”
The inevitable question arises, “What do you do if you can’t land and you have to continue in violent weather?” The answer is to slow the airplane to 10 to 15 knots below maneuvering speed (the published Va usually is not the optimum turbulence penetration speed) when flying in moderate turbulence. For most GA airplanes, Va is a calculated value that applies only to operation at gross weight, and unless you depart well over maximum weight, you’re not likely to be flying at gross. Va decreases with any reduction in weight.


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