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Low Blows

Did you know that flying at maneuvering speed when encountering turbulence might not be such a good idea? Wait a minute, you may have said to yourself; I know that flying above maneuvering speed when penetrating turbulence can definitely be non-habit forming, so…since when should I not slow down when things get bumpy? Don’t worry, I haven’t been sniffing one too many fuel samples. All I mean to say is that…even maneuvering speed might be too fast.Did you know that flying at maneuvering speed when encountering turbulence might not be such a good idea? Wait a minute, you may have said to yourself; I know that flying above maneuvering speed when penetrating turbulence can definitely be non-habit forming, so…since when should I not slow down when things get bumpy? Don't worry, I haven't been sniffing one too many fuel samples. All I mean to say is that…even maneuvering speed might be too fast.

First, which is better, flying high, or down low where you can watch the scenery whiz by? Some of the high road perks often include generally smoother air, less conflicting traffic and more direct routing, vastly increased options for safe havens in an emergency, better radio communications, higher true airspeeds and more miles per gallon, the near impossibility of getting lost, very much lower odds of a bird strike, as well as cooler temperatures and better visibility above haze layers, especially in the summer. On the other hand, even though you can see so many more people who aren't having as much fun as you are, you can also find ferocious headwinds and icing up there, too. Of course, in the real world, the wind isn't always at your back. If it's not howling at you from the front, it's usually shoving you sideways.

Then again, flying down low has its good points, as well. You get a real sense of motion, which is quite exhilarating. Cross-country navigation can be a bit more of a challenge, but at least you always have a close-up look at your potential emergency landing sites, even though that may mean only what's right in front of you. (And in a helicopter, if you get lost, you can always read the highway signs.) Also, unfavorable winds are less unfavorable, down in the friction layer. Someday I'm going to strap on a parachute and hang a fan on my back and see the countryside from more of a Wright Brothers perspective. I imagine that it's like an IMAX movie on steroids, but it's probably going to have to wait until either the kids are grown or we're on an exotic vacation somewhere where cell phone towers and parcels of special-use airspace are few and far between, perhaps where some roadside entrepreneur happens to have a catchy sign for free spirit adventurers who would like to see what its like to soar over the countryside with the wind at your back-always at your back. Of course, depending on your local climate, the type of gentle winds conducive to such low-level aerial brachiating might number all of thirty or forty days a year.

But whether you take the high road, or the low road, real life doesn't always bring balmy air and sighs of contentment. The wind isn't always at your back, and it isn't always free of orographic, frontal, or convection-induced turbulence. In fact, low-level flight has several of its own unpleasant attributes. When low-level winds flow over and around even small hills (and buildings), potentially dangerous downstream vortices can result, even at altitudes several times that of the geography (or structure) that spawned them. Fast-moving cold fronts, especially those that we see in the wintertime, can generate some pretty severe rip-snorting rides down low. Especially at lower altitudes, the effects of differential solar heating, which sailplane pilots seek in the form of thermals, can be the cause of a bumpy ride as you punch through an invisible forest of them. You can encounter wind shear near the top of haze layers or the upper boundaries of stratus clouds when you fly through temperature inversions. The biggest inversion of all of course is flying up towards or into an advancing warm front, which brings the possibility of shear related turbulence, as well as icing in the clouds or possibly worse, freezing rain when precipitation falls into the colder air below. When you fly near the boundaries of earth and water, the result of differential heating such as land and sea breezes are like miniature fronts, with the same (although usually reduced in extent) potential for bumps. Upper level troughs affect the air below, and surface lows can be strengthened by their upper-level counterparts, more so even than you might guess from looking at a low-level chart.

The point of all this isn't a point-counterpoint essay about the relative merits of flying high or buzzing along down low. The point is, whichever road you take, you're going to be flying low soon after you take off, and again just before you land, and this just happens to comprise the most risky phase of any flight you will make. And in the grand scheme of things, we usually encounter roiling air more often at lower altitudes, which might be where we can least afford it.

So how do you minimize your exposure to the risks of flying through turbulence? This might sound about as profound as 'look both ways before you cross the street' but aside from some of the things I already mentioned, the first thing should always be a good weather briefing. If it's a long trip, or you start feeling less than absolutely certain about what's in front of you, a call to Flight Watch (or simply eavesdropping for awhile) on 122.0 never hurts. Listening to AWOS reports and ATIS updates from airports in your vicinity is always helpful; if you hear reports of strong or gusty winds, you'll have fair warning of possible challenges to come.

Another mitigating measure is actually one of the simplest: slow down, especially during your descent from cruise altitude. I realize that descending and slowing down at the same time might seem to be mutually exclusive…but that is your assignment, Mr. (or Ms.) Phelps. The most important thing is to fly slower than maneuvering speed (or your estimate of what it should be, based upon your approximate weight at the time). If your airplane is pushing 30 (years that is) it may have just one published VA speed, so remember that maneuvering speed decreases proportional to the square root of your airplane's decrease in weight below maximum gross weight, if that is the only number you have. Flying at or below VA means that any commanded abrupt and full motion of a control surface (conceivably while one is reacting to sudden attitude upsets during a rollicking ride) will simply stall the airplane before it breaks. However…that number is (was) for a then brand-new airplane, and if the airplane isn't all yours, you never know who might have done what with it before you got into it, so there actually may already be some not-insignificant degree of airframe structural fatigue. The bottom line here is, tread gently. But that's not what I was getting at when I said at the outset that maneuvering speed might be too fast. I wasn't thinking about scaling down VA .

There's another consideration besides your stressing the airframe while reacting of course, and that's what gusts do to your aircraft before you have a chance to react. There's actually another V-speed called VB or gust-penetration speed which aircraft manufacturers assess in regard to sudden sharp changes in wind speed and direction. By definition, the control surfaces are not fully deflected-the pilot presumably hasn't had time to react-however the result is similar: increased load factor (although the stresses may be less). The current requirements as stated in CFR Title 14, Part 23.333 are for tolerances to vertical gusts of 50 feet per second up to an altitude of 20,000 feet (which equates to about 30 knots), going down to 25 fps at 50,000 feet. (To provide a rough yardstick, the British Royal Air Force did a study that said airspeed fluctuations greater than 25 knots constitute 'severe' turbulence.) In general, recommended gust penetration airspeeds are lower than maneuvering speeds (although if you find VB values in your POH, you're lucky). Maneuvering speed is obtained by multiplying the power-off stall speed by the square root of the limiting positive design load factor (3.8 g's, for normal category airplanes, which gives 1.949 times VS). However, we usually fly-or so we would expect-with the power on, and the power on stall speed is significantly less than when the power is off. When the stall speed decreases, maneuvering speed decreases. If a pilot encounters turbulence that subjects the airplane to a gust from the front, this would have the effect of increasing the airspeed beyond the value of maneuvering speed. The closest approximation I've seen for VB is 1.7 times the clean power-off stall speed at maximum gross weight. This approximate number would be good at all weight conditions. (But of course if your POH gives you a number for VB, use that, instead!)

So…if you know (or suspect) that bumpy air may be on the program, the one thing you mustn't do is let airspeed go above VNO (the top of the green arc on your airspeed indicator). I know: well, duh (double duh, if you've been following the logic in the previous paragraph). The first thing that probably would have occurred to you right off the bat is to make an immediate power reduction-start one quickly that is, but do it gradually (unless you happen to be flying a turbine) and let airspeed decay while still in level flight. Then when it gets below VFE (or VLE) you can lower the flaps (or landing gear if so equipped). That will slow you down more, of course. If your airplane has spoilers or speed brakes, you have more options than I do. The whole point of this is to attain slow, stable flight in a descent, before you hit the bumps.

I don't happen to dislike aerobatic flight, but I do harbor some distaste for, and distrust of, flight in turbulence (for some of the prior-ownership related reasons above). All I can say is to grit your teeth and know you're doing the best you can; focus on your target airspeed, and on keeping the wings level. Use small corrections for those inevitable attitude disturbances; large control surface movements just invite pilot-induced oscillations, and worse, excessive speeds as well as large attitude excursions. Obviously, inform ATC of the source for your duress, and if they ask for an expedited anything, use the 'U' word (as in 'unable'). And forget the autopilot!

Once you make it to the runway, don't let your guard down. Low-level turbulence might not let you off the hook until you're on the ground, and it would be an embarrassing ignominy indeed to make it through the gantlet, all the way to the numbers, only to have some cantankerous crosswind give you the bum's rush into the weeds alongside the runway.

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