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Deviant behavior: when convective weather is about, the shortest distance between two points rarely is a straight line.

It's been said that experience is what you get when you don't get what you wanted to get. Well, taking significant liberties, a pilot might then say that a deviation is where you go when you don't get to go where you planned to go. Got it?

We pilots are typically a robust and determined bunch. We don't like to admit we can't travel our planned route. Once we're on our way and it looks like Mother Nature doesn't want us on our planned route, the common technique is to simply go have a look before deciding on a different, longer route. To understand the fallacy in this, let's cover a little basic geometry to see that it's better to deviate early for the shortest practical addition to your route while maintaining your sanity and keeping your blood pressure in check with a conservative deviation.



Imagine you're on a 400-mile leg from A to B. At 200 miles there is a developing line of thunderstorms. Undaunted, you press on to "take a look." After about 150 miles, 50 miles from those cells, Nexrad still shows a 100-mile line bisected by your course. You deviate 50 miles to the left as you nervously compute your new distance, hoping you have enough fuel. Basic geometry tells you if you're 50 miles away and you now change your course to deviate by 50 miles, you've added about 21 more miles, just to get to the deviation point, then presumably another 21 miles to get back on track, for a total of 42 miles out of your way. This scenario is illustrated on the opposite page as Lesson 1.

Had you not insisted on taking a look, you would have simply begun to deviate all the way back at point A. You'd steer a course to a halfway point 50 miles off your original route. Assuming the opposite course adjustment on the other side to get back on course, you would have traveled a total of 412 miles vs. the original 400, compared with the 442 miles required by the "take a look" approach. Moral: Deviate early and save gas. Again, this is depicted on the opposite page, as Lesson 2.

But rather than deviating at 50 miles out, super-bold pilots will often still push on, hoping to find an opening as they get closer. At 25 miles out that line of thunderstorms is looming quite large in the windscreen and Capt. Young N. Bold can see what might be a little gap in the line big enough to sneak through. Nexrad confirms it and he goes for that gap with just a 15-mile deviation. With the same thing on the other side to return to the course, Capt. Bold has only added about eight miles to his flight. See Lesson 3.

Yup, instead of the conservative early deviation, he was able to achieve a total savings of four miles with the shorter course that came with all the stress, uncertainty and turbulence bouncing through that little gap, not being entirely sure what might be beyond. Moral: Deviate early and reduce your stress.


We tend to think of deviations only in two dimensions--left and right of course. The reality, though, is that deviations can take place in any of four dimensions. We've got left/right deviations, but we can add the third dimension of vertical deviations: It's sometimes possible to climb or even descend to avoid weather.

Finally, there's the dimension of time. Frequently if you just linger a little longer over lunch, that line of thunderstorms will have drifted out of your way. To figure out how best to deviate requires a good mental picture of the entire situation. You can best get this, of course, from real-time data and imagery.

Deciding how to deviate around the weather is largely a matter of knowing where the weather is and isn't, where it will and won't be. Simple, eh? Actually, with the right resources it's not that difficult. It's not necessary to be a weather expert to properly plan your deviation. You just need to have the weather big picture. For that early deviation we're recommending, you must know if those cells are moving, in what direction and speed they're moving, what they're leaving behind them as they move, and the rate of development or dissipation. You can easily get this information from the time-lapse view of the radar images available from many sources.

Let's revisit our simple example of the leg from A to B with the 100-mile line of storms at our midpoint. Good planning and weather awareness requires knowing the direction of travel and development of that line. It's quite possible that it's moving, say, southeast at 15 knots and that the whole line is moving rather than just building, and leaving good IMC behind it. If you're zooming over the ground at 150 knots, a little action with the whiz wheel (remember those?) will tell you the line will have moved southeast about 20 miles by the time you get there, or say about 14 miles out of your way since it's not moving perpendicular to your course. So, with this information you can plan to deviate only 36 miles instead of the 50. Of course, you should monitor the line's progress to make sure it hasn't unexpectedly stalled. Fine tuning this plan once on your way is essential, and the distance added by any possible en route adjustments should be negligible.

Maybe the line of storms isn't showing much vertical development and you can top it. Be sure to do so with enough room to avoid any potential nastiness coming out the top, though. Or, perhaps if you just wait a bit longer, the line will have moved far enough that you don't have to deviate at all. Is the line building or dissipating? In addition to its movement, is it getting stronger, requiring a wider margin, or is it weakening, allowing you to cut it closer? Can you use the wind to help your deviation, perhaps giving you a good push?


Of course, there's one more aspect of all this we haven't yet discussed: ATC. Obviously, if you're flying VFR you don't need to worry (much) about getting a clearance from ATC to deviate. However, if you're on an instrument flight plan, you need approval or a bona fide emergency to deviate from your clearance.

The difficulty of working with ATC for a deviation ranges from trivial to impossible. If you're simply at cruise and not at a crowded altitude, it's usually just a formality. By "crowded altitude" we mean anything below 10,000 where most of GA will be flying. We also mean altitudes above FL290 that can get pretty crowded. In between, it's generally reasonably quiet. So, if you can get your normally-aspirated single above 10,000 feet you should be in good shape. With a turbo go a bit higher, say in the low teens (13,000-16,000) but you'll need oxygen. At these altitudes you should have a smoother ride above much low altitude weather and you won't be competing with many folks for airspace. If you can, picking an altitude in the low flight levels will be even better. In most of these un-crowded altitudes there's not much traffic to worry about and if you don't have any terrain issues, ATC will usually tell you, "Cleared as requested."


Then, there are departure and arrival airspaces. They can be a real challenge at any altitude. It's common, for instance, to be descending into a major terminal and ask for a deviation to the left of a visually obvious build-up and be told, "Unable left, but I can give you up to about 10 degrees right." The common reason for this is that terminal controllers have their hands full trying to mix descending traffic arriving into the terminal from the traffic trying to climb out of the terminal. Adding some nasty cells in the area can really bollix things up. The best advice here is to simply listen and see what others are doing, and be prepared to request the same thing.

Don't be a lemming, though. If those jetliners with their digital weather radar are threading the needle into the terminal through a cluster of cells right over the arrival gate and your 172 isn't even equipped with a Strikefinder, don't be afraid to insist you need to go way around that dark ominous mass in front of you that's stretching from your 10 to your 2 o'clock.

What if the deviation is causing fuel worries? Say you've got two hours of fuel remaining and one hour of planned distance yet to travel, but you're looking at a 15-minute deviation. You're still looking at landing with 45 minutes in the tanks, but it should start increasing your pucker factor. Don't fixate solely on that, though, lest it lead to bad deviation decisions. Worry about the immediate threat--weather--first, then deal with the fuel that's left. Keep your options open in the back of your mind, though, and be thinking about a diversion to a closer airport for fuel, or deviate around the weather to an airport with fuel. Again, keeping the big picture will dramatically improve your peace of mind.


Let's see how this can be applied. Say you're IFR, going into Big City Hub airport on the Dviat 1 Arrival, Outwest transition. When you departed it looked like there might be some weather developing southwest of Dviat, moving northeast, so you delayed your departure an hour to let it move enough to give you some room. As you pass Outwest you begin to hear some complaining about the weather on the way in from Dviat. Some of the folks are asking to delay the turn at Dviat, preferring to go further east before heading down to Big City, and others are turning early.

As you expected, Nexrad shows a nasty low and a bunch of weather on the arrival just past Dviat, and it is moving northeast. Tops are too high and there is terrain below, so you've got to turn. You remember your basic weather that says the wind is counterclockwise around a low pressure area. Combined with the weather moving northeast, you conclude that you can best avoid the weather and might get some favorable winds if you start your turn well before Dviat. ATC makes it work for you. You note as you head down the new course towards Big City that you've picked up about 15 knots of groundspeed. You also see some real nasty stuff off to your left, but it looks like you'll remain VMC on the edge of it.

This oversimplified textbook example illustrates a few things: deviate early (in this case it saved distance), get the big picture so you know how the weather is changing and what the wind is doing, use time to your advantage, then pick a deviation that will best keep you out of the weather and maybe even get a little help from the wind.


Naturally, each situation is different, presenting different challenges. However, if you keep the big picture in mind, monitor that big picture and remember that you have four dimensions available, you can very often fly around the weather without too circuitous a route, sometimes even with favorable winds.

Much to our dismay, we have discovered Frank Bowlin deviates early and often.

RELATED ARTICLE: And When It Was Bad, It Was Horrid ...

A couple months back I was flying into Nashville. The TAF was VMC, so we were dispatched with our standard fuel: anticipated fuel burn plus about an hour. We didn't need an alternate and there was no obvious need for "just in case" fuel ... until you looked a bit closer, which neither our dispatcher nor I did. There was a mention in the TAF of thunderstorms in the vicinity. The airport's good, so why worry, right? Wrong!


Indeed, there were significant thunderstorms developing in the area. The airport remained VMC. But, one by one, storms closed arrival gates. This required rerouting the arriving traffic to the remaining gates. By the time we got there we were told to expect about 30-45 minutes of low-altitude holding just outside the arrival gate while they fed all the traffic through the one open gate. Down low a jet engine gulps fuel like a frat party goes through beer during pledge week, and we simply couldn't afford the delay. I wished for another keg!

So, I asked my newbie first officer what our alternate was and how much fuel it would take us to get there. He fumbled through the release and finally apologized that he couldn't find the alternate. D'oh! Our destination was/is VMC; we don't need no stinkin' alternate! Bouncing around in the clouds I messaged our dispatcher for an alternate. What I got back said that we could proceed only for another few minutes before we'd have to divert and still have enough fuel to get to our new alternate. So, we soldiered on, dodging cells, alternately playing with some bad bumps and flying through a few lakes.

Then we got an airspeed alarm. Seems that my airspeed indicator wasn't reading the same thing as the FO's. We opened the "When things go wrong and you need to know what to do" book. It said to compare each airspeed indicator with the standby one to see who went tummy up, then throw some switches to source the incorrect indicator to the correct source. Okay, lessee: My airspeed said about 205; the FO's said 220 or so. Next I look at the standby airspeed indicator--"Oh, this is gonna hurt!"--it read zero. There's no such chapter for this in the "When things go wrong and you need to know what to do" book, so I figured this is where "Captain's Authority" was supposed to kick in. I made the obvious choice to believe my indicator, figuring that it's the more conservative of the two.

Well, we didn't end up in USA Today, so I guess it was okay. We just tried to stay high, kept going slower to conserve fuel and delay the hold, complaining to ATC that we couldn't wait long, and we got right through the gate with no holding. Made a nice visual approach with a smooth landing (the landing is all the pax notice anyway), grounded the airplane in front of the next crew, went to the hotel and had a few adult beverages, bragging about our heroism. (Meanwhile, maintenance extracted a few acre-feet of water from the three pitot systems.)

Colorful story telling aside, there is an important lesson here, and that's to know the big picture. Sure, Nashville was forecast VMC. But, had I stopped to gather the big picture and see all those nasty storms developing around Nashville, I'd have put on at least 30 more minutes of fuel and insisted on an alternate. "Just in case" is a very healthy game for a pilot to play when flight planning.
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Author:Bowlin, Frank
Publication:Aviation Safety
Geographic Code:1USA
Date:Sep 1, 2008
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