Training and Safety

The bottom line on the art of avoidance

BY THOMAS A. HORNE (From AOPA Pilot, June 1997.)

Summer's here, and with it come equal measures of pilot joy and dread. Joy, for long, warm days and trips to vacation spots. Dread, for the threat of thunderstorms, haze, mean fronts, and turbulence. But by preparing yourself with the right kind of preflight and in-flight weather information, you can keep those ugly weather surprises to a minimum. By having an avoidance or escape strategy in mind before you encounter these adverse phenomena, in-flight anxiety can be kept under control when confronting a dicey weather situation.

Thunderstorms

Information about any thunderstorms likely to affect your flight is all around you. Flight service briefers are ordered to tell you about any convective activity, The Weather Channel broadcasts status reports several times an hour, DUATS providers spew sigmets and airmets, and local television weathermen also do fine jobs of analyzing the strength and movements of storm complexes. So it's a rare pilot indeed who unwittingly barges forth into an area of convective weather. This is why there are only 15 or so fatal thunderstorm-related accidents every year.

Even so, many pilots do blunder into convective weather. And since each encounter has crash potential, it's worthwhile to review some of the tactics that can help to keep you well away from convective hazards. Here are a few that have stood the test of time.

Obtain a good preflight weather briefing. This goes without saying, but it's still worth emphasizing. The accident record shows a depressingly high number of thunderstorm-related accidents involving pilots who didn't bother to get a briefing.

Determine the type of thunderstorm situations affecting your flight. Thunderstorms are either of the air mass variety, associated with fronts, grouped together in mesoscale convective complexes (MCCs), or tied in with the lifting activity generated by air flowing up and/or around terrain. Frontal and MCC thunderstorms are the most difficult to circumnavigate safely because they cover so much ground and can be so powerful. Air mass storms, on the other hand, tend to be smaller and more easily avoided. Air mass storms in Florida seem to be the easiest to spot and avoid visually. The lesson: don't even try to take on strong frontal storms or MCCs. A few airliners have tried — and failed.

Stay visual. In other words, don't fly into clouds or haze that could impair your view of any nearby storms. The best way to avoid thunderstorms is to fly at least 10 nautical miles away from them — 20 nm away if the tops are above the tropopause (about 30,000 feet). You may have radar, and you may have lightning detection, but even if you have both, nothing will keep you away from storms as well as a pair of eyes and the will to resist any shortcuts through clouds associated with storm cells. Should you inadvertently enter these kinds of precursor clouds, perform a 180-degree turn and get out of there. Many accidents have come about as a result of inadvertent penetration of clouds in convective weather, and not all of them involved non-instrument-rated pilots.

Check convective outlooks in the preflight briefing. This forecast product describes the areas with the highest probabilities of thunderstorm activity. These forecasts are seldom incorrect.

Check radar reports prior to departure. Radar reports (rareps) define areas of precipitation and thunderstorm activity in terms of ranges and bearing from selected fixes. Rareps are updated more often than radar summary charts. The downside is that they are textual information that you must copy down and apply by using your orientation skills. With a DUATS briefing, however, you can skip the copying requirement.

Use flight watch, ATC, ATIS, ASOS, AWOS, and HIWAS. Your weather briefing doesn't end the moment you put down the phone or log off DUATS. Once in the air, use the many weather resources your radios offer. An hourly update of the en route and destination weather is an absolute minimum on any cross-country flight. Updates should be more frequent if you're facing an active weather situation. If you are flying under IFR, air traffic control can also help to pass on vital information, such as how airplanes ahead of you are faring, whether your destination airport is affected by weather, or if delays are to be expected. Even flying VFR, consider calling up an ATC frequency and asking for VFR flight following. Almost as good as having an IFR clearance, flight following can be critical to staying up-to-date on convective weather.

Depart early in the day. Real early. A thunderstorm's lifting forces are fed by the heat of the day, and so is turbulence. By 10 a.m. the earth begins heating and vertical air currents begin in earnest. By 1 p.m. areas of convection are identifiable on radar. By 4 p.m. storms typically enter the mature phase, and by 5 to 7 p.m. thunderstorms are at their most powerful. By flying early you avoid these dangerous times and have a chance to obtain a thorough weather update during an en route stop before 10 a.m. By then, forecasters will have a much better idea of what to expect for the rest of the day.

Avoid night flying when thunderstorms are around. This, because it can be more difficult to see and avoid clouds in which storm cells may be embedded. By the time you see the lightning, it may be too late. It can be surprisingly easy to enter a thunderstorm when flying at night or on instruments. First, there's a little turbulence. Then it gets worse. Then comes the rain, and it gets worse. Each time, you think, "I can live with this. It'll be over soon," and then you see the lightning and hear the thunder. You don't want to believe it, but you're in a thunderstorm, all right.

If caught in a storm, keep your cool. That's easy to say but harder to do when chaos is reigning. Once you are in a storm, the primary directives become:

• Slowing to maneuvering speed (VA). This will help to prevent structural damage to the airframe. Lowering the landing gear of a retractable-gear airplane is one very good method of slowing to VA; this will also aid in preserving directional stability, as extended landing gear act rather like strakes.
• Tightening the seat belts and shoulder harnesses to the max.
• If it's night, turning up the instrument panel lights to full bright. Lightning flashes can blind you temporarily. Having the panel lights turned up means that you can see them better when your vision improves. Some even suggest closing one eye to minimize any vision problems.
• Keeping the wings level.
• Disengaging the autopilot. Don't try to maintain altitude, as you could overstress the airplane. Ask ATC for a block altitude if you cannot maintain an assigned altitude.
• Not turning around. Once in a storm, it's usually best to try to stay on the same heading. Turning around could put you in an unusual attitude or impose excessive G-loads.
• Flying instruments. Trust their indications and keep up your scan speed. It will save your life. Too bad if you don't have an instrument rating.

Fast-moving cold fronts

A word about fast-moving cold fronts is definitely in order. Every once in a while aggressive cold fronts will shove their way through, moving along at groundspeeds of 25 knots or more. Ahead of these fast-movers, turbulent gust fronts filled with severe turbulence and rotor clouds may pave the way. Once again, the rule for coping with these phenomena is strict avoidance. A good briefing, full use of in-flight weather information, and a pair of educated eyes should keep you out of trouble. Don't try penetrating a fast-moving cold front, even if there's no evidence of thunderstorms. The turbulence could prove too much.

Haze

Haze occurs when a large, stalled high pressure system creates a temperature inversion that traps pollutants and other airborne particulates beneath it. Visibilities can drop to IFR levels after a few days of dense haze, and change will come only when one of those fast-moving cold fronts blasts through and washes the air clean.

Combine escalating surface temperatures with the lethargy of haze-producing highs and you'll see that the top of a haze layer rises with each passing day. Ultimately — just before the high breaks down — haze layer tops can reach to 10,000 feet msl or more. Obviously, the safe answer to dealing with dense haze is to file IFR or postpone flying until conditions improve. A VFR climb to VFR-on-top conditions will not only be illegal, it could also take a long time (remember density altitude effects) and leave you exposed to traffic conflicts at a time when your forward visibility is practically nil.

Turbulence and wind shear

As we all know, turbulence isn't something confined to the summer months. But turbulence caused by localized surface heating most certainly is. The thermals created by this heating can cause light to moderate turbulence, and the inflows and eddies caused by these thermal propagations can also create wind shear problems for pilots taking off or landing. Wind shear is a condition where rapidly fluctuating wind speeds and directions can cause abrupt losses or increases in airspeed. Gusty and shifting surface winds are dead giveaways that wind shear will be a factor when flying in the airport traffic pattern.

The best way to handle wind shear on approach is to fly the final leg with an extra amount of airspeed as a safety margin against wind shear-induced stalls. One rule of thumb recommends adding one-half the gust value to the airplane's recommended final approach speed. So if the surface winds are 15 gusting to 25, add 5 knots to your normal, published approach speed.

You'll also want to keep your crosswind takeoff and landing skills brushed up for those gusty summer days when a front is approaching — or has just passed by. As surface heating warms up the relatively cooler air behind a cold front, expect all kinds of turbulence as the newly arrived air comes under attack from the thermals below.

Enjoy the season

So far in this article and all the rest in this month's summer weather series, the words about summer flying are scary ones. Yes, thunderstorms will be out there, lightning detection and radar have shortcomings, density altitude robs your performance, and sometimes you can't see for the haze. But if you stay well-briefed, keep an eye on The Weather Channel, and use a grain of common sense, you'll realize the other truth about summer flying: Nothing beats it. Enjoy the season. Go to the beach. Go to the mountains. Hitch a ride in an open-cockpit biplane. Fly more often. It is, after all, flying season.