Wx Watch: Windwise

Scoping out surface winds

April 1, 2005

I once did a study of general aviation weather accidents for the AOPA Air Safety Foundation. It had many goals, but one objective was determining which weather phenomenon caused the most accidents. Was it thunderstorms? No (just 20 or so accidents per year, on average — although two-thirds involved fatalities). Icing? Nope (about 60 accidents per year, with almost 30 percent being fatal). No, the hands-down winner was a category defined as "wind-related" and consisting principally of loss-of-control situations during landings in gusty, crosswind conditions. These accounted for 2,835 accidents in the 1982-to-1993 time frame — 48 percent of all 5,894 weather-related general aviation accidents!

Here are some more details about this "typical" accident scenario:

  • Half happened in the landing phase. Only 22 percent happened during takeoffs.
  • 8 percent of these accidents were fatal; 71 percent involved no injuries.
  • Private pilots with more than 1,000 total hours had 35 percent of the accidents; those with 100 to 500 hours had 31 percent; and those with fewer than 100 hours had 18 percent.
  • 95 percent of accidents happened in day-VFR conditions, and on weekends.
  • In 68 percent of the cases, there was no record of a weather briefing.
  • Tailwheel airplanes were involved in 371 recorded wind-related accidents.

A manual search of archived surface analysis charts later revealed that many of these accidents occurred in the time frame ranging between a day before and a day after a cold front's passage.

What can we infer from all this? Apart from the usual assumptions about crosswind landing skills, we can easily concoct a typical scenario. A pilot who limits his or her flying to visual conditions learns that the week's lousy weather will give way to improving conditions on the weekend. With pattern practice in mind, the pilot heads for the airport for some touch and goes. If a weather briefing was skipped, it was for good reason: It will be a local flight, and under FAR Part 91.103 a briefing isn't required for flights "in the vicinity of an airport...."

That may be the letter of the law, but it's certainly not its spirit. Common sense dictates that you check the weather before any flight. When it comes to dealing with adverse surface winds, there's some special information that's worth remembering.

Warning signs

Cold fronts — especially fast-moving ones — can create the strongest and gustiest surface winds during their passage. There will be a wind shift from the southerly to the westerly quadrants of the compass, and you should be ready for it if a cold front is predicted to affect your flying. Be ready to change runways to suit the prevailing winds, or divert to another, alternate airport that has more favorable runway alignments. Airport designers are supposed to do surveys to determine the most common prevailing wind directions, but complications can arise. Nearby housing, property limitations, local terrain, utility lines, and other obstructions (some of them political) can give some airports less high-wind-friendly runways than others.

Thunderstorms can always cause strong surface winds. Anyone not keeping an eye on any nearby storm-cell progression is asking for a bad surprise — even if the center of the storm cell is miles away. Downbursts from thunderstorm cells hit the ground, then spread out, causing strong horizontal and vertical winds. When severe thunderstorms are near, you can expect surface winds in excess of 50 knots. In fact, 50-knot surface winds are part of the definition of a severe thunderstorm (tornadoes and three-quarter-inch hail are the others).

Fair-weather cumulus clouds can also cause trouble. These are signs of a much milder form of convection, one that works opposite to the dynamics of a full-blown thunderstorm's downbursts and microbursts. Fair-weather cumulus clouds are created by surface heating. The heating sends air upward, and the air cools as it rises. When it cools to its dew point, moisture condenses and a cloud forms. And what do you suppose happens when all that heated air leaves the surface — let's say an airport's surface, with all its nice, warm asphalt? The departing air leaves a vacuum of sorts, and in rushes the surrounding air to fill it. Presto, you have gusty, shifting surface winds! So a really nice flying day can, in actuality, prove to be a challenging one — even if no fronts or thunderstorms are present. The antidote here is to fly early in the morning or late in the day, when surface heating and convective currents are at their weakest. Unless, of course, you need that high-wind landing practice.

As a general rule, the stronger the wind and the higher the gusts, the greater the turbulence near the surface. That holds true with or without fronts or thunderstorms, and it's especially true if terrain or buildings and other structures are in the area. These act as obstructions in the wind flow, and cause eddies much like those seen in boulder-strewn rapids. Better have quick, correct control inputs of the right duration and deflection when flying down final.

Low-level wind shear (LLWS) is another possibility when fronts, thunderstorms, thermal convection, and strong winds hold sway. LLWS implies winds that quickly change in both strength and direction at multiple altitudes. Halfway down final, and everything seems right with the world. Approach the runway threshold, and a crosswind gust hits you from the right, causing you to lose alignment with the extended runway centerline, balloon upward, and gain airspeed. You correct, but the crosswind abruptly stops and you lose altitude and airspeed. Then, inches from the runway, the wind changes again, causing yet more weathervaning and yet another balloon. That's a typical wind-shear encounter in a piston single. We're lucky because the airplanes most of us fly respond quickly to power and thrust inputs, so altitude loss is minimal — if you are prompt in responding. Pilots of large Transport category business jets and airliners have a more difficult time with wind shear because their turbofan engines are slow to spool up from reduced power settings, and slower to accelerate. A sudden loss of headwind component — as might happen in a classic microburst encounter on final approach — can cause a rapid loss of altitude and airspeed in these kinds of airplanes. Several disastrous wind-shear accidents have happened under these circumstances.

Briefing sources

A flight service weather briefer should fill you in on wind hazards, but if you like to check Internet sites for wind information, be sure to look at:

  • Area Forecasts (FAs). These mention LLWS, and also forecast strong surface winds in the six-hour categorical outlook (OTLK) section at the end of each 12-hour VFR Clouds/Wx statement. A "VFR WIND" statement is a tip-off. It means ceilings greater than 3,000 feet and visibility greater than five miles, but also means sustained surface winds or gusts 20 knots or greater for 50 percent or more of the outlook period.
  • TAFs. Wind speeds and directions are posted, and so are anticipated gust intensities. For example, "30025G35KT" means winds should be out of 300 degrees at 25 knots, with gusts to 35 knots. Nonthunderstorm LLWS is also mentioned in TAFs. This refers to shear within 2,000 feet agl. The notation "WS015/27045KT" means wind shear is forecast at 1,500 feet agl, with winds above that altitude out of 270 degrees at 45 knots. This information is derived from pireps, another source of adverse-wind reports.
  • METARs. Wind information on these hourly reports is right up front, and wind direction is reported relative to true north. The difference between wind direction relative to true and magnetic north can be significant in those locations where magnetic variations are large. (The only time you'll hear wind directions mentioned relative to magnetic north: from a control tower, from an FSS (flight service station) airport advisory, from an ATIS (automatic terminal information service) recording, or from an ASOS (automated surface observation system) broadcast. The G (gusts) in a METAR wind report means rapid fluctuations in wind speed that vary by 10 knots or more. Expect moderate turbulence if gusts hit the 25-to-30-knot range. A V (variable) in the wind report means that wind direction is swinging through an arc of 60 degrees or more, so "270V350" translates into "winds out of 270 degrees variable 350 degrees." Other METAR wind notations of interest in challenging landing situations include PK WND, WSHFT, and FROPA. PK WND (peak wind) indicates the time and nature of a peak wind occurrence; PK WND 29050/35 means the peak wind was 290 degrees at 50 knots at 35 minutes past the hour. WSHFT (wind shift) reports a change in wind direction greater than 45 degrees that took place in less than a 15-minute time frame; WSHFT 15 means the wind shift happened at 15 minutes past the hour. FROPA (frontal passage) is included in METARs that are made or augmented by human observers. This means that observers have concluded that a wind shift signaled the passage of a front. By the way, if the wind sensors at a METAR or automated observing station are kaput, then the wind information will be missing from the report.

On the airport

All those text forecasts and reports are great, but the rubber meets the road (sorry, couldn't resist) when you show up in the traffic pattern and get ready to land. At towered fields, you'll be assigned a runway (but don't hesitate to challenge a landing clearance if it puts you on approach to a runway having too great a crosswind component for your airplane or skill level). At nontowered airports, you'll have to make the runway decision yourself.

AWOS (automated weather observation system) and ASOS broadcasts can help you decide on the runway with the most headwind component, as can other direct-observation cues (smoke, tree movement, flags, dust plumes). There's also the wind tee and segmented circle to help you determine the proper runway, as well as the radio check via the CTAF (common traffic advisory frequency) or unicom.

But hey, don't forget the best indicator of all — the venerable windsock. No fancy meteorology or threshold algorithms here, just plain, old-fashioned, real-time wind information — straight from the runway surface. One quick check and you know it all — wind strength, direction, gusts...and second-by-second updates!

Now that spring is approaching full bloom, let's resolve to use all these tools to help us make better adverse-wind landing decisions. Hopefully, none of us will end up on the next weather-accident review. A botched crosswind landing may be the least dangerous of weather-accident categories, but it sure can be damaging to the ego — and the airplane.


E-mail the author at tom.horne@aopa.org.