WX BRIEF

Knowing how to handle adverse blows on approach and landing

Strong winds and unexpected gusts are leading factors in weather-related ground accidents.


Dust kicked up by the gust front from a strong line of storms as it approaches IND (Intl, Indianapolis IN) is visible on this radar image and is several miles ahead of the main cells. This gust front produced winds in excess of 60 kts and killed several people when it collapsed a concert stage at the Indianapolis Fairgrounds.

Extreme winds, such as may pass through with the occasional storm system or behind a front, may well be completely misaligned with any of the airport's runways, leading to a substantial crosswind.

In places where space is a limiting factor, airports often do not have the luxury of even orienting their runways relative to any local wind patterns. Coastal and mountain airports, in fact, are often oriented perpendicular to the prevailing flow, meaning a 90° crosswind just about every day. However, at most of these airports, the crosswinds are usually relatively light and manageable, although strong crosswinds are possible at just about any airport in the right circumstances.

So how do you figure your crosswind? A crosswind chart, such as that found in every operating handbook, will give you the right answer every time. Short of referring to the POH, however, you can use your E6-B or other flight computer.

There are even apps for your smartphone that will help you figure it out on the fly (eg, CFI Tools Crosswind Calculator). In the absence of a concise crosswind calculation, the crosswind component is obtained by breaking down the wind into its vector components and determining what proportion is crosswind and what is head or tailwind.

First, simply determine the angle between the wind and the heading of your aircraft/runway. The greater the angle, the greater the crosswind. At 0° (direct headwind/tailwind), your crosswind will be zero, and at 90° the wind will be entirely crosswind.

For the angles in between, you can obtain a reasonable approximation of the crosswind speed by using the rule of sixths. For every 10° of angle from the aircraft's heading (or runway) line, the crosswind will be that many "sixths" of the overall windspeed.

So, for Runway 18, when faced with a wind from 150° at 20 kts, the difference is 30°—which makes the crosswind 10 kts (20 x 3/6 or half). At 60° pilots can get a better estimate by using 90% rather than 100%, but at angles greater than 60°, 100% (6/6) should be used, meaning that, at those angles, your crosswind is within a knot or so of the full windspeed.

The next question, though, is whether your aircraft can handle the crosswind. Turn again to your POH and take a look at the maximum demonstrated crosswind component. Unless you have an aircraft built before 1962, that number is set by FAR 23.233 at 20% of Vso.

This is a fairly conservative speed—it means, for example, that a Learjet 35A with a Vso of 100 kts has been demonstrated by the manufacturer to handle a direct crosswind of up to 20 kts safely—and, in general, your aircraft's controls can handle a crosswind above that limit.

Because of the relatively low crosswind limits imposed on some aircraft by their stall speeds, I have known some pilots to use a rule of thumb that sets their crosswind limits to 65% of Vs1.

(Many GA pilots use this as a go/no go criterion for determining whether to pull their airplanes out of the hangar.) But, in exceeding the manufacturer demonstrated limit, you are becoming a test pilot and you can't blame the manufacturer if you wind up in the weeds next to the runway. Also, the ensuing enforcements against you will probably be quite unpleasant.

Dust kicked up by strong vortices associated with a thunderstorm gust front. Surface winds in a gust front can gust upwards of 100 kts and can hit a landing or departing aircraft with little or no warning. If possible, waiting until storms move on or dissipate is the best course of action to avoid a loss of control at low altitude.

Another factor that must be considered when faced with a crosswind is the condition of the runway. Those crosswind performance numbers were based on landing on a nice, dry, paved surface. Add a little water (the universal lubricant), or its frozen cousins snow and slush, and even a minor sideways load on landing can send you into a dangerous skid.
Finally, the moment before touchdown in a crosswind situation is a critical one.

Most pilots will tend to transition from a crab to a slip in order to align the aircraft with the runway and minimize any side loading on the landing gear. The problem is that, as soon as the crab is removed, the crosswind starts to blow the aircraft off the centerline. Corrective action is often accomplished by further lowering the upwind wing. And this presents 2 dangers.

The first is that, in a stiff crosswind, the pilot not only has to concentrate on keeping the aircraft lined up, but at the same time has to keep an eye on the lowered wing to ensure it is not too low. In 2-pilot cockpits, the pilot not flying the aircraft should be monitoring the bank closely and call out if it exceeds an angle that would put the wingtip in jeopardy, allowing the flying pilot to concentrate on landing the aircraft.

The second danger is that high crosswinds tend to be relatively gusty. Many of these winds are born of nearby storms or fronts, and gusts are the physical manifestation of invisible vortices that have spun off of the flow due to shear.

They can be tiny—no more than a sidewalk leaf tornado—or a kilometer in dia­meter. If a pilot has encountered an eddy near the touchdown zone and compensated for it only to have it dissipate, the response is usually a rapid turn toward the wind followed by opposite aileron to correct.

If this is done too abruptly, it can cause the downwind wing to strike the runway. Similarly, a pilot who has lowered the upwind wing to maintain a slip might suddenly encounter a strong gust that both pushes him off centerline and pushes the upwind wing closer to the ground.

All of these conditions are reasons to be very wary of crosswinds that exceed your aircraft's maximum demonstrated limit.

Storms and low level shear

Another way in which surface winds present a terrible danger to pilots is in low-level windshear (LLWS). Windshear is simply a change in wind speed and/or direction in space, either horizontally or vertically. We see windshear and the resulting turbulence it creates at all flight levels. Aloft, we call it clear air turbulence (CAT), near mountains it is rotors, and around airports it is called a downburst or LLWS.

A number of aircraft have been lost to LLWS over the years. One of the most common causes is thunderstorm outflow. Most of us know well enough to avoid penetrating a thunderstorm, but many pilots continue to fly beneath or next to them as they either depart or approach an airport.

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