Wind Really Matters Over the Runway

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When the FAA changed its weather reporting format to match the international metar standard several years ago many of us with more than a few years in the logbook scoffed. For one thing, what was a "metar" anyway? We had grown up, and grown old, with the sequence report that used to clatter out of old low-speed teletype machines at flight service stations spread across the country. The word "sequence" made sense because it was a sequential observation of the weather conditions over the airport on the hour, or when rapid changes made a special report useful.

Metar is an acronym of some international terminology having to do with meteorology, I think. I don't really remember. It contained new abbreviations for weather phenomenon such as the letters BR for mist. What's up with BR? Derived from French, I think. And low ceiling heights were replaced with a report of "vertical visibility" in hundreds of feet. We always thought of visibility as a quantity you measured looking straight ahead, not up. If you're looking up instead of ahead when landing, it's not going to work out very well.

But most galling about the metar format was the change in order -- and apparently change in emphasis -- of the observed weather conditions. For many decades American pilots were used to seeing the ceiling and visibility come first in the sequence report. That is what we all talked about, and wanted to know. Two hundred and a half was the sequence report that separated the real pilots from others. That meant the ceiling was 200 feet and visibility one half mile, the typical minimums for an ILS approach, and ceiling and visibility always came first in the report.

The international aviation authorities who developed the metar format many years before the FAA adopted it had a different priority -- wind. The first item in a metar, after the airport identifier, is wind direction and velocity. If it is gusting, that value is reported after the speed of the steady state wind. And if the wind is variable in direction, the range of variation in degrees is also reported. Clearly wind is a big deal in the metar.

In the old sequence report, wind got the ranking it deserved; at least pilots at the time thought so. We worried most about ceiling and visibility and that was first, followed by the reason for any visibility restriction such as haze, fog, rain, snow, dust, smoke and so on. After that in the sequence report came the atmospheric pressure in millibars, information of little use to most pilots in the U.S. with altimeters that could only accept barometric pressure in inches of mercury (hg). Next was the temperature and dew point, and it was in degrees F. Temperature aloft forecasts were in C, but on the surface in the sequence report the temperature was in Fahrenheit so we didn't need to translate to guess how comfortable we would be after landing. Mostly we worried about the spread between the temp and dew point because a small spread is a good indicator of the likelihood of fog forming.

Finally, after all of that came the wind direction and speed in the sequence report. The only thing less important than the wind was apparently the altimeter setting -- that was abbreviated so you needed to translate -- and remarks, if there were any from the observer. Wind was bringing up the rear in the worries of pilots who relied on the sequence report for all of those years.

But, looking back, I have to admit those international types who created the metar got it right. Wind belongs at the front, and should be foremost in a pilot's thinking and planning when considering takeoff or landing. Yes, you need enough ceiling and visibility to see the runway to land, but it is wind that will have the final say on how that landing works out.

I started thinking about all this while reading a metar on a particularly breezy day shortly after reviewing a bunch of preliminary accident reports. Without question the most common accident scenario for all airplane types involves running off the end of, or side of, or hitting short of, the runway. Often these accidents bruise only the pilot's ego and bend his airplane, but there are also many disasters, particularly in larger and heavier airplanes. And in many of these accidents strong and gusting winds are an important factor.

It's now popular for aviation safety experts and pilots to discuss setting "personal minimums" that restrict a pilot's operations to something higher than FAA minimums. These recommendations invariably center around ceiling and visibility for a VFR pilot or a new IFR pilot, or a pilot new to a type. That makes a lot of sense because you have to be able to see terrain to avoid it while flying VFR. Under IFR you need to be competent as an instrument pilot to fly into a position to see the runway with low ceilings and visibility, so a higher personal minimum gives you a little more margin for error. But I have not seen wind included in these discussions. It's as though wind doesn't matter as long as you fly the approach successfully and see the runway. It's that old sequence report mentality, I think. But wind should be near or at the top of every pilot's operating concerns because it will greatly affect every takeoff and landing no matter how bad or good the visibility, or how high or low the ceiling.

The personal minimum implies that it is the experience, skill or currency of the pilot that matters, because under the concept some can have lower minimums than others while flying the same airplane. A pilot must be able to correctly, and fully, use all flight controls to defeat the effect of strong winds, and recent practice and total experience help one do that. But the best pilot can only do so much to counteract strong and gusting winds before reaching the limitations of his airplane. In the end there is such a thing as too much wind for every airplane and pilots need to know what that limit is and approach it with caution.

The greatest factor in limiting the ability of an airplane to handle strong wind is stalling speed, which is largely a function of wing loading. Obviously, for an airplane to remain safely on the pavement whether during taxi, takeoff or landing, it must not be flying. But many light airplanes have stalling speeds low enough that strong gusts combined with the apparent wind created by even slow forward movement can cause the wing to momentarily lift, and then stall again. A pilot can position the controls against the wind, but if the airplane actually flies momentarily in a gust there is nothing left to do but hope for the best.

The impact of wind on light airplanes is becoming apparent to the people at Avemco Insurance, who have studied claims reports on the LSAs they insure. A huge majority of the accidents have happened during takeoff or landing on a windy day, usually with a pilot who has logged considerable time in a larger and heavier conventional piston single at the controls. The required low stalling speed, and thus low wing loading, of the LSA means it can be tossed around by a gust that wouldn't be a problem in say a Bonanza or Cessna 210. The pilot who has plenty of skill and experience to handle those conventional piston singles in a breeze may suddenly find himself out of control authority and ideas in the LSA.

Another wind limiting issue for many light airplanes is the angle of attack of the wing while the airplane is on its landing gear. A taildragger is the most obvious case where its nose is pointed skyward and the wing angled into the breeze ready to turn any gust into lift. Taildragger pilots are extremely aware of this situation and are the first to pay attention to the windsock, and usually the first to keep the airplane tied down when the breeze kicks up. But most piston airplanes with tricycle gear sit with at least a little nose up attitude so the wing is ready to lift in a gust. That means during takeoff roll a gust can momentarily lift the airplane before it accelerates to sustained flying speed, or can toss it back into the air during landing rollout. In either situation the pilot is left with little control authority because airflow over the control surfaces is low, and the airplane will stall when the gust subsides. In that situation the airplane drops back onto the runway, probably not aligned with the pavement, and it darts for the ditch, or bounces and starts the process all over again. Even with the ace of the base at the controls there is little that can be done to salvage the situation other than to recognize that the airplane is up against its wind operating limits and avoid the conditions.

A typical jet is able to operate in much stronger winds than piston airplanes, or even most turboprops. Among the jet advantages in the wind are high wing loading and stall speed, negative angle of attack while on the ground, and large and effective ground spoilers that dump residual lift from the wing.

The higher stalling speed of a jet forces its pilot to approach for landing, or accelerate on takeoff roll, at a higher speed. That means a gust is a smaller percentage of the actual airspeed of a jet than for a lighter airplane. The greater mass of the jet, and higher drag from its flaps and high lift devices, make it more susceptible to wind shear because it takes longer to accelerate when a gust subsides or shears to a new direction, but the wind can't toss the heavier airplane around as much as the light airplane. The wind can only work on the wing area it can impact, and if every square foot of that wing is lifting say 20 pounds, as in a typical piston single, the wind can do a lot compared to hitting the square foot of a jet that is lifting 80 or 100 or more pounds.

Jets sit at a negative angle of attack on the ground because they must be able to remain firmly planted on their wheels, so the brakes are most effective during an aborted takeoff when it must stop on the remaining runway, or when it is braking to stop after its much faster touchdown speed. That means jet pilots have good steering authority during the takeoff roll on a windy day, and as soon as they lower the nose on landing the angle of attack of the wing goes negative and lift is essentially gone. The hard part for the jet pilot is to get the airplane over the runway at the target approach speed, more or less aligned with the center line, and get it down. Unlike the pilot of the light airplane, the wind isn't going to toss the jet back into the air, though it can sure make directional control a challenge during the rollout when jet pilots earn their pay.

The third big advantage of jets in the wind are big ground spoilers. The spoilers -- which deploy automatically on all larger jets -- dump any remaining lift from the wing, and by design, actually push the airplane down on the pavement for more effective braking and directional control. On many jets the spoilers deploy when the main landing gear wheels start to spin. You can bounce a jet after touchdown, but it won't last long, and it won't bounce again if it has autospoilers because there won't be wing lift to sustain it. In contrast, a landing bounce in a light airplane on a windy day can become divergent and actually lead to a bigger bounce as the airplane stalls and falls and then repeats the process to an ever greater height. Anybody with experience in Cessna taildraggers with their spring steel main landing gear legs has bounced through what is inelegantly called a "crow hop," for obvious reasons.

When it is really blowing the wind is never steady down the runway, unless you are landing on an aircraft carrier or some remote island surrounded for miles by open water, because at any other airport the terrain around the runway distorts the wind. The wind friction effects of the terrain around a runway vary greatly and the more hills, valleys, trees and buildings there are the more the wind will be disturbed. That means every windy landing will have at least some crosswind component and every airplane has a crosswind limit. Actually, it's not an operating limitation like maximum airspeed or weight, but there is a point where full corrective controls are applied in a crosswind and the airplane is still blowing sideways and there is nothing the pilot can do except try to go around.

Runways located in really rugged terrain are particularly risky on a windy day. Large mountains and deep valleys not only cause the direction of the wind to vary drastically, but they can create powerful up- and downdrafts that may overcome the capability of a light airplane. Strong wind, light airplanes and mountains -- even relatively low ones such as those found in the eastern United States -- are a terrible combination and must be avoided.

Large transport airplanes can handle direct crosswind components of greater than 25 knots for all of the reasons noted above. But the demonstrated crosswind of a light airplane, which is the strongest crosswind handled by test pilots during certification flying, is less, and should be approached with caution. The demonstrated crosswind advice you find in the POH is an after the fact report. The test pilots flew on a windy day, landed, and then calculated the highest crosswind during a successful landing. They don't report how many times they went around, or if the airplane could handle one more knot of wind, or if the wind was reasonably steady or gusting. But you can be pretty sure that the demonstrated crosswind reflects the actual limits of the airplane and you should treat that number with respect and build in a large margin for gusts and less than perfect pilot technique.

We are now in the windy season in the Northern Hemisphere and I expect the gusts will take their usual toll on airplanes taking off and landing. Mix in a slippery runway contaminated by snow, ice or rain and the ability of the airplane to handle strong winds goes down dramatically. But we have the tools to avoid a windblown runway accident and they are right there, front and center in the metar. Wind matters on every takeoff and landing, and if we are going to reduce the terrible toll of runway accidents, pilots need to recognize how much wind their airplane, and they, can handle and stay away from winds any stronger than those limits.