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.

By Karsten Shein
Comm-Inst, Climate Scientist


Bombardier Challenger 300 departs ORL (Exec, Orlando FL). Gusts, crosswinds and other surface winds should not be underestimated when operating near the ground.

About a month ago, the evening news broadcast I was watching played an amateur video (www.you­tube.com/ watch?v=mMvLuUJFHYk) of commercial aircraft landing at DUS (Düsseldorf, Germany) in high winds. These aircraft—Boeing 777s, Airbus A340s and other heavies—were keeping pretty severe crab angles right down to the concrete due to substantial crosswinds over the airport.

A Metar in effect at the time (Jan 5, 2012) read, "EDDL 051150Z 29025G40KT 9999 FEW030CB SCT040 09/01 Q0989 TEMPO TSRA," which gives Runways 23L/R at DUS a crosswind component of 21.6 kts (gust crosswind of 34.6 kts). In fact, peak measured gusts associated with nearby thunderstorm outflow were around 50 kts. Although the crosswind limits of some of these larger jets were not reached (the Boeing 777 limit is 38 kts), those speeds would have spelled trouble for many business or regional jets that have limits in the upper-20-kt range.

While no aircraft were harmed in the making of the video and the pilots exhibited a great deal of skill, the fact that none of the aircraft could maintain a static centerline approach and the tires all billowed thick puffs of grey smoke on touchdown spoke clearly to the vicious forces at work. What's more, there are dozens of similar videos posted on YouTube, including imagery of several aircraft that struck their wingtips as they transitioned from the crab to ailerons in an attempt to stay over the runway.

Unfortunately, there are even more discussions in the NTSB files of pilots who chose not to go around or divert, but rather attempt a landing in high crosswinds only to wind up cracking a wing, collapsing the landing gear or simply being blown off the runway.

Fortunately, most of these landing accidents result in only minor injuries, but other accidents involving adverse or unexpected winds, such as encounters with downbursts and mountain waves, have been fatal. What's more, although we may think of icing, thunderstorms or fog as leading factors in aviation accidents, it is the unseen force of the wind that leads the list of contributing weather factors.

Winds play a very important component in all aspects of aviation. They determine what runway we'll use for takeoffs and landings. In fact, except where an airport's layout is limited by the surrounding environment, runway orientations are determined primarily by the prevailing winds in order to minimize the occurrences of significant crosswinds. Winds are also a primary factor in planning our cruise.

We need to know the winds aloft to figure out our groundspeeds, relative headings and fuel burn. Regions of the sky where winds change direction or speed over short distances are usually avoided due to the likelihood of clear air turbulence.

For similar reasons, pilots tend to steer clear of corridors beneath or downwind from mountain ridges. But, while we can easily avoid cruising turbulence-prone areas of the sky, we still must take off and land, so a healthy understanding of what winds await is critical to our safety.

Of winds and runways

Cessna 172 lies inverted on a taxiway at CIN (Carroll IA) after being flipped by what was reportedly a 60-kt crosswind gust. The Metar just prior was reporting winds from 310° at 34 kts gusting to 48. Even when taxiing, pilots must remain aware of the wind speed and direction, and recognize that some winds may simply be too strong.

One of my fonder memories of flight training, though not one I would advocate anyone trying, occurred shortly after a front had passed through the area. The winds were howling directly down the runway at over 20 kts. While I preflighted my aircraft for the morning's lesson, I watched in amazement as one of the local pilots taxied out in his Piper J3 Cub, gave it full power and became airborne in what seemed like just a few feet.

More astonishingly, as he reached a few hundred feet, he reduced power, and the wind actually blew him backwards as he descended. As he neared the ground, he reapplied power and brought the aircraft to a touchdown on almost the same spot from which he had lifted off. He did this twice more before departing the area—apparently an enjoyable novelty for him. My instructor told me later he was an aerobatic pilot who did that every so often when the winds were just right.

While none of us will be practicing stationary touch-and-gos in our Gulfstreams or Falcons, a strong and steady wind directly down the runway is also a good thing. Since the lift the wings produce is entirely a function of the velocity of air flowing over and under them, the influence of a strong headwind is identical to a similar speed of forward motion by the aircraft.

As the Cub pilot knew, his aircraft was already "moving" through the airflow at perhaps 20–25 kts, which meant he just needed another 10 or so to become airborne. That 10 kts of groundspeed could be achieved in a far shorter distance than had there been no wind and the Cub would have needed to gain 30–35 kts of groundspeed before lifting off.

For all of us, the stronger the headwind, the shorter the takeoff distance. A rough rule of thumb for this is that your takeoff (and landing) distances will be reduced by about 1% for every 1 kt of headwind. So a 2000-ft takeoff run would only require 1800 ft in a 10-kt headwind (2000 x 0.9). Unfortunately, if you find yourself in the rare situation of needing to take off or land with a tailwind, the distance penalty is increased to about 5% per knot.

So a 2000-ft landing with a 10-kt tailwind would require at least 3000 ft (2000 x 1.5) of runway. For this very reason, tailwind takeoffs and landings should be avoided if the winds are more than 10 kts.

Crisscross

Bombardier CRJ lands at SDR (Santander, Spain) in an 80° crosswind gusting to around 40 kts. When faced with strong and gusty crosswinds, plan to make several approaches before a lull in the gusts lets you land within your aircraft's operating envelope.

Large airports in wide open spaces are wonderful because they often have multiple runways that ensure that at least one of them is oriented more or less into the wind at any given time.

This is because many of these airports have existed since the dawn of aviation. Initially they were simply large grass fields, and aviators would need only orient themselves into the wind for takeoff and landing. But, being airports, wind data was collected.

When it came time to pave runways, the airport planners looked at the wind roses to figure out the best orientation for primary and secondary runways. Unfortunately, one drawback to this sort of planning is that the runways are oriented to the prevailing wind flow, which usually means the lower speeds of the normal wind patterns.

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