February 26, 2019.   The Guts of a Gust.

The big story this week has been wind. First we had an unusually strong jetstream that was pushing planes across the country, and across the North Atlantic, at record-breaking groundspeeds — as discussed here. Now, powerful low-level winds have been wreaking havoc at airports around both the U.S. and Europe.

The first thing people are asking is whether these two phenomenon are connected. The answer is not necessarily. Hey, it’s winter, and things tend to be colder and windier across the board. Certain patterns and systems do affect both high and low-altitude weather, but with wind there’s usually a pretty clear distinction between the goings on at higher altitudes and those close to the ground. Abnormally strong winds at 35,000 feet do not not, by themselves, foretell abnormally strong winds at 350 feet.

The other thing people want to know is how dangerous windy conditions can be when taking off or landing. Yesterday at airports in the northeastern U.S., gusts were topping 60 knots, causing widespread delays and cancellations. And maybe you caught the video of the British Airways A320 flailing overhead as it attempted a landing in Gibraltar before diverting to Malaga, Spain.

Most of the logistical snarls we’re seeing are due to runway alignment. Aircraft take off and land into the wind. There’s some flexibility here — crosswinds and even tailwinds are tolerable to some extent — but if the wind is blowing hard enough, certain runways become unusable. This can vastly restrict the number of arrivals and departures an airport can accommodate. That’s mostly what we’ve been dealing with. If your flight is delayed or canceled, it’s not because conditions are dangerous. It’s because of congestion.

When the wind comes at you from the side, that’s a crosswind. All planes are certified to a maximum crosswind component, and these tend to be quite high. The 767 that I fly has a max allowable crosswind component of 40 knots.* With respect to headwinds, meanwhile, it’s the more the merrier, from a pilot’s perspective. A strong headwind keeps your groundspeed low and allows you to use less runway. Rarely, though, is the wind perfectly stable or consistent from a particular direction. As anybody can feel when outside on a blustery day, it can shift around and intensify unpredictably. Seldom is this a problem; planes are surprisingly stable even in very rough air, and on landing we calculate buffer speeds to account for gusts and small shears. But, every so often, things become so gusty, and so unpredictable, that yeah, conditions can be unsafe. Powerful low-level gusts can bring on windshear or severe turbulence. On landing this can make it difficult to maintain what a pilot calls a “stabilized approach.” There’s no fixed number, in terms of miles-per-hour, when this might happen — but if an approach becomes unstable or windshear is an issue, you’re liable to find yourself diverting to another airport.

As for that British Airways video from Gibraltar, winds that afternoon were reportedly no more than about 20-30 miles-per-hour, which is fairly tame, with no significant crosswind. So why was the plane rocking back-and-forth like that? Most likely it had something to do with the famous, 1,400-foot “rock” that rises nearby. So-called “mountain waves” can form when cold, dense air spills from a peak, alternately sinking and rising again, creating significant turbulence. There’s also an unusual phenomenon called “Von Kármán vortex shedding,” whereby a mountain can spin off alternating, upward/downward whirlpools of air — vortices that may have caught the Airbus as it flew past.

* That’s right, you can land a 767 with a direct crosswind of up to 40 knots. I’ve seen this in the simulator several times, but in the real world it’s something I’ve experienced only once, about eight years ago in Madrid. I can’t remember the runway, but the wind was almost exactly 90 degrees from our left, at 35 knots. It was the captain’s landing, and he nailed it. Luckily the wind was very steady, with only a slight gust factor. Had it been more blustery, we’d probably have needed a different runway.


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