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Wake turbulence: why pilots stay far behind the big jets

Oct. 07, 2019
5 min read
Wake turbulence: why pilots stay far behind the big jets
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It was a wild ride.

In 2017, a Challenger 604 business jet was passing under a much larger and heavier Airbus A380 over the Indian Ocean. The A380 was flying 1,000 feet above in the opposite direction. (This may seem dangerously close, but a separation of 1,000 feet is standard procedure.) The smaller jet rolled over at least three times, injuring several passengers. Its cabin looked like a bag of Halloween candy dumped out by a kid, and while it managed to make it safely to an emergency landing in Oman, it was written off. The G-forces it endured exceeded the structural limits of the aircraft.

What had happened?

Two words: wake turbulence. The smaller jet had been caught in the vortices generated by the huge A380. Fortunately, pilots actively avoid situations like this one, and wake-turbulence incidents are very rare. But they are serious.

Before and after, aboard the Challenger (Image via Flight Global Services from News Corp. Australia)

I checked in with Chris Brady to learn more about wake turbulence. Brady is a long-time Boeing 737 and A320 pilot, and the man behind the AvGeek haven that is the Boeing 737 Technical Guide.

“Wake turbulence is pretty much exactly the same thing as happens in water when one boat passes through the wake of another, except that on water you can see the wake as waves. In the air you can’t see the wake. However, there is a clue where it might be, and that is to look for the vapor trail," Brady said.

"Flying through wake turbulence can be quite alarming, especially for passengers who can’t see when we have crossed the path of another aircraft and therefore aren’t expecting it," he added. "It feels like a sudden burst of instant heavy turbulence but usually only lasts for less than a second."

The danger is that an aircraft may suddenly and violently roll when passing through the wake.

"Most wake turbulence events result in a roll angle of less than 30 degrees, which is less bank angle than you use during a normal turn. This again shows that a wake-turbulence event is fairly benign but the suddenness of it is what makes it feel much worse," Brady said.

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"When pilots involved in a wake turbulence encounter are asked to estimate the angle to which the aircraft rolled, most over-estimate the actual figure by around 50%, which just goes to show that it feels worse than it is to both pilots and passengers alike!"

Every airplane generates vortices that extend outward and curl upward from the tip of the wing as it generates lift.

Wing vortices: the wake of an airplane (Image via FAA)

"The strength of the vortex is governed by the weight, speed, and wing shape and span of the generating aircraft," the Federal Aviation Administration explained in guidance for pilots. The heavier the aircraft is and the slower it is moving, the stronger the vortices. That means that "heavy" aircraft such as a the Boeing 747 — and of course the Airbus A380, which is even bigger and heavier — are most prone to generating this kind of effect. (As an aside, the Boeing 757 was alleged to have much greater vortices than expected for its size.)

In the video below from NASA, a jet aircraft passes above smoke trails in a visual test of wingtip vortices. You'll see how much turbulent air is generated and how slow the vortices dissipate.

The telltale sign for pilots like Brady are the vapor trails of other aircraft. But eyeballs aren't the only tool they use.

"Apart from looking out for the vapor trails of other aircraft, [air traffic control] helps us as well," Brady explained. "Fortunately wake vortices dissipate quite quickly, so part of the function of ATC in keeping aircraft apart is not just so that they don’t collide, but also so that they don’t encounter each other's wake."

Brady looks out for vapor trails as a visual clue (Image by Getty Images)

He explained that Air Traffic Control uses tables published for wake vortex separation minima to keep aircraft a safe distance apart.

"These tables consider the size of both the preceding aircraft and the size of the following aircraft and give a safe distance in either miles or time depending upon the situation," he said. The A380 is given a spot all on its own in the tables, together with the Antonov An-225 cargo plane. Those two are so much bigger and heavier than anything else flying that they are referred to as "Super" by air traffic control, not just "Heavy."

Source: FAA, research paper by GE Global Research

"As a general rule, narrow-bodied airliners like the Boeing 737 or A320 have to leave a one-minute gap between each other or a two minute gap if following a wide-bodied airliner," Brady said.

Mike Arnot is the founder of Boarding Pass NYC, a New York-based travel brand, and a private pilot who studiously avoids wake turbulence.

Featured image by Getty Images