Caution, Wake Turbulence

Hour 66

"What is that thing?" a disembodied voice asked through my headset.  It took a moment before I realized the voice was coming from inside the plane, not over the radio. Gary, as usual, was instructing me from the co-pilot's seat.  I'd forgotten that my friend Alberto was also sitting behind me.  He'd wanted to do some aerial sightseeing and had always loved airplanes so I'd invited him to ride along on one of my lessons.

"I don't know," I said. "But it's big."

We'd flown from Santa Monica to Long Beach where we were waiting to take off again.  An enormous plane was landing -- some sort of military transport, I guessed, so massive that it was hard to believe it was even capable of flight.  Yet there it was, coming down the last few feet and kissing the runway right in front of us.

We overheard the tower controller giving an instruction to a small airplane like ours that was still in the air: "Bonanza 65T, runway 30, clear to land. Caution, wake turbulence."

A wake vortex made visible in
a NASA test using colored smoke.

Wake turbulence is a disturbance in the air that is generated by any wing that's flying.  Wings go up by forcing air down.  The process generates swirling vortices at each wingtip that slowly fall to the ground, a little bit like the wake that spreads behind a speedboat planing through water.  Unlike a boat wake, wingtip vortices are usually as invisible as the air they're in. The heavier the plane, and the slower it's going, the more air must be sent hurtling towards the ground to keep the plane aloft -- and the larger these invisible menaces become.

A small plane caught in the wake of a much larger one can suffer serious consequences.  The plane may roll uncontrollably.  The FAA's Aeronautical Information Manual, in its typically understated style, warns that "in rare instances a wake encounter could cause inflight structural damage of catastrophic proportions." Pilots are trained to avoid this -- for example, by taking off before and landing later on the runway than a much larger plane that has just taken off or landed.  Controllers warn small-plane pilots of larger aircrafts' nearby wakes using the standard phrase, "Caution, wake turbulence."

"I was at an airport a few months ago and a Caravan was coming in," Gary recounted as we waited for our takeoff clearance. A Caravan is a plane several times bigger than our little Cessna 172, but still much smaller than a heavy transport plane like the one we'd just seen land.  "A big fella like that was landing before him.  The Caravan pilot decided to abort the landing, and the controller said on the radio 'Good idea.'  Wake turbulence can be scary stuff.  No one will think less of you for trying to avoid it."

It's one thing to hear stories about wake turbulence and read about it in books.  The real thing is quite another matter.


Hour 157

The dot-com boom lured many of my Southern California friends up north to Silicon Valley to try and make their fortunes.  Even after the tech bubble popped, most of them stayed. It was always fun to go visit them for a weekend of poker or a birthday party, and in my airplane it was a just two-and-a-half hour flight up through California's wine country.  One fine weekend afternoon I was on my way up for a visit.

Silicon Valley
photo by Michael

True to its name, the Valley is a long, narrow basin just a few miles wide sandwiched in between two parallel mountain ranges.  Aircraft arriving from the south typically fly straight up it. On that day, I was no exception, flying at 4,500' as I made my way north-westbound on my way to the small Palo Alto airport.  The air was clear and calm -- one of those days that makes flying feel so easy and carefree.

I was talking to air traffic controllers at NorCal Approach. Even though it wasn't required, I always like to have an extra pair of eyes on me to warn me if I'm about to screw up.  The airspace is complex and there are big jets coming in and out of the San Jose and San Francisco airports... better safe than sorry!

"Cardinal 97H, traffic at your 6 o'clock will be passing 500 feet above you, a 737 restricted to 5,000," the controller told me.  "Caution, wake turbulence," she added perfunctorily.  I'd heard that warning so many times in the past year it had virtually lost its meaning.

Photo by Bill Abbot

I leaned forward and looked up to see an awesome sight: a big commercial jet painted with Southwest Airlines livery passing right over my head!  It was fantastic getting a close-up view of such a big machine in motion. They were flying the same track I was, right up the center of the valley on the approach into San Jose. I got a nice long look as he lazily pulled ahead of me.  I had a big, dumb grin on my face, thinking this is one of those cool sights that only pilots get to see.

My reverie was abruptly interrupted by the realization that the plane was in a 20 degree bank to the left.  I turned the yoke to the right to correct it.  Then something shocking and terrifying happened: absolutely nothing.

I've experienced almost every form of both elation and fear while flying airplanes over the last decade, but there's nothing more frightening than not having control of the airplane.  All the fancy-pants aeronautical decision making in the world isn't worth much if the airplane just won't do what you tell it to do!

I kept turning the yoke further and further to the right but the plane continued rolling left.  When I'd turned the yoke to its limit, the plane was still banked about 30 degrees.  We were in a standoff: the plane was not rolling any further left, nor was rolling back to the right.  It seemed quite indifferent to my control input, stubbornly flying with the right wing up as if it were stuck.

I realized I must be inside a huge, swirling wake vortex from the jet that had just passed overhead. Since the jet had been only 500 feet above me, the vortex hadn't had much time to disperse.  It was strong since the jet was about to land and was going slowly. And since he'd been flying parallel to me, I was flying straight down the long axis of the vortex like I was in a spinning tunnel. 

The controls abruptly returned to normal as my plane exited the vortex. Since the yoke was still turned all the way to the right, the plane rolled right quite enthusiastically, putting me wings-level in an instant.  I straightened the yoke and the plane kept on flying as if nothing had happened.

It felt like an eon, but the whole encounter lasted perhaps two or three seconds.  I'm lucky I was able to react so quickly.  If I'd been, say, trying to pee, the plane might have gone wings-vertical or even inverted.  Lesson learned: now, when I hear "Caution, wake turbulence," I keep my hands on the yoke!