I wondered, too, about wake turbulence hazards. SNA is where a fatal wake turbulence accident several years ago led to a change in air traffic control in-trail separation standards. That accident involved a corporate jet that was spun out of control by deadly wake turbulence while following a 757 too closely on approach. How do the instructors and students flying 152s and other light aircraft deal with the very real threat of wake turbulence from the many airliners that operate out of SNA?
The answer is training. Light aircraft operating off the short parallel runway at SNA fly angled departure and approach paths to minimize proximity to vortices streaming off airliners' wingtips. SNA tower controllers also are sensitive to the potential for wake turbulence encounters because of the mix of traffic and the close spacing of the parallel runways. For example, a touch and go is considered an intersection takeoff for ATC separation purposes. If an airliner has departed the main runway within two minutes of the time a light aircraft would land on a touch and go, the tower won't authorize it. The pilot must make a full-stop landing, exit the runway, and taxi back for another takeoff.
SNA pilots are familiar with wake turbulence-avoidance procedures because it's a big concern there. Few airports have the mix and volume of flight training, other light aircraft, turbine-powered business aircraft, and airliners - all operating off closely spaced parallel runways - that describe SNA.
Of course, SNA is not the only airport where the pilot of a light aircraft has to be concerned about wake turbulence encounters. All fixed-wing aircraft generate wingtip vortices, the raw material of wake turbulence. It results from higher-pressure air beneath the wing mixing at the wingtip with lower-pressure air above the wing. The vortices rotate outward from the wingtip, and slowly sink and spread behind the aircraft.
Even small, light aircraft leave a twin-vortex trail that can affect an airplane following closely in trail. We were photographing the new Piper Warrior for Flight Training, and were in the pattern for a formation approach behind the airplane carrying photographer Paul Bowen. Rolling out on final, the Warrior flew through the lead aircraft's wingtip vortices and did a rock and roll dance before I regained full control. Though brief and relatively mild, the encounter was a wake-up call about the potential for loss of control due to wake turbulence.
Weight, speed, and wing design determine the nature and extent of wingtip vortices, but in general, heavy airliners produce the strongest wake turbulence. And the worst vortices occur while they are flying slowly and the wings are producing maximum lift - in other words, just after lifting off the runway, and on final approach.
That's why wake turbulence is a big concern at airports where there is a mix of light and heavy aircraft. No doubt, pilots based at SNA can recite wake turbulence avoidance procedures by chapter and verse. But what about those of us who operate primarily out of general aviation airports, and occasionally fly to larger airports with airline operations? What do we need to know to stay safe?
That's what I was thinking sitting in the left seat of my Cessna 172 awaiting takeoff clearance at Memphis International. It was a busy weekday morning, and I felt like a tiny hummingbird caught up in a flock of giant, hungry condors, all of us waiting to launch into a brilliant blue sky.
At times like these it helps to know something about the behavior of wake turbulence vortices, and about ATC rules regarding wake turbulence avoidance. Sure, it's the tower controller's job to know the rules and abide by them, but remember, you're the one in the little airplane about to take off or land behind the big jet. You're the pilot in command.
Two minutes from the start of the heavy jet's takeoff roll - that's the basic separation standard controllers use when clearing a small aircraft for takeoff behind a departing heavy aircraft. The rule applies when both aircraft take off from the same runway or from parallel runways separated by less than 2,500 feet - as at SNA. The wait time increases to three minutes if the small aircraft makes an opposite-direction takeoff after a heavy jet departure or low missed approach.
If you're not comfortable with a two or three minute wait because of conditions, it's your prerogative to ask for more time. You also can ask to waive the wait time, unless you're making an intersection takeoff.
Why do such a thing? It's difficult to imagine a circumstance where you'd be in such a rush that waiting two minutes or more would be a huge problem. Perhaps an isolated shower is approaching that could cause the airport to go IFR, and you'd like to be on your way before it arrives. If you can take off and turn before flying through the departing jet's flight path, you may be able to safely waive the two-minute wake turbulence departure restriction. But beware the blast from the departing jet's engines. It can toss a small airplane like a tumbleweed before a desert gust. Why not just take the two-minute wait.
Wingtip vortices sink at a rate of several hundred feet per minute. If the airplane is close to the ground, as on takeoff and landing, the vortices move laterally over the ground at a speed of two to three knots. Atmospheric turbulence helps break up wake turbulence. Based on those characteristics, the worst-case scenario would be a no-delay intersection takeoff behind a jumbo jet on a relatively calm day with only a slight quartering tailwind.
Taking off from an intersection instead of using the full length means you could rotate beyond the point the big jet rotated, which could put your airplane squarely in the path of the wingtip vortices slowly descending behind the climbing jet. A slight quartering tailwind would tend to cancel out the upwind vortex's lateral and backward movements and hold it over the runway. That's bad news for you.
In this case avoidance is relatively easy. Use the full length of the runway, or better yet, ask to use the runway facing into the wind, if one is available.
You probably won't be making many takeoffs in a light aircraft behind a jumbo jet. But if you plan to fly into airports used by jets, especially heavy airline-size jets, be sensitive to the potential for wake turbulence, and be prepared to use ATC separation standards and flying techniques to avoid it. Visualize wingtip vortices - tight, conical tornadoes - streaming out from behind an aircraft. If you understand how they behave and visualize where they might be, you can adjust your procedures and flight path to avoid them, just like the wake turbulence-savvy students and instructors at SNA.
Related Articles
