We've all heard wake turbulence warnings from ATC during our takeoffs and landings behind large aircraft. The dangers of wake turbulence aren't limited to takeoff and landing, however. The departure, en route, and descent phases of flight pose a unique risk when you're following aircraft miles ahead of you that you can't even see.
A Sudden 60 Degree Roll
The following NASA ASRS report describes what happened when a Falcon 2000 private jet followed a Boeing 777 on a STAR into the Washington DC area:
We were descending on HYPER7 arrival in VMC conditions, smooth air, and winds at our altitude were nearly nil. Just as Center called to assign hand-off to POTOMAC TRACON we were jolted by wake turbulence. The airplane suddenly rolled 60 degrees to the right. The SIC (pilot flying) disconnected the autopilot and stabilized the aircraft. As quickly as the encounter occurred, it was over.
ATC called to repeat the frequency change. I replied, including report of wake encounter. By the time I switched to POTOMAC they were calling us. I checked-in and acknowledged the new descent clearance (to 10,000 ft). After investigating TCAS (in BELOW mode to see aircraft at our altitude and below that might be on the arrival ahead of us), I asked ATC to identify the type aircraft ahead of us. It was a B777, also flying the profile STAR. After communicating our wake encounter, TRACON issued vectors to take us off the STAR and away from the path of the proceeding aircraft. Minutes later the POTOMAC controller inquired about the incident, flight conditions, aircraft damage, and whether anyone was hurt. That was good.
The heavy 777 was nearly 16nm ahead of us. That seems to be plenty enough separation to avoid a wake encounter. Even with winds aloft nearly zero, the standard rules regarding descent and dissipation of wake did not keep us from getting violently tossed. I am not aware of minimum separation requirements for en route and arrival aircraft, but it was insufficient for these conditions.
How Wingtip Vortices Are Created
What are wingtip vortices? They're swirling tunnels of air that form on your wingtips. High pressure air from the bottom of your wing escapes around the wingtip, moving up towards the lower pressure area on the top of the wing. This movement creates a vortex or tunnel of air, rotating inwards behind the wing.
They're strongest when the air pressure difference between the top and the bottom of the wing is the greatest - which happens when you're generating the most induced lift. This occurs when you're at high angles of attack.
During takeoff and landing, you're slow - so you're at a high angle of attack and generating strong wingtip vortices.
When you're cruising at high altitudes, like a jet in the flight levels, the air is thin. So, you need a high angle of attack to generate enough lift to stay level, even though you're moving fast. Your wingtip vortices are stronger here, too.
Even at 16 miles in trail, the Falcon 2000 crew experienced strong wake turbulence. Wingtip vortices hang in the air several minutes after an airplane passes. This is aggravated in calm wind conditions with no force acting to dissipate the vortices. Additionally, with the wind calm, there was nothing to "push" the vortices away from the STAR arrival path that both aircraft were following.
In the video below, take a look at how strong these vortices are, minutes after a C-5 Galaxy passes overheard...
The Avoidance Strategy: Offsetting
According to FlightGlobal, strategic lateral offsetting is a technique where aircraft fly slightly off the centerline of an airway to reduce the risk of a mid-air collision or wake encounter, particularly in regions outside normal air traffic control surveillance. Here are some examples of en route passes:
Earlier this year, a Bombardier Challenger crew narrowly escaped a fatal accident when rocked by the wake turbulence of an A380. The A380 was flying in cruise at 35,000 feet, which was just 1,000 feet above the Challenger jet. The aircraft passed each other, head-on, with the minimum vertical separation allowed in that airspace. Just over 40 seconds later, the Challenger jet experienced 3 sudden, simultaneous rolls, a double engine flameout, and was forced to descend thousands of feet to perform a restart. En route offsetting procedures were not available for this airway in the Middle East.
What You Can Do
If you're flying in busy airspace or around large aircraft, keep track of the windspeed and direction. If winds are calm, remember that wingtip vortices will have a tendency to remain along an aircraft's flight path and will not dissipate quickly.
According to the FAA, vortices descend at an initial rate of about 300 to 500 feet per minute for about 30 seconds. The descent rate decreases and eventually approaches zero between 500 and 900 feet below the flightpath. Flying at or above the flightpath always provides the best method for avoidance.
En route, avoid flight below and behind a large aircraft's path. If a larger aircraft is above you and on the same track (converging or overtaking), adjust your position laterally, preferably upwind. If needed, request a lateral deviation from ATC.
The best strategy for wake turbulence is to position your aircraft to avoid it altogether.
But it you do hit it, report your wake turbulence encounter to ATC as soon as possible, and take the necessary steps to get out of the wake.
Have you ever experienced wake turbulence? Tell us about it in the comments below.
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