For what it's worth
Here is a scenario worth keeping in mind.
Numbers to remember: 1,000 feet, 500 fpm, 16 miles, 2:40 minutes
Operating with RVSM in the North Atlantic has greatly increased the usable airspace for the swarm of aircraft that cross the Atlantic twice each day. But, it also exposes you to the very real hazard of encountering the wake turbulence of the aircraft that is 1,000 feet above you on the same track. It is inevitable that as
the number of crossings you encounter increase, sooner or later you will experience this phenomenon.
Wake turbulence comprises two components, engine exhaust and wing tip vortex. The engine exhaust is not significant for this discussion since it does not descend as fast as the vortex and is weaker and dissipates rapidly.
The wing tip vortex, however is powerful and does not dissipate quickly, and descends at approximately 500 feet per minute. In smooth air, not necessarily still air, but uniformly moving air, wing tip vortex remains intact and for the two minutes it takes to descend to your altitude, it is still violent enough to knock a person of their feet in the back of the aircraft.
When will you encounter it?
It takes two minutes for the wing tip vortex to fall the 1,000 feet to your altitude, but deceivingly, you will not encounter it until approximately 2:40 minutes after the offending aircraft has passed overhead.
The math involved (1,000 feet, 16 miles, 2:40 minutes) can be a little confusing, but need not be. A B747, or other similar aircraft cruising at FL340 is doing M.84,
or approximately 495 KTAS. Your Bizjet is at FL330 and assigned M.83, or 489KTAS. His 6 knot speed advantage will put him 16 miles ahead of you in 2:40
minutes after he passes overhead. You can watch him on the TCAS. At that moment he generates a wing tip vortex that begins falling at 500 fpm. In the two minutes that it takes you to cover that 16 miles, (approx. 8 miles per minute) the vortex has descended to your altitude. If there is no crosswind, or if you are downwind of his track just the right amount you will be in his vortex.
What’s the effect of the wind?
Wind obviously has a great effect on these encounters. In short a headwind or tail wind will have no effect, but a crosswind has a major effect on the outcome.
A headwind or tail wind component has no effect because we all are flying the Mach Number Technique prescribed for MNPS airspace. The vortex moves fore or aft with the air mass just as we do.
Our speed relative to wing tip vortex does not change. Therefore the approximate times and distances discussed here remain valid, regardless of a headwind or tailwind component.
A crosswind component is different because we fly a prescribed track over the ground. We allow our aircraft to crab into a crosswind component in order to maintain the desired track over the ground. So, while the vortex will blow the offending aircraft’s wing tip vortex two miles downwind from his track during the two minutes it takes to descend to your altitude, clearly, if your tracks are identical you will not encounter his vortex but, if your track differs from his by two miles down wind, you will may be in for a large jolt.
On the westbound North Atlantic crossings in good weather, and contrailing conditions, you can visualize the vortices coming, in other conditions you must be aware of where the airplanes are above you. Listen on Air to Air and watch the TCAS, watch the crosswind components for signs of impending wake turbulence.
Guidance:
The Jeppesen AT (H/L) 1 & 2 chart has guidance. The guidance begins with “Notify ATC and request a revised clearance. If a revised clearance is not practical....” You can safely interpret that to mean that you should request a new clearance only if you want a clearance on a different track. It does not mean that you should request a new clearance to fly an offset from your cleared track. You may offset up to 2 miles without a clearance. Although you are required to advise ATC of your offset, they will not normally issue a clearance, nor will they normally respond to you through the radio facility. If and when your aircraft becomes ADS
equipped, ATC will be able to see your offset just as if they were using radar, so
any failure to acknowledge your actions will be handled as if you had deviated without the proper clearance or communications.
