Descending wingtip vortex ?

[B2N2]

In avoiding wake turbulence I've always been told the wing tip vortex spreads out and descends behind the airplane due to the fact that the vortex is using up its energy.
Does anybody know whether there's a formula or a rule of thumb to determine a safe distance behind a Heavy should you fly at a lower altitude?
A while ago I was flying right-seat in a Pa-28 with radar-vectors over Miami Int. when I spotted a Heavy at my 2o'clock.And about 2000' higher.
By the time we crossed behind it was about 5mi to our left.I was kinda worried about it but didn't do anything besides thinking of the question.
Anybody?

[yxcapt]

If I remember correctly, the vortex sink at several hundred feet per minute and normally level off between 500 & 1000 feet below the offending aircraft. vortexs stablize around that altitude while they expand and disipate. They tend to remain about a wings span apart and all so drift with the wind. Strong winds and turbulance aloft tend to brake up the vortexs faster than if a no or light wind condition.

My personal rule is no closer than 1000 feet vertically and 5 miles in trail during cruise. Takeoff and landing minimum of 3 min. seperation and 5 to 6 miles depending on type I'm flying.

[Genghis the Engineer]

There are ICAO seperation minima. You can find them at...

<a href="http://www.ais.org.uk/Uk_aip/pdf/aic/4P188.PDF" target="_blank">http://www.ais.org.uk/Uk_aip/pdf/aic/4P188.PDF</a>

on page 2.

G

[john_tullamarine]

There was a very interesting paper in the AeroSoc Journal on trailing vortices some time ago.

Unfortunately, I don't have the files to hand but it would have been 1-2 years ago. Shouldn't be too hard to find.

Made for extremely interesting reading.