My brown pants moment (wingtip vortices)

AF wrote:

We had flown into it about 10 minutes after the giant prop plane had passed our nose. The vortex hadn’t settled much at all…

In clear air with no wind shear (both calm and steady wind meet that definition) there is no driving force but viscosity to dissipate energetic wingtip vortices, and it ain’t much. They can subside for a long time, they will increase in diameter and move downwards. Near the ground there is friction with trees and stuff, which increases dissipation rate a lot and prevents growth and descent (by definition), but in altitude they just stay put until ultimately decaying a lot later.
In your case the prop vortices create their own problems. Along those lines, I’d be wary of potentially severe downdrafts behind an A400M with its counter-rotating 1-2 and 3-4 props. Let’s hope the aerodynamicists at Airbus have done their job, but I’ll still keep my distances.
Peter wrote:

so 400kt would be 7 times greater

Intuitively I would disagree. It’s back to the triangles of speeds, for a TB 20 the vertical component would modify your aoa a lot more, on top of which you’d stay in the wake for a longer amount of time. Turbulence is worse at higher speed because you pass through more cells per unit time with little change in altitude. In a small plane you have time to follow the airflow more, gain or lose 250ft in the process like @AF or turn upside down like the DR400 behind an An-2 or the SR20 in the rotor wash. Plenty scary and dangerous in its own right, but not shaky like in a fast jet.

FWIW crossing the wake of a JAS39 Gripen is like hitting a curb head on in a car at 50kph. Your butt hurts.

The BFU published a “Zwischenbericht” (intermediate report) on the incident between the A380 and the Challenger C604:

URL Local Copy

(in German, relevant pages 12 to 36)

Even for those not able to read German, the pictures as well as parts of the text in english are pretty interesting. The destruction caused by the A380’s wake turbulence is palpable in a photo of the interior of the C604.