I am getting a grid pattern in my contrail clouds, looking more like a tic-tac-toe than plane flights in Vue5 Infinite. How do I change the pattern?

Change the scale? R

I can expand the contrails so that the pattern is not that evident but the pattern reamins the same. And the contrail effect is lost.

What cloud pattern do you have loaded in your atmosphere? This looks like the "Busy Aliens" pattern. "Airplane Trails" is a lot more subtle.

(To change the pattern, go to the Atmosphere editor, select Clouds, right click the preview picture to load a material, and look under Clouds - Other Clouds.)

Have you tried going into the function editor for the clouds material and changing the wavelength of the lines function (either the x or y to zero)?

That is a snapshot of Airplane trails with a scale of .10.

I made progress by setting the X value of the function editor to 0. But the lines were now symmetrical in one direction, as if a whole squadron of jets flew side by side.

It's quite easy to reproduce - just add the Airplane trails clouds - I'm not doing anything complicated.

Actually you've posed an interesting challenge. Until now I hadn't paid much attention to how clouds are placed in Vue since I've always treated them as random. But to create a single, well-defined contrail in the sky: hmm...

The contrail effect is mostly created by the Lines noise node that's connected to the Transparency in the function editor.

We don't have much control over how the pattern is repeated because it's a noise node. So try replacing it with an image map consisting of a fuzzy, thin horizontal black line on a white background. Vue repeats the image map across the sky, but you can adjust the UV scale to achieve a single contrail like the above example.

Here's the settings I used in the atmosphere's Transparency function editor for the above example. I blended the image map with a perlin noise node to soften the line. I also bumped up the UV map Y-scale to 3.0 (that function is located in the half-round icon above the image map node) to thicken the line and eliminate the visible repeats of the image map across the sky.

With a little work on the image map, the blending of the noise node and the UV mapping, I hope you can use this idea to create the effect you're looking for.