It hasn't cropped up in any campaign I've been in but I just wondered how would I handle an Imperial character trying to pilot a xenos vehicle. Say an Eldar Darkstar fighter captured intact. If it is possible, I'm thinking it would at least initially involve some combination of Forbidden Lore (xenos), Tech-Use, and the appropriate pilot/drive skill.

Eldar tech, in particular, is also psychoactive, which might present a problem to most people. I'd consider adding some kind of willpower check in there. I wouldn't worry about tech-use unless they trying to do something other than operate it's motive ability.

Advanced Eldar tech is 100% unoperable for a regular human. It is controlled by mind using psychic power. A psyker could theoretically controll it, but it would be extremely hard for an untrained non-Eldar, probably a -60 and a fail would indicate something like flipping the fighter over and falling down.

It would be pretty similar to your great-grandma trying to write a computer program without any knowledge about programming.

The Eldar grow up around all these things (heck, they even open doors with their brains) so they just do it naturally (much like a modern teenager doesn't have to read a manual about using his new smartphone).

On top of all that, the Eldar have supernatural reflexes, which are pretty mandatory to control their vehicles at the extreme speeds they acheve.

If the pilot has the requisite piloting skills. And if it's even possible (this fighter is controlled by psyker dolphins who worship chaos), then depending on what types of vehicles it is and who it belongs to. What I'll likely do is, give them a tech-use or forbidden lore tests to reduce their minus. Unless it's something simple and a very obvious fit for humans or it fits the pacing or tone of what's going on, then I may just let them go for it. Like depending on what's going on, I may decide that the Rogue Trader plays around with the controls of a DE skimmer for 5 seconds intuitively gets it, and takes off like a bat outta hell as a couple dozen Kabalite close in pouring on vigorous fire.

But mostly I'll make them roll to see how far they can mitigate say a -10 to -30. And depending on what it is and how much they play with it. I may drop the negative entirely, perhaps with a really small 100 xp spent for that type of vehicle if it's really fancy.

It gives me ideas anyhow. Cheers.