Ok, so I'm aware that if you attempt to do something basic (like climb, dodge, or whatever), and don't have the appropriate skill, you attempt it at half the corresponding characteristic.

Are there any rules for attempting an Advanced skill without training? For example, if a character says they want to attempt to drive a vehicle, or try and fiddle with some machinery, even though they don't have Drive: Ground or Tech Use, I feel I should still allow them to make the attempt, albeit with absolutely no idea what they're doing.

So far I've been using the rules for using untrained Basic Skills, but throwing an additional -20 on top of it (similarly to if you try to use a weapon type you're untrained it), but I'm wondering if I've overlooked rules for what I should be doing if an Acolyte attempts a skill they have no training in?

Basic skills can be used untrained. Advanced Skills cannot be used untrained.

Basic skills can be used untrained. Advanced Skills cannot be used untrained.

So what do I tell my Acolyte when they say: "I try and figure out how to fly the Valkyrie." or "I try and use the complicated looking machine"? That: "Sorry, you don't have Pilot: Military Craft so you can't just get into a vehicle and push a bunch of buttons and pull a bunch of levers to find out which one makes it go"?

Similar deal for skills like Blather or Acrobatics. It says they're Advanced Skills, but I feel that I should at least give the Acolyte the benefit of the doubt if they tell me: "I want to try and somersault over the enemy's head, to block his escape route" or "I try and yammer on for ages about nonsensical things to confuse and stall the guards"? Do I tell them: "Nope you don't have the required skill for that, so you can't?"

In such an instance, they're basically attempting an Advanced Skill they may not have, and it feels wrong as a GM to A. Say they can't do it, or B. Let them use a different skill for it/make it a general characteristic test, because they're basically attempting something they haven't the first clue how to do, but they're still gonna try for it anyways.

(Incidentally, the catalyst for this thread, was a Cleric in one of my campaigns without Tech Use, trying to find a way to turn off a Vox Caster that normally required mechandrites to turn off, so I allowed him to take a Tech Use test at half his Characteristic, with an extra -20 on top).

By RAW you would indeed say: "You have no training, I'm sorry you cannot." In this system especially I see little reason a PC, acting in character, would even attempt such a thing. One big reason jumps out at me: The Machine Spirit. If you don't go through the proper rituals to start up or work with such a piece of equipment, you are unlikely to be able to activate it, much less run it if you do simply because you have angered said spirit. This is an easy, and simple explanation for all pilot/drive/tech-use/similar tests. Not to mention the likely anger of the Mechanicus for breaking such a piece of machinery (As you all too likely will.)

For some of the others, no training in the skill means you automatically fail. I would personally go with letting them roll it, but they only succeed on a 01, regardless of their actual characteristic score. IF I even allowed them to roll it. You are after all running on pure luck, since you have no idea what you're doing, and just hoping for the best.

Our houserule for stuff like this, is that if the player is willing to Burn a Fate Point then their character can make a Challenging/Skill Rank 1 attempt at whatever they're trying to do, as long as they offer an interesting explanation for why the character just happened to know how to do this one thing.

At least in our games, situations like this almost always involve one of two things: either the player is trying to do something completely logical and consistent with the fiction, but the rules are getting in the way. Or they're trying to do something really awesome that would both carry the fiction forwards and be hilarious, but the rules are getting in the way.

In either situation we find that it works better to give the character a decent shot at success and call for a bit of roleplaying, than it does to give the character a nearly impossible chance of success while assuming there's a good in-fiction reason the character can't do whatever they're trying to do.

Mind that if you do adopt this houserule, it's important that you ask your players to Burn rather than just Spend a Fate Point. Otherwise it's possible they'll end up gaming the houserule.