As it was said earlier, no Aspect has their integral parts locked to helmet or faceplate. All Eldar helmets have a separate chin part that is hardlocked with the suit and thus is worn all the time. Both Scorpions' mandiblasters and Banshees' psycho-sonic amplifiers are attached to this chin part (you may look at my avatar for reference). The rest of the helmet can be taken off or put on, and when put on, it locks to the chin part.

Is that actually found in the lore? I thought that was just a DoW invention.

That's hard to say if it's GW canon lore or DOW artistic license. Having seen every eldar codex (except the most recent) and having read the novels which focus on eldar, I can't remember an instance of them going with out a helmet when in battle. But I could be wrong. It would be great to get answers from GW itself but the odds of that happening...not so good. Anyways, great discussion though.

Can't see your problem here. Such construction makes perfect sense.
And why you ask how can Eldar go into battle without helmets.. SM and CSM do it all the time, Orkz too, so why can't Eldar?
But yeah, if I played, I would have my helmet on in battle for sure.
And I'd like to see how SM heroes catch stray shurikens with their bare heads.

Because they're not the same race, people, traditions, technologies, ...

Like asking "How can Tyranids go into battle without lascanon. All the other do it all the time (except maybe Necrons).".

No, it's not the same. There are no tyranids with lascannon, but there are Eldar miniatures without helmets, they are quite official, so just deal with it.

And yes, the thing with the chin piece may be a late design, but it's quite logical and very cool!

It's not logical at all, there's absolutely no reason to not wear the helmet.

#downwithchinarmour

It may be not logical but its awesome so it might find its way into the game.

DoW shenanigans. There's a specific stuff in Munitorum and IA vol 11 that goes into how the war masks don't come off. DoW is a non-canon 3rd party product.

SM do it because they have iron halos and orks do it because they are idiots.

I don't say not wearing helmets is logical. I say having a chin part to the helmet is logical, by the way Jes Goodwin's concepts show it for Exarchs and Autarch.

See pictures above. And war masks are not helmets.
But quote your specific stuff, please.

I saw hundreds of images and miniatures of helmetless SMs without any iron halos.