UKC

Recently we climbed the highest peak of Iceland Hvannadalshnúkur from 100m to 2110m located on the largest glacier in Europe Vatnajökull. It's an 15 hrs walk both ways, ice starts from 1000m. Everyone had 1 ice screw attached to belt for crevasse rescue, we had an discussion in our team if we should leave caps on ice screws or take them off. From one side you want to have ice screw ready to use and taking cap may take precious seconds, but from other side on 15hrs walk you may not use it at all and it's pretty dangerous to have ice screw swinging next to you hip. If you stumble and fall it may damage your leg. Actually we had 5 situations where guys fell into crevasses, 4 of them only with one leg, in one case guy went up to the hip but managed to get out by him self.

Never seen anybody having the caps on, except while carrying them in the bag protecting the tips.

And have you seen people walking 25km on ice covered with thick layer of snow ?

Oh yes, since 1990...

Blimey 5 people falling in a crevasse on Hvannadalshnúkur, must have changed a bit since I went up there.

To answer your question, in reality the chances of being able to use an ice screw to set up a belay should someone fall into a crevasse are tiny. If the glacier is covered in soft snow such that you could fall into one without seeing it was there, then an ice screw would be useless as a belay and conversely, if the ice/snow was good enough to hold a screw you'd be able to see the crevasses in the first place or at the very least it would hold your weight. In that respect it doesn't matter if your screws are capped or even in the bottom of your rucksack!That said, we all dutifully carry a screw 'just in case'.

There is a possibilty that the person who actually falls into the crevasse might be able to place it in the crevasse wall and clip into it to make life easier for those on top while they set up a snow anchor but in if you're in a position to do that pulling a plastic cap off first will be the least of your worries, getting close enough to the sides and getting the screw to bite in bullet hard ice and then clipping in and weighting it will all be more difficult than removing the cap.

So my advice would be do your salopettes a favour and keep the screw covered.

Yeah - I'm with pec on this - leave the caps on.  If you're in a slot a have enough upper body movement to place a screw then you'll easily get the cap off.  The teeth will just make holes in all your good gear otherwise.