Poland, the United Kingdom and other partners extend their air defense systems to cover western Ukraine
Planes don't fly there, only missiles do (and some long range drones). I don't think this would be very meaningful, because Ukraine already has very capable anti air systems.
But that's not really the issue, is it?
You say this is unusual, because the Europeans are doing it without looking over their back at the United States. This is misleading.
You are part of the same alliance. If a Polish or British jet fighter is shot down over Ukraine, what happens next? I believe the Russian government would view the US as being directly involved, since Poles and Brits can't fly anywhere and can't bomb anything for more than a day, without requiring US assistance. They tried to do it over Libya, but quite literally ran out of munitions within a week and had to request urgent resupply from the US.
Having Brits and Poles there is as good as having Americans there, and we know America doesn't want to be there. So they will simply tell you not to do it.
If you insist on proceeding with your brilliant plan regardless, I think it would be only fair for the United States to say: "In that case, Article V protections cannot be invoked through a potential further widening of the war".
You can't drag America into this war, by entering Ukraine quickly - losing a couple of planes - and then running back to daddy to complain. That's a transparently extortionate policy of "ally blackmail".
But would you do anything if you knew there wasn't an American backstop? 100% no.