I can't discount the possibility that we are already secretly supplying aircraft to Ukraine; it's entirely possible. It's not inconceivable we have covert transport and surveillance aircraft in Ukraine airspace right now. We, the public, won't know until years after the conflict ends.

But doing such openly would be begging for escalation. Also, it would be begging for a "black swan" event. For instance, what if an unarmed plane openly flown across the Polish border by a Ukrainian pilot saw an air attack on a refugee train, and chose to ram the attacking Russian aircraft, contrary to orders?