It seems that the russians have effectively no air defence detection capability accross Russia and they could not detect a C172 sized aircraft.
Now imagine if the Ukrainians could take a few clapped out airliners that were heading for the scrap yard. A 737 sized airframe could easily carry 10 tons of explosives and 50,000lbs of fuel. Now that would make one helova mess it it plowed into an oil refinery. Surely 5 or 6 barely airworthy airframes could be scrapped up around the world for one final flight.