The Russian air force lost another Sukhoi Su-34 fighter-bomber on Thursday, the Ukrainian air force claimed. If confirmed, the Thursday shoot-down would extend an unprecedented hot streak for Ukrainian air-defenses.

The Ukrainian claim they’ve shot down 11 Russian planes in 11 days: eight Su-34s, two Sukhoi Su-35 fighters and a rare Beriev A-50 radar plane.

But those 11 claimed losses are worse than they might seem for the increasingly stressed Russian air force. In theory, the air arm has plenty more planes. In practice, the service is dangerously close to collapse.

Exactly how the Ukrainians are shooting down so many jets is unclear. It’s possible the Ukrainian air force has assigned some of its American-made Patriot missile launchers to mobile air-defense groups that move quickly in close proximity to the 600-mile front line of Russia’s two-year wider war on Ukraine, ambushing Russian jets with 90-mile-range PAC-2 missiles then swiftly relocating to avoid counterattack.

But the distance at which the Ukrainians shot down that A-50 on Friday—120 miles or so—hints that a longer-range missile system was involved. Perhaps a Cold War-vintage S-200 that the Ukrainian air force pulled out of long-term storage.