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.