How Saab's Viggen Jet Fighter Locked Onto the US SR-71 Blackbird. The Lockheed SR-71 Blackbird is the fastest air-breathing manned aircraft in the world. It is also one of the most fearsome reconnaissance aircraft, able to outrun threats from the Soviet Union at speeds of over Mach 3.0. Not once did a Soviet interceptor lock onto an SR-71. However, Sweden did manage to lock onto the aircraft with their Saab 37 Viggen fighter jet, making for one of the most remarkable tales in Cold War aviation history. The Viggen remains the only fighter to ever lock onto the SR-71. Thanks to knowing the patterns of SR-71 flight paths and sophisticated radar systems, the SR-71 would be locked onto by the Viggens multiple times, showing just how strong the Swedish fighter was, despite its speed disadvantage. The Viggen finally intercepts the SR-71 Blackbird. An alert came through that an SR-71 was approaching Swedish airspace. So Eldh scrambled in his Viggen to try and intercept the American aircraft. He climbed as fast as he could before eventually reaching the altitude that the SR-71 was flying at. He started the head-on attack and lined himself up with the Blackbird. Then, Eldh managed to gain full missile lock on the SR-71 as it rapidly approached him, the missile fully armed and ready to fire. Naturally, the Swedish pilot wouldn’t fire, and the two aircraft passed by one another, with visual contact, also gained. But the intercept had proven one thing. The SR-71 wasn't invincible. While none were ever shot down by the Soviet Union, the Swedish would lock onto the SR-71 several times. Eldh himself would do so another five times. To this day, the Saab 37 Viggen is the only aircraft to lock onto the Blackbird successfully.