The first of the two bridge raids took place on 10 August 1941. For this mission, the I-16s were fitted with additional 95-liter (25 US gal) underwing fuel tanks for an additional 35 minutes of flight time. Of the three Zveno-SPBs, one had to turn back due to mechanical problems. The other two launched their fighters 15 km (8 NM, 9 mi) from the Romanian coastline. The fighters successfully dive-bombed from the altitude of 1800 m (5,900 ft) and returned home with no losses despite heavy anti-aircraft fire. The second raid took place on 13 August 1941. This time, all three Zveno-SPBs reached the target. The six fighters scored five direct hits on the bridge and completely destroyed one of the spans. On the way back, the fighters strafed Romanian infantry near Sulina and returned to Eupatoria with no losses. Following the successful sorties, two additional Zveno-SPB were brought to operational status, bringing the total to five. The main limiting factor was the lack of high-output Mikulin AM-34FRN engines, as the other versions were not powerful enough to get the aircraft airborne. On 16 August 1941, Admiral Kuznetsov asked Joseph Stalin for additional AM-34FRN-engined TB-3s from the Air Force so they could be converted to Zveno-SPB carriers, but the request was denied as the Air Force had suffered heavy losses in the opening days of the war. In the meantime, the five aircraft continued flying operational sorties, destroying a dry dock in Constanţa on 17 August and a bridge across the Dnieper River on 28 August, losing one I-16 in the process. During the repeat attack the next day, four Zveno-launched I-16s engaged several Messerschmitt Bf 109s, shooting down two. Despite the high success rate, Zveno missions ended by 1942 due to high vulnerability of the obsolete TB-3s and I-16s in the face of enemy air superiority. It is estimated that Zveno-SPB flew at least 30 combat missions.