 The Su-27 served with both the V-PVO and Frontal Aviation.Operational conversion of units to the type occurred using the "Su-27UB" (Russian for "Uchebno Boevoy" - "Combat Trainer", NATO designation 'Flanker-C') twin-seat trainer, with the pilots seated in tandem.When the naval Flanker trainer was being conceived the Soviet Air Force was evaluating a replacement for the Su-24 "Fencer" strike aircraft, and it became evident to Soviet planners at the time that a replacement for the Su-24 would need to be capable of surviving engagements with the new American F-15 and F-16.