This rare Mi-17 variant, the Mi-17PP, is an electronic warfare version of the Mi-8MT, equipped with the 'Bizon' jamming system. I wasn't aware the Bulgarian Air Force operated it. Aircraft 431, one of four (431-434) delivered from 1984, served as an ECM communications jammer. Its advanced antenna array surpasses that of the Mi-8 'Hip-K.' Key features include a 32-element array aft of the landing gear, a four-element array on the tailboom, large radomes near the cabin, triangular containers replacing rear windows, and six heat exchangers under the front fuselage.