Complex sentence: The R-77 can be used by most of the Russian Air force fighter aircraft, since many of their aircraft, primarily MiG-29, Su-27 and MiG-31, were upgraded recently.
Split into simpler sentences: The R-77 can be used by most of the Russian Air force fighter aircraft. Since many of their aircraft, primarily MiG-29, Su-27 and MiG-31, were upgraded recently.