Originally Posted by
megan
Many of the E series were 707's, in as much they are converted ex airline aircraft, E-8C 90-0175 being ex QANTAS for example.The US changed the designation system in 1962, any 707 delivered after that date changed from C-137 to C-18
Not strictly true, VC-137 72-27000 was delivered 10 years after that, and a further two C-137s were obtained second hand in the 80s. They adopted C-18 for the second hand 320 series airframes they purchased. If you were strictly following the 'rules' as these were all variants of the original C-137 they should have had C-137 derived designations but there are logical reasons for not doing so. (Upgrade status, fleet management, avoiding confusion!). Same thing goes for the various 737 and 747 military versions, there would be no logic in the Posideon being the PT-43H
What it does show is that the system doesn't always follow strict rules, so even a chang in role may not result in anoevrall change of designation