While I'm sure this topic may have been discussed before can anyone advise if the JT3C turbo-jet to JT3D turbo-fan 707 upgrades were mandatory or descretionary? (the fan's merit being of course lower polution & noise, higher power & efficiency)
Why? Becuase considering that the intoduction of the fan engine was quite early in the 707s career (early 60s) whence all new 707s were pressumably fitted with the better fans, the jet powered 707 remained in service well into the late 70s maybe even 80s. Were those original engines upgraded to see service in parallel with the fan?
The JT3C nacelle and exhaust nozzlse being very distinctive (easy to confuse with the RR Conway, even though the Conway was a fan) and arguably nicer to regard than the later P&W fan version. Photo from 1972 but these engines are seen into late 70s.
