Gaspath is absolutely correct. The VBV's modulate and are not closed at idle. (The VSV's when "closed" are obviously not closed shut like the VBV's can be, but they're as "closed" as they can get.)
I've probably run as many CF6-50's and -80's as many who fly with them have, and I never once experienced -nor even heard of - the bowed rotor effect with them... many other troubles, but not that!
As for your doubts Easyduzzit. What can I say.... GE didn't issue an AD, but they did issue the recommendation ( probably had a fancier name -SB? EI maybe? I forget) to cool the GE90 for 20 minutes after the throttles were off the stop. (never saw any such recommendation for any other GE motor)... and as you've noted, the noise/vibration on a GE90 start is not an
every time occurrence, it ONLY occurs when there's been insufficient cooling time prior to shut down... and as it takes 20 full minutes, it is very often the case.
If you're ever running at idle for that long and you have the opportunity -waiting for deice, or great lines of congestion or whatever, try shutting one down and restarting.... you'll see. Smooth as a babies bum! Guaranteed! (but don't cheat! 20 minutes at idle! Not 18!, and not 20 minutes which included just a little breakaway throttle use ...it IS that fussy.) --and again, that's the GE90-115 (&-110) I'm referring to.... Don't know about the lesser GE90's.
CF6-50's were always smooth starters, especially when we adopted our PP engineers' recommended method of putting the fuel on at 10%.... Seemed odd at first, but made for a
very smooth start, as the thing would light off at about 15% and, they claimed, the thermal shock was was much less that way compared to motoring to max motor before lifting the fuel cock. Haven't run a -50 for about 10 years now... I miss those beasts! Great engines to work on!
CF6-80's (been fiddling with them since about 1988 or 89, and still am) are also quite smooth during start.... Unless something's wrong of course.
