Originally Posted by
k.swiss
Hello All Question:
Descending with OP DES and green-dot-speed gives a very shallow ROD, (less than 700FPM with a standard weight) try and use speed-brake and the VLS shoots up, even worse on 321.
Instructor once told me better to use V/S if you want to get down faster when at green-dot-speed but in some cases if selecting for example -2,000FPM at green-dot the speed trend starts to creep up.
The physical interaction between flowing air and the fuselage/wings/speedbrake/etc of the airplane, are not effected by the autopilot mode or FMA annunciation. If X airspeed and Y vertical speed are maintained at idle thrust and Z speedbrake setting in one mode, it'll be the same in all modes.
Question 1:
When in this situation of high V/S selected and speed trend is shooting up why am I told speed-brake is useless in V/S mode? Surely if the speed is shooting up because of the high ROD speed-brake will have some effect.
Because whoever told you this was making an offhand exaggerated remark, that taken literally went beyond his real point. Or, his real point could be plainly in error, too. Even if they're an instructor.