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.
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.
Question 2:
What is the true meaning of "speed" mode on FMA when in V/S, it keeps the ROD but how much is the speed trend allowed to fluctuate?
OP DES is a fixed thrust(idle) mode it will pitch up and down to maintain speed and resulting ROD has to be accepted. To increase ROD you can use speed brakes keeping the desired speed above the increased VLS. Nothing more can be done unless descent speed is increased. SPEED/VS is fixed VS mode at desired speed which is maintained by variable thrust. You have the option of selecting higher VS till the thrust reaches idle. Any higher VS will result in speed increase. Let's compare the two modes, say you are in OP DES with thrust idle(obviously) and now you pull VS it will open at present VS and present idle thrust. There will be no advantage of selecting higher ROD without resultant speed increase. However in OP DES you had the option of increasing ROD with some speed brake that is not there in SPEED/VS because use of speed brake will bring the thrust out of idle to maintain the speed.