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?
In addition to the info above:
First of all, you will not descend faster without the speed increasing if you use V/S instead of open descend. Either the instructor was talking about V/S giving a faster response when starting a descend, or there was some other misunderstanding/miscommunication.
1)
If you select a high V/S and the speed goes over the selected/managed speed, the speedbrakes will work. The only reason I can think of that you were told the speedbrakes are useless in V/S is in the situation that you are descending in V/S, speed is at the selected speed. Extending the speedbrakes in this situation only adds power without changing the ROD or speed.
As far as the difference between open descend versus V/S:
You stated you were descending at green dot speed, and I assume you have selected speed at green dot. In open descend, giving you a descend rate of about 800FPM. You can't really use the speedbrakes, because of the increase in VLS. If you select V/S nothing relating to the aerodynamics with the airplane changes, the speed and V/S will remain the same. If you increase the ROD by selecting a higher V/S, your speed will increase, and you could have achieved the excact same thing in open descend by selecting that higher speed. Open descend would IMO be the better mode, because if through variations in wind during the descend, your speed momentarily drops below the selected speed, in open descend the autopilot will correct that by increasing the ROD, and in V/S the autothrust will correct that by adding power.
2)
Speed (or Mach) on the FMA means the autothrust is responsible for maintaining the speed, and the autopilot is responsible for maintaining the vertical path. Possible modes for this situation are ALT, V/S, GS. In open descend the thrust is idle, in open climb the thrust is at climb power, and for both of those the autopilot pitches to keep the speed at the managed or selected speed bug.
If you climb in V/S and the speed is higher that the selected/managed speed, the autothrust will reduce power to decelerate to the selected speed. If the speed is lower, the autothrust will increase power up to climb power, to try to get to the selected speed, but it might not be able to if a high V/S is selected.
If you descend in V/S, and the speed is below the selected speed/managed, the autothrust will add up to climb power to accelerate to the selected speed. If the speed is higher than the selected/managed speed, the autothrust will reduce the thrust to try to decelerate to the selected speed, but it might not be able to if a high V/S is selected.
(not an all inclusive list of possible modes, and explanation based on AP/AT on with PL in the climb detent)