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ahinterl

MD-11 (FS9): Some questions on descend/approach (2)

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Some more questions (they keep on coming) :( :1.Unfortunately, I've chosen FL100 for the cruise level. Since the plane slightly deviated from that altitude, it messed up with the speeds, and I guess this was the cause that the automatic descend was initiated wit a "VERT ALERT" instead of being started silently.Anyway, the descend begun, but at some point the plane started to dive below the vertical path more and more, and then, V/S was announced instead of PROF, see screenshot 1. A fixed v/s rate was present as long as I watched the screens, then I decided to manually intervene. I kind of recovered by opening the vertical speed window and selecting a lower V/S, so when the I/P was reached, the FMS transitioned back to PROF.Why does the FMS choose V/S (in magenta) at all, and why doesn't it adjust the v/s rate to intercept the vertical path again?2.Screenshot 2 shows a symbol which I cannot clearly identify (red arrow). From what I see on the FMC, I could imagine this is the point where the predicted 1500ft would be reached...Andreas

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Why does the FMS choose V/S (in magenta) at all, and why doesn't it adjust the v/s rate to intercept the vertical path again?
I think because you deviated from the decent path too much.
Screenshot 2 shows a symbol which I cannot clearly identify (red arrow). From what I see on the FMC, I could imagine this is the point where the predicted 1500ft would be reached...
That's the point where you (with the current v/s) intercept the decent path calculated by the FMC.

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I think because you deviated from the decent path too much.
So, when does the FMS switch to V/S and why doesn't it try to recapture the path?
That's the point where you (with the current v/s) intercept the decent path calculated by the FMC.
That's unclear: Why is this symbol so different (hollow magenta cirlce with "x" symbol in it, no label) from the normal intercept symbol (hollow magenta circle with label "I/P") then?Andreas

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So, when does the FMS switch to V/S and why doesn't it try to recapture the path?
I think this is quite well explained at FMS.50.2 Early Descents and FMS.50.5 Below Path. You can read over there that the FMS actually is trying to recapture the path.
That's unclear: Why is this symbol so different (hollow magenta cirlce with "x" symbol in it, no label) from the normal intercept symbol (hollow magenta circle with label "I/P") then?
Indeed, you're right. I don't know and can't find something about this in the manuals. Maybe you can try some different situations and find out by trial and error the difference between the two dots.Edit: the dot revers to the other dot you have on your FD: 2200ft. The FMS wants the plane to be at 2200ft, displayed on your FD. At the ND you can see where the FMS expects you to be at that altitude.

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I think this is quite well explained at FMS.50.2 Early Descents and FMS.50.5 Below Path. You can read over there that the FMS actually is trying to recapture the path.
Thank you. I just wonder why recapturing isn't performed more aggressively. When terrain becomes an issue, I think it can get problematic being much below profile for a long time...Andreas

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Thank you. I just wonder why recapturing isn't performed more aggressively. When terrain becomes an issue, I think it can get problematic being much below profile for a long time...
Because the FMS is trying to fly the plane as economical as possible. It costs less fuel to let the plane slowly get back on the flightplan than to maintain the current height or even climb to the path. The FMS doesn't know about the height of the terrain: it's your job to keep an eye out for the terrain!

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Unfortunately, I've chosen FL100 for the cruise level. Since the plane slightly deviated from that altitude, it messed up with the speeds, and I guess this was the cause that the automatic descend was initiated wit a "VERT ALERT" instead of being started silently.Unlike the Boeing, the MD doesn't descend silently. VERT ALERT will appear every time the FMS commands a pre-programmed climb/descent to the MCP altitude for the T/D or S/C.Paul

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Why command VS ? Because thats how they designed it.Manual FMS.50.3 - FMS.50.4 In a nutshel : In descend phase PROF will revert to VS (magenta) at a rate of -750 ft/min when below path by more than 300 ft until (close to) re-intercepting the path. If PROF is engaged from another pitch mode under-the-path, commanded VS will be the last target/current VS. If flaps are extended, however, and below path, PROF will command level flight until re-interception (for obvious reasons, the interception point might be below the ground !). In the opposite situation if the aircraft is over-the-path by more than 300 ft then PROF will revert to PITCH/IDLE (i.e. level change down). In addition if the active descend mode is not EDIT speed and target IAS is unrestricted (waypoint restriction or below 10000), PROF will command a +20 or 0.02 Mach speed increase (without exeeding VMax - 5 though) to facilitate more rapid path re-capturing. Regular PROF mode will be re-engaged once the aircraft is close to re-intercepting the path.Finally PROF will abandon path and hold at the speed limit altitude (usually 10000) if at 500 ft above that and at the present rate the descend speed limit (usually 250) will be exceeded. PROF will command level flight until indicated airpseed becomes less than 3 knots of the speed limit (i.e. usually 248). It will then resume path.It is all modelled to the dot. It does a few other things all in the FMS manual.Best,Vangelis

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Edit: the dot revers to the other dot you have on your FD: 2200ft. The FMS wants the plane to be at 2200ft, displayed on your FD. At the ND you can see where the FMS expects you to be at that altitude.
Thank you a lot!And thank you all the others as well for the explanations, I'll go over the manual once more.Andreas

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