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Benjamin J

Auto Throttle suddenly off?

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This puzzled me just now: I resumed a flight I had saved several days ago, and to my great dismay, the A/T was for some unknown reason off. I'm sure it had been set before takeoff and that it had functioned, but now it had gone. Anybody an idea how this could have happened? As an aside, I redid the flight from takeoff, only to see that A/T did not suddenly switch off. It remained on and functional through the entire flight.I do have some other questions concerning the throttle and VNAV. I know this is also mentioned in the manual and in the type rating course, but at this point, I simply need somebody to tell me "this is that", rather than somebody telling me to read the manual, so here goes:- When VNAV is ON, and the MCP contains an altitude above the VNAV calculated altitude, the plane will climb to the VNAV calculated altitude first, right (showing VNAV PTH as the operational VNAV mode)?- When the waypoint has been passed that holds the previously calculated VNAV altitude, VNAV will make the plane ascend towards the height that is defined in the MCP altitude window (so, it goes through VNAV SPD and than to VNAV ALT), right?- Now, when I change the MCP altitude to my cruise altitude, which is above the currently flown altitude, and then press the MCP ALT knob, the plane will ascend to cruise altitude and VNAV PTH will be displayed, even though the MCP ALT window holds the same altitude, right?- Now, here comes something I don't seem to grasp fully, even from the type rating courses: when top of descent is reached and you do not change the MCP ALT window's setting (so, you leave it at your current cruise altitude), the plane will continue flying on cruise altitude saying VNAV ALT, right?So, now what? I don't really seem to understand what should be done now. How do I make the plane follow the calculated descent path? I mean, at T/D, the plane will keep the MCP altitude, apparently, instead of following the calculated path downwards. So, say that from FL350 I have to descend to FL250, and so my MCP altitude is on 35000, to what value do I have to set it, to make sure the plane will follow the calculated path downwards without me needing to interfere at each waypoint to reset the MCP to a lower altitude?I hope somebody can help - I'm really trying to understand this, but I don't seem to really understand what I'm actually doing...


Benjamin van Soldt

Windows 10 64bit - i5-8600k @ 4.8GHz - ASRock Fatality K6 Z370 - EVGA GTX1070 SC 8GB VRAM - 16GB Corsair Vengeance LPX @ 3200MHz - Samsung 960 Evo SSD M.2 NVMe 500GB - 2x Samsung 860 Evo SSD 1TB (P3Dv4 drive) - Seagate Barracuda 2TB 7200RPM - Seasonic FocusPlus Gold 750W - Noctua DH-15S - Fractal Design Focus G (White) Case

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On descent, you would normally set the MCP altitude to your clearance limit. Real world, that means hi sector will clear you to the limit of his airspace, mid sector same and finally lo sector. You would be getting clear to FL330 then FL230 then probably the arrival gate altitude of 10-12,000. You might get a limit based on traffic. Usually, the decent never has to level off and clearance to the next limit comes before you reach the current limit but not always. Sim world I just set it to the arrival gate limit per STAR.You don't need to intervene at each fix with a constraint, you could set the MCP ALT to the FAF glideslope intercept and VNAV should get you there.Most descents are not done with VNAV. ATC starts giving you limits for traffic and speed for sequencing and maybe even vectors then MCP SPD ALT and VS rule the way. The only PMDG aircraft that has vertical navigation that I really like is the MD11.


Dan Downs KCRP

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Thanks for the explanation! So, if I don't get instructions from ATC (I don't have any good ATC at the moment, but I'll be buying PFE hopefully sooner than later), I can just set the MCP ALT to the lowest altitude possible before ILS interception, as indicated on the charts, and leave it at that? VNAV will than follow the programmed/calculated plan?


Benjamin van Soldt

Windows 10 64bit - i5-8600k @ 4.8GHz - ASRock Fatality K6 Z370 - EVGA GTX1070 SC 8GB VRAM - 16GB Corsair Vengeance LPX @ 3200MHz - Samsung 960 Evo SSD M.2 NVMe 500GB - 2x Samsung 860 Evo SSD 1TB (P3Dv4 drive) - Seagate Barracuda 2TB 7200RPM - Seasonic FocusPlus Gold 750W - Noctua DH-15S - Fractal Design Focus G (White) Case

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It works for me as you echoed. You can scan your LEGS pages and find the final constraint before the runway and set it to that value, which is usually the ILS FAF. Once on approach you should reset the MCP ALT to the MA altitude in case of go around.


Dan Downs KCRP

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Okay, thanks for the help! Much appreciated!


Benjamin van Soldt

Windows 10 64bit - i5-8600k @ 4.8GHz - ASRock Fatality K6 Z370 - EVGA GTX1070 SC 8GB VRAM - 16GB Corsair Vengeance LPX @ 3200MHz - Samsung 960 Evo SSD M.2 NVMe 500GB - 2x Samsung 860 Evo SSD 1TB (P3Dv4 drive) - Seagate Barracuda 2TB 7200RPM - Seasonic FocusPlus Gold 750W - Noctua DH-15S - Fractal Design Focus G (White) Case

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