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LeifApex

737 descend before reaching T/D and mid-flight

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I'm beginning to fly on Vatsim with the PMDG B737-800.
I've planned my route and altitude (and Descend point) and configured the FMC, now when i fly towards the destination airport, ATC will descend me before T/D, and i'm struggling with descending using the FMC and VNAV. I can't seem to find a proper "descend now" option, and i can't choose an altitude at the next waypoint (in the legs page) as i've been directed to a waypoint about 50NM away.

 

So, to put it in other words, i can't figure out how to descend using VNAV and the FMC while on a planned route before reaching T/D. Whether its an altitude change in mid-cruize, or initiating the final descend and approach. Any help?

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49 minutes ago, LeifApex said:

So, to put it in other words, i can't figure out how to descend using VNAV and the FMC while on a planned route before reaching T/D. Whether its an altitude change in mid-cruize, or initiating the final descend and approach. Any help?

If you click ahead to the DES page of the FMC, there's no <DES NOW prompt?

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25 minutes ago, scandinavian13 said:

If you click ahead to the DES page of the FMC, there's no <DES NOW prompt? 

I find that now, on my next flight.
So how does the DES NOW work?. Kets say i'm at FL390, and ATC instructs to descend FL370. Would i program MCP to 37000 Feet, and then just click DES NOW?

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6 minutes ago, LeifApex said:

I find that now, on my next flight.
So how does the DES NOW work?. Kets say i'm at FL390, and ATC instructs to descend FL370. Would i program MCP to 37000 Feet, and then just click DES NOW?

If you need to descend early then you hit des now. 

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To supplement my last message. When i use the DES NOW function from (using my last example) FL390 and descending to FL370, when that descent was finished, i couldn't do it again, and my VNAV was stuck on FL370, and i had to use LVL CHG. Its not that the DES NOW button didn't work, it wasn't there.

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1 minute ago, ph-cxz said:

If you need to descend early then you hit des now. 

Will i be able to engage VNAV again after the in-flight descent is finished or is it more complex than that?

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Just now, LeifApex said:

Will i be able to engage VNAV again after the in-flight descent is finished or is it more complex than that?

Vnav is still engaged because I imagine you did not change the Vnav to speed.

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Just now, ph-cxz said:

Vnav is still engaged because I imagine you did not change the Vnav to speed.

Yeah i didn't know what that message was about. Sorry.

When i have descended via the DES NOW, will i have to change cruising altitude in the FMC to the new FL, og will i just continue cruize at my new FL automatically? (i'm talking about a mid-cruize descend on this question).

 

Another question:

To supplement my last message. When i use the DES NOW function from, for instance, FL390 and descend to FL370, when that descend is finished, i can't use the DES NOW function again, and my VNAV is stuck on FL370, and i'm required to use LVL CHG. Simply because the DES NOW button isn't visible anymore. So, how can i fix this, so i can use the DES NOW again, or could you explain to me how it works?

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22 minutes ago, ph-cxz said:

Yeah for a mid descend to a new level requires you to re enter the cruising level. 

Ok.

 

Got an answer for this question?

29 minutes ago, LeifApex said:

When i use the DES NOW function from, for instance, FL390 and descend to FL370, when that descend is finished, i can't use the DES NOW function again, and my VNAV is stuck on FL370, and i'm required to use LVL CHG. Simply because the DES NOW button isn't visible anymore. So, how can i fix this, so i can use the DES NOW again, or could you explain to me how it works? 

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1 hour ago, LeifApex said:

Got an answer for this question? 

DES NOW is only an option to start descent before reaching T/D. In the FMC logic cruise is over and descent is now the phase of your flight so the DES NOW option is gone. If ATC just needs you to go down 2000ft it's not really the start of the descent but a change of flight level (due to traffic). You don't do that by using the DES NOW option. Set the MCP to the new flight level and put it into the FMC cruise page and VNAV will descend. When you finally begin your actual descent before T/D you can now use DES NOW.

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19 minutes ago, threegreen said:

DES NOW is only an option to start descent before reaching T/D. In the FMC logic cruise is over and descent is now the phase of your flight so the DES NOW option is gone. If ATC just needs you to go down 2000ft it's not really the start of the descent but a change of flight level (due to traffic). You don't do that by using the DES NOW option. Set the MCP to the new flight level and put it into the FMC cruise page and VNAV will descend. When you finally begin your actual descent before T/D you can now use DES NOW.

That clears it up.

Thank you to everyone who took time to answer, I appreciate it.

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5 hours ago, LeifApex said:

I'm beginning to fly on Vatsim with the PMDG B737-800

with vatsim (& any atc programs), it would be much wiser to use heading and flch / vs modes along with mcp speed so that you can efficiently adjust to directions ..... and refer to lnav / vnav for "awareness" rather than chasing fmc "updates".

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9 minutes ago, Bluestar said:

Real pilots would just disco the A/P and hand fly it to the next altitude

not the first choice near "coffin corner"... the OP's scenario.

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"Real pilot would disco(nnect) the autopilot….."

Why should they disconnect the a/p?

If I was ordered to descent from FL390 to FL370 I would simply use V/S (vertical speed) mode by setting -1000 ft per minute descent rate (by keeping the A/P on)...why make things complicated when the solution is easier?

And if I think that I'll will have to mantain FL370 for a while I'll adjust the new "cruise level" into my CRUISE PAGE of my CDU and, of course reset also 37000 ft into my overhead pressurization cruise altitude window…

Best Regards

Andrea Buono

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All you have to do is select the new altitude on the MCP Altitude Window and hit ALT INTV. The airplane will descend on VNAV PTH at a fixed rate of 1000fpm. If you plan to stay long enough at that new altitude, just set up a new CRZ ALT in the CRZ page and change the FLT ALT on the presurization panel. If that's not a new cruise level and you expect to continue descending, when they give you a new cleared level, just set it in the MCP, the airplane will catch the new VNAV PTH and fly it.
Sometimes the instruction is "descend now" and some other times it's "descent at discretion". In one case, you don't have to start your descent until you hit your TOD. In the other case, ATC expect you to descend with at least 500fpm.

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On 12/25/2018 at 4:59 PM, vadriver said:

not the first choice near "coffin corner"... the OP's scenario.

Even at max altitude you have a fairly decent range of speed and maneuver margin. I've had to hand bomb it from cruise a few times due to failures causing us to be non-rvsm. 

 

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On 12/25/2018 at 4:59 PM, vadriver said:

not the first choice near "coffin corner"... the OP's scenario.

I think it was a joke. Hence the laugh emoji.

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1 hour ago, Spin737 said:

I think it was a joke. Hence the laugh emoji.

Knowing his faible for the 707 and shooting the IGS13 into kai tak manually with raw data it definitely was one 😄 

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I'm not sure if it's the proper procedure, but what I do is dial in the new alt on the MCP, then change the cruise alt in the FMC.
Once in a while the airplane will not seem to respond. If/when that happens I'll just click the VNAV button on the MCP off then on again.
That seems to wake it up.

If ATC has instructed me to 'descend via the XXXXX approach' (STAR) I'll select one of the waypoints where the alt restriction is in the 4-7,000' range AGL (for example 6,000' on app to KLAX would be equivalent to 11,000' on app to KDEN). I'll set the MCP to that alt. I like to start my descent 10-15nm before I reach the calculated TOD in the FMC, especially if I have a healthy tailwind.

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