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Slick9

737NG (FS9) VNAV descent

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I fly both the PMDG 737NG and the 747-400 and Freighter. On the 747s usually once I've been given descent clearance from cruising altitude I'll tune in a lower altitude on the MCP, then I hit the DES NOW button on the FMC and basically I'll let the VNAV function control the rate of descent (using the throttle to adjust as needed) all the way down through 10,000 feet. where I'll swith to the V/S. This normally gives me about a 1800-2000 ft/min descent rate give or take depending on speed adjustments. I used to use this same method on the 737NG until recently, now when I hit the DES NOW button on the FMC the 737 will start descending, but the problem is that the descent rate is only between 1000 and 1300 ft/min (I retard the throttles also) which leaves me too high to make my crossing restrictions most of the time. Is there a way to increase the rate of descent that the FMC calculates on the 737NG? Thanks for the help!!Richard

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I'm a little rusty, but I think it is correct that the 737 will descend at about 1000fpm until intercepting the VNAV path then increase descent to stay on path. I thought the 747 would do the same.

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I'm a little rusty, but I think it is correct that the 737 will descend at about 1000fpm until intercepting the VNAV path then increase descent to stay on path. I thought the 747 would do the same.
Yep, Dan is right. The DES NOW function is used for those times when you want or need to start your VNAV descent early. Normally you would just dial in the lowest altitude constraint in the MCP and let the plane fly to the top of descent (TOD). At which point the FMC will command an idle rate of descent that will meet the constraint. If you want to descend early then you can use the DES NOW function which will initiated a 1000fpm descent immediately and then upon joining the originally computed VNAV descent path the FMC will then command the normal descent rate to reach the constraint. There is normally no reason to use DES NOW unless there is a compelling need to start down early.

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I fly both the PMDG 737NG and the 747-400 and Freighter. On the 747s usually once I've been given descent clearance from cruising altitude I'll tune in a lower altitude on the MCP, then I hit the DES NOW button on the FMC and basically I'll let the VNAV function control the rate of descent (using the throttle to adjust as needed) all the way down through 10,000 feet. where I'll swith to the V/S. This normally gives me about a 1800-2000 ft/min descent rate give or take depending on speed adjustments. I used to use this same method on the 737NG until recently, now when I hit the DES NOW button on the FMC the 737 will start descending, but the problem is that the descent rate is only between 1000 and 1300 ft/min (I retard the throttles also) which leaves me too high to make my crossing restrictions most of the time. Is there a way to increase the rate of descent that the FMC calculates on the 737NG? Thanks for the help!!Richard
Richard, it sounds like you may not understand how to properly use the different VNAV modes:DES NOW is used when you want to start a VNAV PTH descent before the computed optimal top of descent (T/D) point, but eventually intercept the normal best economy path down. It commands a rate of descent of 1000FPM until intercepting the path. Watch the path deviation indicator on the right of the ND and you'll see when this happens... VNAV PTH mode is essentially a computer created virtual glideslope, which will take into account the altitude and speed restrictions that exist along the descent. It sounds like you may not have this programmed correctly if you're trying to use DES NOW and VS to hit restrictions - the plane will do it all for you if you have VNAV PTH information correctly set up. There's several good tutorials out there that explain the programming and use of this system within the FMC... (including our Type Rating Course documents for the 747 - the system works in essentially the same way between the two planes)

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Richard, it sounds like you may not understand how to properly use the different VNAV modes:DES NOW is used when you want to start a VNAV PTH descent before the computed optimal top of descent (T/D) point, but eventually intercept the normal best economy path down. It commands a rate of descent of 1000FPM until intercepting the path. Watch the path deviation indicator on the right of the ND and you'll see when this happens... VNAV PTH mode is essentially a computer created virtual glideslope, which will take into account the altitude and speed restrictions that exist along the descent. It sounds like you may not have this programmed correctly if you're trying to use DES NOW and VS to hit restrictions - the plane will do it all for you if you have VNAV PTH information correctly set up. There's several good tutorials out there that explain the programming and use of this system within the FMC... (including our Type Rating Course documents for the 747 - the system works in essentially the same way between the two planes)
Thanks to everyone for all the help here guys, much appreciated. I am starting my descents early which is why I have been using the DES NOW function, I use Radar Contact IV which always gives me an early descent order, usually the descent order comes about 40 to 30 miles ahead of the FMC calculated TOD. When programming the FMC, I import my flightplans from FS Build, and then enter my performance data, takeoff data and go flying. I have seen the descent forcasting page in the FMC but I haven't used it before, I guess from what I'm reading here some sort of manipulation of the FMC descent page is necessary to generate an earlier TOD which would eliminate the need to use the DES NOW function. I'll check out the type rating documents for the 747 and see what's in it.Jack C Posted Today, 06:35 AM In the real plane we use ALT INTV on the MCP. Can you give me a little more info on how you work with ALT INTV. I see the button on the MCP but the only time I've used it on the 737 is when I have a different altitude dialed in on the MCP and I need the autopilot to fly to that altitude. Again thanks for helping in the relentless pursuit of realism!!!Richard

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Hey Richard, how are ya? The ALT INTV operates just like Ryan explained how the Descend Now Line select key works. We just use the ALT INTV in the real thing. The ALT INTV button can also eliminate crossing restrictions on the CDU. By this I mean lets say you are on the descent into an airport. Pretend we are on an Arrival into some airport (I do not have any charts with me right now and the only ones I know off the top of my head are Canadian ones). If the next waypoint on the arrival say cross at 10000 feet, and lets say the controller says "crossing restriction cancelled". You can eliminate the restriction two ways. Go to the CDU and delete the crossing restriction in the 6 LSK or you can just push the ALT INTV which will delete the entry in the 6 LSK for you. Lets say the next 5 waypoints on the arrival have crossing restrictions, for each time you push the ALT INTV button it will eliminate the crossing restrictions on the CDU one at a time. NOW saying that, that is how the real plane works and not sure if it was simulated on the PMDG but I wouldn't be suprised if it was.Also when it comes to planning you descent, if you can place the descent winds on the descent page of the CDU. You should have a wind entry for the TOD, around 20000 feet and one for 10000 feet. Why this helps is the FMS thinks the winds decrease at a linear rate on the descent. SO IF we were cruising at FL300 and the winds were 90 knots the FMS will automatically think the winds at FL200 will be 60 knots and 30 knots at 10000. But what happens if they werent. About a month ago I was flying from Calgary to Vancouver and the winds were about 330/190 knots at FL380. At 27000 they were still 175 knots. I think at FL210 they were still 145 knots. You can see how these winds are not decreasing on a linear scale. This can mess up your descent profile on the way down. In addition to winds you can also throw the local altimeter in for the descent too. This really isn't necessary if the altimeter doesn't vary much from 29.92. But if the altimeter is 30.52 then that is a difference of 500 feet when you switch from a standard altimeter setting to a local pressure setting. This also can through you off a bit when transitioning through FL180 and your FMA might switch from VNAV PTH to VNAV SPD and if you do not catch this in time it might bite you a little bit later on in the descent. Again, I am not sure how accurately this was simulated on the PMDG. Hope this helps a wee bit. Let me know!Jack

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Hi Jack C.Man, thanks for the detailed explanation. I will do a test flight this weekend with the 737 and mess around with the ALT INTV and with the descent planning page and see how it works. I really appreciate the help here. I'll report back after testing over the weekend.Richard

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Guys, I find this subject informative,seems most airlines have SPD INTV & ALT INTV, but it is an option (or at least was) and many airlines, using photos on airliners.net as evidence, don't actually have SPD/ALT INTV on their 737s, classics or NGs.if I understand the above discussion, if you don't have SPD/ALT INVT, and you must modify either VNAV computed speed/alt on the descent, you either modify the FMC entries for speed/alt, staying in VNAV mode or you switch to SPEED & V/S mode, turning off VNAV. Is that correct?so for NDB/DME approaches, without SPD/ALT INTV, these can't be completed using VNAV, must use V/S ?thanksjlm

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ALT INTV should be modelled but probably only if you have the 800/900 addon, I think that was one of the things we added in that along with soft constraints.

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ALT INTV should be modelled but probably only if you have the 800/900 addon, I think that was one of the things we added in that along with soft constraints.
yes its modelled on 800/900, but if using 600/700 and simulatibg real world withou spd intv then how does one operate?jlm (john)

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You know in Radar Contact you can ask for pilot's discretion when you are told to descent? Usually they give it. Select the "PD" option.

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You know in Radar Contact you can ask for pilot's discretion when you are told to descent? Usually they give it. Select the "PD" option.
The Radar Contact controller usually tells you to be at a certain altitude 40nm or so from a waypoint. As another option, you could set a custom waypoint with its altitude restriction making this part of the vertical profile. RegardsDavid Vega

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