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787WannabePilot

ASN + PMDG + Descent Issues

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Greetings,

 

So, I am feeling pretty comfortable with flying the 737 following the tutorials and other guides except for the descending segment. (I love manual landing with the HGS. I rarely use the ILS anymore)

 

I am still having trouble with descent. I am OK (speed and flight path follows the guidance) until I drop under 18k. Then the plan speeds up and calls for "more drag".  It compensates by switching into VNV Speed mode, which levels the plane but I am then way over the flight path and need to drop altitude. I find one trick by increasing the speed via LVL CHANGE, which helps somewhat.. but only until I get into the 10K area, where I need to drop under 250k.

 

When that happens, I am riding the speed brake like my life is depending on it. I tend to extend flaps to '1' pretty early (maybe earlier than I need to) just to keep the speed under control.

 

I did some further reading and found the forecasted winds input into the DES section of FMC. I screamed "That has to be it!" (I was fortunate to have the upload link from 777 to help me, but 737 doesn't have it). I then put the "Windows Aloft" data into the FMC using ASN's addon tab in FSX. However, I realized that the information I was putting in wasn't from the waypoints of where I was descending, but the winds at various flight levels on where my plane currently was (duh!). 

 

I then remembered that the flight plan I was exporting from PFPX, didn't include the STARS. So, when I loaded the flight plan into ASN, it was missing key waypoints that I needed. I attempted to manually add the waypoints, but the majority of them ASN didn't recognize. The last waypoint for ASN was one or two right after the TOD, nothing more.

 

So, what do I need to change? Or where do I go from here?

 

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So, what do I need to change? Or where do I go from here?

 

First - the use of wind data is highly subjective. Even though ASN gives you "waypoint by waypoint" wind, it's using estimates based on interpolation. Why? Wind data is only sparely collected. Note the huge gaps between points here:

fball.gif

 

This means that anywhere between the red dots is technically "unknown." Given the fluid dynamics of air, however, it's unlikely that there are drastic differences, so interpolation usually gets you close estimates of the wind over points in between, but it's worth mentioning because your T/D wind (or final core route - non SID/STAR - fix) is likely a close enough estimate.

 

The 737, as mentioned in the manuals (and I believe in the tutorials), is a very slippery aircraft, so it requires you to be on top of things. Keep in mind, as well, that you need to pay attention to your hardware. If you've left it where you set it during the takeoff roll, this will influence your descent path, as A/T can be overridden in HOLD/ARM mode. You can either pull your hardware back to the idle stop prior to T/D, or you can change this setting to ignore your hardware at all times in the PMDG SETUP> menus.

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Everytime I try to upload wind data into the 777 FMC with ASN running, it returns: "invalid forecast link" or words to that effect...

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Everytime I try to upload wind data into the 777 FMC with ASN running, it returns: "invalid forecast link" or words to that effect...

 

Check the intro manual for the requirements to get this working. I believe it's also covered in Tutorial #1.5. Either way, there are extra steps other than simply having ASN and the 777.

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My two cents from the plane:

 

I flew the 737 for years without putting in a descent forecast. The plane pretty much does a good job of handling it. A third of the descents you're adding power at some point, a third getting fast, and a third of the time it's on speed.

 

Now we have fancy datalink that automatically gives us the descent forecast winds. Guess what: A third of the descents you're adding power at some point, a third getting fast, and a third of the time it's on speed.

 

Slippery does not equal speedbrake. Slippery is (?) accounted for in the drag coefficient that's hidden in the FMC. You can actually go into the box and change it in the plane, RW.

 

Maybe people add power if they get slow, but it seems like they never complain about it here. If the models were correct, you should sometimes be getting slow and sometimes getting fast.

 

So, it's most likely hardware or the profile.

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I will second Matt's post.  As a matter of habit once I reach cruise I enter the forecast winds for FL340, FL240 and 14,000 feet.  There have been times where I have entered no winds at all and honestly do not see much difference.  The only parameter on the descent forecast page that really has much impact on the descent profile in my experience is the anti-icing on and off altitudes.  I have seen many techniques for entering data on the forecast page and the end results are all pretty much the same.  This is one aspect I think simmers are focusing more attention on than they really need to.

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Then the plan speeds up and calls for "more drag". It compensates by switching into VNV Speed mode, which levels the plane but I am then way over the flight path and need to drop altitude.

 

Robert:  Both Matt and Joe above are experienced 737 jocks so listen to them.

 

All I can add is the switch from path to speed pitch mode occurs long after the need for drag arises. You might be able to avoid that with experience and keeping an eye on trends.

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Thanks guys.

 

It's definitely not a hardware issue. I checked the PMDG setup, and I also put the throttle at idle prior to descent. (It's on the checklist I have)

 

So, if it's a profile, what should I check?

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the drag coefficient that's hidden in the FMC. You can actually go into the box and change it in the plane, RW.

Can you actually change it by making a entry in the CDU or does maintenance need to make the change inside the CDU?

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Robert:  Both Matt and Joe above are experienced 737 jocks so listen to them.

 

All I can add is the switch from path to speed pitch mode occurs long after the need for drag arises. You might be able to avoid that with experience and keeping an eye on trends.

 

True.

 

Is it common to *slam* on the speed brakes while descending from that far up?

 

In some cases I probably would be riding them on the way down.

Can you actually change it by making a entry in the CDU or does maintenance need to make the change inside the CDU?

 

Curious myself as well.

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Robert,

 

Fly the VNAV profile to 10,000 feet then FLCH to landing.  Make sure the A/T is at or near flight idle during descent phase of flight.

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Can you actually change it by making a entry in the CDU or does maintenance need to make the change inside the CDU?

It seems on my companies planes, the boxes differ a fair amount. Some have prompts for all sorts of things and some are missing. I think the ETOPS birds have a lot more menus available. But, I do believe one could change it via the fmc.

 

I don't like to poke around in those boxes much for fear of messing a setting like this up.

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Is it common to *slam* on the speed brakes while descending from that far up?

 

You said you were okay until below FL180, which I don't consider "that far up" so I am a little confused by your question.

 

It's normal to use speedbrakes above the 10,000 250 kt speed constraint up to early phase of descent.  I use them on a minority of my descents, maybe 1/3 as Matt suggested but could be more or less. Several places I visit routinely descend at mach number until 280 Kts then 280 kts (per STAR) and not even that low of an indicated presents a problem most of the time.

 

If you are having a consistent problem then lets get some specifics from you such as final cruise altitude, arrival route, FMS descent speed, CI and description of actions required as pilot.  I suspect that once you are able to recognize speed trends and respond with speed brake before it goes into VNAV SPD pitch mode you will be fine.

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You said you were okay until below FL180, which I don't consider "that far up" so I am a little confused by your question.

 

It's normal to use speedbrakes above the 10,000 250 kt speed constraint up to early phase of descent.  I use them on a minority of my descents, maybe 1/3 as Matt suggested but could be more or less. Several places I visit routinely descend at mach number until 280 Kts then 280 kts (per STAR) and not even that low of an indicated presents a problem most of the time.

 

If you are having a consistent problem then lets get some specifics from you such as final cruise altitude, arrival route, FMS descent speed, CI and description of actions required as pilot.  I suspect that once you are able to recognize speed trends and respond with speed brake before it goes into VNAV SPD pitch mode you will be fine.

 

My apologies.

 

I viewed anything above 10k as "far up".

 

I will put together a flight, and record that information and post it.

 

It has happened on all my flights.. not just 1/3rd.

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