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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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don't wait until TOD to go down most pilots request descent earlier, or if you don't use atc, start earlier. And put in hard altitude restraints for the approach so the TOD is more realistic. Also put the speed up higher to 300 and above rather than vnav descent - David Lee

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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 know you claim it's not a hardware issue but from the above quote it sure sounds like the thrust is not at idle.  If the throttles were at idle there is no way the plane is going to level off in VNAV SPD mode.  The only way it would do that is if you were carrying a bunch of power.

 

Edit:  If the speed increased to the point where it reaches the redline the mode will change from VNAV PATH to VNAV SPD and it may level off temporaily until it gets the speed back under control.  Although to me it still sounds as if you are not at idle.

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don't wait until TOD to go down most pilots request descent earlier, or if you don't use atc, start earlier. And put in hard altitude restraints for the approach so the TOD is more realistic. Also put the speed up higher to 300 and above rather than vnav descent - David Lee

I'll start my descent a bit early if I'm above FL350, but vnav really should work with the rest.

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I'll start my descent a bit early if I'm above FL350, but vnav really should work with the rest.

 

vnav descent on a 737 can sometimes be around 250 knots! No thanks! I like a general 300 or better even on the 777 . David Lee

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I know you claim it's not a hardware issue but from the above quote it sure sounds like the thrust is not at idle.  If the throttles were at idle there is no way the plane is going to level off in VNAV SPD mode.  The only way it would do that is if you were carrying a bunch of power.

 

Edit:  If the speed increased to the point where it reaches the redline the mode will change from VNAV PATH to VNAV SPD and it may level off temporaily until it gets the speed back under control.  Although to me it still sounds as if you are not at idle.

 

The settings in the FMC are correct.

 

I move the joystick throttle to idle.

 

The engines are around 40% (which is normal according to what I have read on here). If I hit F1, it overides the system and puts it down to around 30%... which causes issues upon descent.

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vnav descent on a 737 can sometimes be around 250 knots! No thanks! I like a general 300 or better even on the 777 . David Lee

Higher speeds are good, but that's independent of the mode. You can do 300 in LVL CHG or VNAV or VS.

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I did two different flights this morning. (Short trips)

 

This time, anytime the FMC msg came on saying "Drag Required" I activated the speed brake until the speed dropped to under the target speed, then released it. The speed continued to slowly climb up on at least two occasions, requiring the speed brake to be used.  However, I didn't have any major speed or outside the flight path issues come approach time.

 

I never put any winds in the forecast DESC page, as some here have suggested not to do.

 

So, thanks to all who helped on that one.

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The procedure that I use for descent from cruise.

 

Descend at 280/.78 until speed restrictions.
 
Start descent 10 miles before T/D using DESCEND NOW on the DESCENT page. The NGX descends at 1000 fpm until it is back on profile and at a 1000 fpm the speed stays right below 280 kts.  This will usually eat up about 3-4000 ft of the descent.
 
Descent forecast is all tailwinds no matter what PFPX, ASN, or AWC Forecast Winds may indicate. An example would be a STAR heading 155 degrees. I would enter three level of wind at 335 degrees with the wind speed increasing with each lower level entered. This will require thrust to be used pretty much during the whole descent if you try to keep the speed close to the target speed. I prefer thrust to the speedbrakes.
 
Turn A/T off it looks like FMC Speed is going to kick in and blow the descent to hell. Sometimes FMC Speed can see 5 miles in front of itself. An example is descending at 11000 ft and the speed has decayed to 243 kts. and a 240 kts/10000 is only 4 mile ahead. What happens?  ARM switches to FMC Speed and the NGX accelerates to 250 kts. By the time it reaches 250 kts it is time to slow to 240 kts. That why I turn it off sometimes. Just remember to turn A/T back on before it’d too late and you fall out of the sky.
 
At 11-12000 ft. use V/S until touchdown.

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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.

There is a problem with the NGX which causes thrust to stay above idle as the aircraft descends. Using F1 will override this and give you the correct idle value. Last time this issue came up in the forum Ryan said it was on the list of things for the next update. Using F1 may help your problem. However as Dan said, if the CDU says DRAG REQUIRED add some drag. Don't expect VNAV to sort it out for you. As you've seen it will eventually reduce speed but at the expense of not being on profile.

 

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?

Adding more waypoints to ASN is a complete waste of time. In the descent phase wind predictions are based purely on altitude using the profile that PFPX has calculated. Simply insert the predictions as PFPX gave them to you. ASN doesn't need a flight plan to give you the current wind at a given position and altitude and adding more waypoints to its route will not give you better results. Note, the current wind ASN computes may well be different to the forecast wind PFPX gave you.

 

In essence the key is not to imagine VNAV can do everything and that speedbrake should not be necessary. Monitor airspeed and if it's looking to be increasing add some drag to keep it in check. That way you shouldn't need a lot of drag all the time later on. Also bear in mind that the 737 speedbrake is rather ineffective so you may need more of it for longer than you might expect.

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Turn A/T off it looks like FMC Speed is going to kick in and blow the descent to hell. Sometimes FMC Speed can see 5 miles in front of itself. An example is descending at 11000 ft and the speed has decayed to 243 kts. and a 240 kts/10000 is only 4 mile ahead. What happens? ARM switches to FMC Speed and the NGX accelerates to 250 kts. By the time it reaches 250 kts it is time to slow to 240 kts. That why I turn it off sometimes. Just remember to turn A/T back on before it’d too late and you fall out of the sky.

 

Why not simply add thrust before you get to FMC SPEED? That way you don't have to kill the A/T.

 


At 11-12000 ft. use V/S until touchdown.

 

VNAV is best if you're still on a STAR with restrictions. LVL CHG is best if you just need to get down. V/S is nice and smooth for shallow descents or smoothing out a bumpy ride. I fly with a few guys who use V/S quite a bit, but they are in very small minority and are apt to complain about missing the MD-80. ;)

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