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rmhoman

PMDG 747X Porpoising

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I was searching the forum to no avail to find out if anyone else is having this problem. I have ASX and FS2Crew installed for my 747. The problem is that I get to altitude and the plane starts to pitch up and down quite vilently (1200-2000 fpm) I have also noticed that sometimes after my initial climb out that the plane wants to climb to the cruise atl at a very steep rate 5000fpm then it will drop the nose pick up speed and then climb steeply again. any ideas on what is causing this. I want to keep ASX and FS2crew running but was wondering if there is a setting that needs to be tweeked that I am missing. Thanks for your timeRussell Homan

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ASX isn't the problem, but the FSX wx engine probably is... use a registered copy of FSUIPC to smooth the winds instead of that feature in ASX. To prove it is wx related, try it with clear wx (no wx). I dampen the FSX winds down to 1 deg/kt per 8 seconds... doesn't help the slow shifts in wind direction I still see (the jet over NAmer swings from 300 to 020??) but at least the violent stuff is gone.

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ASX isn't the problem, but the FSX wx engine probably is... use a registered copy of FSUIPC to smooth the winds instead of that feature in ASX. To prove it is wx related, try it with clear wx (no wx). I dampen the FSX winds down to 1 deg/kt per 8 seconds... doesn't help the slow shifts in wind direction I still see (the jet over NAmer swings from 300 to 020??) but at least the violent stuff is gone.
I'm right to assume that it's pretty much impossible to do FULL CDA approaches, even with a correct setup of the VNAV FORECAST, because of FSX wind shifts? For me it's always hard to maintain thrust at IDLE/HOLD from FL100 to final approach when on a "CDA STAR". From CRZ to FL100 it is better though.I haven't got FSUIPC myself registered (thinking of buying), but I recall a thread (actually look here) about wind smoothing on Peter Dawson's support forum. He said this (note: it was two months ago):"The earlier posting on this was actually mentioning a stupidly slow smoothing rate of some 1 degree /knot every 4 seconds. At such a rate the wind will almost never be what it should be. It's a wonder the AI traffic at airports aren't conflicting all the time with your own flights.With more sensible smoothing rates like 1 for 1, the Wind should never get that far behind what it should be that it is going to matter so much."Could someone comment on the statement above?

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Sammy, thanks for sharing that. Pete knows of what he speaks... I've been very frustrated with wind directions so maybe I have been too agressive with my smoothing values. I need to play with that, maybe that will resolve the Easterly jetstreams.I recommend you spend for the registered copy of FSUIPC. Most would agree that this is the single most important add-on to have for MSFS. You should not have a problem flying a CDA approach in any of the PMDG aircraft, in fact this was discussed in a thread in the past month or so.

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"The earlier posting on this was actually mentioning a stupidly slow smoothing rate of some 1 degree /knot every 4 seconds. At such a rate the wind will almost never be what it should be.
I have a hard time with the above statement. If you look at any typical weather charts with winds aloft, 1 degree/knot/4 sec rate of change seems like it should be plenty enough to handle any realistic change of upper winds, including the most drastic changes (outside of major storms).

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Sammy, thanks for sharing that. Pete knows of what he speaks... I've been very frustrated with wind directions so maybe I have been too agressive with my smoothing values. I need to play with that, maybe that will resolve the Easterly jetstreams.I recommend you spend for the registered copy of FSUIPC. Most would agree that this is the single most important add-on to have for MSFS. You should not have a problem flying a CDA approach in any of the PMDG aircraft, in fact this was discussed in a thread in the past month or so.
I'm sorry to ask this, but could you point me to that thread? Would be very greatful as I can't find it.

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For me it's always hard to maintain thrust at IDLE/HOLD from FL100 to final approach when on a "CDA STAR".
But being at IDLE thrust is not an absolute requirement for a well executed CDA though would be ideal. You should not be far from idle but some adjustments in thrust are inevitable.There are some good articles but a bit technical about Continuous Descent Approach for example into Louisville airport written by Boeing and others (easy to Google). For example I am looking at: Continuous Descent Approach: Design and Flight Test for Louisville International Airport. From the many graphs you can clearly see how often and when the auto-throttle system had to increase power during such an approach. There is plenty of data with explanations why it was the case. There is also a fair amount of discussion about influence of winds.

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But being at IDLE thrust is not an absolute requirement for a well executed CDA though would be ideal. You should not be far from idle but some adjustments in thrust are inevitable.There are some good articles but a bit technical about Continuous Descent Approach for example into Louisville airport written by Boeing and others (easy to Google). For example I am looking at: Continuous Descent Approach: Design and Flight Test for Louisville International Airport. From the many graphs you can clearly see how often and when the auto-throttle system had to increase power during such an approach. There is plenty of data with explanations why it was the case. There is also a fair amount of discussion about influence of winds.
Well, I'm actually taking the auto throttle inv. into account and consider CDA approaches with them a success. So I get HOLD | LNAV | VNAV PTH on the FMA which is what i strive for and get the occasional SPD | LNAV | VNAV PTH in order to be in ECON DES SPD and that is okay (but I try to limit them by filling VNAV FORECAST page).But from FL100 to final approach, when the SPD sets to 250 or less due to the CDA STAR instructions... then I get:SPD | LNAV | VNAV PTH all the way down (that is not good). I think it's due to FSX wind shifts, which makes it hard for the FMS to calc. the best descent path (even with a correct VNAV FORECAST). But I'll study, try to perfect my technique and will get back to you - because the fault might as well lie in there as I'm a newbie. Thanks for the docs suggestions, I'll look at them tomorrow. Notice, the STAR I'm talking about is a specifc CDA approach.

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I think it's due to FSX wind shifts, which makes it hard for the FMS to calc. the best descent path (even with a correct VNAV FORECAST).
Perhaps it is simply due to poor wind prediction (higher head wind?) rather than "shifts". You would think that frequent shifts would cancel each other and overall had neutral effect. This is all easy to test - fly with no weather and see how it goes.

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Perhaps it is simply due to poor wind prediction (higher head wind?) rather than "shifts". You would think that frequent shifts would cancel each other and overall had neutral effect. This is all easy to test - fly with no weather and see how it goes.
Sure, will try it next time.

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Also try to slow down.. ATC speed restrictions in busy terminal areas are common. In fact, some newer RNAV STARs have many of them that slow you to 280 before entering approach airspace (40nm out) then further decreases to 210 at about 10 nm and 190 kts at 5 nm. The nice thing about going slower is it makes it much easier to dump altitude with moderate descent rates. The updated RNAV STARs for KPHX that come out tomorrow are a good example of this flow control on published charts.Keep 250+ KIAS all the way to the airport will result in high descent rates.

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Also try to slow down.. ATC speed restrictions in busy terminal areas are common. In fact, some newer RNAV STARs have many of them that slow you to 280 before entering approach airspace (40nm out) then further decreases to 210 at about 10 nm and 190 kts at 5 nm. The nice thing about going slower is it makes it much easier to dump altitude with moderate descent rates. The updated RNAV STARs for KPHX that come out tomorrow are a good example of this flow control on published charts.Keep 250+ KIAS all the way to the airport will result in high descent rates.
Shall I alter the VNAV DES ECON SPD (will this really be economical?)? If I use SPD inv - VNAV goes to VNAV SPD (I want to avoid that).

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I use ECON descent speed until 40 nm out or below 10,000, which is normal. At 10,000 you should be no closer than 40 nm anyway, since this yields 250 ft/nm (to sea level). The key point is to slow down within 40nm.

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I use ECON descent speed until 40 nm out or below 10,000, which is normal. At 10,000 you should be no closer than 40 nm anyway, since this yields 250 ft/nm (to sea level). The key point is to slow down within 40nm.
Yes, how do you do this? Speed inv, altering ECON SPD, manually entering every WPT in LEGS page to set 250 SPD, alter 250/10000 VNAV SPD/RES (and/or TRANS)? Sorry for nagging but I find this topic very interesting as I'm trying to perfect my own technique.

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Depends. In the 737/747 I start using MCP SPD and V/S for descent/speed control below FL180. In the MD11 it depends on the arrival. If it doesn't have a speed constraint in the STAR I will either add one or start using FCP SPEED, usually the STAR constraints are all the MD11 with it's superior PROF verses the VNAV in the Boeings.

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Depends. In the 737/747 I start using MCP SPD and V/S for descent/speed control below FL180. In the MD11 it depends on the arrival. If it doesn't have a speed constraint in the STAR I will either add one or start using FCP SPEED, usually the STAR constraints are all the MD11 with it's superior PROF verses the VNAV in the Boeings.
Okay, but when switching from VNAV/PROF to anything else in a CDA approach totally messes up the profile and then it isn't really a CDA anymore? CDA approaches that I fly requires P-RNAV, which are suppose to "automatically determine aircraft desired flight path by a series of waypoints held in a database". Isn't the desired profile/path to be "selected" by the civil aviation authority (like FAA) in the country and just flown automatically by the P-RNAV equiped aircraft (using profile mode in order to be as close to the desired descent path)?Naturally, I'm a bit confused. Can you please explain further?

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Are we talking about descents prior to the final approach or the final approach itself?Isn't a constant descent approach a technique rather than a type of approach such as RNP, RNAV LPV, etc? (I think P-RNAV is the European equivalent of the US RNP)I thought your problem was descending from 10000 to the approach, once on the approach fly the technique and type of approach you wish and/or are cleared for by ATC. I have limited my recommendations to before IAF/FAF. Re-reading your comments, I see you referred to a CDA STAR. I've never run into one of those... got an example?The FAA RNP approaches have the altitudes included in the approach procedure used by the FMS and in this case yes, you want to be in the VNAV/PROF mode. Agreed.

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Are we talking about descents prior to the final approach or the final approach itself?Isn't a constant descent approach a technique rather than a type of approach such as RNP, RNAV LPV, etc? (I think P-RNAV is the European equivalent of the US RNP)I thought your problem was descending from 10000 to the approach, once on the approach fly the technique and type of approach you wish and/or are cleared for by ATC. I have limited my recommendations to before IAF/FAF. Re-reading your comments, I see you referred to a CDA STAR. I've never run into one of those... got an example?The FAA RNP approaches have the altitudes included in the approach procedure used by the FMS and in this case yes, you want to be in the VNAV/PROF mode. Agreed.
Ah, misunderstanding. Yeah, I was talking about Continuous Descent Approach (CDA) STARS or Optimized Profile Descent (OPD) STARS (like the ones used at London Heathrow Airport, Stockholm-Arlanda Airport and Los Angeles International Airport).

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Which ones at EGLL are OPD?At KLAX, I suppose the CIVET arrival could be thought of as a CDA STAR, never heard it put that way. I notice they bracket the altitude constraints and my custom sidstar has the calculated crossing altitude to maintain a reasonably constant descent rate, but the STAR isn't designed for RNP use. On this STAR, I let the MD11 do it's majic all the way from DAG VOR to rollout but in the 747 I use MCP SPD and VS using the descent rate to approximately following the verticle deviation indicator that appears on the ND. But, I never considered this a special kind of arrival except KLAX is one of the few US locations that have different legs to each one of the parallel runways on the STAR.

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Which ones at EGLL are OPD?At KLAX, I suppose the CIVET arrival could be thought of as a CDA STAR, never heard it put that way. I notice they bracket the altitude constraints and my custom sidstar has the calculated crossing altitude to maintain a reasonably constant descent rate, but the STAR isn't designed for RNP use. On this STAR, I let the MD11 do it's majic all the way from DAG VOR to rollout but in the 747 I use MCP SPD and VS using the descent rate to approximately following the verticle deviation indicator that appears on the ND. But, I never considered this a special kind of arrival except KLAX is one of the few US locations that have different legs to each one of the parallel runways on the STAR.
Here, I found a CDA-approach. You can see that there are alt. constraints all the way to FAF (so that VNAV can calculate ILDE/HOLD approach I think). Here the goal is to make an IDLE thrust approach all the way down to final approach.

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Hey, I need to try this one soon. Thanks.Okay, the instructions to pilot request a constant descent from 10000 to FAP at 2500 but none of the altitude constraints are hard constraints (they are at or above). The MD11 should pull this off quite easily but in the 737/747 I would definitely not use VNAV but the MCP SPD and V/S as discussed earlier. I notice the constraints allow a descent angle of about 250 ft/nm, just as a point of interest.

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Here the goal is to make an IDLE thrust approach all the way down to final approach.
Complete idle though full duration of this CDA is simply not achievable in real life (as Boeing's study indicates). The goal is to eliminate or reduce number of "step downs" - when engines have to rev up.

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Complete idle though full duration of this CDA is simply not achievable in real life (as Boeing's study indicates). The goal is to eliminate or reduce number of "step downs" - when engines have to rev up.
EDITED: Oh, I'm not saying that it should be COMPLETE IDLE. It's the goal. I read in the docs you suggested above, how for example a sudden tailwind can make the autothrottle to become active. And as I said before, from own experience in replies above, that the occasional "autothrottle intervention" is NOT the problem. But the main goal is to have IDLE thrust as long as possible all the way down (even to the threshold, but that depends from airport to airport). That's what I have a problem with.

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