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Guest Andrea

Unable to follow descent path

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Guest Andrea

Mmmm... it's the third time that I try to approach (737 600 with 47784 Kg Zero Full Weight and 10500 Kg Fuel) LIML airport using a proper STAR without success. It's supposed that the the FMS is able to compute a feasable descend path before starting it and to follow it afterwards, and is what I do before arriving to the T/D point. Ok, as soon as the "drag required" message appears I turn on the spoilers, but it is not enough... few seconds later... "dling dlong"... I receive the "unable to follow descent path" message, LVL CHG button switches on, VNAV switches off and descent speed appears in IAS indicator.Andreahttp://www.precisionmanuals.com/images/forum/800driver.jpghttp://fs2crew.com/linepilot.jpg

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>Mmmm... it's the third time that I try to approach (737 600>with 47784 Kg Zero Full Weight and 10500 Kg Fuel) LIML airport>using a proper STAR without success. It's supposed that the>the FMS is able to compute a feasable descend path before>starting it and to follow it afterwards, and is what I do>before arriving to the T/D point. Ok, as soon as the "drag>required" message appears I turn on the spoilers, but it is>not enough... few seconds later... "dling dlong"... I receive>the "unable to follow descent path" message, LVL CHG button>switches on, VNAV switches off and descent speed appears in>IAS indicator.>>Andrea>>http://www.precisionmanuals.com/images/forum/800driver.jpg>http://fs2crew.com/linepilot.jpg 1] There is a BUG that relates to having certain HARD restrictions on the PATH. At times you may need to cycle the A/P or remove as many "restrictions" as possible. 2] Sometimes it is not this bug but asking the FMC to perform mind bending physics while descending. Remember that the real FMC (and this one too) judge the *path* wpt to wpt and if that path angle exceeds a given degree for two given altitude ranges it will cause VNAV to disconnect. 3] It makes since to set the MCP to the next cleared altitude insted of the miss app altitude that many set it to in descent. 4] Try VNAV SPD. 5] Try not putting any restrictions close to the Airport restriction (240/10000). 6] In rl and the sim flying a descent does not always entail VNAV alone, when workload becomes more of a burden to program the FMC than simply reseting the MCP and pushing LVL then the choice is clear. 7] It's not perfect, hopefully this one item gets fixed sometime in the future since it is the only remaining thing that really bugs the **** out of me ;-)Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/southparkcartmad.gif[h3]PMDG 747![/h3]Caution! Not a real pilot, but do play one on TV ;-)ASUS KV8 DLX | AMD 3200 64 | 1 GIG PC 3200 DDR | GIGABYTE 5700 ULTRA | ViewSonic VP192b 19" |

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Guest Andrea

Many thanks Randy,your specifications make me sure that my problem is due to the bug.By the way, hey PMDG... when do you think to fix this issue?In addition to the 240/10000 restriction (I can see the corresponding green circles along the path: the first where the plane starts to reduce the speed towards restricted 240 Knots, the second where the plane should have reached the 240 Knots) I have only two restrictions: one 8000B, another 160/3000 near to the final path (I can see the "Decelerate" green spot along the path). I tried to remove all restrictions (except for the 240/10000, but the plane arrives too high at the glideslope ILS interception and rejects to enable ILS APP after having catched the localizer.PS: what do you think about Ariane Boeing 737-900 Next Generation. It seems to be better than PMDG, at least on the paper. Do you know if and where they have a support forum? Andreahttp://www.precisionmanuals.com/images/forum/800driver.jpghttp://fs2crew.com/linepilot.jpg

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The ariane does not have 1/10th of the systems this bird does, nuff said about that. As for the path, you cannot expect the FMS to DEScend and slow at the same time, at times you MUST help it to accomplish this and means using the boards. If you are coming at 240 and all of a sudden have 160 knots just how in the world do you expect an aircraft to slow on it's own and on top of that KEEP a restriction? VNAV speed will do this better, that is why VNAV SPD DEScents are perfered when one has RESTRICTIONS ON THE PATH. VNAV SPD will not keep PATH, it keeps speed, VNAV PATH keeps path and not SPEED. While there is ONE bug concerning VNAV here the other issues have to do with you the pilot. As been stated more than a few times on this forum by rl pilots, DEScent is the hardest phase of flight, one cannot simply program the FMS and sit back idle and wonder why the plane is not doing what one thinks it should. Try using the FORECASTS page for head and tail wind path calculations to help VNAV plan the descent better. Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/southparkcartmad.gif[h3]PMDG 747![/h3]Caution! Not a real pilot, but do play one on TV ;-)ASUS KV8 DLX | AMD 3200 64 | 1 GIG PC 3200 DDR | GIGABYTE 5700 ULTRA | ViewSonic VP192b 19" |

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Guest nattydread

Use the in-game ATC, it always tosses your STARs out the window, forces you to bypass all that stuff while you are Radar Vectored. It gets you using the Heading Bug, LvL CNG and V/S functions instead. Now you dumping speedbrakes and just flying vectors and while you do all that you can MOD your plan and have the ILS up ready to be executed/engaged...I hear thats what the do in real life :) It seems your radar vectors will mirror your STAR closely, but ATC will modify to fit their need to cram as many planes as they can on approach in 30 sec-1 min intervals.I personally enjoy it...but then again I like to do manual approaches and no auto-pilot once they start radar vectoring, sometimes no auto-pilot after the Top of Descent.After a flight I talked to a SWA Captain who was training the F/O, he said they hand fly it from the start of descent from cruise all the way down to touchdown...if thats how they do it, so will I. I

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Guest Andrea

Thanks Randy,I tried to remove all restrictions along the descent path to LIML and to leave only the standard 240/1000 and a 160/3000 at the latest point before glideslope interception. I haven't been idle during VNAV descent. I've activated spoilers every time the FMS asked "drag required" (but there were no tailwind) and everything worked well. It remains the problem that a standard STAR with real restrictions isn't reachable due to a known FMS "bug".Hey PMDG... hey Robert, when do you think to fix this issue?PS: Ok Randy, I can imagine that an aircraft cannot decelerate and descend contemporarily at the same rate as when it descends at constant speed (it's physics).Andreahttp://www.precisionmanuals.com/images/forum/800driver.jpghttp://fs2crew.com/linepilot.jpg

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Guest gustav.m

"PS: what do you think about Ariane Boeing 737-900 Next Generation. It seems to be better than PMDG, at least on the paper. Do you know if and where they have a support forum?"Andrea. Suggest you do a search on Avsim for, "ariane".Regards,Gustav.

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Its probably not the right thing to do but I select a waypoint on the STAR that is, say, 12NM from the runway and enter in a value of say, 210/3000. This forces T/D to be recalculated to a point earlier along the path. (Obviously do this well before T/D). The aircraft will then follow VNAV. Spoilers may still be required but it wont disconnect VNAV. Not pretty and maybe a cheat but it works.

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Andrea,Normally I would skip trying to explain VNAV PTH descents on the forum because of the complexity of the subject but I actually have nothing to do right now.Just remember that VNAV PTH descents are not mastered overnight. Also, VNAV PTH descents can occasionally whip the butt's of seasoned airline pilots. It's not something that can be learned easily in a forum. It just takes a lot of practice by flying your PMDG SIM and reading the proper manual. There are just too many possible variables for the reasons that you can not fly a successful VNAV PTH descent in the STAR you mentioned above.But wait, some of these things may help you:- During a VNAV PTH descent, the airplane will pitch up or pitch down as necessary to stay on the PATH regardless of the TARGET SPEED in the FMC. It doesn't try to maintain TARGET SPEED. The Autoflight/FMC controls the PATH and the pilot controls the SPEED. Too fast, SPEED BRAKES up always when below 10000 feet and optional above 10000 feet unless there is a DECEL approaching. Too slow, 10-15 knots below TARGET SPEED manually add thrust or the auto throttles will add thrust to get back to target speed if the speed gets to far below target speed. -Above 10000 feet the airplane can go above target speed to VMO/MMO before it VNAV DISCONNECTS.-Below 10000 feet, it is imperative that you do not let the airspeed go more than 10 knots above TARGET SPEED or you will get a VNAV DISCONNECT every time. Use the SPEED BRAKES or FLAPS or LANDING GEAR for DRAG to keep airspeed under control. VNAV PTH can be reengaged again when airspeed is within 10 knots of target speed. - Set the winds of the SIM so that most of the VNAV PTH descent is flown into a headwind or if you know how, go to the FMC Forecast page and insert know descent winds. That will help with the FMC's PATH descent prediction. -Keep is mind, flying a VNAV PTH descent in a tailwind is much more difficult than flying into a headwind.- Get rid of all the "At or Above" altitude constraints, Ex: 3000A. Make the restraint a hard altitude, Ex: 3000.-Get rid of any ridicules SPEED and ALTITUDE CONSTRAINTS in the LEG's page. Impossible physicist.-Anticipate DECELS (little green circles on the MAP) that are ahead, throw out the drag, SPEED BRAKES, FLAPS, or LANDING GEAR as needed when you do get to the DECEL point to controll airspeed especially below 10000 feet.- Monitor the PFD's IVSI during a VNAV PTH descent. When you see a magenta target dot on the IVSI this is wrong, this should never be there while in a VNAV PTH descent, that is the "bug". Immediately push the VNAV button twice then check the IVSI to see if the magenta target dot is gone. - Always, always, always, always scan the FMA (Flight Mode Annunciator) to insure VNAV PTH is still engaged.I suggest you purchase a copy of the "FMC User's Guide B737" by Bill Bulfer or something comparable. His manual has a lot of good information about VNAV PTH descents and everything about the B737NG FMC you ever wanted to know. I have no affiliation with MR. Bulfer, just like his manual.GOOD LUCK and HAPPY NEW YEAR!Floyd

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Great post! One correction if I may -"-Below 10000 feet, it is imperative that you do not let the airspeed go more than 10 knots above TARGET SPEED or you will get a VNAV DISCONNECT every time." It's actually 15 knots over TARGET not 10 ;-) BUT you will get the "Drag REQ" in the scratchpad at 10..Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/southparkcartmad.gif[h3]PMDG 747![/h3]Caution! Not a real pilot, but do play one on TV ;-)ASUS KV8 DLX | AMD 3200 64 | 1 GIG PC 3200 DDR | GIGABYTE 5700 ULTRA | ViewSonic VP192b 19" |

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Guest Andrea

Many thanks Floyd,your explanation is very clear. You and some other guys in the forum (like Randy) are "living 737 manuals".I think that PMDG team should also treat this kind of broadenings in the manuals to help people to understand correctly how to interpret some aircraft behaviors that otherwise, in a superficial manner like mine, seem to be inexplicable. And this adds more value to the work done by PMDG.Andreahttp://www.precisionmanuals.com/images/forum/800driver.jpghttp://fs2crew.com/linepilot.jpg

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I thought of doing my own "How to" for the 737 but for one reason or another I never got around to doing it. For the 744 it will be different as I have been doing nothing but studying the various aspects of the systems to be able to provide a user friendly way to use the FMC and AFDS in flight sim. It's not too far off now so stay tuned for some goodies ;-)Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/southparkcartmad.gif[h3]PMDG 747![/h3]Caution! Not a real pilot, but do play one on TV ;-)ASUS KV8 DLX | AMD 3200 64 | 1 GIG PC 3200 DDR | GIGABYTE 5700 ULTRA | ViewSonic VP192b 19" |

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>as I have been doing nothing but studying>the various aspects of the systems to be able to provide a>user friendly way to use the FMC and AFDS in flight sim. It's>not too far off now so stay tuned for some goodies ;-)Randy,You should reconsider doing such a thing. You should live up to your image as a mean spirited cop of this forum. Please do not mellow and ruin your reputation. :-lolMichael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

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