[B744] VNAV desc path / use of speed intv knob

[horace 3]

Hi folks,yesterday I used a CI of 80 ...well I was a little bit behind my schedule so I used the speed intervention knob to bring the speed up to M086 with VNAV left engaged. About 30nm before T/D at FL380 ATC instructed me to descend to FL240 so I went to the FMC desc page and activated desc now. I expected a 1200fpm descend but what I got was >3000fpm so I had to use vert spd and forgot about VNAV. I think the VNAV spd for the CI of 80 was about M0844.Now my question: Is this behaviour correct or did I do something wrong? I assumed that when I use spd intervention that this value is given to FMC and used for the VNAV desc calculation.Or do I first have to cancel the speed intervention and afterwards activate the desc now function?I hope you guys understand what I'm trying so say ;)Thanks in advance.

[crvm 0]

Hello Hendrik, Speed intervention while in VNAV is akin to VNAV SPD, in that the aircraft is pitching to maintain the SPD not pitching for the path and if I am correct in my theory :( should well overtake the shallow descent on DES NOW to maintain the SPD. So IMO if you desire to use the DES NOW with the proper descent for that mode then do not use spd intervention...[h4]Randy J. Smith[/h4]AMD 64 4000+|ASUS K8V DELUXE|SAPPHIRE ATI X800XT PE|MUNCHKIN 3200|80 gig SATA|DELL 1905FP 19" LCD|TRACKir PRO|PFC JEPPESEN MOONEY YOKE|CH PRO PEDALS|

[Check_Airman 3]

For such a situation, where u are changing the speed for a prolonged period of time, it is better to enter the speed on the CRZ page of the FMC, that way, your predictions will be correct. When in SPD intervention mode, the FMC calculations assume an immediate return to the FMC speed. Speed intervention is for use over short periods of time.

[horace 3]

Ah yes, sounds logical to me. Thank youI tried two other flights now - with econ crz and econ desc, so nothing modified. 24nm prior t/d i selected desc now to start a gently 1200fpm descend. But unfortunately the aircraft started pitching down tremendously to almost 4000fpm ...then slowly returned to 1200fpm. That happened on both flights I did and doesn't look normal in my opinion. Can anyone comment or confirm this? Thanks

[crvm 0]

Well I cannot say other than what I "think" and take that for what it is worth. IMO the aircraft should keep the shallow descent rate but how the aircraft behaves when it "initially" starts the descent I am not certain, something a real 747 pilot should comment on.[h4]Randy J. Smith[/h4]AMD 64 4000+|ASUS K8V DELUXE|SAPPHIRE ATI X800XT PE|MUNCHKIN 3200|80 gig SATA|DELL 1905FP 19" LCD|TRACKir PRO|PFC JEPPESEN MOONEY YOKE|CH PRO PEDALS|

[emvaos 13]

Hi,When within 50 miles of ToD, resetting MCP altitude and pressing either altitude intervention OR DES NOW orders the FMC to transition from cruise to descend phase....If you press SPD INTV the plane goes into a mode simlar to FLCH down. Throttles are retarded and the AP adjusts pitch for speed. Here is another useful feature : in the PMDG 747 plane you can use an advanced FMC feature related to CRZ phase "reactivation". This is a common and very useful feature for all FMC equipped Boeing planes. All you need to do is perform the ATC ordered early descent, then go back to the now "blank" CRZ page (VNAV pg 2) and enter the new (lower) altitude at L1...Check LEGS and press EXEC...You are now back in CRZ phase at the lower flight level. The FMC calculates a new ToD etc etc. That will save a kilo or two of fuel since you have again the FMC working / calculating optimum descent point/profile. Note that you can use the same feature if a STAR procedure includes an extremely long shallow leg.Best,Vangelis===================================== E. M. Vaos Precision Manuals Development Group www.precisionmanuals.com=====================================

[Guest A32X]

That is a very handy feature! -Paul :-)

[horace 3]

Thanks for your answers guys.>When within 50 miles of ToD, resetting MCP altitude and>pressing either altitude intervention OR DES NOW orders the>FMC to transition from cruise to descend phase....If you press>SPD INTV the plane goes into a mode simlar to FLCH down.>Throttles are retarded and the AP adjusts pitch for speed. Is it really that way and in so far different from the 737 and 767? When I use DESC NOW there the plane reduces speed to ECON DESC and begins with a smooth initial descend of 1000fpm (737) and 1200fpm (767) until it catches the VNAV desc path and increases descend. In the 747 it really behaves like Vangelis said - like FLCH retards Throttles to idle and AP adjusts pitch for speed. I can't image why this should be different on the 747 compared to the 737,767.And i dare to say that this was not that way before Service Update.Can anyone of you try this next time when flying?Maybe there is something corrupted in my installation? I used the PMDG747_400_V1R10.ZIP which I downloaded on 10th December.

[emvaos 13]

Dear Horace,FYI : In the 767-400 if you press DES NOW and "open" speed window via SPD INT then it will do exactly what the 747-400 does...i.e. VNAV SPD thrust to IDLE, then HOLD and behave like FLCH down.Best,Vangelis===================================== E. M. Vaos Precision Manuals Development Group www.precisionmanuals.com=====================================

[Guest D17S]

We have a 747-400 FO on the board and he saw this too:". . . When nearing the T/D I decided to test a function we often use, the "premature" descent. I entered a lower alt in MCP, and pushed on the altitude button, which is the same as hitting the "descend now" button on the FMC. The plane should enter a slow descent, a little over 1000fpm, until the precalculated descent path is reached, then VNAV PTH should reengage. This aircraft started a fairly steep descent, around 4000fpm intitially, then shallowed out to around 50 fpm ROD. It yanked out way too much power, so with speed decaying too much the SPD mode reengaged (fortunately). In this state, with very little rate of descent and almost normal cruise-thrust, the airplane stays until VNAV PTH is reacquired. (Major issue)." (Janov)I have seen it as well. The airplane achieved a rate of descent of 4000 fpm before the system caught up and began to mitigate the event. It was a real E-Ticket ride. Jan thought the event was initiated by too much power being pulled out. But it looked to me like the excessive rate event happened too quickly to have been initiated by thrust. It's a very aggressive event. I saw an abrupt pitch down, followed by an excessive thrust reduction, followed by a pitch and thrust normalization to a ~1200 fpm descent. I've been playing with manual "cruise descent" transitions from VNAV PATH pitch mode to descents in both FLCH and VNAV PATH/SPD pitch modes . . .with and without a Vnav speed intervention. I can't get near this aggressive of an immediate ROD.

[horace 3]

Hi Sam,>". . . When nearing the T/D I decided to test a function we>often use, the "premature" descent. I entered a lower alt in>MCP, and pushed on the altitude button, which is the same as>hitting the "descend now" button on the FMC. The plane should>enter a slow descent, a little over 1000fpm, until the>precalculated descent path is reached, then VNAV PTH should>reengage. This aircraft started a fairly steep descent, around>4000fpm intitially, then shallowed out to around 50 fpm ROD.>It yanked out way too much power, so with speed decaying too>much the SPD mode reengaged (fortunately). In this state, with>very little rate of descent and almost normal cruise-thrust,>the airplane stays until VNAV PTH is reacquired. (Major>issue)." (Janov)>>I have seen it as well. The airplane achieved a rate of>descent of 4000 fpm before the system caught up and began to>mitigate the event. It was a real E-Ticket ride. Jan thought>the event was initiated by too much power being pulled out.>But it looked to me like the excessive rate event happened too>quickly to have been initiated by thrust. It's a very>aggressive event. I saw an abrupt pitch down, followed by an>excessive thrust reduction, followed by a pitch and thrust>normalization to a ~1200 fpm descent. I've been playing with>manual "cruise descent" transitions from VNAV PATH pitch mode>to descents in both FLCH and VNAV PATH/SPD pitch modes . .>.with and without a Vnav speed intervention. I can't get near>this aggressive of an immediate ROD.You're my hero ...you got it. THAT's what I'm talking about! I guess I've been a little bit misunderstood. Sorry that I haven't found this thread on the forum here.Vangelis, I do know how speed intervention works now and I see that it was my fault in the beginning so thanks for clearing that up. But after my first post I mentioned that I see this strange behaviour in spite of not using SPD INTV.

[Guest D17S]

There was some initial discussion that a VNAN PATH, speed mode intervention might contribute to an initial excessive rate of descent. Here's my best guess about the Vnav/AT teamwork that is going on in this situation. I'm observing that if speed is decreased in VNAV PATH, pitch focuses entirely on maintain altitude. If a pitch up is necessary to maintain altitude at a lower targeted airspeed, it occurs. Conversely, if target speed was increased, pitch might need to decrease a bit to maintain altitude at this increased airspeed. For lack of something better, I would call this Altitude (or path) on Pitch. (Or Vnav PATH mode?) When a VNAV PATH speed intervention is commanded, I see the ATs powering up/down to target the new

[Guest D17S]

I really hadn

[horace 3]

Sam,I thank you for your comprehensive investigation. I'm pretty sure, the PMDG team will take care of it :( ...huh? I would have failed due to my limited english vocabulary when trying to report in that detailed level. Very appreciated.

[Guest D17S]

That was a great find. Not that we found something wrong, but that we 'Serious Simmers' are able to operate the system at this depth of detail. That's the real story. It's a tough system, but we're gonna have this Vnav stuff down cold before we're through.