Very High Vertical Speed When Using VNAV

[kmanning 12]

Hi all,There's one thing that is frustrating to me. When I use VNAV as my veritcal profile, the 747-400s rate of climb is from 3500fpm to 4500fpm, and this is way too hight. It should climb from 1500fpm to 2500fpm, an average of about 2000fpm. Has anyone experienced this and what is causing it? It's like it's trying to hurry and get to it's cruising altitude. Does the position of the T/C have anything to do with this? I use the flight plan from Routefinder.com. I've tried this at different gross weights and the climb rate is still way too high. Ken.

[michal 44]

As was pointed out here many, many times (you can easily search this forum) your climb rate is very much dependant on your weight. There is no such thing as 'abnormally' high climb rate if your speed is in the correct range. You, as a pilot, can always derate your engines (up to 25%) to get lesser climb rates - this is what professional pilots do. 744 is known to climb like a rocket if it is light and engines are working at their max.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

[jon b 2,046]

Hi Ken,No the T/C is just an FMC generated prediction on the top of climb point using forecast winds and performace database calculations,slightly different from the green arc which is based on instant readings of ROC and winds.Although 3000-4000 fpm would be considered a very healthy rate of climb for most conditions it is not unusual to see such climb rates,temp has a big part in it as wellregardsJon

[Guest D17S]

Also you may want to get a bit technical. Vnav's climb mode is 'VNAV SPD.' This is a 'speed on pitch' climb mode. This means that thrust will simply go to the EICAS thrust limit (TO, CLIMB, whatever). The airplane will maintain a fixed airspeed too, but it will use pitch to stay at this airspeed. It will do this by lowering the nose to speed up a bit and raising it to slow down. The rate of climb will simply be whatever it ends up being. Like Michael said, if you have a light airplane, the nose will have to really get up there to keep the speed under control. With that very high 'angle of attack', a very high rate of climb will occur. And up you'll go . . . like a rocket! In a Vnav climb, ROC is not controlled, it just happens.

[kmanning 12]

Hi Michael,Thanks for the information. But the FMC thrust limit only goes to 15% derate. How do I derate the engines to 25%.Ken.

[michal 44]

>Hi Michael,>Thanks for the information. But the FMC thrust limit only goes>to 15% derate. How do I derate the engines to 25%.>You can use the temperature derate - most of this topic is very technical and most information on the subject is probably still in the old forum. But no matter which forum you search "temperature derate" should give you plenty of hits.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

[Guest A32X]

Regarding very high descent rates, using the "DES NOW" function. I hit (initially) -2500fpm. I believe it should hover around -1000fpm?Have noticed that the PMDG744 commands flight idle, and perhaps that is the cause?-Paul :-)

[Guest A32X]

Also using the "DES NOW" function the aircraft didn't maintain height to let speed bleed off, unlike a normal VNAV descent. However I don

[michal 44]

Ken,I realized that temperature derate works only in the takeoff phase (someone correct me if I am wrong). Once you are in the climb-out mode you only can do CLB1 or CLB2. But there is nothing to prevent you from just disconnecting the AT and pulling back on the throttle.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

[crvm 0]

>Also using the "DES NOW" function the aircraft didn't>maintain height to let speed bleed off, unlike a normal VNAV>descent. However I don

[rsrandazzo 6,353]

Paul-The optimum profile for descents is flight idle. This reduces operating costs to the greatest degree, and allows you to get some payback for the potential energy you have added to the airplane's flight profile via step climbs, etc.In the eastern US where most of my flying takes place, you RARELY every see this because of all the conflicting airspace needs. Instead ATC steps you down.Out here in the western US where I live now- I am always surprised that fellows are getting idle power descents..... but we have quite a bit more open space out here too.... ;-)

[Guest A32X]

>>>>The optimum profile for descents is flight idle.But is it Robert? Is the optimum descent flight idle if descending before the calculated idle descent path? ;-) Very pedantic I know.Paul :-)

[kmanning 12]

Hi Paul,I've never used the DES NOW function but you said the aircraft did not maintain altitude to allow the speed to bleed off. When you used DES NOW function, is the FMC still in VNAV mode or does it switch to another mode? By the way, it's the excessive climb rate that I was having problems with. But my decent rate is around 2000 fpm, which is not excessive. Like you, a 1000 fpm would seem to be a smoother decent profile. I should have listed my weight in my previous post. My gross weight at take-off is 658,000 pounds. Of course, this is fairly light but it still doesn't seem it should climb from 4000 fpm to 4500 fpm. I know it uses thrust ref when in this climb mode and it will use whatever pitch to maintain the programmed speed in the FMC. I'm not trying to put down the programmers at PMDG because I think they have done a good job desiging this aircraft. But I was just wondering if the thrust is programmed correctly. I'm sure it probably is. Even the acceleration at take-off seems to be very rapid compared to the real thing. Maybe there's something wrong on my PC that's causing these problems. At 658,000 pounds, I suppose the real thing could probably climbs that fast but it seems that this would be very uncomfortable for the passengers. I've did a search and I haven't found much information on this subject and it takes too long. Ken.

[crvm 0]

I suppose the real thing could probably climbs that fast but it seems that this would be very uncomfortable for the passengers. If you keep in mind that the AFDS does not take pax comfort into consideration then this makes sense and it's true. Also keep in mind that one guy in this thread actually flys the real 747-400 and has not mentioned this to be a real issue.[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|

[michal 44]

>but it still doesn't seem it should climb from>4000 fpm to 4500 fpm. How do you judge whether it "should" or should not? Do you have any operational experience with a real 744? If you have a mathematical proof that it should not we are all ears here ;)a quote about the 747:Sometimes a lot of power reduction is necessary to keep 250 kts in a light 747. Climb rates as long as speed is at 250kts...can be very high, go ahead and let climb rates be at 4000 - 6000 fpm until 10,000' is reached. Airlines like to hustle up to smoother air rapidly, so if she's climing rapidly, let her.[/]Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg