Very High Vertical Speed When Using VNAV

[kmanning 12]

Hi Michael,I don't have any mathematical proof of these performances, but I checked at other websites and found a chart that showed real world 747-400 climb performance. It seems to be faily close. I noticed that when I enter a new higher altitude into the MCP, in VNAV mode, the V/S seems to really peak out and then it settles back, if you know what I mean. I assume the real 747 does this as well. The only thing I was going by is the performance I saw in the Virgin Atlantic video. Not one time did the airplane's V/S reached 3,000 fpm and higher in VNAV mode. If I remember correctly, the most I saw is 2,300 fpm. But we have to remember that the airplane's gross weight was around 800,000 pounds at take-off, 362 tonnes times 2200 = 796,400 pounds. Thanks for your tips. Ken.

[michal 44]

.>But we have to remember that the airplane's gross weight was>around 800,000 pounds at take-off, 362 tonnes times 2200 =>796,400 pounds. exactly - so load your 744 to match their weight and see what climb rates you get ;)Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

[kmanning 12]

Hi I've already tried it several times in the past. It performs faily close to the one in the Virgin Atlantic video. Ken.

[Guest D17S]

As Robert said an Idle descent is the holy grail, but as Randy pointed out the Des Now function in the PMDG model may be still a bit buggy. There are some discussions around in other threads. but here's the jest: The DES NOW maneuver begins from level flight with pitch in VNAV PATH and the AT in SPD. When Des Now is selected, the path deviation indicator displays on the ND and show the airplane to be

[emvaos 13]

Think 4500 is high ?Well I have news for you. A 747-400 at full climb power and 200 tons (about 440 lbs) will do 6700 ft/min at sea level and 5900 at 10000 ft (deck angles 14.5 and 11.5 degrees respectively). And that is with C2B1F engines. C2B5F engines produce 3000 lbs of thrust more (per engine !!) Note also that momentarily VNAV SPD may command a higher rate to decelerate the plane if current IAS is greater than target.Two general comments :(a) There seems to be a "belief" that climb at this or that rate is correct...Nope it is not. VNAV SPD commands whatever vertical speed required to maintain target IAS given commanded thrust. WHat law/rule/regulation says right or wrong ? Only flight performance as quoted (usually in detail) by the manufacturer.. Which brings us to...(:( An extraordinary number of users/reviewers think that "good" flight dynamics are related to how easy and "stable" is the aircraft to fly. Sorry that's complete GARBAGE and a nice excuse for "developers" selling add ons to all of us that fly like something other than what advertised. To begin with who says that ALL aircraft are easy to handle ? For the record 99% of time with respect to this area goes into accurately modelling flight performance. Making an aircraft fly "easy" for the novice simmer takes 10 minutes flat. And if one does not know how to do it then he should not be charging people (I am *that* desperate with some of the stuff that I have bought over the years yes...) The remaining months go into things like this...So if you see a PMDG plane doing a 4500 fpm climb rate then yes it should be 4500, period.Happy new year guys,Vangelis===================================== E. M. Vaos Precision Manuals Development Group www.precisionmanuals.com=====================================

[Guest neilb]

Indeed. As I recall on the Virgin ITVV DVD they go into VS mode to reduce the climb rate, and that was with a full load and a lot of fuel.

[michal 44]

>Indeed. As I recall on the Virgin ITVV DVD they go into VS>mode to reduce the climb rate, and that was with a full load>and a lot of fuel.Actually being almost fully loaded they did not "suffer" from some oustanding climb rates. The reason they went into VS were the frequent vectors for traffic and other climb limits north of Heathrow that prevented them for climbing unrestricted. You can easily see on the video that the max climb (in VNAV SPD) they could squeeze out of this aircraft was 2000 fpm but usually they even stayed round 1500 fpm.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

[Guest D17S]

Well said Vangelis. Once again for those techno-droids, (like me): In Vnav climb mode, VNAV SPD does not target 'Rate of Climb.' VNAV SPD does not increase a rate of climb target to decrease airspeed. The technical execution of this mode is exactly the opposite of this. The target is airspeed, not ROC. VNAV SPD will increase pitch to slow the airplane down or decrease pitch to speed the airplane up. The ROC will simply increase / decrease as a result of the aerodynamic characteristic of an increased / decreased angle of attack (AOA) at a fixed airspeed. No magic here and it has a name. This is called a "Speed on Pitch" or "Speed on the Elevator" climb or descent mode. Your initial take-off climb, TOGA, is a Speed on Pitch mode too. So is an idle descent. When you are climbing, anytime the AT is in THR REF or IDLE, look out. The thrust is simply fixed at a thrust level. Something has to be controlling airspeed. What does the AFS have left to work with? Drogue chutes? Anchors? There's only one thing left that the AFS has authority over. That's pitch. If you don't have a pitch mode engaged with THR REF or IDLE controlling thrust, you will overspeed or underspeed. IMO, the AFS should not even allow these

[kmanning 12]

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. Hi Michael,I read up on the temperature derates and I understand it much better now. I see what's taking place here because temperature affect N1. Yes, I think you're correct. It only works in the take-off phase.Ken.

[kmanning 12]

Indeed. As I recall on the Virgin ITVV DVD they go into VS mode to reduce the climb rate, and that was with a full load and a lot of fuel.Hi Neil,Yes, you are correct. Allen went to V/S mode to avoid other traffic in the area. You probably noticed the traffic warning coming from TCAS. This is when Allen went to V/S mode.Ken.

[kmanning 12]

Two general comments :(a) There seems to be a "belief" that climb at this or that rate is correct...I did not say that climb at this or that was correct. I know as much about the performance of the 747 as anybody at PMDG. I posted it because it was exceeding those performances. I happen to know that the real 747 is a very stable aircraft to fly. Ken.

[michal 44]

>it was exceeding those performances. :-violin

[Guest D17S]

Do you have the chart that specifies:X (fixed) thrustbyY (fixed) airspeedbyZ weight=A ROC or RODI'm sure there is. It really must exist somewhere. This had to be part of Boeings certification data set. If you have access to it, It would sure be interested to see.

[jon b 2,047]

> I know>as much about the performance of the 747 as anybody at PMDG. I>posted it because it was exceeding those performances. I>happen to know that the real 747 is a very stable aircraft to>fly. That`s a pretty bold statement don`t you think?Jon

[emvaos 13]

Sam,This what you want ?===================================== E. M. Vaos Precision Manuals Development Group www.precisionmanuals.com=====================================