Problems getting up to cruise speed

[Fatback 84]

I have had this problem before and must be doing something wrong. I was up to cruise speed but noticed that it had dropped back to M .676. The TAT is -23 and I have Opus but tried turning off weather updates. There is a pretty significant headwind, 83 kts or so, but I should still be able to get to cruise speed. Here is a link to a photo of my panel.

 

https://www.dropbox.com/s/4k5guwjqu6vf434/fsx%202014-05-08%2019-13-15-05.bmp

 

Any help appreciated. 

 

EDIT: about 30 minutes after making this post the speed started to increase to cruise speed. I didn't touch anything and the only change I can see is that the temperature went to -17C. 

[martybuzz 2]

The 747 when heavy and in the cruise will not accelerate in real life.  You have to change VNAV to V/S and start a slow descent and take a run up so to speak.

 

Buzz

[indyrocks 2]

I was able to copy your situation pretty closely and, as you can see in the following screenshot, I had no problem at all holding a M0.855 speed at FL340, which is just slightly faster than the speed your 747 was trying to reach. The screenshot is a bit blurry, but you can make out the most important things.

 

http://i711.photobucket.com/albums/ww112/boilerbill/B744_zpsdc9caee2.png

 

Here are some comparisons between your aircraft and mine...

• Both aircraft have roughly the same weight. Since you didn't specify it, I used the clues that are visible. To get the same Optimum and Maximum Altitudes that are shown on your FMC's VNAV page, I had to remove all passengers and cargo from the aircraft. According to the PMDG Aircraft Configuration tool, an empty B744 has an Actual Take-Off Weight of 722,714 lb. Since the Maximum T/O Weight is 875,000 lb, that means both aircraft were ~150,000 lbs short of the maximum possible weight. So I think it's safe to say that the aircraft is nowhere near is weight limit.
• Your screenshot shows that you had 327,200 lbs of fuel in your aircraft. It's a bit difficult to tell from the screenshot, but my aircraft has 330,200 lbs of fuel. Although these fuel loads are a bit high, they are still well short of the B744's maximum fuel load of 382,336 lb.
• Not only are the weights of both fuel loads equal, their distributions are identical as well. This is because the PMDG Aircraft Config tool distributes fuel among the various fuel tanks in the same proportions.
• The temperature wasn't a factor since our TAT values are only 3° different (-21°C vs. -24°C).
• The wind in your case was 84 kt @ 278°; in my case it was 86 kt @ 302°. So the wind was roughly the same speed, with only 32° difference in direction. However, the wind was from the left front in your case and the left rear in mine. I can't see any way that could cause the difference in our two aircraft, but maybe I'm missing something.
   
• The one thing that is noticeably different in the two aircraft is the pitch. Your screenshot shows that your aircraft's nose-up pitch is close to 6.5°, while mine is around 2.5°. I have no idea why our aircraft would have such a different pitch, considering they are roughly the same weight and the fuel loads are just about identical in every fuel tank. Whatever is causing the difference in pitch may be the explanation for why the aircraft fly so differently.

To be honest, I don't know why our two 747s are flying so differently considering how alike they are. Perhaps someone can spot some details that we've missed.

 

 

[Fatback 84]

thx for the detailed reply. The pitch is due to the low speed--as it started to resume normal cruise speed the nose came down. Could it be something else with the weather? I use Opus but can't try stopping it because my OAT rockets upwards without real weather. I'm lost as to what the problem could be but at least it is very infrequent. 

[Don t moderate me this way 882]

Center of Gravity within limits?

 

Any trims out of limits (aileron/rudder trim)?

 

PMDG load planner or FSX default loading or any external "loader" like FSPassengers or FS Captain that could 'spoil' the PMDG load map?

 

Not so efficient wind smoothing/a sudden wind shift, the autothrust couldn't compensate for?

 

What was your thrust limit (CLB/CRZ/MCT) and did the A/T max out the thrust accordingly?

[Fatback 84]

Everything was loaded with the PMDG load manager and I had no CG alerts. I also use Acars and it will pick up CG that is out of limits. Didn't see any trim out of limits and I tried disengaging and reengaging the AP so if it had been knocked out of whack by a wind shift it should have been ok after being reset. 

 

I also tried manually increasing thrust in the FMC and there was no noticeable difference in speed. 

[indyrocks 2]

If you'd like to try a weather engine other than Opus then I would suggest FSrealWx. It's very easy to use, has a number of different options, and best of all ... it's free! You can check it out for yourself at http://www.hanse-coders.net/index.php?page=self&id=1. Although the creators are German, there are English-language versions of almost everything, including the forum.