Hi all,

This is my first post and topic on the forum.

I have recently bought the 777 and noticed something strange. The Reference and VREF speeds do not seem to be correctly computed in the FMC. 

Example: For a Landing Weight of 170.0t, the FMC gives me for VREF30:

- 134kts on the -200F/200LR while the FCOM states circa 125kt. Also, by forcing the GW down, the VREF doesn't go below 134kts

- 122kts on the -300ER which seems to be correct

I have not found any topic mentioning the issue. Apologies if this has been raised before.

Many thanks for your help.

Thomas

 

Hi Thomas and welcome to the PMDG forums, please note their rules ask that we sign our full names on all posts.

Where specifically in the FCOM are you finding the V speed?  I've scanned the Performance Dispatch and Inflight tables and I'm not an expert on what's in those tables but I don't recall any Vref tables.  PMDG used Boeing engineering data not readily available to us in creating how the FMS provides performance data... be interested in what your source is.

Hi,

I have realised that I made a mistake. I misread the VREF30 for the -200LR and read instead the one for the -200ER. It's Page 813 of the PMDG FCOM V1 PDF.

The VREF30 does show 134kts for a pressure altitude of 2,000ft. I assume this is due to VMCG speed in case the aircraft bounces/lands on the runway before going around but if someone could confirm that would be good.

Also, to explain why the reference take off speeds are that high, it's due to the minimum VMCG speed which is higher than V1 for this specific weight and therefore V1 is adjusted to VMCG.

So I guess, nothing is incorrect, it does work! hehe

Many thanks.

Thomas Pujoulet

16 hours ago, Parkbreak said:

I have recently bought the 777 and noticed something strange. The Reference and VREF speeds do not seem to be correctly computed in the FMC. 

What is the AOA telling you?  That is the real indication of the correct speeds. 🙂

8 hours ago, Parkbreak said:

It's Page 813 of the PMDG FCOM V1 PDF.

I'm looking at pdf page 813 and all I see are  charts for clearway and stopway V1 adjustments for inflight performance.  The FCOM page that corresponds to pdf page 813 is PI.30.1.  So what is the FCOM page that you are looking at (not the pdf page)?

PI.30.2?

6 hours ago, Bluestar said:

What is the AOA telling you?  That is the real indication of the correct speeds. 🙂

I had circa -0.5/-1 degrees pitch on approach so didn't feel right. The 777 is pretty flat on approach and flare, +1 to +2 degrees feels more comfortable 🙂 

4 hours ago, downscc said:

I'm looking at pdf page 813 and all I see are  charts for clearway and stopway V1 adjustments for inflight performance.  The FCOM page that corresponds to pdf page 813 is PI.30.1.  So what is the FCOM page that you are looking at (not the pdf page)?

PI.30.2 

13 hours ago, Parkbreak said:

The VREF30 does show 134kts for a pressure altitude of 2,000ft. I assume this is due to VMCG speed in case the aircraft bounces/lands on the runway before going around but if someone could confirm that would be good.

Your logic about loss of control is correct, but it’s actually a VREF floor related to VMCA in the event of a single engine go around.  Lower altitudes equal more engine thrust, this is why the VREF gets lower at higher altitudes.

2 hours ago, dmwalker said:

PI.30.2?

Thanks.... I don't use the performance tables often enough to be able to find stuff quickly.  Notice that 134 kt Vref holds true from 170 T through 200 T before it starts increasing to this tells you at the lighter weights that Vref is not a function of weight.  Your suspicion that Vmcg is a factor is borne out by the table on PI.30.28 where at 2000 pressure altitude and 20C Vmcg is 134 kts at maximum thrust, decreasing to 124 kts at TO2 (20% reduction)... illustrates nicely why using derate can lower your V1.  Notice too that Vmcg is not a function of weight.

Thanks a lot guys for your contribution!