With TOPCAT, I usually use its V1 and just verify that the VR and V2 are similar and use the FMC value for those.

Thank you

Gentlemen,

 

I have now tried to alter my V speeds for my flight OMDB-EGLL.

 

FMC calculated 163,174 and 181. TOPER 777 calculated 176,180 and 186 for runway 30R.

 

Without confirming the FMC speeds, I was able to change the values to those by TOPER no problem.

 

Untested still, confirming the speeds and then trying to change them. Another time!

I got some additional info for those interested.

 

The EFB performance software we use not only calculates the Vspeeds and assumed temp for thrust reduction.

 

It additionally calculates the optimum EO clean up altitude (usually between 1000-1200ft AGL)and presents a text description of the EO SID that needs to be flown if an engine fails during T.O. or shortly thereafter.

(As said in another thread, these EO SIDs are stored in our FMC and pop up automatically in case of Engine fialure).

 

The software is build on an obstacle database so that you are granted obstacle clearance while following the EO SID and while accelaring to clean up (very shallow flight path!)

 

So besides the FMC only being able to calculate balanced take offs, it also can not take the terrain into account!

 

Further more we can input all kinds of runway contamination parameters (braking action/snow depth/standing water depth/etc) and calculate a safe max take off weight.

(I never tried TOPCAT, so I dont know if it allows for that).

 

We can also select MEL items from a dropdown list if they have an impact on performance, select a shortened runway at either end if there if work in progress (KJFK R22R for example is shortened at the moment) and we have a couple of tricks in there that we can use to increase max take off weight (packs off take off for example)

 

 

We used to compare the EFB calculations with the FMC calculations to make sure they were similar. To do so we entered the same parameters for wind and temp in the FMC and EFB. The EFB calculation was mainly for max take off weight calculation plus how much thrust could be reduced, taking the obstacles along the EO SID into account. The EFB speeds were pretty much the same as what the FMC calculated.

 

Since a recent software update that is not required anymore.

There is no need anymore to enter any performance data into the FMC either (wind/temp)....it does not matter what the FMC calculates. There is quite a split between the FMC and EFB calculated Vspeeds sometimes now (more that 5kt) and we overwrite all three of them with the EFB speeds.

To still be able to compare our calculations to something, both pilots have to now calculate the take off performance independantly on his/her EFB.

 

 

I never tried TOPCAT, so I dont know if it allows for that

 

TOPCAT will allow runway shortening and basic dropdown selection of dry/wet etc but we've got the basic constraints of MSFS. The takeoff calculations then identify which limitation is affective such as climb, performance or obstacle. It uses departure data from navdata files from same hobbiest sources as PMDG navdata so simulation is consistent.

TOPER 777 supports runway shortening,  wet/dry, wind, OAT, flaps, A/C, A/I and QNH but not obstacles, E/O or derates. So it is an improvement on the FMS calculated numbers, but not comprehensive.

This morning on EGLL-CYYZ I confirmed the FMS-calculated V speeds and was then able to overwrite them with the TOPER-derived values without trouble.

 

So now I am not sure why I originally had the overwrite problem.

@ Richard,

 

I often get this too,

The best way around it is going to takeoff page 2, and inputting some wind and wet runway for example. And that will align the v speeds to your calculated ones (i use a real world 300ER calculator and the values are sometimes off quite a lot if the runway is wet etc. etc. )

 

Hope that helps