The following is with regard to a SimBrief flight plan (21 JAN 2024, 07:49UTC; LIDO format, KORD to ELLX using Aerowinx PSX and aircraft model LX-WCV), in particular to wind data included within the OFP. Discrepancies exist between Flight Log and Wind Information pages, especially on long legs with a wind change – either in wind direction or wind speed, or both.
I will first start with the definitions, provided by Simbrief. Then continue with details from the OFP mentioned above.
In the Briefing Preview section “How do I read this briefing?”, for the LIDO format and the Flight Log page, I read under WPT CEFOU at the wind section (FL330 279/071):
“The forecast wind direction over this waypoint, in degrees true and knots”.
On the Wind Information page, under the title Wind Information, I read:
“This section provides forecast winds for every waypoint in cruise. Values are also provided for 2 flight levels above and 2 flight levels below the planned altitude.”
Under CEFOU (FL 330 279/071) it says:
“The waypoint name associated with this wind data”, and highlighting the wind data itself:
“The wind direction (in degrees true), wind speed (in knots), and temperature (in degrees Celsius) associated with this wind data.”
Here, CEFOU Flight Log and Wind Information winds are identical at FL330 (279/071), and so are the temperatures. From the instructions I understand that whether I take wind data from the Flight Log or from the Wind Information pages, they both apply to overhead that waypoint.
In the SimBrief Flight Log of my flight plan, looking at waypoint DOVEY, wind is forecasted as 311/040 (and M49); on the Wind Information page it is forecasted as 278/044 (and -48). Calculated Track Wind component [TWC] of P25 and Ground Speed [GS] of 514 kts are - almost, however not quite - correct for a wind of 311/040, but for a wind of 278/044 (-48 C) this would have been a TWC of P41 kts and a GS of 530 kts. It does not matter whether the TT towards, of the TT from DOVEY is used; 278/044 will always yield a 15-16 kts higher GS over this WPT.
With the Wind Barbs enabled on the Navigraph route chart, over DOVEY they show ± 280/045, hence in accordance with the data on the Wind Information page. Therefore, the aircraft would leave DOVEY at True Track [TT] 078 (“towards the next waypoint”) with a GS of 530 kts instead of 514 kts. This, of course, will slightly influence the calculated time en-route to the next waypoint.
For the next two waypoints - 42N060W and 44N050W - similar discrepancies occur, resulting in incorrectly calculated groundspeeds (wind barbs are always in line with data on the Wind Information page).
From an earlier discussion (with Derek I believe…) one of the forecasted winds should be regarded as the average wind on a previous leg where the other should be regarded as the forecasted wind overhead the waypoint. I cannot find this e-mail discussion anymore, so my memory may be incorrect here.
Although the definitions are using slightly different phrasing, they do both indicate that the forecasted wind are to be associated with a specific waypoint and therefore should be the same, as is the case in the CEFOU example. Then there will be no confusion as to what wind is used to calculate TWC & GS.
Sometimes I see a temperature difference as well, for instance M52 in the Flight Log, and -54 on the Wind Information page. This would influence both M-number & TAS and subsequently GS.
Apart from all of the above, calculation of TWC and GS sometimes is incorrect. Since the SimBrief calculation is done using data on the Flight Log page, I now refer to WPT EVRIN: for TT 098, M0.85, TAS 487, M57 & 254/115…: SimBrief calculates a GS of 601 kts and TWC of P114. This should have been a GS of 590 kts and a TWC of P103 (for M0.85 - TAS 487,26 - M56,7 & 254/115).
One final remark on this OFP: for no obvious reason the planned M-number (or TAS) drops significantly between WPTs EVRIN and INFEC. From M0.85 to M0.82, or from 487 kts to 470 kts. The ATC flight plan page does not reflect this speed change over INFEC. It indicates changing from a fixed M-number - crossing the Atlantic at M.85 - to a TAS (487 kts) at WPT ATSUR, which is the previous WPT before EVRIN.
If this SimBrief flight plan is no longer visible to you: it is on my computer, and so are screenshots of the Navigraph en-route chart for WPT DOVEY and WPT 44N050W, with wind barbs.