Is the ISA Dev calculated correctly

In the flight plan summary for my flight DLH 253 on 21/02/2023 there is a calculation of M04 for ISA deviation. If you look at the attached PDF of the flight / wind information on page 7, the temperature at my cruise flight level of FL320 is predominantly M54, whereas ISA at this FL is M49, giving an ISA deviation of M05

I know it is a small difference, M05 versus M04 but I get a difference on every flight, so wanted to check if I was doing something wrong.

LIMCEDDF_PDF_1676996286.pdf (589.8 KB)

Hi, thanks for the report!

The ISA DEV is actually based on the entire flight, including the climb and descent. Not just the cruise segment.

So in your example OFP, looking at the Flight Log section, you have some deviations as high as P03 during the climb phase and P02 during descent. This is pushing the average deviation for the total flight up to M04, despite it being colder than that in cruise.

Initially I wasn’t sure if this is correct or not, but having re-checked other real LIDO OFPs, it seems they also are considering the climb/descent portion for the ISA DEV.

Hope this helps,

That’s very helpful, thx, for some reason the FMCs refer to Top of Climb OAT and ISA deviation and it looks like the wind too (CRZ wind) - see attached. I presume the wind given in Simbrief (it’s in the 1st or 2nd section of your simbrief plan, not the PDF) is also average for the flight and not cruise. I wonder why FMCs only calculate for the CRZ phase and not the whole flight