TOC ISA and TDV in an OFP
 

Hi all, I would like to ask a question regarding some data found included in an OFP for a flight from FCO to BRU. In particular the OFP has a first FL STEP at FL380. In the first page I found that TOC ISA is M003, but when I go to the flight log page where is the TOC point I found OAT = M61, TDV = M04 and TRP = 434. So I have some doubt.
Following the rule to calculate the ISA deviation at FL380 it should be: (38*(-2))+15=-61°, so being the TRP (tropopause height) at FL434, why the TDV is indicated as M04, I was expecting that TDV should be 0? Also why the TDV at the TOC point is different from the TOC ISA indicated in the other page of the OFP?
Where am I making some mistake?

You are mixing a bit of apple and oranges here.

Temperature in ISA is defined as 15°C at sea level with a lapse rate of 1,98°C/1000ft until 36000ft, when it stops at 56,5°C and it is then the same for quite a while.

So at FL380, ISA temperature is always -56,5°C. In your case, ISA deviation at FL380 is -4°C, which matches the -61°C (if you round the numbers) and is correct. Life - fortunately - is never standard, so forecast for the actual atmospheric conditions during the flight are that tropopause is at 43400ft (as opossed to ISA's 36000ft), so if you climb higher than FL380 in that atmosphere, the temperature should continue to drop.

Ok, thanks. The height of the standard troposphere is exactly the concept I missed. All the other things you wrote are perfectly clear.

The only thing that remained unsolved is why the TOC ISA is M003 (in the first OFP page where are also reported CI, GND Distance and so on) instead in the Flight LOG page the TOC point report M04 under TDV

Thanks in any case for your help.

Hi,

most likely a rounding / mathematical issue. Precisely why are the displayed numbers not identical can be probably only be answered by a software engineer from the OPF provider.

regards, FD

PS: it is often hard to see from an OPF whether the displayed data (wind, temp, MORA, speed) is valid for a waypoint or for the leg between the waypoints. Could be the case here, that you are looking at two different physical places.