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ISA deviation in OFP

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Hi all,


I'm trying to learn more about flight operations. What is the relevance of including the ISA deviation on each leg in the OFP? (I'm asking because some of the OFP templates I've been looking at have the ISA deviation listed at each waypoint in the Navigation Log.)


Prior to departure, I'm aware that the temperature at various altitudes is important for performance calculations, so PFPX certainly takes it into consideration. But once the flight has been dispatched, I don't see the advantage of putting the ISA deviation on every leg of the OFP. It would seem to only be relevant if performance numbers were varying from predicted. (If the performance numbers were spot-on, then I doubt a flight crew would ask for a different altitude just because the OFP's temperature was two degrees warmer that the SAT measured by the aircraft.) 


But assume the performance is off a bit, the aircraft is using more fuel than predicted, so you want to find a more economical altitude.  That would usually depend on the winds more than the temperature, but let's say for the sake of argument that we're in completely still air. Specific fuel consumption drops as temperature drops, so you'd want to climb higher if able, and the only thing arguing against a higher altitude (remember we're disregarding winds here) is a temperature inversion or else the plateau you see at the tropopause. 


But giving the ISA deviation for a particular waypoint doesn't rule in or rule out a temperature inversion, and the ISA is corrected for altitude, so the deviation alone doesn't tell you where the tropopause begins. 


Another argument for the ISA deviation might be that you could tell if your flight plan is crossing isotherms. Fine, that may be interesting information to have, but PFPX has already calculated the optimum route, and besides, the ISA deviation is only given on along-route waypoints, not throughout the atmosphere, so it's not like the pilots could use the ISA deviation information on the OFP to pick a parallel track that has better performance.


What am I missing? How might knowing each waypoint's ISA deviation influence operations once airborne?





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Never mind, I think I can answer my own question. The ISA deviation isn't given so that you can change your flight while enroute, instead it's given so that you can feed it into your FMC (or equivalent) before you take off. PFPX will calculate the performance according to given parameters, but for your FMC to calculate performance, you need to pass those same parameters to the FMC. Hence the OFP includes wind and temperature data. Obviously most FMCs get their flight plan data downloaded through something like ACARS, but you could still enter everything manually, and if you do enter all the data manually, then you'd need to enter the weather manually too, and that includes the temperature at the flight planned altitude.

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  • 5 weeks later...

And, in addition to what you have correctly figured out, the ISA deviation can greatly affect the optimum flight level, or even the maximum reachable flight level.


Last summer, ISA deviation on the way home from the Canary Islands to mainland Europe was rather high. And with fully laden and fully fueled narrowbody aircraft, even FL360 was unreachable, not to mention sensible in terms of fuel burn, so it was indeed smart to start out on a lower level as it would even save gas compared to aiming for FL360 or 380 right away.

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And adding a little more: I mostly fly Airbus (Aerosoft and FSLabs) PMDG 777 and the Milviz King Air 350i (can't wait to put my hands on the Eaglesoft C750 once it comes out for P3Dv4/v5), and if there's such thing as a flightplan uplink or alikes in the Buses and Boeings, the King Air doesn't have such a thing even in reality and it is simulated that way in the Milviz pack (one can fill a flight plan on the FMS and save it for later use though). As such, in the King Air (simulator or reality) one has to fill the wind and the ISA deviation as well in order to have the fuel on the FMS predicted as closely as possible to real life or to the flight plan.

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