by design "888" Optimal FL in the PMS, v1.0.3

[mark75]

Hi,

even after filling GW, Fuel and OAT in the flight init page of the PMS on the ground, I get OPT flight level as 888. This looks to me like a regression bug, not present in the previous versions.

 

Edited Thursday at 09:44 AM by mark75

[Prof_Plumpy]

Getting the same.  1.0.3 seems to have added back a bunch of old problems 1.0.2 fixed a pile of new ones. 

 

[richboy2307]

Hi

As of 1.0.3, the PMS behaviour was changed to show "888" when it cannot calculate any valid result. It is also actively using the current SAT value to determine OPT. As such, it will update in flight, particularly when current SAT updates to sensible values where a valid result can be achieved.

SAT of +40c at those weights won't net any valid results, hence the 888. Team is still investigating whether you can force a prediction on the ground, references we have are unclear but the behaviour currently is reflecting best the "known" reference logic.

[mark75]

21 minutes ago, richboy2307 said:

Hi

As of 1.0.3, the PMS behaviour was changed to show "888" when it cannot calculate any valid result. It is also actively using the current SAT value to determine OPT. As such, it will update in flight, particularly when current SAT updates to sensible values where a valid result can be achieved.

SAT of +40c at those weights won't net any valid results, hence the 888. Team is still investigating whether you can force a prediction on the ground, references we have are unclear but the behaviour currently is reflecting best the "known" reference logic.

Hi,

thank you for the reply. That was a real SAT, in New Delhi. Anyway, thanks for the explanation.

Edited Friday at 12:54 PM by mark75

[MD82]

Makes sense. A SAT of +40°C at FL360 would fly off of any chart 😉

[mark75]

Maybe. But anyway, that SAT on Cruise page is taken from Flight init page. I entered a correct value, according to the PMS guide.

[richboy2307]

Yes, you entered it correctly, and the current SAT was +40c on the ground. The limitation is that the PMS only knows "current" SAT, not predicted SAT at FL360. Hence its running the OPT calculation with a +40c Temperature, which is way too high at the cruise levels and hence nets in "888" or invalid result.

As mentioned above, the PMS references available do not indicate any PMS capabilities to predict SAT at different flight levels, or calculate any ISA deviation on its own. All the logic shows it using "current" temp probe sensor data directly, hence making it unserviceable if temp probes are INOP or malfunctioning even. The SAT entered during pre-flight setup is used only for Takeoff EPR advisory on the ground.