cruise fuel flows in a warm atmosphere

[torque2]

I am not (yet) an owner of PSX but an owner of PS1.3a.  One of the things that bothered me with that program was the high fuel consumption in the cruise phase I observed when the temperature was set warmer than standard.  According to the performance tables included with the manual fuel consumption should increase 3% per 10 degrees C above standard TAT at cruise altitude.  At the same time TAS should increase 1 kt per degree C above standard (at the same Mach speed).  But I saw much greater increases in fuel flow than that, on the order of 10% or so per 10C, even using a lower Mach number to keep TAS the same.  As a result my predicted arrival fuel level continuously decreased on the progress page as the plane was burning more fuel than the FMC was predicting.  This forced me to always program standard ISA temperatures on the cruise weather pages to have accurate fuel consumption.

Can anyone tell me how PSX handles warm temperatures (ie TAT 10 - 20 degrees above standard) in cruise?  This is a very common situation in the summer or near the equator.

David

[Hardy Heinlin]

I just made some random checks for you with all engine models (GE, RR, PW) using Mach .82 at FL330, 300 tons, changing OAT from -57 to -47°C.

The FF increased by ca. 3% (indicated on EICAS by 2.4 to 2.5).

When changing OAT from -57 to -56°C, the TAS increased from 469 to 470 kt.

When changing OAT from -56 to -55°C, the TAS increased from 470 to 471 kt.

Please note that PSX is rewritten from scratch. There is no PS1 anymore in PSX.


Regards,

|-|ardy

[torque2]

Thank you very much Hardy for taking the time to run the tests for me.  The results look promising.

David