Winds/Temps Aloft Discrepancy

[simonijs]

Good morning,

But I think you're now talking about the effect of weight loss, not of density loss (air warmer than ISA). The original question was whether the FMC increases the ECON speed when the temperature rises above ISA while gross weight, altitude, and cost index are fixed.

Actually I did write a lot about rising temperatures and it's effect on performance at first, but then deleted it before posting because it was - once again - not avoiding the things that should be avoided.

I will see, if I can make contact with an ex-student who has worked as a performance engineer for Martinair Holland and ask him if he knows the answer to that question. But Martinair has ceased to exist, and I have no idea where he is at this moment.

Kind regards,
Simon

[Markus Vitzethum]

Hi Hardy,

thanks for the answer.

I haven't done enough reading yet to have a good reasoning for a good answer - yet. Anyway, I understand that in PSX you are trying to achieve a constant TAS - I have to admit that I am surprised by that because I expected that TAS is not a relevant parameter for aerodynamics - or performance.

But let's focus on facts. I came across this Youtube video of a Qantas 747-400 over the Philipines:
https://www.youtube.com/watch?v=_LKMCn7EbA8

It's a bit shaky but good enough to extract all relevant parameters (except CI which is not visible). So, the 747 is cruising inbound RPMZ at FL370 and M.859, determined by ACT ECON CRZ with a ZFW of 203.3 and a fuel load of 61.1 tons. Looks like a classic 747-400 with RR engines to me.

I've created a situation file with exactly the same parameters and also with fixed weather parameters such that I get the same winds and temperatures, see http://www.vitzethum.de/psx/psx-sitatuation_qantas_phil.zip

What I find is that I need to use a CI of 9999 (in PSX) to achieve the same ECON CRZ speeds as in the video.
True, the CI is not visible in the window. But is 9999 a reasonable CI for a real world Qantas flight? I would have assumed from previous discussion that for a routine flight a CI of 100-300 is more likely.

Markus

p.s.
Note that I also some discrepancies with respect to engine EPR and N1. (Oh ... and the limit is CLB, not CRZ)

[Hardy Heinlin]

Hi Markus,

there's not much difference between CI 9999 and values far below 1000 (actually, I don't know if a CI above 999 has any effect at all and whether it's just a "valid entry"; on other aircraft types CI 999 is the max). E.g., if you enter 500 instead of 9999 you will also get a Mach number above .850 -- in fact, in some airlines (e.g. Qantas) the third digit of the MCP Mach number is blank because they probably prefer uncluttered instruments over peanuts :-)

TAS is a flight planning speed. Cost index is a flight planning parameter. We're talking about cost index stuff, not just about aerodynamics alone. By the way, IAS or CAS are not the only aerodynamic speeds; EAS is used in aerodynamics as well (e.g. in pitch control systems).


Cheers,

|-|ardy

[Hardy Heinlin]

Warm-air Mach drop in ECON SPD is now reduced in PSX 10.14:

http://aerowinx.com/board/index.php?topic=4191.0


|-|ardy