Mach difference at equal cost index comparison

[ZeeDoktor]

Hi all,

I'm interested in comparing the mach numbers your FMS's calculate for a given cost index. Can some Boeing and Bus (+other) drivers please post what their typical figures at a given cost index and altitude are?

If you need a reference, try a cost index of 35 at FL350 (assuming ISA).

Thanks!

[Old Smokey]

For a B777-200/200ER/300 at CI=85 at OPTIMUM level and Nil Wind, a Climb / Cruise Mach of 0.835 is typical. In the same conditions, M0.84 is more typical for the B777-300ER.

Naturally, cruise at Off-Optimum levels, and wind component, will change these 'ball-park' figures.

Regards,

Old Smokey

[ZeeDoktor]

Thanks, old smokey! that's a great start.

Any others?

[alexban]

737 NG with CI 30, FL 350 M 0.788
737-300 with CI 30,FL 350 M 0.752

[divinehover]

A319 with CI of 25 will give about M0.76-0.77. at FL350. At FL390 this will go up to M0.78.

[Beakor]

B757 200 CI 30 just under M0.79, CI 42 just under M0.80 at FL 350 - 370. Increases marginally above that.

[Five Green]

FMC er

Don't forget that (at least on the boeings) Cost index is also adjusted by the company, as are optimum and maximum flight altitudes. So you may not be comparing apples to apples. It depends on how the rotating components are paid for compared to the price of fuel. Also different engine installations get different ratings for the cost index. Even within the same engine you can have different cores ie Rolls Royce G and H cores which have slightly different preformance characteristics.

The bigest determinate of speed is still weight. In the 744 at 385tonnes (assuming max take off wieght) optimum is around FL280 and at CI 80, (nil wind ISA) it is around .85 and gets slower as you get lighter.

cheers

FG