cost index/mach number/weight

[sdlc]

Hi,

Actually the cost index drive the speed/mach target for cruise unlinked to the actual weight of the aircraft.

For example with CI of 45 you can start the cruise at FL360 and 240T with mach at 0.85 and finish the cruise at the same level and  190T still at mach 0.85 instead of ~0.830 

 Is it possible to have a more accurate speed/mach/weight versus Ci management ?

Thanks

 

Stephane

[Swisspilot1986]

Indeed, my understanding of how CI is handled is that the resulting (cruise) speed is dependent on the CI for the given conditions: altitude, weight, wind, and ISA Deviation. If the conditions aren't the exact same, the cruise speed for a given CI won't be the same. Wind and weight are two factors that have a direct influence on the speed the airplane calculates.
In a headwind, the speed will become higher, in a tailwind lower as the plane tries to maximise the gain from the wind. Weight will also play a role since the plane will aim to fly at a certain AoA. A lighter plane will fly slower. A heavier one faster.