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Old 16th May 2013, 18:15
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EMIT
 
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The above quote about the workings of the engine control computers is nice - it is the process by which your idle will be at slightly higher thrust and RPM's with anti-ice ON than with anti-ice OFF. This quote however, does not explain why the aircraft seems to fall short on energy on most managed descents.

The descent path as stated a few posts above, is calculated by the FMGC, which, for its calculations, assumes the elevated idle thrust will apply for the whole descent. If, in reality, anti-ice is OFF and consequently, the lower level of modulated idle is technically available, the autothrust will have to maintain slightly more thrust to maintain speed while the pitch mode keeps the aircraft glued to the managed descent trajectory.

For fuel economy, a real idle descent would be ideal, all the way from cruise level to the point where your dirty configuration on the final approach slope necessitates thrust to keep speed at the desired approach speed.

Needless to say, real world constraints make it hard to achieve that lofty ideal, but theoretically ... think about the alternatives:
If you descend too soon, you will need level flight in thick air - you're worse off
If you need speedbrakes down low, you have stayed at cruise thrust too long - also a waste.

Descent paths calculated by FMC's (or FMGC's for Airbus folks) are just a means to help you to approximate the ideal situation.
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