Hello,

 

after reading a thread about cost index and some thinking... there is a LRC Cruise setting within the FMC. But what is the main reason for this? I mean... you can enter a pretty low CI and the airplane will cruise more economical (with greater range) than with LRC setting. So when it is used and why?

LRC (Long Range Cruise) is not the theoretical maximum range speed, that is MRC (Max Range Cruise) which is not an explicit option in the FMC, but is mentioned in the FCTM. MRC is the equivalent of a cost index of zero. However, at such speeds the airplane is just on the edge of the back of the drag curve, meaning that it is more speed unstable than normal. In practical terms this can mean quite a lot of autothrust activity which adversely affects fuel burn and hence reduces range.

 

LRC is actually 99% of the specific range of MRC, this is enough to make the aircraft more speed stable, so although theoretically results in 1% less range, overall, it is more efficient.

 

However, on a specific day, we have increased our arrival fuel by setting cost index to 0 in the cruise, so the MRC option is still available.

Thanks for clearing thing's up.