737NG and Smiths FMC Climb Calculations

[duster1]

Pehaps some of you engineer types could help with this -

I have noticed some interesting anomalies when entering various climb speed schedules in the FMC - this on 737 600 and 700s flying Cost index 13-17.

as an example:

Econ Climb gives speed of 274 Kt/.77 - time at TOC of 1948.3Z -distance to top of climb 105nm.

Max rate gives speed of 256/.77 time at TOC of 1948.5Z distance to TOC 99nm

Climb speed of 320/.77 gives time at TOC 1948.5Z - distance to TOC of 112nm.

My question - why do max rate and high speed climb take the same amount of time to TOC. Would it not make sense to climb at high speed ie gaining an extra 7 miles down range for 12 seconds more at climb thrust? Or is this an FMC error? Or am I missing something blindingly obvious

[Cough]

[guess mode]A 320kt climb will spend much more time at lower altitude, so whilst the climb is only slightly longer, you will spend much more time with a higher fuel burn at a lower altitude. Hence your actual fuel burn will be higher[/guess mode off]