Originally Posted by
Escape Path
Any one knows what the difference in full consumption is if you modify the descent speed from CI (around 270kias) to say 300 on a medium size aircraft? You'll keep cruise power a few minutes longer but your descent will be shorter, taking advantage of the higher GS/TAS.
I've done this to save a couple of minutes when going behind schedule but don't know how does it affect fuel economy (haven't done the fuel gauges vs computer flight plan math to be honest).
Work out the fuel flow for your aircraft type in cruise (~2200/2500 kg/hour for 737);
Work out time spent extra in cruise (check against calculated TOD to get minutes extra) then based on above work out additional cruise fuel requirement;
Work out the fuel flow for your aircraft type in descent (~500 kr/hour for 737);
Work out time spent less in descent (use RTA for runway prior and after KIAS increase (earlier arrival therefore time gained, then reduce it by additional time spent in cruise), this gives descent saving;
You WILL find out that a minute extra in fuel in cruise (~40 kg) can only be regained by reducing descent time by >6 minutes which means this was it is to generate a fuel saving.
Starting an idle descent as soon as possible at best glide speed (lowest CI) is most fuel efficient.