Descent speed

[super 27]

DO you save fuel by descending for the same weight faster on glide or slower and earlier before the glide?
let say a B727 at FL 350 descending to 1000ft (airport elevation) at 300kts started the descent at 105 nm,will it burn more or less than the one who started at 125 nm with 250 kts (both at maximum landing weight)??:confused: the same question applies for the MD80

[Rainboe]

250 is closer to the min drag speed and gives a more efficient descent. You also save burning 20 miles worth of cruise fuel, so more efficient at the slower speed.....but boring- it seems so slow.

[Shanwick Shanwick]

The longer time spent with the thrust levers closed = less fuel burnt

[Piltdown Man]

The closer you fly to your minimum drag speed, the lower your fuel burn. I recon that as 250kts is closer than 300kts to min. drag that speed would give you (all things being even) the lowest fuel burn.

[vwreggie]

If your thrust is idle then your fuel flow is the same regardless of whether or not you fly fast or slow. What changes is the time spent in cruise and the fuel burn in the cruise. If you descend at an earlier point at min drag type speeds then you have spent less time in the cruise and therefore burnt less fuel. If you descend at high speeds ( ias) then you have a steeper descent profile which means that you have flown closer to the field which in turn means you have spent longer in the cruise and burnt more fuel. how much depends on type. 737 about 40kg /nm cruise. 747 at end of cruise about 20 kg /nm. On a practical level if you are too slow or fast compared to other jet traffic your vectors will eat up any savings as atc fit you in behind others.

[Catabolic IBS]

Longer spent at altitude=Less fuel spent for jet engines
Longer spent at altitude=Greater descent angle=greater descent speed
This increased descent speed equates to less time spent in the descent

Most economical descent speed equals one where the ATC speed restrictions (commonly 250kts below fl100) can be met with thrust levers closed from TOD to touch down. Roughly slightly higher than Vmd (around 280 kts)

[Rainboe]

Craig Freier, your fuel figures aren't quite right. Before the 'Green Nazis' latch onto them as evidence of profligate carbon emissions by 'evil aeroplanes' (works of Satan and all that garbage), I think you meant
737- 40kgs/min ie 2400kgs/hr
747- 200kgs/min ie 12000 kgs/hr

In fact we were looking at 1900kgs/hr at the end of cruise in a 737-700 this week, and the 747 comes down to well below 10,000kgs/hr at the end of cruise.