Slingshots??

[Wake Turbulence]

Can someone please tell me why it is more efficient for jet aircraft flying short sectors, to climb as high as possible and then descend, leaving a very small cruise sector.

Do the speed gains at higher altitude, and the higher engine operating RPM overcome the extra fuel burnt in the climb?

Thanks for any help able to be given

[john_tullamarine]

A turbine engine has a comparatively narrow design operating envelope. The main problem looking at height is having to operate at lower rpm at the lower levels to keep the EAS under control.

If you look at the fuel flows against height you will note that the lower levels have fairly horrendous burns compared to the higher.. although, generally, at the extreme limits for an aircraft, the very high levels will show a further slight decrease.

Hence, it is usually better fuel burn policy to climb and then descend, with a minimal cruise.

Apart from that, it is a great mental exercise (and good fun) to fly a (fair weather, by day, ferry or freight) short sector with no fancy JB gear (ie only the DME for profile planning and monitoring), straight up and then straight down, especially if the runways are aligned and the descent runs straight into the ILS. Beers are on him (or her, these days) who has to spin up before 1500 feet. Great fun over about an 80-90 nm sector.

[OzExpat]

And it's not just the gain in fuel economy in the higher levels. The fact is that a jet engine is quite unlike a piston in that your descent can be at flight idle the whole way back down - if you've worked out your profile just right! This helps to save even more fuel, which translates to a lower cost for the operation and, therefore, more profit to apply to the days when you need to go missed approach of an ILS!

Besides, as j_t says, there's the added incentive of not being the one to buy the first round of drinks! :)

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