More fuel for climb or low altitude cruise in a jet ?
 

Does a jet generally use more fuel in the climb, or if cruising at at low altitude ?

E.g. For a short flight, i.e. 100 nm, what altitude be chosen?

If low-alt cruise is more expensive than climb, I guess the plan would be to "zoom" up to FL240 or so and then glide down ?

If the fuel burn is less for a cruise of ~300 kts at ~11,000 (North America), ~6,000 (Europe ???) than climbing, then I suppose you'd climb to minimum "above 250 kts" altitude ?

I have the book "Fly The Wing" by Jim Webb. He mentions the rule-of-thumb of multiplying the distance by 100, e.g. 10,000 for 100 mile flight, but he doesn't explain where that number comes from.

Mike

AFAIK, the 757 FMC computes an almost parabolic vertical profile for short flights...

I remember in the good old days (70's) that Lufthansa B727s used to go straight up to F390 and then back down between Frankfurt and Hamburg and vv , something like 35 mins flying time - if that's any help !

Fuel flow decreases quite rapidly with increased altitude. Also, when in descent, the engine(s) idle with minimal f/f. The optimal short distance profile is a climb until top of descent. A triangle.
Brgds,
Charles

Agreed - up as high as you can, and then idle thrust descent to landing.

Calls to mind the almost audible shock from a Clacton sector controller once... LTN-AMS was capped at FL230, but it was a quiet day, and he asked whether we'd like to cruise any higher, in our 737-NG. 'Yes please', we said, 'What would you like?' came the reply. A quick glance at the FMC revealed our ECON cruise level as FL410 (aircraft ceiling). The controller, somewhat taken aback, arranged to give us 350, and we were there in no time.

Thanks to everyone for the replies !

My only jet PIC time for the forseeable future will be in MSFS :)

Mike

Although the triangle may be the 'ideal' profile, it is a little impractical on such a short flight. We tend to have a fairly 'sterile' cockpit (chit chat ok, but no other paperwork etc) up to top of climb and from top of descent. This is to try to prevent level busts due to pilots attention not being completely on what the aircraft is actually doing.

What wee tend to do is try to get about 80nms of 'cruise', which obviously wouldn't possible on a 100nm sector otherwise we would only cruise at about 3000'!!! But that is the idea, so that we get a little bit of time for setting up the aids, getting an updated ATIS, completeing bug card and the like before we start descending.

PP

.. no sense of adventure ...

Hobart to Launy, if not much traffic, we would wager a beer on a nominated maximum level ... just enough time at that level to roll over into the descent ... wager then depended on idle thrust until normal spin up on the ILS ..

made for some iterative mental calculations throughout the sector .. and, no using the FMS .. and, to make it interesting from time to time, we would do the whole thing single pilot with the other pilot keeping a watchful eye

On the A330, the OPT ALT for short sectors takes into account 15 minutes of cruise time. I can't really say what that does to your net fuel burn but I think it was planned with giving the cabin crew an easier time serving drinks (less climbing uphill).

In the end, with either triangle, parabolic or even long cruise segment, total fuel burn might all be depend on air traffic congestion. We can only do so much....

In the computerised Flight Planning programmes which I've written, unless an adverse wind gradient exists, for short flights, the programme seeks the Level where the Climb Profile intersects with the Descent Profile.

The chosen cruise level, then, is the next lowest hemispherical level to keep ATC happy, or that giving 2 minutes minimum cruise time, to keep the crews happy.

In short - The 'user friendly' slingshot manoeuvre.

Regards,

Old Smokey