What do you think about this inflight fuel saving technique

[SloppyJoe]

Wondering if burning off more fuel by flying less economically is a better idea for saving fuel, in a 400, on an oceanic track. I would place a bet that this is a PACOTS track and not over the Atlantic. The airline where I work like to put people with no aviation experience into 400s on oceanic (PACOTS) tracks. They think it saves money and with fuel strategies like this they are surely onto a winner.

[parabellum]

The airline where I work like to put people with no aviation experience into 400s

What function do these people with no aviation experience fulfill on your company's aircraft?

[A Squared]

OK, lets try a thought experiment. Assume that temp/wind/etc is constant everywhere at 330 and constant everywhere at 350.

Lets say that in order to be able to climb to 350, you have to burn down to 200,000 lb of fuel *.

Now lets say that if you go to LRC, you burn down to 200,000 lb 100 miles sooner than you would have otherwise.

Now lets say that to climb from 330 to 350 burns 10,000 lb of fuel (same disclaimer about number applies)

So, either way, you arrive at 350 with 190,000 lb of fuel.

So, which do you think saves fuel: arriving at 350 with 190K lb of fuel 100 miles sooner, or arriving at 350 with 190K lb of fuel 100 miles later? In which scenario will you land with more fuel, all else being equal?










* I have never flown a 747-400 and have no idea what the fuel loads and burns are so I'm just picking numbers at random. If you don't like the numbers substitute in ones you do like. The principle remains the same.

[Wizofoz]

Clearly the second one, as you've spent the time at 330 operating efficiently.

You're own scenario shows that- you arrive at the fuel to climb 100 mile later- meaning you got that extra 100 miles for free compared to flying faster.

You use less fuel flying a route that is 100 miles less at 350 after all, don't you?

[A Squared]

Originally Posted by Wizofoz

Clearly the second one, as you've spent the time at 330 operating efficiently.

Wizofoz, yeah it's obviously less efficient to burn more fuel now to climb sooner. I wasn't trying to dispute the correct answer already give by you and others. I was offering the thought experiment for the OP who seemed a little unclear on the deal.

[enicalyth]

for hawk37

Rather hasty sums for you. All ISA and a "pristine" 400 assuming the CF6 80 C2B1F and M0.85 cruise. You need at least a current FCOM and FPPM. Write to PO box 3707 Seattle for these. Then look at the clever bits of the FMS.

This is not a very professional job and only an overview. Do NOT bet the farm on the figures, especially with 20 year old aircraft!

Loaded 396000
Empty 183000
Fuel 174000
Payload 40000
Ldg 239000
Cont 8000
Alt 5000
Hold 3500

7200nam 890mins trip

FL/Dist/Wt/SAR
290/120/388/37
310/240/385/38
330/1500/353/41
350/2850/322/45
370/4400/289/50
390/5950/206/55

[Wizofoz]

Thanks A Squared- Yes I was giving the bleedin' obvious answer to your well thought out way of illustrating the fact!!

[hawk37]

Enicalyth wrote

and M0.85 cruise

I was actually hoping for the max range speed(s), in order to compare specific range with LRC and the aforementioned ECON.

If thats too much work, ok, thanks for the effort

[taufupok]

I wonder what has happened to basic fundamentals, as far as the proponents of such a technique are concerned. Cruising at a higher LRC speed ( assuming the econ speed is somewhere between LRC and MRC ), at a lower level whilst at a higher gross weight is probably not a sensible idea unless if are really going to be stuck for a long long time at lower levels, or the wind trades are better at higher levels a little later down the route. Just wondering.........

[enicalyth]

for hawk37

Write for the FPPM and FCOM. There are bootleg versions, some brazenly offered on the web. The labourer is worthy of his hire, don’t feed the pirate.

One popular US distributor sells works with an “impressum” endorsed by Boeing itself but with the caveat that the “design” is frozen around 2008 with an engine model that is showing its age. At about $300 honestly acquired I think that’s Okay.

Maximum range occurs when zero-lift drag and induced drag are coincident. Broadly speaking that occurs at 473kts, FL360 ISA and 300 tonnes mass with residual climb >600fpm.

Remembering that force is measured in newtons I have estimated L/D as 2942000/159000 or 18.5 in which case if the instantaneous value of sfc is 0.65 it follows that SAR is 45-44nm/tonne over the range M0.825-0.860