What do you think about this inflight fuel saving technique

[JammedStab]

Flying along in the -400 at 330 on an oceanic track and the flight plan calls for a climb to 350, but we are a bit too heavy, so the FMC is changed from Econ to LRC in order to burn off fuel more quickly so we can get higher sooner.

Seems odd to me but I am kind of new to this glass cockpit stuff.

Didn't work anyways as we closed in on someone 1 FL above us and ahead of us so we stayed where we were.

[EMIT]

On an oceanic track - aren't you supposed (read: required) to maintain assigned Mach, rather than fool around with speeds as pleases you?

Otherwise - if you are heavier than planned, then OFP climbs are not exactly valid anymore. Crazy to try to burn fuel faster so you can then make the climb as close to OFP point as possible.

[pudoc]

On an oceanic track - aren't you supposed (read: required) to maintain assigned Mach, rather than fool around with speeds as pleases you?

How do you know he didn't request it?

Jesus, people love to jump down others throats on here.

[Ian W]

Not all oceanic tracks require specific Mach this is only normally important in busier traffic or if you are in trail to another aircraft or another aircraft is in trail to you on that track at that level. The tracks above and below with slightly different wind and temperature will result in different ground speeds in any case.

[SR-22]

Flights should not be planned nor flown with speeds such as ECON or LRC in MNPS airspace at least.. below a copy/paste from MNPS Manual:


Pilots must recognise that adherence to the assigned Mach Number is essential. No tolerance is provided for. Pilots must not utilise Long Range Cruise or ECON FMC modes when transiting NAT MNPS airspace

[FE Hoppy]

Don't know which airspace you were in so can't comment of mach technique. However, unless your econ speed was MRC then "changing" (accel or deccel?) to LRC would not burn more fuel but less!

[RAT 5]

You burn more fuel so you can climb higher to save fuel. No disrespect to the Irish, but you know what I mean. Is this a 1 step back to take 2 steps forward scenario? I suppose it would depend on how long you would cruise at the higher level.

[EMIT]

PUDOC, not jumping any throat here, but the OP wrote about flying on an OCEANIC TRACK and he supposedly flew it in FMC ECON, then perhaps, FMC LRC.
OCEANIC TRACKS are flown with ATC assigned fixed Mach numbers, period (maybe the OP is a Flight Simmer, that I don't know).

Yes, you can request another Mach number, but ATC never assigns "ECON" or "LRC".

The FL the OP mentions, F330 and F350, is in the band where MNPS rules apply.

Yes, if you are an operational maritime patrol aircraft, you will fly far below RVSM and MNPS, and then you can do as you please for operational reasons. However, the designation -400 as mentioned, and the OFP and economic considerations make it clear it must be a commercial aircraft operation the OP is writing about.

Ian W - geographically speaking, flying from Portugal to the Canary Islands is a stretch over Atlantic Ocean water, but is not an Oceanic track in the meaning of ATC, that stretch is just another airway. On an oceanic track, you are not free speed, unless it is busy - as every operational long hauler will know, your oceanic clearance will contain route, flight level and assigned Mach, often also an assigned time for crossing the oceanic entry point.

[BuzzBox]

OCEANIC TRACKS are flown with ATC assigned fixed Mach numbers, period

Not true. There are plenty of oceanic routes where MNPS rules don't apply. Ever heard of the Indian Ocean? The OP didn't specify the route he/she was flying.

[waffler]

No disrespect to the Irish.

Rat 5, what are you talking about ?

The poster clearly mentioned he was in a -400, I presume a 747.

[EMIT]

Buzbox - Indian Ocean, same as Atlantic stretch between Portugal and Canaries.
Yes, it over oceanic water, but it is just an "ordinary airway" , not what is commonly referred to as AN OCEANIC TRACK by the professionals that fly in oceanic tracks for a daily living.

[Denti]

@FE i would think that depends. For example we fly CI 10 on our 737NGs (and 7 on the airbus) which is usually slower than LRC. LRC indeed uses a bit more fuel, however not all that much. Over a 5 hour flight it is about 50 to 100kgs more and 15 minutes saved depending on wind of course.

[parabellum]

Sorry Buzz Box and others but in this case I think EMIT is right on the money, to most professionals 'Oceanic Track' refers to the Atlantic tracks. Pacific uses airways, not tracks, as does the Indian Ocean. There are specific rules for flying the Oceanic tracks. ATC assigned Mach no. is but one.


You burn more fuel so you can climb higher to save fuel.


At the beginning of a thirteen to fifteen hour leg, for example, if you think you are going to get trapped in the lower levels, I can see that working.

[latetonite]

You could also dump a few tons. Then cheat a bit with the cruise CG, and go straight to your required level. Easy.

[BuzzBox]

to most professionals 'Oceanic Track' refers to the Atlantic tracks.

Point taken, but there ARE other oceanic organized track systems that aren't subject to the same rules as the North Atlantic system, such as the PACOTS tracks between Asia and North America. PACOTS is made up of flexible tracks, not airways.

I guess we'll never know which 'oceanic track' the OP was flying unless he or she pipes up and tells us!

Getting back to the original question, selecting LRC in the 744 results in a faster cruise speed (M0.85-0.86 from memory) than ECON and will burn off the fuel more quickly. That MIGHT allow you to climb a bit earlier and save fuel overall on a long sector, but only if you would otherwise end up being stuck at the lower level, as parabellum said.

That said, with the use of CPDLC on a lot of oceanic routes, it can be difficult (impossible?) to keep a picture of the other traffic. You could end up being stuck at the lower level anyway and screw yourself out of fuel, as you found out.

[Wizofoz]

WRT to actual OP question- No, that isn't going to save you any fuel, unless, as has been mentioned, it stops you being stuck at a lower level by ATC.

[framer]

However, unless your econ speed was MRC then "changing" (accel or deccel?) to LRC would not burn more fuel but less!
Is that statement true for the 747 ? It is not necessarily true for the 737 as Denti pointed out.

[AtoBsafely]

Framer,

LRC on the B744 corresponds to CI 250. I expect the OP would have been using a cost index somewhat less than that, as most operators are pretty fuel conscious these days. So, yes it would be an acceleration from ECON to LRC.

OP,

When you said you were too heavy for F350, I assume that your FMC max altitude was less than that and your flight plan was based on the correct weight. You may be able to make a cruise CG correction to increase your max altitude because it usually defaults to a conservative value. When pushing close to max altitude, be aware of the buffet boundary/expected turbulence, and OAT trend.

Other than that, you are best off to just wait until you burn down to an appropriate weight. In fact, unless there is a strongly increasing tailwind you should be close to optimum altitude anyway.

Basically the idea proposed is just fiddling with cost index, which doesn't make any sense because the fuel/time costs remain the same for the entire flight.

[enicalyth]

I just happen to have an example before my eyes on my PC.

Case 1 is M0.825; case 2 M0.845 and poses a fictitious instantaneous step climb FL330-350 at 341560kg. The “econ” specific air range computes as 43nm/tonne; "LRC" choice reduces to 42nm/tonne.

[hawk37]

Enicalyth, those are interesting figures. Are you able to give us an estimate for what max range speed would be, and it's associated specific range?

[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]

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]

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]

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