Jet Fuel Consumption

[WIDN62]

My question concerns the difference in fuel consumption if an airliner is held down at a lower level than the planned level.

For example, if crossing the Atlantic you are held down 2000 or 4000 feet lower than planned how significant is the increase in consumption?

[18-Wheeler]

If you keep the same speed then yes it'll hurt you by a percent or two.
Usually all you have to do is slow down a little and you'll get much the same burn for the leg. I've flown a 747 for about six hours on a ten hour flight, stuck a good 6,000' - 8,000' low and only lost a tonne or two. Also lost a bit of time but in a ten hour flight not enough to worry about.
FWIW the rule of thumb with a 747 Classic is to keep the TAS to a maximum of 500kts or an IAS of about 320, whichever is lower, that seems to work fairly well down low.

[HazelNuts39]

EDIT:: Typically, depending on airplane, engine, weight, and Mach:
2000 ft below optimum: 2%; 4000 ft 5%; 6000 ft 9%

[CabinMaster]

A319 with CFM56-5B engines:

Using In-Cruise Quick Check table of the FCOM:
2000nm to go, when you want to cruise FL370 but can only fly FL310, you'll burn 9820kg instead of 9045kg. That is 8.6% more.
If you have only 1000nm left, it is basically same percentage.

Long range cruise assumed, that is, the previous statements on how to adapt true airspeed are applied. Close to 9% is quite a lot and shows how altitude-hungry turbofans are. The more the better.

[Nubboy]

I concur.

Comiing back to UK earlier this year from the middle east and was held down by 6000' for several hours. Made a big hole (400kgs or so) in the contingency fuel. But that's what it's for.

[CabinMaster]

Using FCOM of A330-300 (Lufthansa, PW engines), assuming again 2000nm distance to destination and FL310 instead of FL370 (this quietly assumes that FL370 can be reached at current gross weight), the fuel burn difference is 3200kg.

FL370 M0.8 - 20053kg
FL350 M0.8 - 20968kg or 4.6%
FL330 M0.8 - 22034kg or 9.9%
FL310 M0.8 - 23427kg or 16.8%

One can say, roughly 4-5% more per 2000ft.

[BOAC]

5.3.2.3 of "Getting to grips with Fuel Economy" from ABI will tell you indicative figures.

[HazelNuts39]

CabinMaster;

I noticed that you get bigger percentage differences from the 'Quick Check' tables than from the tables giving momentary cruise fuel flow. Compare for example FCOM 3.05.15 p.11 (cruise M.80) and 3.05.20 p.3 (cruise M.80). I don't understand why there is so much difference. Any ideas?

regards,
HN39

[CabinMaster]

I just checked 170t gross weight, FL310 versus FL370, Mach 0.8.
Difference in fuel flow is 16.9%, so in line with the numbers I previously provided.

The slight difference is due to the different ground speed at constant Mach, as lower altitude means higher ground speed (this note is aimed at the people without ATPL - I don't even have a PPL).

We must consider that the Quick Check table is a bit unrealistic as 170t Gross Weight (reference for Quick Check I used) for an A330-300 is pretty low, actually only possible on a ferry flight or with very low payload.

[Maurice Chavez]

Try Long range Cruise, problem solved....

[Old Smokey]

I've just completed a relatively short sector (1518 nm) in a B777-200 with a Lido Flight Plan showing a TIME of 3:28 and FUEL BURN of 21767 Kg.

Lido add a very nice little estra to their CFPs, quoting directly -

ONE FL BELOW BURN ADD 447KG / TIME 03:27

That's as near as dammit to a 2% increase for 1000 ft below planned (Optimum) level. Over a reasonable range of lower levels, it would be safe to extrapolate this figure, e.g. for 6000 ft below Planned Level, add 12%. This seems to be in line with previous responses.

I'm about to launch on a Trans-Pacific crossing with a B777-300ER, and with this this thread in mind will post if, for a much longer flight, the percentage increment changes significantly. (I suspect that the percentage increment will be much the same).

Maurice Chavez, Assuming that RSA indicates Saudia Arabia (and not South Africa), I suspect that your CIs from your oil-rich country would be above LRC, so not a bad suggestion. For we mere mortals from oil-deprived countries, our CIs are already below LRC, a wiser suggestion might be to reduce to Maximum Range Speed (CI=0)

Regards,

Old Smokey

[Wizofoz]

OS,

I operate from one of those "Oil rich" countries, and OUR CIs are ALSO below LRC!!

One point, I think the "One level below" figure on a LIDO plan means one APPROPRIATE level below- thus 2000' lower in RVSM.

[Thridle Op Des]

Wiz it is 4000 below as non-RVSM is considered (see OMC App J P15:47) I thought it was 1000 once until a smart cadet pointed it out to me!

TOD

[HazelNuts39]

CabinMaster;

Your reminder that the change in TAS must be taken into account is appreciated.
Apparently it matters where you look in the table. It would seem that the smallest percentages are found near the optimum altitude for each weight, as illustrated here.

regards,
HN39

[WIDN62]

Thanks for the interesting replies. It would appear that, like us down in turbo-prop country (mid to high FL2xxs), you will burn more, but not an insurmountable problem.

[CabinMaster]

The turboprop is less susceptible to lower altitudes. That's why turbo props rule on short range. It is both due to optimization of the designs at hand, and due to the physical limitations of a prop.
Usually an ATR will gain some advantage when going higher, but it is not as significant as for the jets, also due to the lower distances.