Imagine I have two identical 747s flying from Detroit to Tokyo. One has 400 passengers + luggage, the other has 401 passengers + luggage. How much more fuel does the second plane, with its additional passenger/luggage, use during that 6,000-mile trip?
How much does the efficiency of the 7478-400 change by an increase or decrease of 250 # (passenger & baggage) in the weight of 910,000 total.
Or 2.747252747252e-4 %
Or, would a slight change in barometric pressure or small temperature change make a bigger difference?
Paging LSL Guy
I wonder if fuel consumption is consistent enough to make any calculation meaningful. Maybe humidity, temperature, load distribution, and many other factors could change fuel consumption more than the difference in weight of one passenger in actual conditions.
Off by two orders of magnitude, 0.0274% (need to multiply by 100 to convert to %).
Probably, but you can still come up with a theoretical number.
I don’t know if anyone here will be able to answer for a B747 but LSLGuy should be able to answer for other big Boeings. To give you an idea, before he gets here, the type I fly burns around 2200 kg of fuel per hour in the cruise. On a 1:00 sector (chosen because I know the figures off the top of my head) an increase in weight of 1000 kg costs about 50 kg in extra fuel burn. So for us an extra passenger would be about 4-5 kg more fuel on a short sector.
So that would be about $1.30 worth of jet fuel. (Price went down by about 40% the past year, did we passengers benefit from that??)
I’mg guessing “1:00 sector” means an hour at cruise altitude? So multiply by 10 or so, but then you need to add the fuel consumed by climbing to cruise altitude, which is a lot.
So if they serve a decent meal the cost of that could be higher than that of the fuel.
The 50 kg extra fuel includes the climb, so for a longer sector the percentage increase would be slightly less.
Yeah, but because so many other factors like wind, humidity, temperature, how clean the exterior of the plane is, how long they may have to hold before landing/taking off/taxiing etc. are all as or more significant (in terms of fuel usage) than an extra 250 lbs. of weight that theoretical number isn’t going to be too meaningful (i.e. it will vary by over 100% every time you actually calculate it…)
I suspect it’s quite meaningful. It directly relates to decisions that Boeing & Airbus make about aircraft design, e.g. “How much more/less would an airline be willing to pay for our new aircraft if it were 100 kg lighter/heavier.”
You might be surprised by what the airlines consider to be meaningful.
In my own company we are encouraged to start the APU after landing instead of before as it saves about 5-10 kg of fuel. They don’t fill the potable water tank above half full because we never use more than half and it saves 1kg of fuel on a one hour sector. We shut down one engine during taxi after landing to save 10 kg of fuel. We have recently been instructed to plan to land with 2000 kg of fuel instead of 2500 kg to save around 25 kg.
Some of it is a bit ridiculous IMO, such as the potable water, but it all adds up over many flights and on big jets the dollar savings are higher than on our baby jumbo.