# What is a (rough) rule of thumb for calculating fuel consumption in a modern jet airliner like Boeing 737 - 300?

I'm wondering what would be a rough rule of thumb for calculating the fuel consumption of a modern airliner like a Boeing 737 - 300.

I am not asking for anything from the plane's manual or even a rule of thumb to be used by pilots, but rather a "general idea", to be able to compare the fuel consumption of different modes of transportation.
I am citing the Boeing 737 - 300 as an example as it is a pretty common type of airplane, but feel free to answer for another type of passenger aircraft.

• Dec 26 '16 at 22:42
• The 737-300 isn't particularly modern -- it first flew more than 30 years ago. Dec 26 '16 at 22:46

Here are some very rough values for the 737-300, that a certain airline uses to compute manual flight plans in case the computer system fails.
Climb - 2950lbs / 15mins
Cruise - 5500lbs/hr
Descent - 600lbs / 20mins
Hold - 2650 / 30mins
Alternate - 1950 / 20mins
Min arrival fuel - 6300lbs
Recomended arrival fuel - 8300lbs
These numbers were based on a 27000lbs payload (approx 130pax plus bags)

[EDIT: I've edited the answer to remove any mention of the 737-300 being one of the most common 737 variants, thanks for the constructive comments!]

• I would bet my house that the 737-300 is no longer the most common 737! That thread is 16 years old now. Still interesting data though.
– Ben
Dec 26 '16 at 22:38
• @Ben According to Wikipedia, there were 1113 -300s built, delivered between 1984 and 1999. Since the introduction of the Next Generation 737s (-700/800/900) in 1997, nearly four times that many -700s have been built and a good number of the Classics will probably have been retired. So, yeah, the -300 is nothing like the most common today, and Boeing is cranking out new -700s at a rate of more than one a day. Dec 26 '16 at 22:45

For a first approximation, use 55 to 65 g of fuel per Newton-hour or 15 to 18 g per kN of thrust per second with modern jet engines. Please do not use the static thrust at sea level, but the actual thrust at the right flight Mach number and altitude. If you lack this figure: A modern airliner needs thrust equivalent to between 1/15th (6.67%) and 1/20th (5.0%) of its weight. In cruise flight at Mach 0.82 a value of approximately 1/17th (5.9%) should be used.

Thrust-specific fuel consumption nearly doubles between the static case and cruise at Mach 0.82, and the maximum thrust of the engines is roughly proportional to air density, which is a quarter of its sea level value at cruise altitude (approx. 10 km). The best turbofans achieve 9 g per kN and second in static tests!

If you want to calculate the fuel consumed over a longer trip, use the Breguet range equation and reformulate it so you get the mass ratio between take-off ($m_1$) and landing mass ($m_2$) for a given range $R$. For jets this equation is $$\frac{m_1}{m_2} = e^{\frac{R\cdot g\cdot b_f}{v\cdot L/D}}$$

Nomenclature:
$m\:\:\:\:$ mass in [kg]
$R\:\:\:\:$ Range in [m]
$g\:\:\:\:\:$ gravitational acceleration in [m/s²]
$b_f\:\:\:$ thrust-specific fuel consumption in [kg/Ns]
$v\:\:\:\:\:$ ground speed in [m/s]
$L/D\:$ lift-to-drag ratio

• Great, very detailed answer. A little tough to do without a calculator (or at least pen and paper), though. Dec 28 '16 at 22:00
• @Peter Kampf, you are just too smart for your own good! Seriously though, do you think someone looking for a "rough rule of thumb" figure is going to use that formula to come up with a number in "newton hours"?! I will spare you a downvote because I trust that this answer is substantially correct, but I really think answers should mirror the relative complexity of the question. Jan 12 at 18:00
• P.S. The fact that I am probably using "newton hour" out of context actually confirms my point: That even a relatively intelligent, college educated, military and commercial pilot doesn't think in these terms, much less a casual aviation enthusiast asking a very basic question. Jan 12 at 18:03
• @MichaelHall So you suggest I downgrade the answer by switching to Imperial units? And for thrust 1/17 of weight isn't rule-of-thumby enough? Jan 12 at 18:33
• Nope, I'm not suggesting anything. Just offering an observation... Jan 12 at 20:06

For an A330-200 operating in the region of FL200 its approx 100kgs/min or 6T per hour. Less at higher levels. I'm an instructor on the military version of the A330 and this is our mission planning rule of thumb. Add 2 tonnes for the climb and a suitable reserve at destination and your fuel planning is done!

The same principle can be used for any large aircraft, fuel flow per minute in the cruise is a known quantity(either from published figures or the fuel flow guages). The length of time in the cruise is a known quantity so if you add on the minimum fuel required at destination plus a little bit extra to allow for the initial climb you'll have a ball park figure for fuel required at the start of the trip.

Despite the fact that you'll have a computer generated flight plan before getting airborne, this simple rule of thumb allows you to do a gross error check to spot any mistakes. When airborne the same principle applies.

• Hi Nic, and welcome to Av.SE. It's helpful to know what an answer is based on. For instance, if you're an Airbus pilot & that's a rule of thumb that you use, adding that gives your answer credibility. Otherwise, it amounts to "somebody on the internet said..." which as you can imagine doesn't really carry much weight. Alternatively, if you can link to an authoritative source, that works well also. Jan 12 at 20:49
• Hello Nic, welcome to aviation.stackexchange.com. This is one data point for a different aircraft. It doesn't answer the question. It would be better to post this as a comment or to expand you answer so that it addresses the question. Jan 13 at 8:09
• The question stated 'modern airliner' which the A330 is. It's relevant if you want a rough idea of fuel consumed with regards to flight time.
– Nic
Jan 13 at 22:10
• Upvoted, the question mentions "I am citing the Boeing 737 - 300 as an example as it is a pretty common type of airplane, but feel free to answer for another type of passenger aircraft. " so this is a good answer to the question. Jan 14 at 9:42