Working out the fuel consumption rate of aeroplanes
By MUNGAI KIHANYA
The Sunday Nation
Nairobi,
11 November 2018
Shadrack Mwangi says
that a newspaper story about the first by Kenya Airways (KQ) direct
flight to the USA claimed that the aeroplane will curry 85 tonnes of
fuel. In another paragraph, the writer wrote 85,000 litres. So, Shadrack
wondered: is one kilogram of jet fuel equal to one litre?
The answer is no.
Water is the only substance I think of for which one litre is one
kilogram. In the case of jet fuel, one litre is about 840g, or 0.84kg.
This is the reason why it floats on water.
Therefore, 85,000
litres of jet fuel weigh 71,400kg, or 71.4 tonnes. On the other hand,
the 85 tonnes are equal to 101,000 litres. Now this is a significant
difference. Indeed, there was an aircraft that crashed several years ago
because it ran out of fuel.
The problem was that
the pilot ordered fuel in kilograms but the ground crew were using a
filling machines that dispensed in litres. Instead of working out the
conversion correctly, the technicians assumed that one litre is equal to
one kilogram. The plane got 15% less fuel than required so it couldn’t
complete its journey!
Now you might wonder
why the pilot didn’t just fill it up to the brim. The reason is that
aeroplanes have very large fuel tanks. If it is filled up for a short
journey, it ends up carrying a lot of dead weight – and wasting fuel.
The Boeing 787-8
Dreamliner use by KQ on the Nairobi – New York service has a tank
capacity of 127,000 litres, that is, 106 tonnes. It is one the most
fuel-efficient commercial jets consuming about 5.5kg/km.
So, on the 12,000km
journey from Nairobi to New York, it will need about 5.5 x 12,000 =
66,000kg or 66 tonnes of fuel. To be on the safe side, the pilots add
another 19T to make a total of 85T
An interesting
comparison might be made between the aeroplane and a family car.
5.5kg/km is equal to 6.55L/km.
An average family car
easily does 15km/L on the highway. This is equivalent to 0.067L/km. It
seems that the car is about 100 times more efficient that the aeroplane:
but is it?
The aeroplane carries
234 passengers excluding the crew. So, its consumption per passenger is
0.027L/km. The car takes 4 people excluding the driver. Thus, it
consumes 0.017L/km.
Even though the car
is more fuel efficient, the difference is not so big. And considering
that the car cruises at just 100km/h while the aeroplane does 1,000km/h,
I think the little extra fuel is worth it.
***
On a related matter;
I was shocked to learn that there are people out there who still think
that a bus doing 100km/h is faster than a car also at 100km/h. They say
that this has to do with the size of the wheels.
If you come across
such a person, ask them this: Since an aeroplane does not fly on wheels,
does that mean that it has no speed?
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