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# Fuel Burn A/c vs Automobile

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# Fuel Burn A/c vs Automobile

31st Aug 2000, 01:47
Tim Hall
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Fuel Burn A/c vs Automobile

Today a passenger asked me a question about the fuel consumption of the aircraft vs that of a car (I'm a Cabin Manager with a UK airline). I told them that our miles per gallon were more efficient than a car, but later wondered if this was accurate.

Is there any way of working out, on a 757-200 operating a 2 hour flight what the (approximate) burn per mile per pax is, in terms of either gallons or litres (just in case I'm ever asked again). I appreciate that the answer will be infinitely variable, subject to winds, temp, route and altitude, but a rule of thumb answer would be welcome.

Thanks for any help that may be available.

31st Aug 2000, 03:41
HPSOV
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Ok, I'll try to come up with some very round numbers and see what happens.
These will be for a 767-300.
Fuel burn 6000kg/hr=8300ltr/hr.
Cruise speed=1000kph.
So we're burning 8.3ltr/km.
If we have 200pax that's 0.0415ltr/km/pax.
Where I come from we measure fuel efficiency in ltr/100km. A good car might do 8ltr/100km, the aircraft is doing 4.15ltr/100km per passenger, making it about twice as fuel efficient as a car. That is a car with one person in it, put four people in the car and the car becomes twice as fuel efficient.
I'm way too lazy to look up how many litres in a gallon etc, so you'll have to work that out for yourself.
Meanwhile I'll just wait for someone to prove me totally wrong

1st Sep 2000, 16:48
Tim Hall
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Thanks - If asked again I'll come back with "roughly twice as efficient as a 1 person car" and leave it at that. Your help was appreciated.

1st Sep 2000, 20:21
casual observer
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Comparing fuel efficiency of a personal automobile to an aviation mass transport is not meaningful. A bus will sure be a lot more fuel efficient than an airplane. However, a bus travels probably ten times slower than an airplane (which means a bus uses a lot less energy, that's why it's more fuel efficient). Furthermore, surface transportation requires significant more infrastructure than air transportation. Thus, taking everything into account (time, infrastructure, fuel, etc.), air transportation is most definitely a more efficient means for *long-distance* travel (here long can be just a few hundred kilometers).

2nd Sep 2000, 01:31
Mr Benn
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You should know both your fuel burn for the trip and also the distance.
Convert your fuel burn into gallons. Say 1600 gallons.
Divide this by the number of passengers. Say 230.
This is c7 gallons per person, say.
Convert the distance from nautical miles to statute miles. Say 1000 miles.
So each person would use 7 gallons to go 1000 miles. Thats 142 miles per gallon.
You just need the basic conversion rates and a calculator. Check it out on the next long flight, it is always roughly the same.

2nd Sep 2000, 06:41

Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 5,233

Remember also that the aircraft is flying in a straight line, while the car has to follow all the wriggles and turns of the road!

2nd Sep 2000, 10:16
tailwinds
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Any idea how much a litre of Jet A-1 costs?
When you have the average Fuel/km (mileage) then you have the info that he is really looking for.
How much fuel (litres) do you burn from A to B and how much it costs and the efficiency of airplanes. Why does my Ticket cost so much? etc. These were asked once when I was an F/o and the Capt came up with some weird answer. Can't remember the cost. It was a long time ago. Besides, fuel prices have gone up!

2nd Sep 2000, 17:14
Tim Hall
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Thanks everybody for all the help - you're right, the missing pieces would be useful i.e. conversion rates for tonnes to gals/litres and the price of a gal/litre.

I'm well impressed with these responses - I was a bit nervous posting in this forum as a non techie.

[This message has been edited by Tim Hall (edited 02 September 2000).]

2nd Sep 2000, 18:18
Shanwick Shanwick
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Just worked out the other day, the 747-400 does approx 6.5 nautical miles per US gallon.

That's with 407 pax at 540 kts.

2nd Sep 2000, 21:44
Mr Benn
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Kgs to Litres, depends on SG, but I think it is c1.25 to 1.35, ie. multiply your "kgs" by c1.3 to give litres. 1 gallon = 4.54 litres if you want to do the gallon thing.
I think jet fuel is c\$200 per tonne (1000 kg) but I could be wrong....it has been known

3rd Sep 2000, 01:52
twistedenginestarter
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The figures quoted seem about right. Aircraft are two to three times less efficient than a car with 100% load factor. However this is generally a pointless statistic. You wouldn't drive to New York. And that's the problem. The aircraft is doing a 7000 miles round trip. Whether or not it is efficient is probably less relevant than whether it is ecologically advisable. To answer that it may be best to think in terms of gallons per hour.

Popping over to New York for a few days break, you're burning maybe half a tonne of fuel. Believe me - you wouldn't find that acceptable for 4 days in Scarborough.

Airliners are in general a nonsense. They only serve the following purposes:

1 Allow people to maintain a personal presence across large geographic territories rather than delegate responsibility.

2 Allow people to live in otherwise uninhabitable geographic areas (eg UK)and still enjoy decent weather now and again

3 Allow pilots to spend their lives avoiding the crushingly boring work that almost everyone else has to do.

Items 1 and 2 are entirely unnecessary. Item 3 keeps the whole thing going. Would you want to work in an office?

3rd Sep 2000, 12:03
Danish Pilot
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HPSOV: Just a little corection. It is not enough to look at the aircraft speed, and fuel burn/pr hr. To get the most true picture use MR BENN's method. When flying from A-B the aircraft will use fuel from first "light up" to fuel cut off. So, when you are at your destination, you have to know how much fuel did we use, how many pax did we transport (including crew), how much balast such a freight/bag did we transport.
When you have theise figures, you put that up against how much "value" you got for your money; over how many miles did we use this fuel to transport all these bag/freight/pax/crew, and how long did it take.

It is not enough to compare fuelburn in cruise.

3rd Sep 2000, 21:41
Noddy Staltern
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Shanwick x 2,

Try those sums again - I think you'll find it comes to about 6.5 gallons per mile, not the other way round!

4th Sep 2000, 11:29
Johnson Bar
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Bit of a pointless exercise. As others have pointed out, the airliner can take you between destinations where the auto is impractical (for instance, to the US from the UK, to Japan from anywhere). But the auto takes you places where the airliner can't, for instance from the drome to the home. For real accuracy, you would have to factor in the travel door-to-door.

Fuel efficiency is a losing argument. After all, peak fuel efficiency was achieved in the clipper ships of the 1850s. The only fuel they used was a bit of whale oil to light the lamps. Yet improvements in Mr Watt's engines left the graceful vessels relegated to history.

=Johnson=

5th Sep 2000, 19:19
Fastpants
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ok....3 tonnes of kerosene at 0.8 SG = 3750 litres....

my Peugeot diesel has a 50 litre tank...on the motorway i can get 600+ miles out of that.

so for 2 hours worth of flying about in a 737, i can get 45,000 miles in the car..

a kind of out of context comparison, but amusing nonetheless..

6th Sep 2000, 06:00
Angle of Attack
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This may be slightly off on a tangent but I once read that full fuel in a 747 could run an average sized car continuously for 87 years! When you think of it its amazing, but I dont think there is any car with a fuel tank big enough!!

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