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I read on a site that a 747 taking off emits the same amount of air pollution in the first 5 miles or so as 3000 cars. This seems extreme (accepting that the cars will be average).

Are there any more precise comparisons for 2 or 4 engine aircraft?

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    $\begingroup$ As 3000 cars would in 5 miles, or in the time that it takes the 747 to travel 5 miles? Driving uphill like the 747, or on a flat road? If you are not precise in your question then the answer definitely is: yes there are more precise comparisons. $\endgroup$ – DeltaLima Jul 2 '14 at 7:27
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    $\begingroup$ I've no idea if the number is extreme or not but remember that there are fewer than 1,500 Boeing 747s in the world but a couple of orders of magnitude more than 1,500x3,000 = 4.5 million cars. $\endgroup$ – David Richerby Jul 2 '14 at 8:23
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    $\begingroup$ @farhan don't forget to account for the retired/lost hulls $\endgroup$ – ratchet freak Jul 2 '14 at 15:33
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    $\begingroup$ Is that a fair comparison? Measuring the 747 at what is admittedly the worst case condition - on takeoff - to automobiles in cruise mode? Put those 3000 cars on a drag strip and measure again... And then break down the answer to compare pollution per occupied seat mile. This sounds like a statistic created to serve an agenda. $\endgroup$ – Skip Miller Jul 2 '14 at 16:48
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    $\begingroup$ Where did you read this? (It's generally good form to cite your source - An old press hack once told me "Never respond to a blind quote.") $\endgroup$ – voretaq7 Jul 2 '14 at 17:06
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To provide more detail, let's look at some actual numbers. Without knowing where you got this information, the term "air pollution" is especially vague. But let's see what we can do. I'll compare CO (carbon monoxide) emissions since I could find that for both cars and jet engines.

The new 747-8 uses the GEnx-2B67 engine. The older 747-400 has a few engine options, one of which is the CF6-80C2B5F. Emissions information about these engines is available from EASA tests, and available for the GEnx and CF6. More info about the tests is here.

Based on that information the two engines have the following CO emissions at takeoff, reported as grams per kg of fuel burned:

  • CF6: .05 g/kg CO
  • GEnx: .17 g/kg CO

The fuel burn rates at takeoff for the engines are reported as:

  • CF6: 2.685 kg/s fuel
  • GEnx: 2.451 kg/s fuel

To find a value of comparison for cars, the EPA standards are here, in units of g/mi. Let's assume that we are using a recent model year, certified to the ULEV (Ultra-Low Emission Vehicles) standard.

  • ULEV: 1.7 g/mi CO

Now we need a way to compare these values. Let's use your initial value of 5 miles as the comparison distance. The jet engines will remain at takeoff thrust over these 5 miles, and the cars will meet the ULEV standard.

So how long will it take the 747 to cover those 5 miles? Let's look at the whole takeoff run, from a complete stop. This means that it will take about 40 seconds to cover the first mile down the runway. After that, lets assume it averages 160 knots, or 185 mph, over the next 4 miles. This means it will take 78 seconds to cover the rest of the distance, for a total of 118 seconds.

Computing the emissions for the takeoff:

CF6:

$$\begin{align*} 2.685\,\mathrm{kg/s} \times 0.05\,\mathrm{g/kg} &= 0.13\,\mathrm{g/s}\text{ CO}\\ 0.13\,\mathrm{g/s}\times 118\,\mathrm{s} &= 15.84\,\mathrm{g} \text{ CO per engine}\\ 15.84\,\mathrm{g}\times 4\text{ engines} &= 63.4\,\mathrm{g}\text{ CO per takeoff.}\end{align*}$$

GEnx:

$$\begin{align*} 2.451\,\mathrm{kg/s} \times 0.17\,\mathrm{g/kg} &= 0.42\,\mathrm{g/s}\text{ CO}\\ 0.42\,\mathrm{g/s}\times 118\,\mathrm{s} &= 49.17\,\mathrm{g}\text{ CO per engine}\\ 49.17\,\mathrm{g}\times 4\text{ engines} &= 196.7\,\mathrm{g}\text{ CO per takeoff.}\end{align*}$$

The car will be easier to compute:

$$5\,\mathrm{mi}\times 1.7\,\mathrm{g/mi} = 8.5\,\mathrm{g}\text{ CO per car.}$$

The 747-400 holds 416 people, and the 747-8 holds 467 (3-class configuration). Let's assume each car will carry 4 people.

$$\begin{align*} 416/4 &= 104\text{ cars}\\ 467/4 &= 116.75\text{ cars.} \end{align*}$$

We'll split the difference between cramming the three extra people into other cars and adding another car, and just leave it as 116.75 cars. Totaling the emissions from all cars:

$$\begin{align*} 104\text{ cars}\times 8.5\,\mathrm{g/car} &= 884\,\mathrm{g}\text{ CO}\\ 116.75\text{ cars}\times 8.5\,\mathrm{g/car} &= 992\,\mathrm{g}\text{ CO.}\end{align*}$$

So comparing the 747 and the cars:

  • 747-400: 63.4 g CO
  • 104 cars: 884 g CO

  • 747-8: 197 g CO

  • 116.75 cars: 992 g CO

So a 747-400 is equivalent to about 7.5 cars, and a 747-8 is equivalent to 23.2 cars, comparing the takeoff run with average vehicle emissions.

I found this pretty surprising. It would be more fair to compare a takeoff with a drag race or a 0-60 test on the cars, which is probably not what the emissions test requires.

If anyone has more info or sees an issue with this please let me know.

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    $\begingroup$ It would be more fair to compare a takeoff with a drag race -- It would probably be more fair to compare the total carbon output per passenger-mile for the entire flight (takeoff, cruise, landing) with driving equivalent distances. This is more important when comparing mileage figures (the EPA Miles-per-Gallon figure for cars are averaged and include time/fuel spent on acceleration (takeoff), cruise, and deceleration (descent/landing)). $\endgroup$ – voretaq7 Jul 2 '14 at 17:04
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    $\begingroup$ I agree that comparing per passenger mile is much more relevant, but based on the question I tried to focus on takeoff alone. $\endgroup$ – fooot Jul 2 '14 at 17:06
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    $\begingroup$ It seems really weird that the GEnx produces 3x as much CO as the older CF6... any idea why that would be? $\endgroup$ – David Jul 3 '14 at 1:50
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    $\begingroup$ So to put it in the relation provided in the question, you don't end up with 3000 cars but 7.5 and 23.1 cars, respectively? I've yet to see a highway where the total average number of occupants per car was higher than (or even close to) 2. Empty car-pool lane during rush hour anyone? $\endgroup$ – falstro Jul 3 '14 at 7:17
  • $\begingroup$ @David I agree that is strange. Maybe it was a compromise to reduce the HC and NOx, possibly related to the TAPS combustor. $\endgroup$ – fooot Jul 3 '14 at 16:59
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While there are more pollutants than CO, but keeping with CO, if we use 1000kg fuel as typical for a 747 taxi and takeoff, and compare that with 2 miles of automobile driving, I get the CO per passenger is much smaller for the 747:

747 0.05 fuel CO g/kg; 1000 Fuel used in Taxi-TO (kg); 50 CO produced (g); 400 Passengers; 0.125 g CO/passenger;

CAR 9.4 CO g/mile; 2 miles; 18.8 CO Produced (g); 4 Passengers; 4.7 g CO/passenger;

So, the CO produced per passenger is about 40 times greater for an automobile.

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    $\begingroup$ Be careful about comparing apples with oranges! The '40 times greater' refers to the amount of CO pollutant produced per passenger comparing generic fuel consumption for a 747 between taxi and takeoff, compared to a car travelling 2 miles. I'd have no heartburn with arguing it would be fairer to have the car travel 10 miles, in which case the ratio would be almost 200:1. None of this can be easily interpreted as statements of engine efficiency. $\endgroup$ – Padraig H Jul 11 '15 at 16:11
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    $\begingroup$ Just looking at CO is misleading. At least add $CO_2$ for a more complete picture. Aircraft gas turbines run very lean, so much less CO per mass of fuel burnt is produced than in a car engine. $\endgroup$ – Peter Kämpf Jul 11 '15 at 22:50
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Uff way to complicated .

747-400 fully loaded 160t of fuel (can take more fuel). 160.000 x 0.87 (fuel index) 140.000 liters approx.

140.000 L / 50 (average tank full per car)= 2800 tank fulls

140.000 gives probably 12 to 14 k nautical miles.

One car filled up 2800 times = about 2 million km. At about 7l/100 km.

This has nothing to do with mile per pax etc. As rarely do aircraft fly fully loaded or even with pax as many cargo aircraft in the air as well.

Just thought I make it a little bit more easy to imagine.

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    $\begingroup$ Welcome to aviation.SE! This is relevant, but the question was about pollution, not fuel consumption. If you can expand your answer to include that, it would be more useful. $\endgroup$ – Pondlife Aug 16 '17 at 17:45
  • $\begingroup$ @Pondlife one could pose that amount of fuel pollution iis proportional to amount of fuel used. $\endgroup$ – Koyovis Aug 16 '17 at 22:19
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    $\begingroup$ @Koyovis Sure, but I personally don't know if that's a reasonable conclusion. It's like asking "I heard that one hamburger has 5 times the calories of a hotdog, is that true?" It sounds like it should be, but if you load up the hotdogs with chili and cheese and compare them to a tofu burger, maybe not. And now I'm hungry :-) $\endgroup$ – Pondlife Aug 16 '17 at 22:45
  • $\begingroup$ @Pondlife Good point, how much does he eat and how good is the food. $\endgroup$ – Koyovis Aug 17 '17 at 8:34

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