1. Popular US-based airlines

to top | next step: 2. Planes in use at these popular airlines

To figure out how much carbon dioxide is created by a plane flying one mile in the air, we're going to need to look at the technical details of a bunch of planes. However, how do we know what planes to look at, and how will we weight the different performance data?

To answer those questions, we decided to select a bucket of industry-leading airlines, see what planes they fly, and weight the technical details of each plane with that plane's popularity.

There are plenty of ways to measure industry leadership, but we used the number of international departures from US airports as our measure of popularity. Here are the most popular US-based airlines in 2002 as reported by the Department of Transportation. (We know it's old data, and the airline industry has definitely seen some ups and downs since 2002. If you have better data, let us know.)

Exhibit 2.1 Popular US-based airlines in 2002

No. international departures from US airports

US-based airline	Number of departures
American	1,695
Continental	1,157
Delta	860
United Airlines	827
Northwest	721
USAirways	543
TOTAL	5,803

2. Planes in use at popular airlines

to top | previous step: 1. Popular US-based airlines | next step: 3. Sanity check: do Boeing's popular planes fly both domestic and international flights?

Now that we have an idea who our industry leaders are (a rough idea because the data is 6 years old!), we can see what planes they use. Determining which planes are used by our industry leaders will tell us what technical data we need to gather later so we can calculate average fuel efficiency.

These figures correspond to planes registered by the airline with the FAA. They may not necessarily be in use because an airline flies different planes on different routes depending upon a number of factors. Some planes don't get used at all. However, we can't really adjust for that, and since it is in the company's best interest to operate close to capacity, we'll run with these numbers.

Exhibit 2.2A Planes in use at major US-based airlines in 2007

No. planes registered with the FAA as of December 2007

Plane model	American	Continental	Delta	United	Northwest	USAirways	Total	as a % of total
B-737	77	255	71	94	0	92	589	21.6%
B-747	0	0	0	30	35	0	65	2.4%
B-757	140	58	123	97	71	43	532	19.5%
B-767	74	26	104	35	0	10	249	9.1%
B-777	47	20	8	52	0	0	127	4.7%
MD-88	0	0	120	0	0	0	120	4.4%
MD-90	0	0	16	0	0	0	16	0.6%
DC-9	338	0	0	0	134	0	472	17.3%
F-28	4	0	0	0	0	0	4	0.1%
A-300	34	0	0	0	0	0	34	1.2%
A-319	0	0	0	55	59	93	207	7.6%
A-320	0	0	0	97	71	75	243	8.9%
A-321	0	0	0	0	0	28	28	1.0%
A-330	0	0	0	0	29	9	38	1.4%
TOTAL	714	359	442	460	399	350	2,724	100.0%

Examining the data in Figure 2.2A, we can see that nearly 60% of the planes used by our industry leading US-based carriers are made by Boeing (versus 20% made by Airbus, Boeing's closest competitor). Let's examine the popularity of Boeing's models against one another.

Exhibit 2.2B Popularity of Boeing planes

No. planes in use

Plane model	Total planes in service	as a % of total Boeing planes in service
B-737	589	37.7%
B-747	65	4.2%
B-757	532	34.1%
B-767	249	15.9%
B-777	127	8.1%
TOTAL	1,562	100%

Now that we know Boeing provides the majority of planes used by our industry leading airlines, we'll assume they provide the majority of planes to the whole US-based airline industry. With that assumption, we can study the technical details of Boeing's planes to estimate the carbon dioxide produced by flying one mile in the air.

3. Sanity check: do Boeing's popular planes fly both domestic and international flights?

to top | previous step: 2. Planes in use at popular airlines | next step: 4. Fuel efficiency of Boeing's popular 7-series planes

Wait a second. We selected our sample of carriers based on international departures, but we're looking for an industry-wide average for both domestic and international flights. What if the basket of planes we're about to use to represent to entire industry only flies internationally.

When we checked Expedia to see which types of planes were used on a hypothetical trip between New York and London versus a hypothetical trip between Greensboro, NC and Atlanta, GA, we found several Boeing models used on both trips. In addition, since all Boeing models are available in domestic versions, we figured we were safe assuming our basket of planes could represent both domestic and international flights.

4. Fuel efficiency of Boeing's popular 7-series planes

to top | previous step: 3. Do Boeing's popular planes fly both domestic and international flights? | next step: 5. Average fuel efficiency for one mile in flight

Now that we're assuming Boeing's planes are the majority of US-based aircraft, we can set about estimating the average fuel economy of each Boeing 7-series of aircraft. Within each 7-series, there are multiple models, and for one model there are multiple cabin configurations. A two class configuration was chosen where available because we believed this was probably their most popular configuration - first or business class and coach.

To compute the fuel economy of each plane as gallons per mile, we divided the stated maximum fuel capacity in gallons by the stated maximum range in statute miles (miles we drive in a car). Since Boeing's reported maximum range would have to include takeoff, flight, and landing we figured this was a simpler approach than using a more complex multivariate equation. In addition, in their technical details, Boeing wants to overstate both the range and the fuel capacity. Hopefully, any overstating will wash out in the division to create a close or conservative figure.

Exhibit 2.4 Fuel efficiency of Boeing's planes

units marked in columns

Model	# Passengers (2 class config)	Max Fuel Capacity (gallons)	Max Range (nautical miles)	Max Range (statute miles)	Gallons per statute mile (gal / mi)
737
737-600	110	6,875	3,050	3,510	1.96
737-700	126	6,875	3,365	3,872	1.78
737-700ER	76	6,875	5,510	6,341	1.08
737-700C	120	6,875	3,205	3,688	1.86
737-800	162	6,875	3,060	3,521	1.95
737-900ER	180	7,837	3,200	3,682	2.13
AVERAGE	129	7,035	3,565	4,103	1.79
747
747-8	467	64,225	8,000	9,206	6.98
747-400	524	57,285	7,260	8,355	6.86
747-400ER	524	63,705	7,670	8,826	7.22
747-100	452	48,445	6,100	7,020	6.90
747-200	452	52,410	7,900	9,091	5.76
747-300	496	52,410	7,700	8,861	5.91
AVERAGE	486	56,413	7,438	8,560	6.61
757
757-200	200	11,489	3,900	4,488	2.56
757-300	243	11,466	3,395	3,907	2.93
AVERAGE	222	11,478	3,648	4,197	2.75
767
767-200ER	224	23,980	6,590	7,584	3.16
767-300ER	269	23,980	5,975	6,876	3.49
767-400ER	304	23,980	5,625	6,473	3.70
AVERAGE	266	23,980	6,063	6,978	3.45
777
777-200	400	31,000	5,235	6,024	5.15
777-200ER	400	45,220	7,700	8,861	5.10
777-300	451	45,220	6,015	6,922	6.53
777-200LR	301	47,890	9,450	10,875	4.40
777-300ER	365	47,890	7,930	9,126	5.25
AVERAGE	383	43,444	7,266	8,362	5.29

5. Average fuel efficiency for one mile in flight

to top | previous step: 4. Fuel economy of Boeing's popular 7-series planes | next step: 6. Amount of CO2 produced by one mile in flight

Now we know the fuel efficiency of each Boeing model. We can combine that fuel efficiency data with popularity data to figure out how much carbon dioxide is produced on average when you fly one mile in any Boeing plane (in geek speak, we're going to create a weighted average). However, since Boeing provides the majority of planes to the US airline industry, we can safely assume that the fuel efficiency for one mile in a Boeing plane is the same or close to the fuel efficiency for one mile in the air in any plane.

Exhibit 2.5 Average fuel efficiency for one mile in flight

units marked in columns

Model	average fuel economy (gallons / mi) (1)	avg number of passengers (2)	as a % of total Boeing planes in service (3)
B-737	1.79	129	37.7%
B-747	6.61	486	4.2%
B-757	2.75	222	34.1%
B-767	2.75	266	15.9%
B-777	5.29	383	8.1%
AVERAGE	2.75	218

Alright! Now we're getting somewhere. Based on our assumptions and calculations, one mile in flight burns 2.75 gallons of fuel on average. Plus, we can see that on average one plane can carry 218 passengers.

6. Amount of CO2 produced by one mile in flight

to top | previous step: 5. Average fuel efficiency for one mile in flight | next step: case study

So now we know on average how many gallons of fuel are burned for each mile in the air. Now we're ready to do a little chemistry and see how much CO2 is released when you burn several gallons of fuel.

In a turbine engine, kerosene is mixed with air and combusted. The hot gases created by the combustion reaction in the engine exit the engine at high speeds and create thrust in the opposite direction. This moves the plane forward.

The chemical formula for kerosene is C₁₃H₁₈. It is burned in a typical combustion reaction where atmospheric oxygen (O₂) combines with a hydrocarbon (C_xH_y) to create carbon dioxide (CO₂) and water (H₂O). This is the same combustion reaction that occurs in our bodies to generate energy from food. Other elements in the air (such as nitrogen and carbon dioxide) are burned in the engine. They create other molecules like nitrates, but they are ignored in our calculations.

Here's our chemical reaction for combusting kerosene: C₁₃H₁₈ + O₂ -> CO₂ + H₂O .

We just need to balance it so that the number of elements that enter our reaction exit our reaction. Unbalanced, the formula has 13 carbon atoms (C), 18 hydrogen atoms (H), and 2 oxygen atoms (O) creating 1 carbon atom, 3 oxygen atoms, and 2 hydrogen atoms. Atoms can't appear or disappear! We need to balance the equation by saying how many molecules of each element enter and exit. We do that by putting coefficients in front of each molecule in our equation that, when multiplied by the number of atoms in the coefficient's associated molecule, create the same number of atoms entering and exiting. Balancing the combustion reaction yields, 2C₁₃H₁₈ + 35O₂ -> 26CO₂ + 18H₂O. 26 carbon atoms, 36 hydrogen atoms, and 70 oxygen atoms combine to create 26 carbon atoms, 36 hydrogen atoms, and 70 oxygen atoms. When you look at the coefficients, our combustion reaction takes in two kerosene molecules for every 26 carbon dioxide molecules it creates. That means for every kerosene molecule, 13 carbon dioxide molecules are created. Scaling up, for every gram of kerosene consumed, 13 grams of carbon dioxide are created.

That's all the chemistry we need. We just need to know the density of kerosene so we can move from a measure of volume like gallons to a measure of mass like grams. The density of kerosene is 0.81762 grams/milliliter or 3,095.25 grams/gallon.

2.75 Gallons of fuel (kerosene) burned each mile(1)

8,521.30 Grams of kerosene burned each mile (2)

110,776.88 Grams of CO2 produced each mile (3)

244.22 Pounds of CO2 produced by one mile in the air (4)

1.12 Pounds of CO2 per passenger per mile (5)

There you have it. To the best of our ability, on average...

one mile in flight creates a little over 1 pound of carbon dioxide per passenger.