Re: Efficiency of commercial planes

From:         kls@ohare.Chicago.COM (Karl Swartz)
Date:         05 Oct 93 01:10:26 PDT
Organization: Chicago Software Works
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>The number I would like to know is the total passenger miles
>of the airline industry divided by the total number of gallons
>of fuel used. That is, how many passenger miles per gallon to
>planes actually get on average.

I can't help with totals, but I do have some info on specific aircraft
which may at least get you in the right ballpark.  The basic 737-500,
for example, has a range of 2,500 nm.  I assume that's maximum fuel,
with payload up to the MTOW.  Empty weight of the aircraft is 68,180
lbs.  Maximum fuel is 5,311 U.S. gallons, which at 6.7 lbs per gallon
brings the total up to 103,760 lbs.  MTOW is 115,500 lbs so that gives
a payload of 11,740 lbs; at an average of 180 lbs each that's about 65
passengers.  Ignoring fuel reserves, that's 30.6 passenger miles per
gallon.

Similar figures for a maxed-out 767-300(ER) are 6,650 nm and 24,140
gallons, empty weight of 196,100 lbs, MTOW of 400,000 lbs, giving a
payload of 42,120 lbs or 234 pasengers.  Bottom line in this case
works out to 64.5 passenger miles per gallon.

Shorter distances should produce better fuel consumption, up to a
point, though I'm not sure how large the effect would really be.  Of
course even a maximum-range flight isn't going to run the tanks dry,
which would improve the numbers above.  On the other hand, I ignored
crew weight, which would drag the numbers down slightly.

These are also modern aircraft.  When talking about the overall fleet
you must consider all the old "gas guzzlers" which are still flying.

For a real-world example, in March of '92 I flew on a United 747-238B
from LAX to LHR.  Figuring a full house -- and we were close to it --
that's 353 passengers, carried 5,456 nm.  The captain posted a number
of interesting figures, including fuel: 38,418 gallons.  Once again
ignoring reserves, that works out to 50.1 passenger miles per gallon.

>Also, how small a fraction of the average fuel cost per passenger
>is  the incremental fuel cost of adding a passenger?

Considering that the payload in the 737 and 767 examples above is only
about 10% of the takeoff weight, the marginal cost per passenger is
pretty small.  On my London flight, over a third of the takeoff weight
was fuel.  The airframe itself is an even greater percentage, nearly
half the MTOW!

>I was told by a reasonably credible person that planes get
>about 15 passenger miles per gallon (worldwide average) but
>that Lufthansa gets about 25 passenger miles per gallon.

Hmmm ... if you consider passengers flown, rather than passengers who
*could* have been flown, then I could easily see this.  Load factors,
at least for domestic U.S. flights, are around 65%.  With marginal
cost per passengers being miniscule, that essentially knocks the
Pmi/gal of the 737-500 (at max range) down to a bit over 20.  With a
large number of gas guzzlers out there 15 doesn't seem unreasonable
at all.

Lufthansa has a fairly young, hence more efficient, fleet, which would
bring their numbers up some.  On average, they probably also have
larger planes flying longer routes, which would seem to be better too.

--
Karl Swartz	|INet	kls@ditka.chicago.com		
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