Re: Efficiency of commercial planes

From:         Robert Dorsett <rdd@cactus.org>
Date:         06 Oct 93 03:50:46 PDT
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In article <airliners.1993.613@ohare.Chicago.COM> you write:
>But other breakdowns would be interesting, say by length of
>flight.  How good are the best planes running optimal distances
>with full passenger load? etc.

In general, it's not as efficient to fly short-haul distances as long-
haul distances.  Most fuel is consumed during take-off and climb; 
therefore, short distances are better served by turboprops, which have
a better lifting and climb capability than jets (but lower passenger-
acceptance, which is probably why they aren't terribly popular in the US).


>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. I
>find this interesting because it meanse that flying, on average,
>is as fuel intensive as driving a fuel-hog car alone. But is this
>true?

I dunno.  But for argument's sake, here are some figures: that, and I
need the practice. :-)

Let's say we look at a late-model 727 Advanced.  Figure a MLW of 160,000 
lbs, max. fuel load (heavy-lifter).  Take-off weight of around 210,000 lbs.  
Fuel load of 56,000 lbs (aux tanks), or 8358 gallons at a density of 6.7 
lbs/gallon.  This yields max-range cruise, about 2700 nautical miles, 
including fuel used for takeoff, climb, cruise, and the reduced fuel demands.
of descent and approach, plus IFR reserves.

If we figure 150 seats, this yields 466,000 seat-miles with an efficiency 
of 466,000/8358 = 56 seat-miles/gallon.  This provides a basis on which
to calculate average load factors for that *particular* route.

For another set of figures, look at a shorter-range flight.  For a target
of 1500 nautical miles, the fuel load to get there will be about 34000 lbs,
or 5074 gallons.  That's 258,750 seat-miles, which gives a yield of
51 seat-miles/gallon, or 10% worse.


Now, let's compare this to a more modern airplane, the 757.  For a 
2700-nautical-mile trip, it'll require about 47000 lbs of fuel.  Figure
an average seat-density of 200 seats, that's (200 * 2700 * 1.15) / (47000/6.7)
= 89 seat-miles/gallon, or about 58% better than a 727.  So a 757
could be used on a similar 727 long-range route with up to 85% of an
old 727 load factor, or 63% full.  A smaller airplane like a 737 can 
do better, and that's what's used.


The major question, though, as Karl points out, is how many seats are 
actually filled.  Many state-run airlines run almost empty.  I understand 
European carriers don't maintain very high load factors, when compared to
similar routes in the United States.  So the question then is routing: a 
charter from London to Madrid will be crammed full of passengers, but a 
737 from Hamburg to Dusseldorf probably won't be.  In terms of practical 
limits, therefore, it's entirely conceivable that some flights run at 15 
passenger-miles/gallon, but I'd suspect that most don't, which means we're 
back to the original question of what the industry load averages are, and 
I've just wasted your time.  Just showing what's possible.

But then again, this is all pretty pointless.  Sure, a car can make a trip
more cheaply: so can a DC-3.  But a DC-3 will take 5 times as long, a car
10-15 times as long--provided either could even manage the more interesting
geographic hurdles.  We pay a premium for flying fast, but it's one that
most people are prepared to accept.  Plus, it's safer. :-)



---
Robert Dorsett
rdd@cactus.org
...cs.utexas.edu!cactus.org!rdd