Re: Cruise Altitude Schedules

From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
Date:         30 Oct 94 21:29:42 
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>Hi, flyers,

There's a *really* bad pun there for an article about altitudes!  :-)

>I seem to remember that somebody on this list (rdd?) once mentioned
>nearly parabolical altitude trajectories, but I can't find the exact

I don't recall the quote either, but I've been on flights from San
Francisco to Los Angeles (337 miles) that had a cruise altitude of
39,000 ft.  These were all on lightly loaded 757s, which have
incredible climb performance.  Even at that, we only spend a few
minutes at "cruise" altitude.  I never thought about it in such
terms, but the profile probably was close to parabolic.

In contrast, I've flown between Los Angeles and San Diego (109 miles)
a few times on 737s, and once on a 757.  The 737s leveled off at
either 10,000 or 12,000 ft, I can't recall which.  I'm not sure about
the 757 but I'd imagine it was about the same.

>Would reasons of purely technical economy - apart from marketing
>strategies and such - tend to support that demand? Are "short hops"
>inordinately expensive?

For a given aircraft type, there are a lot of fixed costs, with rather
small marginal costs for additional distance.  Gate and other ground
costs associated with cramming 126 people on a 737-300 and then getting
them off at the other end don't much care whether you're going 100
miles or 1,000 miles.  Same for landing fees, maintenance and airframe
life costs as a result of an additional takeoff/landing cycle, etc.
It doesn't require all that much more fuel to get to cruise if you're
going to stay there for an hour than if you're just there for a few

Looking at it another way, that SFO-LAX flight was schedule for about
90 minutes.  Add in another 60 minutes at the gate before each flight
(assuming we aren't talking about Southwest-style turnarounds).  So
let's say you've paid for crew, airplane, etc. for 2.5 hours, all for
less than 10 minutes of cruise.  Doubling the mileage will probably
require about 45 minutes more at cruise, a 100% increase in distance
for a 30% increase in time (and associated costs) with somewhat more
fuel and essentially the same fixed costs.

On the other hand, short-range planes are designed for the job, and
have design compromises that presumably reduce the cycle-related
costs.  A 737, for example, isn't capable of the cruise altitudes a
757 or 767 can reach, because the extra weight required wouldn't be
worth the cost given how little time a 737 would spend at those
altitudes.  At the other end of the spectrum, flying a 747 on 500
mile segments will rather rapidly use up the structural life of some
key components, such as landing gear.  (This is why JAL and ANA have
special domestic 747 variants, which are beefed up in the key areas.)

Karl Swartz	|INet
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		|Snail	2144 Sand Hill Rd., Menlo Park CA 94025, USA
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