Re: Slower aircrafts ?

Date:         28 Jun 98 18:47:35 
From:         Tom Turton <tturton@cowboys.anet-dfw.com>
Organization: ANET Internet Services
References:   1
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Frank Muenker wrote:
> I was recently on a Delta B767-ER (-200 I think) flight from Frankfurt to
> Cincinatti and the flight time was almost 10 hours. I could see on the
> monitors that the average cruise speed was only about 480 mph which is
> fairly slow compared to the 550 - 570 it could go. Also on 2 other
> inner-European flights ( about 4.5 hrs) with a B737 and a B757 cruise speed
> was way below 500 mph. I can recall that about 5 years ago cruise speed was
> always 520-570, at least on flights longer than 1 hr.
> I assume it has to do with fuel consumption, but does anybody know if the
> fuel performance is significantly better at lower speeds and if so why did
> they just start *now* to do that ? All aircrafts I was on looked like the
> newest models. Is it that  the latest models have a different curve of fuel
> performance ?

Let me preface this by saying its about 20% knowledge, 80% guesswork :-)

It is my understanding that fuel consumption is indeed the main reason why
aircraft today are flying slower.  In fact, if I recall, it was aircraft from
the B707/DC-8 days which made coast-to-coast flights at speeds bordering on
Mach 1 (probably more like 0.95, but this is part of that 80% guesswork I
mentioned!).

To further speculate, I would guess that the reason more modern aircraft are
flying slower is driven from a purely financial bottom line approach.  As
airlines attempt to cut their operating expenses, fuel consumption is one of
the big variables they have to play with.  So they ask their friendly aircraft
manufacturers to give them a plane that beats the competition (or at least
matches it).  The manufacturers can't do a whole lot more aerodynamically to a
conventional looking plane, so they slide on down the drag curve (lower speed
equals lower drag) and they "design" the airplane to operate at a lower speed
(I believe somewhere in the neighborhood of Mach 0.82-0.86 is typical).  I have
no idea what the actual costs involved are, but I assume the benefit derived
from flying slower and saving fuel makes up for the increased costs associated
with taking longer to fly from point A to B (higher costs for pilots and cabin
crew, higher maintenance costs because of maintenance scheduled by time rather
than mileage, etc).

Well, take the above for what it's worth, and maybe someone who is more
knowledgable can give some more "factual" data.

---Tom Turton