Re: 767 Service Ceiling

From:         drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Organization: Boeing Commercial Airplane Group
Date:         05 Aug 94 03:35:28 
References:   1 2
Followups:    1 2
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In article <airliners.1994.1487@ohare.chicago.com>,
Karl Swartz <kls@ohare.Chicago.COM> wrote:
>>  I was wondering if anyone has information on the service ceiling of
>>any of the 767 variants.
>
>According to a chart in a Boeing 757/767 brochure I have, the initial
>cruise altitude of the 757 at low gross weight is 42,000 ft.  The 767
>(both -200 and -300) is 42,000 to 43,000 -- it's a graph, not hard
>numbers, and I can't tell for sure.  Near the maximum TOW, initial
>cruise is about 35,000 ft.

Initial cruise altitude is weight and thrust dependent for any given
airplane type, until you get to the maximum certified altitude, which is
the highest the airplane is legally permitted to fly with passengers aboard
in commercial service.  Maximum certified altitude is the highest alitude
at which the airplane can maintain an 8,000 ft pressure altitude in the
passenger cabin.  This is generally a structural limit of the body skins.
There are obvious economic tradeoffs that determine what maximum
certified altitude to design for - the higher weight of the stronger skins,
which cost more in terms of fuel burn for the enormous percentage of time
that the airplane is not operating at or near its maximum certified
altitude, must be balanced against the increased fuel burn associated with
poor airways routing in heavy traffic.  The short hop guys (Southwest comes
to mind) don't give a rip about high altitude capability because their
routes don't give them time to use that capability; therefore, they are
carrying around, and paying for, capability that they don't use.  The
general public might care because carrying that extra weight makes their
tickets more expensive.  The long haul guys (like some of United's routes)
desparately want the high altitude capability because they can save gobs of
time and fuel by getting above all the short haul traffic and getting
better routing to their destination.  This becomes a knotty problem because
the airplane is generally built with only one kind of body skin, and that
choice must be all things to all operators.  No one really appreciates the
dilemmas that face the airplane designer.  I know this is true because we
never get the hot babes and no one ever buys us a beer.  :-)

>Higher alititude generally means better cruise performance, with the
>limiting factor being cabin pressurization and not aerodynamics or
>engines, so I'd guess the top cruise altitude is close to or equal to
>the service ceiling.  (With oxygen for the pilots, I suspect either
>aircraft is capable of significantly higher altitudes.)

Higher altitudes up to about 36,000 ft mean better cruise performance.
Flying above the tropopause is done to get better airways routing and to
get out of the traffic (one must get out of the traffic to get better
routing is actually how it works).

Interestingly, service ceiling is a light aircraft type of parameter.  It
is fairly meaningless for heavy commercial airplanes.  The cruise altitude
is generally pretty close to optimum cruise altitude at any given stage of
the flight.  This is an economic issue as it is pretty costly to fly off
optimum.  The 500 fpm climb rate line is generally well above the optimum
cruise altitude, and service ceiling is defined as the altitude where the
rate of climb is 100 fpm.  The lowest climb parameter I've seen is 300 fpm,
and that is well above even the 500 fpm line.

The thought I'd like to leave you with is this:  The heavy commercial
jet transport is designed for operation in a given flight envelope.  That
envelope does not include 100 fpm climb rates.  Therefore, a better
indication of altitude capability is certified maximum altitude.

-- 
Terry
drinkard@bcstec.ca.boeing.com
"Anyone who thinks they can hold the company responsible for what I say has
more lawyers than sense."