Re: differing MGTOW for same model aircraft operated by different airlines?

Date:         17 Aug 97 01:57:23 
From:         kls@ohare.Chicago.NOSPAM.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
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>What differs between the aircraft ordered by the different carriers that
>affects the MGTOW, i.e. why do they not all have the same MGTOW for the
>same variant of the same model aircraft?  Is it just a reflection of the
>different cabin appointments, or are there other
>structural/mechanical/fuel tank/something else differences?

Most airliners can take off with a full payload at MGTOW, or a full
load of fuel, but not both.  More fuel means greater range, so by
increasing MGTOW you either increase range with a full payload, or
increase payload at maximum range.

However, higher MGTOW usually requires heavier structures which add
to the empty weight of the aircraft.  If you don't need the full
capabilities of the aircraft, there's no point in carrying around
the extra weight.  That's why most US carriers have 757s with lower
MGTOW than the maximum -- most are used on domestic flights, so the
longest flights they need to handle are MIA-SEA or BOS-SFO, both of
which are just a bit over 2,700 miles, well short of what a fully
optioned 757 is capable of.

Higher MGTOW may also mean more powerful engines, which cost more
and may require more maintenance because they're being operated at
higher thrusts.  For example, United's new 767-300s intended for
domestic use will have 52,000 lbs thrust PW4052 engines, compared
to their 767-300(ER)s which have 60,000 lbs thrust PW4060 engines.

Aircraft with greater MGTOW may have additional fuel tanks, but
that's more an effect than a cause -- without the higher weight,
the plane can't lift the extra fuel, so there's no point adding
the extra tanks and their associated equipment.

In some cases, the difference may be nothing more than what the
manufacturer charges.  A higher MGTOW aircraft is more capable, so
the manufacturer can charge more.  Other than engine choice, each
777-200(IGW) is essentially identical -- ordering higher MGTOW is
simply a matter of paying more money to Boeing and receiving a
certification for the higher weight.  Some software changes on the
plane, I'd imagine, but that's about it.

Karl Swartz	|Home
Moderator of sci.aeronautics.airliners -- Unix/network work pays the bills