Re: 727 lineage (Re: Soviet Aircraft)

From:         Robert Dorsett <rdd@rascal.ics.utexas.edu>
Date:         20 Apr 93 15:28:57 PDT
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> They needed an airplane which [among other things] would have
>> a higher dispatch reliability in the (then) regulatory environment
>> .... Three engines were used to ensure the dispatch requirement.
>
>Please explain further!

If you lose an engine on a twin, you have to land back at the original
airport immediately.  Minima at that time were fairly restrictive: no
Cat II approaches or anything like that.  So all twins were stuck when
weather went down to 500'/1 mile.

In a trijet, though, an engine failure is more of an inconvenience than
an emergency.  So the rules had more lenient minima (200'/1 mile); the
airplane could turn back and land when it got that bad, or even comfor-
tably proceed to an alternate.

In real terms, trijets would have been about three times more reliable,
an important factor for an airline trying to keep its own schedules straight.

The rules were changed: twins got more reliable (A320 has Cat IIIA capability
out of the box), etc., so it's pretty much an obsolete argument.  But
a real one in the late 50's/early 60's.


--
Robert Dorsett
Internet: rdd@rascal.ics.utexas.edu
UUCP: ...cs.utexas.edu!rascal.ics.utexas.edu!rdd