Re: 727 lineage (Re: Soviet Aircraft)

From:         rdd@cactus.org (Robert Dorsett)
Organization: Capital Area Central Texas UNIX Society, Austin, Tx
Date:         30 Apr 93 02:23:26 PDT
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In article <airliners.1993.360@ohareChicago.COM> you write:
>      I find this very interesting-  Unfortunately I don't have any references
>for what the rules used to be.... but FAR 121.617 says:

Just keep in mind that we're talking about rules that were formed over
thirty years ago.  Higher minima in the case of a twin sounds to me like
the rules were designed for two-man cockpits, which might be expected to
have higher workload (especially in a prop); in addition, jet engines of
the time weren't terribly widely propagated or well-understood, and they
did tend to break or need to be shut down, during the early learning
curve.  No doubt the airlines and the regulatory authorities got over this,
and, by the late 60's/early 70's, viewed twins as capable airplanes.


>      Also, I don't really understand the logic behind the different minima
>for twin vs. trijet operations.  Why should trijet minima be 200/1, when they
>don't have any more of a chance to sucessfully complete the approach than a
>twin (or single, etc.)?  What would make sense is that they would be allowed
>to cruise longer in order to find a suitable alternate- which is what current
>regulations allow.  Even a twin on one engine can go a long way in an hour.

On my first take, I read the comments as a "diversion to minima" requirement
as well, but I think in both cases, it's a "what if" contingency: what if
the airplane has to land at the departure airport?  If it is simply verboten
to land a twin in low minima, then that should be the center of debate...


>      I guess my questions are: Does it really make sense for different minima
>for twin vs. trijet? and Have the rules changed because of better weather 
>capability of twins (ie. Cat III), or have they changed because a twin can 
>now cruise to a new destination much more reliably?

I would speculate that it was a workload requirement, that the
rules were designed for both props and jets, and that the reliability of
jet engines was still a question mark.  I know there's at least one
Olde Farte reading the group; perhaps he'd care to comment. :-)




---
Robert Dorsett
rdd@cactus.org
...cs.utexas.edu!cactus.org!rdd