Re: 727 lineage (Re: Soviet Aircraft)

From: (Mark Rogers        )
Organization: University of Illinois at Urbana
Date:         24 Apr 93 09:28:44 PDT
References:   1 2 3 4
Followups:    1
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Robert Dorsett <> writes:

>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.

>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.

      I find this very interesting-  Unfortunately I don't have any references
for what the rules used to be.... but FAR 121.617 says:
" person may dispatch or release an aircraft from that airport unless
the dispatch... specifies an alternate airport located within the following
distances from the airport of takeoff:  (1) Aircraft having two engines: Not
more than one hour from the departure airport at normal cruising speed....
(2) Aircraft having three engines: not more than two hours from the departure

      You do say that the rules were changed... but from how I understood
your posting, it was because twins had to immediately land at their departure
airport, so the weather had to be better-- but now that we have Cat III, they
can land at the departure airport....  I was just wondering if this is truly
the reason the rules got changed (?)

      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.

      I also haven't heard of any different minima for an alternate than 
600/2 (precision approach)... I thought even if you have Cat II (or III), you
have to have an alternate if the weather is below (2000/3 +- 1 hour rule).

      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?

--Thanks for your help :)