Re: over-automation with glass cockpits

From:         "Mark A. Brown" <mark@optima.nal.go.jp>
Date:         04 Aug 96 16:44:56 
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Speaking of airframe limits reminds me of a book I read on handling
large aircraft by the late CAA pilot D.P.Davies, who gave a good
pilot's interpretation of the structural engineer's talk. However, my
recall is ropy (I left the book in the UK and I am now in Japan) but
I'm sure Robert or some other will correct me.

Anyway, the published design limit is the limit to which you can load
up the airframe and it will bend of course, but when you unload it, it
is guaranteed to bend back to its original shape. In order to
guarantee this behaviour there has to be a margin, of course.

Loading over the design load but under what I recall as the ultimate
load (I may be wrong), the aircraft will bend but when you unload it
is not guaranteed to bend back to its original shape. However, it
shouldn't fall apart.

Over the ultimate load and not only is it not guaranteed to bend back
to its original shape but you run the risk of something failing.
However, even then different bits will fail at different loads, and
you'd have to load pretty seriously before something like the wing
spar failed (although tailplane failure would probably occur at a
substantially lower load).

---

Reflecting on postings a little more, then, by folks more qualified to
comment than myself, the emerging concensus seems to be that pilots
don't tend to gratuitously pull high positive or negative Gs without
good reason; if it's to save one's skin, one doesn't really care if
the machine is going to be a little crooked afterwards. However, with
an FBW computer limiting the airframe to "design limits" this may not
be an option.

Speculation; I suppose on an aircraft with artificial feel/force
feedback in the control system, you could let the pilot pull up to the
limit with a certain amount of control force, but if more is wanted
you could provide a "force barrier" to be overcome. This means that
the pilot would be unlikely to overload the airframe by accident, but
if required, a higher load factor could be achieved with a really
heavy pull of the "both feet on the panel and heave" type. In an
aircraft with no dynamic control loading mechanism (like Airbus
sidestick types, for example), this is not really a viable mechanism,
of course.

---

As a final word, I do recall there was at least one crash of a
military aircraft due to a pilot running out of pitch authority
because of the operation of the departure prevention system. Trouble
is that he was in a tight turn close to the ground, didn't expect the
system to behave the way it did, and was surprised when his control
inputs were getting progressively less effective. (I think he may have
left the departure prevention system inadvertently engaged in the
wrong part of the flight envelope.)

Just my 2p's worth.

-----------------------------------------------------------------------------
Dr. Mark A. Brown  (mark@nal.go.jp)
STA Research Fellow, National Aerospace Laboratory, Tokyo, Japan
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