Re: over-automation with glass cockpits

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         05 Aug 96 04:21:21 
References:   1
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In article <airliners.1996.1565@ohare.Chicago.COM> "Mark A. Brown" <> writes:
>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.

The ultimate load is normally defined as the point at which it'll break.
The ultimate load is calculated by applying a fudge factor to the limit
load.  This fudge factor is normally 1.5, and is normally tested.

>Over the ultimate load and not only is it not guaranteed to bend back

Here's more than anyone ever wanted to know on it.

Sec. 25.305  Strength and deformation.

    (a) The structure must be able to support limit loads without detrimental
  permanent deformation. At any load up to limit loads, the deformation may not
  interfere with safe operation.
    (b) The structure must be able to support ultimate loads without failure
  for at least 3 seconds. However, when proof of strength is shown by dynamic
  tests simulating actual load conditions, the 3-second limit does not apply.
  Static tests conducted to ultimate load must include the ultimate deflections
  and ultimate deformation induced by the loading. When analytical methods are
  used to show compliance with the ultimate load strength requirements, it must
  be shown that--
    (1) The effects of deformation are not significant;
    (2) The deformations involved are fully accounted for in the analysis; or
    (3) The methods and assumptions used are sufficient to cover the effects of
  these deformations.
    (c) Where structural flexibility is such that any rate of load application
  likely to occur in the operating conditions might produce transient stresses
  appreciably higher than those corresponding to static loads, the effects of
  this rate of application must be considered.
    (d) The dynamic response of the airplane to vertical and lateral continuous
  turbulence must be taken into account. The continuous gust design criteria of
  Appendix G of this part must be used to establish the dynamic response unless
  more rational criteria are shown.
    (e) The airplane must be designed to withstand any vibration and buffeting
  that might occur in any likely operating condition up to VD/MD, including
  stall and probable inadvertent excursions beyond the boundaries of the buffet
  onset envelope. This must be shown by analysis, flight tests, or other tests
  found necessary by the Administrator.
    (f) Unless shown to be extremely improbable, the airplane must be designed
  to withstand any forced structural vibration resulting from any failure,
  malfunction or adverse condition in the flight control system. These must be
  considered limit loads and must be investigated at airspeeds up to VC/MC.