Re: Questions about the DH Comet

Date:         17 Sep 97 02:49:26 
From:         "P. Wezeman" <pwezeman@blue.weeg.uiowa.edu>
Organization: The University of Iowa
References:   1 2 3 4
Followups:    1 2
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On 8 Sep 1997, Marc Schaeffer wrote:

> michael piersdorff wrote:
>
> > My understanding of the problem was that most of the in-flight
> > breakups were explosive decompressions of the fuselage resulting from
> > fatigue failure.
           ---snip---
> > Successive pressurisation/depressurisation cycles
> > (takeoff and landing, respectively) in combination with flight loads
> > caused cracks at the corners of the square windows used in the early
> > models.
>
> No that's not correct. The YP incident was *not* due to a crack which
> started at a window but the crack started at a hatch in the upper part
> of the fuselage. To be more precise the crack started at the rear A.D.F
> aerial hatches. As a consequence the center fuselage splitted along the
> top center line. Where the crack of YY started is unknown since the
> fuselage was never recoverd. On the Comet YU, which was used to find out
> the reasons of the YP and YY crashes, the cracks started at the forward
> escape hatch.

   The problem was not just that the structure of the Comet had corners
where fatigue cracks could start, but also that there was inadequate
provision to prevent cracks from propagating once they had reached a
critical length. If memory serves, the fuselage of the Boeing 707, in
addition to being heavier skinned than the Comet, also incorporated
titanium "crack stoppers" that limited the spread of cracks. In a test,
a pressurized 707 fuselage, gashed open with a guillotine-like device,
depressurized but remained otherwise intact. The 707 design had this
feature before the Comet disasters. Other airliners also have provision
to arrest crack growth and it was added to the Comet. This information
is from the book "Structures: or why things don't fall down" an account
of structural engineering for the layman.
   The obvious example of a Boeing airliner fuselage that did lose most
of its structural integrity from fatigue failure was the Aloha Air B737
where some twenty feet of the upper half come off, but I believe that in
that case there was extensive metal fatigue through the whole area, and
not just spreading of a crack from an origin through sound structure.
   Others please correct me where I have it wrong in whole or in part.

                        Peter Wezeman, anti-social Darwinist

                             "Carpe Cyprinidae"