Re: Questions about the DH Comet

Date:         25 Sep 97 01:39:43 
From:         Steve Lacker <look@the.sig>
Organization: Applied Research Laboratories - The University of Texas at Austin
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P. Wezeman wrote:
>    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.

True. Somewhere, I have a picture of the guillotined hull section, split
like a banana peel between the tear-stoppers, but not beyond. Were they
titanium? I don't remember the material specifically.

>    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.

Fatigue, or corrosion? I thought that the problem was traced to the way
the VERY early 727 and 737 fuselage skins were assembled, which allowed
corrosion to propagate undetected between two layers of skin. I've never
heard of a 727 suffering a failure from this cause (The oldest 727 is
still very much in one piece!), but as I recall the design change was
made somewhere before airframe #100 in the 727, but somewhat later in
737 production despite the fact that the 727 was in production earlier.
If anyone remembers the straight story, I'd like to have my facts
corrected. I'd also like to know (assuming my memory is right) how the
other early airframes with the design "feature" are holding up. I also
have a vague memory that the fact that the Aloha Air bird was operated
in high salt/humidity conditions for much of its life may have been a
contributing factor.

Stephen Lacker
Applied Research Laboratories, The University of Texas at Austin
PO Box 8029, Austin TX 78713-8029