Re: Questions about the DH Comet

Date:         25 Sep 97 01:39:43 
From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
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>>    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?

Fatigue, according to the probable cause in the NTSB report, along
with separation of the bonding employed in the fuselage lap joints.

>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.

In the original design, employed on the 727, 737, and 747, fuselage
lap joints were "cold bonded" with epoxy-impregnated cloth, cured at
room temperature, augmented by three rows of rivets countersunk in a
fairly thin skin.  The bonding and not the rivets was intended to
carry the primary pressurization loads.

Unfortunately, this bonding process was found to have problems which
in many cases resulted in a poor bond; poor bonds were susceptible to
corrosion which further weakened the joint and caused transfer of the
stresses to the rivets.  Being countersunk into a thin aluminum panel,
the inner portion of the rivet hole formed a sharp edge which was
particularly prone to fatigue cracking.

The joint was subsequently redesigned to use a thicker skin in the
area of the joint, with the triple row of rivets taking the primary
load and a sealant used solely to seal the pressure vessel.  This
change was implemented on the 737 beginning with line number 292
(first flight January 5, 1972), and at approximately the same time
on the 727 and 747.  (Based on first flight dates, that should be
roughly 727 #877 and 747 #174.)

--
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