Posted for "Valin & Allyson Thorn" <thorn@starflight.aero>:

Hello Stan,

I'm not sure how your question about air conditioning external air vents
became part of the thread for this tragic accident...  Anyway, to address
these questions, I agree that everyone should be very cautious about making
any penetrations in an airframe that weren't part of the structure's
original design and testing.  Without analysis and testing otherwise, any
holes have to be reinforced to at least restore the strength lost from the
cutout.  But strength isn't the only consideration - stiffness is very
important too.  At least the same stiffness has to be preserved or the
structure's natural frequency could change such that it might be vulnerable
to resonance from various forces the structure sees in flight - which can
lead to structural failure.

I'm a space systems engineer and project manager -- certainly not a
composite structures specialists.  I do feel I have enough familiarity,
though, with the principles involved to handle a simple structural
reinforcement for relatively small circular holes in a composite sandwich
structure.  For the air conditioning external air vents for our system I
took a conservative, cost effective approach to their reinforcement to
ensure the strength and stiffness in this area was at least as good as
before the cutouts.  The tradeoff in not optimizing the reinforcement is
that the reinforcement is not as light as it could be - but, it saves the
expense of conducting a new ground vibration survey to ensure the frequency
of the structure did not go any lower.  Note, the stiffer the structure the
higher its natural frequency and higher structural frequencies are in the
safer direction for avoiding structural interaction/resonance issues.

The photos you included of your installation are small so I'm not sure I'm
interpreting them correctly.  From what I can see it does not look like the
hole reinforcement guidelines provided in the installation guide were
followed.  The composite sandwich structure in that area of the Legacy (just
behind the aft wing spar) is what's called "2 core 2 sandwich" - two plys of
carbon, a Nomex core, and another two plys of carbon.  The intended layup
plan bonds the outer and inner plys of the sandwich construction together,
with multiple plys of carbon cloth, which results in greater strength and
stiffness in this immediate area than without the holes.  From your photos
it looks like the reinforcement is only applied to the inner skin with micro
replacing the core.  This could be a problem.

On the positive side, it will only help the strength and stiffness if you
decided to bond the lower case of the condenser to the floor.  But, if you
did not directly bond the outer and inner plys of the fuselage sandwich
structure to each other I'm concerned about the resultant strength and
stiffness of the reinforcement.

I know this is not the best place to work this out and it will be easier to
explain on the phone.  I'll give you a call soon - but, I'm tied up this
Wednesday through Friday in a spacecraft design review that's scheduled for
at least 10 hours each day - so it might be this weekend...

Thanks,

Valin

Starflight, Inc.
1145 Timber Lane
Boulder, CO 80304