How much "rust" is tolerated and landing light question?

Date:         12 Oct 98 00:02:07 
From: (John S. Maddaus)
Organization: MindSpring Enterprises
Followups:    1 2 3 4
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OK number one, flying UAL from Denver to San Antonio in an older 737S.
First I noticed that the top (couldn't see the bottom) of both
ailerons were nice shiny metal.  Thought that was an odd area to be
replaced.  Then as flaps were extended for take-off I noticed rust
stains running back approximately in the middle of the flap.  As we
flew along, I reminded myself that most of the ac is not made from
steel.  On landing, the source became clear.  Looked to be coming from
perhaps a steel cover plate (screwed or riveted in place) on the flap
and rust stains were evident all around it.  Also on deploying the air
brakes, the undersides of those showed rust stains as well
particularly at hinge points and hydraulic actuator connection points.

This implied some corrosion to me. Now my questions.  Did I actually
see rust and are steel cover plates used?  If so how much is
tolerable?  Assuming that it was at least surface rust, reminds me of
my car rusting out due to salt air.  Has UA ever used 737s in Hawaii?
Would surface rust on steel parts imply possible corrosion with
aluminum parts as well (salt air also did a number on my parents
aluminum screens at home)?

Number two, UA flight from San Antonio to Chicago.  As our 733
arrives, I notice a hole in the starboard wing root where a landing
light lens used to be.  As it came closer, I see the remnants of what
looked like yellowed plexiglass shards.  The pre-flight picked it up
and before long, both pilots were looking at it and trying to get a
mechanic.  I knew my connection was shot at O'hare but hung around a
bit to see the outcome.  Apparently, they found a local company that
was going to make a temporary metal patch for the hole and it flew on
a couple of hours late.

Now my questions, I am assuming that the plane flew in at least part
of the way with that condition.  What could have happened had the
pilots either not noticed it (and it was almost missed by the 1st
officer until he decided to take a second look) or decided to fly
anyway?  What's behind that light and would the aircraft have flown
any differently?

Just curious,
John S. Maddaus