747 vibration

From:         jmaddaus@gte.com (Put your name -- WinTrumpet)
Organization: GTE Labs Inc
Date:         23 Aug 96 13:44:54 
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Just a quick question or two.  I was on a ORD-SFO flight earlier this year (UA
747-400) and about 15 minutes into climb from ORD, the plane started to
shudder very badly.  This lasted for about 3 minutes before the captain
announced a problem with a gear door that had decided not to drop down. He
noted the crew had tried a couple of things that didn't work to their
satisfaction and were going to lower the landing gear and bring it up again.
We were still climbing and I'd guess about 17-19,000 ft.  Cycling the gear
apparently solved the problem because the vibration went away and we continued
on in an uneventful flight.  As far as I could tell, I felt no difference in
the vibration during the gear cycling.  Could a door cause a vibration as
significant as the gear?  It did seem that the pilot was applying rudder
during this time.

The following week I took the same flight and we were promptly parked in the
ORD penalty box due to ATC delays in SFO.  The crew left the door open and
several of us walked up to chat.  I mentioned the flight from the week before
and the captain said that they were experiencing unexplained in-flight
vibrations on 747s that UA couldn't solve.  The company brought Boeing in and
at that point, neither could identify the exact cause.  Anyone have further
insight on this?

Also on the same route another time (but equipment changed to a 747-100), we
experienced a violent jolt on climbout from ORD (again I'd guess below
20,000).  This one had the FA's looking at each other.  I happened to be
staring out at the wing and I saw no wing or engine movement, but it sure was
violent.  My instantaneous feeling was that we had just hit something (like
another plane).  No announcements were made and the flight continued.  I fly
frequently and I have never felt turbulence like that, but I'm assuming that is
what occurred.  My question is why wouldn't I have seen a wing deflection or
an engine rock?

Thanks for any input.

John Maddaus