Re: 747 vibration

From:         "Brian D. Smith" <>
Organization: The Boeing Company
Date:         03 Sep 96 01:16:58 
References:   1
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Put your name -- WinTrumpet wrote:
> 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.

Yes, a 350+ mph wind funneling through the gear hatch can create a
significant vibration which can resonate through the entire plane.  The
doors are fairly wide and provide an appreciable lever arm.

If one door was not closed completely, the drag would need rudder to
counter it.

> 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?

An airplane is a lot like a car that has been on the road for 200K miles
-- lots of wear and tear, many different mechanics have worked on it,
minor equipment changes to upgrade it.  Sometimes there are knowks and
rattles that take a while to find.

> 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?

Which direction was the jolt from down, forward or sideways?

An engine "burp" can be startling.  At high angle of attack the airflow
thru the inlet detaches resulting in a brief flame-out.

  These opinions, however excellent, in no way reflect those of The
   Boeing Company.  Opinions expressed herein are strictly my own.

Brian.D.Smith, Boeing Commercial Airplane Group       (206) 342-6652
GNSS Landing Systems