Large transport experiences vibration when speed brakes stowed.

Attributes
ACN	318343
Time
Date	199510
Day	Sat
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : hmv
State Reference	TN
Altitude	msl bound lower : 31000 msl bound upper : 31000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : ztl tracon : cvg
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	cruise other descent : approach landing other
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 200 flight time total : 14000 flight time type : 2500
ASRS Report	318343
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial pilot : atp
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other flight crewa
Resolutory Action	flight crew : declared emergency other
Consequence	Other Other
Supplementary
Primary Problem	Aircraft

Narrative:

A very noticeable vibration occurred during cruise. Vibration stopped when speed brakes were extended even a very small amount from the stowed position. Vibration began again when speed brake lever stowed. So checked wings for spoiler causing the vibration but could not see the cause. I advised our dispatcher and maintenance coordinator of the discrepancy and that we were going to follow a spoiler float checklist which called for a flaps 15 degree landing. I also advised them we would declare an emergency with approach control. The dispatcher said he would call cvg approach control and advise them to expect the situation. The spoiler float checklist called for us to isolate (stop hydraulic pressure to) the spoilers causing the problem but we were never able to do this as we could not visually identify them and also the vibration stopped during descent. We continued to use the spoiler float checklist as much as we could and made a flaps 15 degree landing. We also declared an emergency with approach control. First officer flew the approach and landing which were uneventful. Emergency terminated after landing. I was ready to isolate the spoilers should a control problem develop, which it never did. This could have been called a 'precautionary emergency' if there were such an occasion as we could have used a normal checklist and approach etc, since the vibration stopped during descent and we never had uncommanded control inputs. However I felt to be on the safe side it was better to conduct the flight as we did.

Original NASA ASRS Text

Title: LGT EXPERIENCES VIBRATION WHEN SPD BRAKES STOWED.

Narrative: A VERY NOTICEABLE VIBRATION OCCURRED DURING CRUISE. VIBRATION STOPPED WHEN SPD BRAKES WERE EXTENDED EVEN A VERY SMALL AMOUNT FROM THE STOWED POS. VIBRATION BEGAN AGAIN WHEN SPD BRAKE LEVER STOWED. SO CHKED WINGS FOR SPOILER CAUSING THE VIBRATION BUT COULD NOT SEE THE CAUSE. I ADVISED OUR DISPATCHER AND MAINT COORDINATOR OF THE DISCREPANCY AND THAT WE WERE GOING TO FOLLOW A SPOILER FLOAT CHKLIST WHICH CALLED FOR A FLAPS 15 DEG LNDG. I ALSO ADVISED THEM WE WOULD DECLARE AN EMER WITH APCH CTL. THE DISPATCHER SAID HE WOULD CALL CVG APCH CTL AND ADVISE THEM TO EXPECT THE SIT. THE SPOILER FLOAT CHKLIST CALLED FOR US TO ISOLATE (STOP HYD PRESSURE TO) THE SPOILERS CAUSING THE PROB BUT WE WERE NEVER ABLE TO DO THIS AS WE COULD NOT VISUALLY IDENT THEM AND ALSO THE VIBRATION STOPPED DURING DSCNT. WE CONTINUED TO USE THE SPOILER FLOAT CHKLIST AS MUCH AS WE COULD AND MADE A FLAPS 15 DEG LNDG. WE ALSO DECLARED AN EMER WITH APCH CTL. FO FLEW THE APCH AND LNDG WHICH WERE UNEVENTFUL. EMER TERMINATED AFTER LNDG. I WAS READY TO ISOLATE THE SPOILERS SHOULD A CTL PROB DEVELOP, WHICH IT NEVER DID. THIS COULD HAVE BEEN CALLED A 'PRECAUTIONARY EMER' IF THERE WERE SUCH AN OCCASION AS WE COULD HAVE USED A NORMAL CHKLIST AND APCH ETC, SINCE THE VIBRATION STOPPED DURING DSCNT AND WE NEVER HAD UNCOMMANDED CTL INPUTS. HOWEVER I FELT TO BE ON THE SAFE SIDE IT WAS BETTER TO CONDUCT THE FLT AS WE DID.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.