A B777 in cruise at FL340 heard a single thud in the right engine. Vibration indication went to maximum scale. Reduced power on right engine; vibration decreased. Continued to operate engine at reduced power.

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Attributes
ACN	661576
Time
Date	200506
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zzzz.artcc
State Reference	FO
Altitude	msl single value : 34000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zzzz.artcc
Operator	common carrier : air carrier
Make Model Name	B777 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
ASRS Report	661576
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	aircraft equipment other aircraft equipment : right engine vibration ind. other flight crewa other flight crewb
Resolutory Action	flight crew : overcame equipment problem none taken : anomaly accepted
Consequence	other other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

During cruise flight; we heard a single thud sound similar to a compressor stall. Secondary engine instruments indicated the right N3 vibration was at the top of the 5.5 scale. Power was slowly reduced on the right engine and increased on the left as necessary to maintain altitude. The vibration indication decreased to 4.0. Maintenance control advised that so long as all other indications remained normal that other engines had had this failure and have continued to operate normally. Powerplant engineering believed that an hpc stage 6 compressor blade may have failed.

Original NASA ASRS Text

Title: A B777 IN CRUISE AT FL340 HEARD A SINGLE THUD IN THE R ENG. VIBRATION INDICATION WENT TO MAX SCALE. REDUCED PWR ON R ENG; VIBRATION DECREASED. CONTINUED TO OPERATE ENG AT REDUCED PWR.

Narrative: DURING CRUISE FLT; WE HEARD A SINGLE THUD SOUND SIMILAR TO A COMPRESSOR STALL. SECONDARY ENG INSTS INDICATED THE R N3 VIBRATION WAS AT THE TOP OF THE 5.5 SCALE. PWR WAS SLOWLY REDUCED ON THE R ENG AND INCREASED ON THE L AS NECESSARY TO MAINTAIN ALT. THE VIBRATION INDICATION DECREASED TO 4.0. MAINT CTL ADVISED THAT SO LONG AS ALL OTHER INDICATIONS REMAINED NORMAL THAT OTHER ENGS HAD HAD THIS FAILURE AND HAVE CONTINUED TO OPERATE NORMALLY. POWERPLANT ENGINEERING BELIEVED THAT AN HPC STAGE 6 COMPRESSOR BLADE MAY HAVE FAILED.

Data retrieved from NASA's ASRS site as of January 2009 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.