A Mechanic reports about a CFM-56 engine on a company B737-900 aircraft that incurred a sudden increase in low pressure rotor vibration levels after only 1500 hours of service. Upon dis-assembly of the Fan Rotor section; mechanics discovered the # 3 and # 4 blade spacers had been reversed. Reporter had inadvertently switched spacers during previous re-assembly.

Narrative

January 2011; reference Jobcard 78-XX-XX; Step 49. During the Fan Blade Spacer installation; # 3 and # 4 spacers were inadvertently switched; with the # 3 spacer under the # 4 blade and the # 4 spacer under the # 3 blade. Upon [reaching] 1500 Hours of service and 570 Cycles; an Engineering write-up was created June 2011 [to address] a sudden increase in low pressure rotor vibration levels. Upon disassembly of the Fan Rotor Section it was discovered the # 3 and # 4 spacers were reversed. Typically we run an assembly line; a Mechanic would pull the previously numbered shim; blade or platform from the corresponding slot on the Control Stand; hand the part to another Mechanic or directly to the Mechanic installing the part. Handing the spacer to me; I installed the spacers; although the Step involved does not detail the shim installation in sequential order as removed. I did so. The # 3 and # 4 spacers were inadvertently installed in reverse; as numbered. The fan was also re lubed [during re-assembly] and a new Vibration Survey and Solution was performed. I suggest better cognition of sequential event [installation].