Service difficulty report · Nut · Agusta AB139

Difficulty date
May 26, 2016
Submitted
June 3, 2016
Operation
General aviation
Make and model
Agusta AB139
Aircraft serial
31038
Aircraft time
1,001 hours, 2,021 cycles
Part
Nut
Part number
3G6220A02251
Part serial
872
Condition
Defective
Location
M/R Hub
Stage of operation
IN
How discovered
V
Nature of condition
O
Precautionary procedure
K

As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.

What was found

M/R HEAD WAS REMOVED TO INSTALL A NEW SWASHPLATE BOOT, DURING REMOVAL FOUND 3 EA. RING NUT BOLTS EXTREMELY TIGHT AND 1 OF THOSE HAD SEIZED. PROCURED NEW RING NUT AND 3 EA. NEW BOLTS AND PROCEEDED TO INSTALL. NEARING THE FINAL TORQUE OF 55 FT/LBS, THE TORQUE APPLIED TO 1 OF THE BOLTS FELT DIFFERENT. FURTHER INVESTIGATION REVEALED 1 BOLT WAS NOT CONTACTING THE RING, PN 3G6220A02151. THE FALSE TORQUE WAS DUE TO THE BOLT GALLING INSIDE THE MAST NUT. UPON INSPECTION, IT APPEARS THAT THE RING NUT THREADS HAD BEEN CAD PLATED DURING MFG AND THE PLATING WAS DISPLACING AS THE TORQUE LOAD INCREASED, CAUSING AND INTERFERENCE FIT BETWEEN THE THREADS AND MATERIAL TRANSFER BETWEEN THE RING NUT AND THE BOLTS. FOLLOWING REMOVAL OF NUT, FOUND 6 OF 12 BOLTS HAD SEIZED.

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