B757-200
Maintenance Airframe; Maintenance Powerplant
Maintenance Technician 25
Communication Breakdown; Situational Awareness
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural FAR
General Maintenance Action
Aircraft; Company Policy; Human Factors
[This is in] reference Air Carrier Maintenance Computer System; Item XXX01; on a B757-200 aircraft. One of the # 5 main wheel tie bolts had a broken nut. This is a recurring problem. About five years ago; we had problems with the tie-bolts breaking; now it is the nuts.  Part failure of main tire/wheel tie-bolt lock nuts.
Reporter stated they count their Main Landing Gear (MLG) tire positions from left to right across the front row of tires on both MLGs. So #4 main tire wheel position would be the Forward Right Outboard (O/B) main tire on the right MLG. He has recently seen two tie-bolt lock nuts split open horizontally; with gaps that were paralleled with the tie-bolts at the #5 main tire (left MLG; aft O/B position) and the other at #6 main (left MLG; Aft Inboard (I/B)) position. Previous to that he has found lock nuts completely missing from tie-bolts; but couldn't remember if the threads on the tie-bolts were damaged. Reporter stated even though an MEL deferral is allowed; when a tie-bolt or lock nut has been found broken or missing; the tire is changed; if a replacement tire is available. For Mechanics; tire and wheel assemblies with broken tie-bolts; split or missing lock nuts become a safety concern whenever they have to service a tire for low air pressure. The condition of the remaining tie-bolts and lock nuts become more important under those conditions.Reporter stated that; months ago; his Air Carrier checked their B757 fleet due to numerous reports by mechanics about missing or split lock nuts on the main wheel/tire assemblies. Hydrogen Embrittlement of the lock nuts was found to be the contributing cause for early failure of the lock nuts. The Supplier was supposed to have purged his company's supply of the lock nuts. But now he is not so sure that was accomplished; because mechanics are starting to see the same issue recurring. Many mechanics do not bother filing reports about the lock nut problems; instead they just change the tire and move on to the next airplane.
A Line Mechanic reports finding main tire/wheel assemblies with split or missing tie-bolt lock nuts on their B757-200 aircraft. Hydrogen Embrittlement of lock nuts were noted as the cause of earlier failures and thought corrected; but now; similar failures are recurring.