Date: 201601
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Night
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight phase: Parked
Aircraft component: Wheel Assemblies
Component problem: Failed; Malfunctioning
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Human factors: Communication Breakdown
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Maintenance
When detected: Routine Inspection
Result: General Maintenance Action
Contributing factors: Human Factors; Procedure; Incorrect / Not Installed / Unavailable Part
Primary problem: Human Factors

Narrative: Six months ago we started seeing main landing gear (MLG) wheel mount bolts broken off. At that time it was reported to Management. It started with one or two bolts broken off and in the past month we have found four sets of wheels with as many as five bolts being broken or very loose. Last night; on another B737-800; we found #3 MLG tire with two bolts where the nuts where sheared off and three bolts that were so loose you could push them in and out [of the wheel half]. On this tire; the loose bolts had the same amount of bolt threads coming through the nuts as did the tight ones. As a Mechanic; we see no urgency [by our air carrier] to address this problem since it's now been over six months. These tires are being overhauled by a Tire Repair Station in ZZZ1. What is causing these bolts to loosen and come apart?

Callback: Reporter stated he and other mechanics began noticing an increase in the number of loose and sheared main landing gear (MLG) tire wheel-half tie bolts when they started doing A-Checks on their overnight B737NG aircraft at their Line Station. Part of the A-Check requires removing the large hub cap from the outboard tires on the B737NG MLGs which is not done on overnight checks. With the large hub cap on; the wheel bolts are not visible. Initially; the wheel-half Manufacturer/ build-up Repair Station believed the wrong type of anti-seize compound used on the tie bolts was the problem; but that was later discounted. Reporter stated the tie bolts have been shearing at the inboard side of the nut; near the smooth shank of the tie bolt where the bolt threads begin. The bolts are supposed to be steel.  The steel brake assembly prevents the head of the tie bolt from migrating out; unless the head shears off. Even after all the sheared tie bolts he has seen; he has recovered only one nut that had sheared and that one had three threads sticking out from the end of the nut. Reporter stated no one has figured out; or at least explained if the tie bolts are being over-torqued; or experiencing unknown stresses on landing; or are counterfeit (bogus) tie bolts; or if steel tie bolts are being used with Inconel bolts that have different torque values; or the wheel-half Repair Station is reusing tie bolts without adequate Quality Assurance verifying the tie bolts are serviceable. Reporter stated the potential for an explosive wheel half event to occur becomes more likely when four or five tie bolts shear in sequence to each other. He has over thirty-five years Maintenance experience; many of them in aircraft tire shops.



Synopsis: A Line Aircraft Maintenance Technician (AMT) reports noticing an increase in the number of loose and sheared tie-bolts on B737-800 main landing gear (MLG) tire wheel-half assemblies. He recently found one tire with five tie-bolts sheared near the smooth shank and threaded portion of the tie-bolt with all the locking nuts for the tie-bolts gone.