Date: 201906
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737-900
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Wheels/Tires/Brakes
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: Aircraft In Service At Gate; Pre-flight
Result: Aircraft Aircraft Damaged; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: During preflight walk around First Officer noticed the brake wear bar on left main gear; inboard tire; was worn beyond limits; had come out of guide slot; and appeared bent. He showed me a picture and I went to have a look. I saw what he had described. I noticed what appeared to be carbon shavings in several places on the brake assembly. I also noticed what appeared to be a metal buckle or clip under the tire. This got my attention. I returned to cockpit and we called Operations to ask for Maintenance. Someone from ramp came to the cockpit with some shavings and another buckle/clip in his hand. We made an [electronic logbook] entry; informed the Gate Agent and Station Operations that we would be delayed. We called Dispatch and [Maintenance Control] to ask what we had found on and under the left wheel and brake assembly. As this developed; the picture got progressively worse. We hadn't yet identified the buckle/clip; but we kept finding more carbon shavings and chunks. [Maintenance Control] asserted that the buckle/clip was not part of the wheel or brake assembly; and might be FOD from a cargo strap.When the local contract mechanic removed the left inboard tire another buckle/clip fell out onto the ramp; along with several large chunks of carbon brake material and a copious amount of what were now clearly shavings from the carbon brakes. The brake assembly was a mess. Of the four rows of carbon pucks; only one row was largely intact. One other row was partially destroyed; and the remaining two rows had completely disintegrated. What we had been calling buckle/clips were in fact the steel mounting brackets for the carbon pucks. Most of them were missing. The wheel itself was even more surprising. It showed evidence of great irregular internal damage from FOD; presumably from the steel mounting brackets. The inner heat shield was dented all the way around; including several puncture holes. All of the reinforcement braces were bent and chipped. I could not imagine how this wheel could have rotated and not have failed. The person from ZZZ ramp discovered another steel mounting bracket; plus more chunks of carbon brake up in the left wheel well; which raised further questions about where all of this material had gone. We made an [electronic logbook] entry about possible FOD in the wheel well. There was no [electronic logbook] write up or other note from previous crew(s) indicating a problem. If the First Officer had not noticed the wear bar on his walk around this could have easily been a catastrophic failure with much collateral damage.During this process; I felt increasing pressure from the ZZZ Station; gate personnel; and the pilot crew desk to hurry and get this flight out; despite the glaring and obvious reasons for caution. That kind of pressure was unhelpful; unhealthy and unwelcome; especially given the potential consequences.




Synopsis: B737-900 Captain reported First Officer discovered damaged Main Landing Gear Brake assembly on walk around.