Chicago, IL, USA
N644UA
BOEING 767-322
Flight 949 conducted a normal landing under maximum landing weight and the crew did not receive any warnings or cautions. There were no system anomalies found except for the damaged left-hand brake assembly. Examination of the left brake assembly found that it caught fire due to catalytic oxidation, most likely as a result of the carbon disk being exposed to deicing fluid. The catalytic oxidation allowed the pistons from the brake housing to punch through the softened pressure plate and contact the rotor resulting in piston displacement, possible piston bushing damage and hydraulic fluid leakage. When the 2R slide was unpacked at the manufacturer to determine why it did not fully inflate during the evacuation, the aspirator was found unattached. This was likely removed when the slide was packed for shipping because the slide would not have even partially inflated if it was not installed during the evacuation. When the slide was laid out, the two missing portions of the aspirator spider arms were found within the slide. Analysis of the fracture surfaces on both spider arms found that each had preexisting cracks. These preexisting cracks were stress risers that reduced the strength of the arms. Since the fracture faces contained no evidence of fatigue, the two arms fractured during slide inflation sequence which caused the slide to not properly inflate during the evacuation. The preexisting cracks most likely occurred during the manufacturing process.
On August 4, 2010, at about 1547 central daylight time, the flight crew of United flight 949, a Boeing B767-322, N644UA, initiated an evacuation after being informed by air traffic control (ATC) that the airplane's left main landing gear was on fire after landing at Chicago O'Hare International Airport (KORD), Chicago, Illinois. Twelve crew and 178 passengers were on board, and 6 minor injuries occurred during the evacuation. The flight was operating under 14 Code of Federal Regulations Part 121 as a scheduled international passenger flight from Heathrow International Airport (LHR), London, United Kingdom. Post incident inspection revealed that the left main landing gear brake piston housing had ruptured, and the 2R (right aft) emergency evacuation slide did not fully inflate. The brake assembly and slide were both removed and shipped to their respective manufacturers for examination. Brake assembly The examination of the brake assembly revealed evidence of catalytic oxidation, which occurs when premature oxidation of the carbon disks are exposed to contaminants which can accelerate the oxidation process. According to Honeywell, runway deicer fluid has been known to cause catalytic oxidation on carbon disks. Oxidation occurs in localized regions and can lead to softening and loss of disk strength. Catalytic oxidation is a known phenomenon and is addressed by Boeing service letter 767-SL-32-106, dated January 2009, a Honeywell service bulletin 2612412-32-006, dated February 12, 2010, and FAA SAIB NM 08-27. After the event, United inspected all of their 767-300 installed brakes for evidence of catalytic oxidation. None of the 35 aircraft inspected had evidence of catalytic oxidation. United AMM767-32-41-08 Main gear wheel brakes inspection/check dated June 6, 2009 did not contain any inspection criteria for carbon brake oxidation. Although Boeing AMM dated April 2009 did contain carbon brake oxidation criteria in addition to sketches showing areas and examples of oxidation damage. United revised their AMM to include Boeing AMM content on carbon brake oxidation. United also updated component maintenance manual (CO-65-32-43-06-JD) with the requirements of servicing 767 brakes in accordance with Honeywell SB 2612412-32-006. In addition, installing a backing plate during overhaul as called out in the SB. Slide examination The failed slide was sent to the manufacturer for examination. When the slide was unpacked, the upper chamber aspirator was not installed on the slide and that the "spider" in the aspirator was damaged with two pieces missing. During slide inflation and examination, the two missing pieces of the spider were recovered from within the slide. The upper aspirator assembly and two fractured arm portions of the spider were sent to the NTSB materials laboratory for further examination. Examination of the two fractured arms of the spider portion of the aspirator found that both contained a preexisting crack. Further examination revealed the fracture face contained ductile dimple features typical of overstress separation. The fracture face contained no evidence of crack arrest marks (fatigue cracking).
the brake fire after landing was caused by hydraulic fluid leaking on to hot brakes.
Source: NTSB Aviation Accident Database
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