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).