Incident Overview
On August 4, 2010, at approximately 1547 central daylight time, the flight crew of United Airlines flight 949 initiated an evacuation after being informed by air traffic control (ATC) that the airplane's left main landing gear was on fire following landing at Chicago O'Hare International Airport (KORD), Chicago, Illinois. The aircraft, a Boeing B767-322 (registration N644UA), was operating under 14 Code of Federal Regulations Part 121 as a scheduled international passenger flight from Heathrow International Airport (LHR), London, United Kingdom. There were 12 crew members and 178 passengers on board; six minor injuries occurred during the evacuation.
Post-Incident Findings
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. Both the brake assembly and the slide were removed and shipped to their respective manufacturers for examination.
Brake Assembly Examination
Examination of the brake assembly showed evidence of catalytic oxidation, which occurs when premature oxidation of the carbon disks is accelerated by exposure to contaminants. According to Honeywell, runway deicer fluid is 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 addressed by Boeing service letter 767-SL-32-106 (January 2009), Honeywell service bulletin 2612412-32-006 (February 12, 2010), and FAA SAIB NM 08-27. After the event, United inspected all of their 767-300 installed brakes for catalytic oxidation; none of the 35 aircraft inspected showed evidence of catalytic oxidation. United's AMM (767-32-41-08) dated June 6, 2009, did not contain inspection criteria for carbon brake oxidation, although Boeing's AMM (April 2009) did include such criteria and sketches. United subsequently revised their AMM to incorporate Boeing's content and updated the component maintenance manual (CO-65-32-43-06-JD) with requirements for servicing 767 brakes in accordance with Honeywell SB 2612412-32-006, including installing a backing plate during overhaul.
Slide Examination
The failed slide was sent to the manufacturer for examination. When unpacked, the upper chamber aspirator was not installed on the slide, and 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 the two fractured arm portions of the spider were sent to the NTSB materials laboratory for examination. Both fractured arms contained a preexisting crack. Further examination found that the fracture face exhibited ductile dimple features typical of overstress separation, with no evidence of crack arrest marks (fatigue cracking).