EgyptAir Flight 667

Investigation

Since the area where the fire originated has no fuel lines, oil lines, or hydraulic lines, the investigation focused on the crew oxygen supply system as the root cause or a major contributing factor. In the days after the fire, the area of the cockpit where the fire originated on SU-GBP was examined on all EgyptAir 777-200 and 777-300 aircraft. According to the investigation's final report, the wiring found in the oxygen light plate did not match the original Boeing design:

The reason for the missing clamps could not be determined. Approximately 380 early 777 aircraft did not have sleeves on the wire in the oxygen light. Boeing issued a service bulletin in October 2011 recommending that the oxygen light plate wiring be inspected and, if necessary, have sleeving installed and damaged wires replaced.

Previous testing by the US National Transportation Safety Board following a 2008 cockpit fire on a Boeing 767 had found that flexible oxygen supply hoses could ignite if anti-kink springs along the oxygen supply hose were exposed to an electric current. The hoses in SU-GBP were composed of two layers of silicone with the spring embedded in the outer layer, unlike the hose in the NTSB tests. Testing performed by Boeing on 777 crew oxygen hoses found two of the seven tested to be conductive. Further testing in conditions that increased the likelihood of ignition showed that 5 volts of direct current was not sufficient to destroy the hose. However, the final report notes that these tests were conducted on new hoses and that Boeing planned to conduct similar tests on older hoses from in-service aircraft to see if any age-related changes could have affected the results.

The same test was also performed at voltages and amperage of aircraft wiring found elsewhere in the cockpit. With air in the hose, applying 28 volts DC power at 5 amps of current or 115 volts AC power at 2.5 amps did not rupture the hose or cause ignition, but 28 V DC at 6 amps or 115 V AC at 5 amps resulted in a small leak with "some incandescence". With oxygen in the tube, 5 V DC at 1.2 amps or 28 V DC at 2.5 amps did not result in a rupture, but 28 V DC at 5 amps caused a leak "followed by ignition and complete rupture of hose." Boeing examined the design of the oxygen supply to determine possible sources of electricity:

Adiabatic heating from the sudden release of pressurized oxygen into the hose was another possible source of ignition, but ruled out. Window grease in an oxygen-rich environment was another source of ignition that was investigated, but testing determined that the oxygen level did not impact the flashpoint, which was over 200 °F above the maximum theoretical temperature in that part of the cockpit.

The investigation did not conclusively determine the cause of the fire, only that the fire originated near the first officer's oxygen mask supply tubing and that oxygen from the first officer's supply mask "is suspected to have contributed to the fire's intensity and speed", although it is not known whether a breach of the oxygen system provided a flammable environment for the fire to start or whether the oxygen system breached as a result of the fire, and an electrical fault, possibly in anti-kink springs in flexible oxygen supply hoses, may have caused the fire.