EgyptAir Flight 990 (MS990/MSR990) was a regularly scheduled flight from Los Angeles International Airport, United States, to Cairo International Airport, Egypt, with a stop at John F. Kennedy International Airport, New York City. On October 31, 1999, the Boeing 767 operating the route crashed into the Atlantic Ocean about 60 miles (100 km) south of Nantucket Island, Massachusetts, killing all 217 passengers and crew on board. The National Transportation Safety Board determines that the probable cause of the EgyptAir flight 990 accident is the airplane's departure from normal cruise flight and subsequent impact with the Atlantic Ocean as a result of the relief first officer's flight control inputs. The reason for the relief first officer's actions was not determined.

As the crash occurred in international waters, it was investigated by the Ministry of Civil Aviation's Egyptian Civil Aviation Agency (ECAA). As the ECAA lacked the resources of the much larger U.S. National Transportation Safety Board (NTSB), the Egyptian government asked the NTSB to handle the investigation. Two weeks after the crash, the NTSB proposed handing the investigation over to the Federal Bureau of Investigation, as the evidence suggested that a criminal act had taken place and that the crash was intentional rather than accidental. This proposal was unacceptable to the Egyptian authorities, and hence the NTSB continued to lead the investigation. The NTSB concluded "not determined", while the ECAA determined that the incident was caused by mechanical failure of the aircraft's elevator control system.

The ECAA's report suggested several control failure scenarios as possible causes of the crash, focusing on a possible failure of one of the right elevator's power control units. While the NTSB's report stated the impact was "a result of the relief first officer's flight control inputs", the report did not determine a specific reason for the relief first officer's alleged actions.