Two inertial reference units (IRU) on a United Airlines Boeing 787-8 failed shortly before the jet experienced a rapid altitude shift over Africa in January that injured 16 people.
In a preliminary accident report released on 18 March, the National Transportation Safety Board (NTSB) says the jet’s left IRU failed roughly 1h 5min prior to the altitude upset, followed by failure of its right IRU about 55min later.
The incident occurred at 01:31 Lagos time on 24 January and involved United flight 613 from Lagos to Washington Dulles International airport. It occurred over the West African country of Cote d’Ivoire.
The aircraft (registration N27903) took off from Lagos at about 00:20 and climbed to 36,000ft, according to flight tracking website FlightAware.com.
Following the IRU failures, and just seconds before the altitude upset, the jet’s autopilot automatically disconnected and its cockpit master caution and master warning alerts activated. The pilots took over and flew the jet manually.
The sudden altitude changes occurred for roughly 12min. During that period, the 787-8 deviated from 36,000ft to a maximum altitude of 36,203ft and to a minimum of 35,577ft. Its stick shaker also activated and its auto-throttle disconnected, then re-engaged before disconnecting a second time, says the NTSB.
“At the time of these altitude excursions, meal service was being conducted in the cabin and injuries resulted,” says the NTSB. Of the 254 people aboard the jet (including 11 crew and 243 passengers), 15 passengers suffered minor injuries and one suffered serious injuries, the investigatory agency says.
At 00:40, the pilots turned the jet back toward Lagos, where they landed safely.
The location of the incident means responsibility for the investigation falls to Cote d’Ivoire, though that country granted a request from the NTSB to assist.
The 787-8’s flight data recorders, IRUs and attitude heading reference units were removed from the jet in Lagos and shipped to Washington, DC for examination.
The NTSB was unable to access cockpit audio recording during the event because the jet’s cockpit voice recorder captures only 2h of audio and had been overwritten, it says.
