Two inertial reference units (IRUs) of United Airlines Boeing 787-8 failed just before Jet experienced a rapid altitude shift in Africa, which injured 16 people in January.
In a preliminary accident report released on March 18, the National Transportation Safety Board (NTSB) stated that the jet's left IRU failed about an hour before the altitude agitation, and a right IRU failed about 55 minutes later.
The incident occurred at 01:31 Lagos time on January 24th, involving United Flight 613 from Lagos to Washington Dulles International Airport. It happened in the country of Court Daiboir in West Africa.
According to the Flight Tracking website flightaware.com, the aircraft (registered N27903) took off from Lagos and rose to 36,000 feet.
Following an IRU failure, seconds before the altitude becomes unsettled, the jet's autopilot will automatically disconnect, and its cockpit master's warning and master warning alert will be activated. The pilot took over and flew the jet manually.
The sudden altitude change occurred for approximately 12 minutes. During that period, the 787-8 deviated from 36,000 feet to a maximum altitude of 36,203 feet and a minimum of 35,577 feet. According to NTSB, that stick shaker was also active, re-engaged after the autoslot was disconnected and then re-engaged, then re-engaged.
“At the time of these altitude field trips, there was a meal service in the cabin and I was injured,” says NTSB. Of the 254 people on the jet (including 11 crew and 243 passengers), 15 passengers were minor injuries and one seriously injured, the investigator said.
At 00:40, the pilots returned the jet towards Lagos, where they safely landed.
The location of the incident means responsibility for the investigation. However, the country has granted its request from the NTSB for assistance.
The 787-8 flight data recorder, IRUS and Attitude Astitude Reference Unit was removed from the Lagos jet and shipped to Washington, DC for inspection.
The NTSB couldn't access cockpit audio recordings during the event as Jet's cockpit recorder only captured two hours of audio and was overwritten.
