Seventeen passengers and crew were injured when a co-pilot on a transatlantic flight twice put the plane into a dive over fears it was going to stall.
The United Airlines jet was 80 miles south-west of Dublin on October 20 2013 when cockpit instruments gave the impression of low airspeed after flying through heavy turbulence.
The co-pilot pitched the nose of the Boeing 757 down twice, exceeding advised flight speeds.
Inspectors from the Air Accident Investigation Unit (AAIU) of the Department of Transport said any unrestrained people or belongings in the cabin were likely to have been thrown around by “rapid changes in G loads”.
They found the misleading instrument readings may have been caused by icing of pitot probes which measure the actual speed of a plane.
It was one of the issues identified in the crash of the Air France Airbus A330 which went down in the Atlantic in a remote area off Brazil on June 1 2009 killing all 228 people, including three Irish doctors.
The AAIU report on the United Airlines incident also reported the co-pilot may have suffered startle effect – an uncontrollable fast pulse, muscle reflex and blood pressure in an unexpected event.
It said this may have followed the instruments indicating low speed as the plane came out of the turbulence.
Eight safety recommendations were issued by the AAIU including asking United Airlines to review some of its training and guidance for pilots.
After landing safely, eight passengers and two flight attendants, including one who cut their head, were treated in the airport for minor injuries.
Twelve days later a passenger contacted investigators and described being in a toilet at the back of the plane when it went into one of the dives.
He hit his head on the ceiling before falling and hitting a handrail on the back wall of the unit.
Another four passengers and two crew also reported being injured.
Inspections inside the plane showed a ceiling panel had been damaged and tea and coffee had been thrown around in the incident.
The seatbelt sign in the cabin had been on at the time as the plane was making its descent to Dublin Airport after flying from Newark.
The inquiry found the co-pilot twice pitched the nose of the jet into a dive and increased speed after reading his instruments.
The AAIU said control of the flight was handed over to the commander when the flight crew realised there was a problem with cockpit instruments.
There was no cockpit voice record of the incident available as it had been overwritten.
The AAIU said it accepted the co-pilot’s word that he had unintentionally not pulled the correct circuit breakers to allow for the recordings to be recovered.
