Improved safety checks have been brought in after items fell out of an air ambulance at 700ft (213m), injuring a man on the ground.
The incident happened in June, when the Great North Air Ambulance helicopter had taken off from Middlesbrough's James Cook University Hospital.
A door opened and items fell to the ground, knocking a man unconscious.
The Air Accident Investigation Branch (AAIB) urged additional door checks be carried out on all air ambulances.
The AAIB report said the Aerospatiale SA365N Dauphin air ambulance was returning to its base at Durham Tees Valley Airport after dropping off a casualty in Middlesbrough, when the incident happened.
Shortly after take-off the helicopter's right rear cabin door opened sending a number of items, including a stationery folder, falling to the ground.
A man was struck and was treated by paramedics on the ground.
It later emerged that a doctor on board had not properly secured the door before take off.
A spokesman for Inverness-based PDG Helicopters, which supplies the aircraft to the air ambulance charity, said crews would be retrained.
He said: "All crews have been retrained on the closing and locking of doors.
"Pilots must now ensure that they check all doors are locked themselves prior to take off."
The incident did not result in any major damage to the 18-year-old aircraft and no-one else was hurt.