The Air Accidents Investigation Branch has raised questions over how a Flybe aircraft crew handled a false emergency alarm.
The Dash 8 aircraft began to depressurise after a “spurious” smoke warning forced the crew to follow emergency protocol to remove smoke.
The Newquay-Edinburgh flight with 47 passengers and four crew made an emergency descent on July 21 last year.
The actions of the crew “significantly degraded the operational capability” of the aircraft, an AAIB report said. The airline said it had taken action to prevent similar incidents.
The aircraft was about 25 miles south west of Carlisle when a warning light indicated there was smoke on board.
“The crew assumed the warning was valid and carried out checklist actions designed to tackle fire or smoke from an unknown source,” the report said.
This resulted in power being removed from the co-pilot’s flight displays, the autopilot, the instrument landing system and caused the aircraft to begin to depressurise.
After the aircraft landed safely at Edinburgh, the evacuation was delayed by passengers trying to put on coats or take their belongings with them.
Of the four cabin doors used to evacuate the aircraft, only one had exit steps with the others having neither steps nor slides.
“The rate at which passengers left the rear of the aircraft was slowed by passengers reluctant to jump down from the door sills, which were 1.6m above the ground,” the report said.
The AAIB said the “spurious smoke warning” was probably caused by a short circuit in the smoke detector connector.
Safety action has since been taken by the airline to prevent a similar short circuit and “to ensure pilots received training with respect to aircraft evacuation that reflected company policy”, the AAIB added.