The plane was travelling from Manchester to Jersey when it dropped 15,000ft over Dudley due to a sudden fall in cabin pressure, says report.
The pilot radioed 'Mayday' as he was forced to descend from 25,000ft to 10,000ft.
Crew members had to wear oxygen masks due to a faulty valve on the Flybe plane which was carrying four crew members and 23 passengers.
The incident happened on 8 December 2016.
Air Accidents Information Branch (AAIB)'s report classed it as a 'serious incident'.
The report said:
A Flybe spokesman said:
The report also stated that the valve has been replaced and the aircraft has now returned to service.
Passengers were informed of the incident over the PA and there were no injuries.