It has emerged that an oil bleed, due to a damaged bearing washer, caused a serious incident on a Flybe flight from Knock Airport in July of last year.
Air accident investigators have established the plane was climbing when the crew were alerted by a strange odour after a smoke alarm went off in one of the toilets.
Smoke then became visible in both the cabin and flight deck, and the crew were forced to put on their oxygen masks.
The flight diverted to Dublin Airport and nobody was injured.
The Bombardier aircraft had 74 passengers and four crew on board and was flying from Knock to Manchester.