A report into an incident in which a passenger plane skidded off a runway after landing has found de-icing was likely to have had a limited effect.
The Ryanair plane from Dublin, with 135 people on board, ended up on a grass verge at Prestwick Airport near Glasgow on 23 December last year.
The Air Accidents Investigation Branch report said the airport had tried to de-ice the runway.
But the machine had not been set to the correct level to get rid of the ice.
The de-icing machine was automatically limited to an application of 20 grams per square metre - the rate appropriate for anti-icing.
However, to remove ice already formed, a rate of 30-70 grams per square metre is needed.
The report said: "A de-icing run was carried out on the runway but at an application rate only suitable for anti-icing.
"Therefore, it is likely to have been of limited effectiveness."
No-one was injured in the incident and there was no reported damage to the aircraft.
The report also said the co-pilot did not pass on the exact detail of the "extremely icy" conditions.
He told his colleagues about the wind and braking action and added: "And it's icy obviously."
After landing, the plane taxied towards the end of the runway but did not slow down when the brakes were applied.
The plane came to a stop on the grass and the passengers and crew disembarked.
The AAIB said the airport operator identified a number of areas in its winter operations where its procedures could be improved and made appropriate safety recommendations, with a particular focus on anti-icing and de-icing operations.
And the airline operator has included a training module on operations to or from slippery runways in its recurrent training programme.
The AAIB said it was therefore not considered necessary to make any further safety recommendations.