asked for a late descent because air aircraft had engine intake anti-icing, but no airframe anti-icing ... The met office were aware of the danger and had forecast the icing risk as moderate, ... now we found we didn't have enough power to climb, so we slowed down and immediately entrered buffet, at a speed some 60% more than the usual buffet onset. Now we were in trouble as we couldn't remain level at full power as the intake de-icing wasn't coping under there conditions. ... A mayday was declared
A rather classic example of what happens when you violate an aircraft operating limitation that prohibits flight into "known icing conditions".