The Viscount tailplane was de-iced by hot air ducting from the inboard engines. In the case of the accident mentioned, the aircraft had been on descent from cruise with low power on all engines. At this low power the heat rapidly disssipated during the hot air flow down to the tail area and was not hot enough to stop ice forming.
During my time on the Viscount we would keep the power well up on the inboard engines during descent in icing conditions which sometimes made the descent profile flatten but one had to avoid the temptation of further reducing power to the inboard engines because one didn't know whether the hot air was enough to ensure ice free tail surfaces.