Accident Summary
The pilot of a Cessna 421 reported to approach control that the airplane was accumulating significant ice. He described a severe airframe vibration, though no vibration was felt in the control column. The airplane subsequently stalled and entered a right spiral, from which the pilot recovered. During the landing sequence, the airplane stalled again, struck the runway hard, and veered off the side of the runway.
Examination Findings
A post-accident examination of the airplane revealed no abnormalities with any aircraft component or system. The propeller deice system could not be fully checked due to damage to the propeller blades. Inspection of the wings showed ice buildup measuring between one-quarter and one-half inch in thickness.
Aircraft Information
The accident aircraft is a 1979 Cessna 421. The Pilot's Operating Handbook (POH) for this model states that an accumulation of half an inch of ice may cause a cruise speed reduction of up to 30 knots, as well as significant buffet and an increase in stall speed. A Cessna representative indicated that the 1975 Cessna 421 would exhibit similar buffet characteristics when covered with ice. Both POHs note that uneven deicing of propeller blades is indicated by excessive vibration and that the propeller deice system must be turned off under such conditions.