On November 11, 1995, at 2100 eastern standard time (est), a Cessna 421, N1588G, piloted by a private pilot, was substantially damaged during a collision with terrain while landing. The pilot reported control difficulties prior to landing. Visual meteorological conditions prevailed at the time of the accident. The personal 14 CFR part 91 flight was operating on an IFR flight plan. The pilot and four passengers reported no injuries. The flight departed Pontiac, Michigan, at 2037 est, en route to Toronto, Canada. During a telephone conversation on November 15, 1995, with the pilot he said that during preflight he checked the operation of all deice systems. The pilot stated that the airplane had been at 5,000 feet mean sea level (msl) for about 10 minutes, and said the autopilot was on. The pilot said the windshield was clear of ice, he estimated the wing tips had one quarter inch of ice accumulation. The pilot stated that the indicated airspeed was between 150 and 180 knots. A severe airframe vibration was reported by the pilot, at which point he said he disengaging the autopilot and took manual control of the aircraft. The airframe vibration was described by the pilot as high frequency and sounded like the propellers were out of sync. The pilot commented that the shaking of the instrument panel was very noticeable. The pilot said he did not feel any vibration in the control column. After the airframe vibration the pilot said the aircraft seemed to stall, and enter a right spiral. The pilot said the aircraft descended uncontrolled from 5,000 feet msl until he recovered control of the airplane at 1,800 feet msl. After the altitude loss the pilot was given a radar vector by Selfridge approach control to the Romeo Airport. The pilot said he was going to land on runway 36. However, after putting the landing gear down the airframe vibration reoccurred. The pilot said he decided to abort the landing attempt, added power and retracted the landing gear. The pilot reestablished the aircraft for landing on runway 18 at the Romeo Airport. On this approach the pilot said he waited until the aircraft was almost ready to touchdown, before extending the landing gear. The pilot reported the airplane stalled and hit hard on the runway. Propeller slash marks were found on the runway, and three propeller blade tips were found at the Romeo Airport separated from their blades. The airplane went off the side of the runway after landing. Examination of the airplane by a Federal Aviation Administration principal maintenance inspector (PMI) revealed no abnormalities with any airplane component or system. The propeller deice system could not be checked past the distribution block at the back of the spinner, because the deicer elements on the blades were damaged when the blades contacted the runway. The PMI estimated the ice build up along the leading edges of the wings at one quarter to one half inch. The investigator in charge (IIC) obtained the radar data, and voice transcripts for the flight of N1588G. Twice during the flight the pilot told Selfridge approach, that the airplane was picking up quite a bit of ice, and an altitude change would be needed. N1588G descended 2,500 feet in 18.5 seconds. Two Cessna 421 POH's were referenced by the IIC. One for a 1979 Cessna 421, the other for a 1975 Cessna 421. Both POH's recommend to activate the deice boots "... when ice accumulates between 1/4 to 1/2 inch. Repeat as necessary allowing at least 45 seconds between actuations." The POH for the 1979 Cessna 421 states "accumulation of a 1/2 inch of ice may cause a cruise speed reduction of up to 30 knots as well as a significant buffet and stall speed increase." A Cessna Aircraft Company representative said that the 1975 Cessna 421 would exhibit similar significant buffett characteristics when covered with ice as the 1979 model. For the propellers both POH'S state that uneven deicing of propeller blades is indicated by excessive vibration. The POH's propeller warning for both aircraft is: "When uneven deicing of the propeller blades is indicated it is imperative that the deice system be turned off. Uneven deicing of the blades can result in propeller unbalance and engine failure." The airplane's left tip tank was crushed and all six propeller blades were bent during the landing. The landing gear collapsed and the lower fuselage skins were wrinkled.