A First Officer and Captain report they noticed their engine power levers on their P-180 Avanti were sticking after ninety minutes into their flight. Because of heavy rains the night before; they decided to disconnect the autopilot and manually check the flight controls. The P-180 immediately pitched nose down; ice had apparently frozen all six primary flight control cables.

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Reporter stated the P180 Avanti was parked outside and exposed to heavy winter rains all night in approximately 40F degrees temperatures. When he and the Captain first entered the aircraft; they both noted how wet the cabin aisle floor carpet was. They had just finished their external walkaround; but since the carpet was so wet; they decided to check the external fuselage drains again; very little water came out. Reporter stated there are six small drain holes approximately one to one and a half millimeters in diameter that run along the belly centerline. They use a paper clip or something similar in size to push into the drain hole and lift each rubber flap to check for water accumulation. The reporter stated water tends to accumulate during takeoff and climb in the last 1/3rd of the cabin under the floorboards just forward of where the fuselage skin starts to curve up. The rubber flaps over the drain holes are NOT spring loaded to stay open on the ground to let water drain out. They are always closed unless pushed up. Reporter stated there are six primary flight control cables moving the ailerons; elevators and rudder. No system hydraulic actuators. The primary cables and the engine power lever cables run under the floorboards through the fuselage frames and wing spar. There is approximately four inches of space between the fuselage skin inner surface and the primary control cables and engine cables. Reporter stated the Captain was not satisfied with the little amount of water they could drain from the fuselage drains; so they decided to check the flight controls every 10 to 15 minutes in-flight by disconnecting the autopilot. About ninety minutes into the flight; when they disconnected the autopilot again for another manual check of the flight controls; the aircraft pitched nose down. Reporter stated the drain holes are too small to properly drain the fuselage belly. No hot air is ducted under the floorboards in the same area. The rudder was still frozen until they landed. Mechanics opened up the cabin floorboards and showed them the ice that had built up in the belly; all the way up to the flight control cables. Maintenance kept the P-180 in the hangar for three days. He was later told by Mechanics that approximately twenty gallons of water had drained from the belly. Reporter stated the Avanti P-180 has a 41;000 FT ceiling; can fly approximately 400 KTS or 0.7 M.