Flight BE 046 departed Birmingham at 12:52 for a flight back to Belfast. During the climb, the flight crew noticed that the aircraft was oscillating in pitch more than was customary. The autopilot remained engaged for significant portions of the climb but the level-off at FL240 was flown manually. Shortly after establishing level flight, the autopilot was re-engaged but the aircraft began to oscillate in pitch and diverge from its assigned flight level. The captain disengaged the autopilot and was immediately aware of a strong pitch-up tendency. He applied an increasing forward pressure on the control column and supplemented this with electric elevator trim in the nose-down sense. At FL242 the aircraft was reported to have pitched down at a marked rate. Having pitched to below the straight and level attitude, the captain then tried to counter this with a progressive rearwards force on the controls. Both pilots then pulled back with considerable force. The control column suddenly moved aft, the aircraft pitched up and the flight crew felt a violent shudder through the whole airframe that lasted for two or three seconds. After this the control forces returned to normal and they were able to level the aircraft at FL240. During the pitching manoeuvres, two of the three cabin crew had fallen in the cabin aisle, sustaining serious injuries. The elevator control difficulties might have been caused by natural icing; investigation is still ongoing. PROBABLE CAUSE: “Exposure of the aircraft to precipitation on the ground in near freezing conditions prior to take off, resulting in accumulations of snow, rain or hail in the elevator/servo tab gaps which then freeze in flight.”