Abstract: To solve the attitude control problem of aircraft with variations in mass and center of gravity (CG), a general model, including the dynamics and mass/CG variation properties, for such aircraft was established with the variations converted to disturbance forces and moments. This model was able to reveal the dynamic couplings between the CG shifts and aircraft motions, and the induced disturbances. For the attitude control of such aircraft, with variations in mass and CG treated as unknown disturbance inputs, an observation and compensation framework was proposed. The longitudinal attitude controller was then designed within this framework based on the proportional-integral observer (PIO) which estimates the disturbance force and moment inputs caused by mass and CG variations. The PIO compensation command was added to the regular autopilot. In the example of heavy load airdrop on cargo aircrafts, computer simulation demonstrated the proposed model could accurately reflect the aircraft-s dynamic responses in every stage of mass/CG variations, and pitch attitude response was greatly improved compared with controller without PIO compensation command and met precise attitude control requirements