The operational requirements of a large jet transport aircraft  necessitate large pitch trim changes. Some requirements are:  These requirements are met by the use of a variable incidence  horizontal stabilizer. Large trim changes on a fixed-tail  aircraft require large elevator deflections. At these large  deflections, little further elevator movement remains in the  same direction. A variable incidence horizontal stabilizer  is designed to take out the trim changes. The stabilizer is  larger than the elevator, and consequently does not need to  be moved through as large an angle. This leaves the elevator  streamlining the tail plane with a full range of movement up  and down. The variable incidence horizontal stabilizer can  be set to handle the bulk of the pitch control demand, with  the elevator handling the rest. On aircraft equipped with a  variable incidence horizontal stabilizer, the elevator is smaller  and less effective in isolation than it is on a fixed-tail aircraft.  In comparison to other flight controls, the variable incidence  horizontal stabilizer is enormously powerful in its effect.  Because of the size and high speeds of jet transport aircraft,  the forces required to move the control surfaces can be beyond  the strength of the pilot. Consequently, the control surfaces are  actuated by hydraulic or electrical power units. Moving the  controls in the flight deck signals the control angle required,  and the power unit positions the actual control surface. In the  event of complete power unit failure, movement of the control  surface can be affected by manually controlling the control  tabs. Moving the control tab upsets the aerodynamic balance,  which causes the control surface to move.  In order to sustain an aircraft in flight, a pilot must understand  how thrust, drag, lift, and weight act on the aircraft. By  understanding the aerodynamics of flight, how design,  weight, load factors, and gravity affect an aircraft during  flight maneuvers from stalls to high speed flight, the pilot  learns how to control the balance between these forces. For  information on stall speeds, load factors, and other important  aircraft data, always consult the AFM/POH for specific  information pertaining to the aircraft being flown.  Chapter Summary  • A large CG range  • A large speed range  • The ability to perform large trim changes due to  wing leading edge and trailing edge high-lift devices  without limiting the amount of elevator remaining  • Maintaining trim drag to a minimum  5-51 