usually in the upper pulley in a piston helicopter or mounted  on the accessory gearbox in a turbine helicopter. When the  engine is driving the rotor, inclined surfaces in the sprag  clutch force rollers against an outer drum. This prevents the  engine from exceeding transmission rpm. If the engine fails,  the rollers move inward, allowing the outer drum to exceed  the speed of the inner portion. The transmission can then  exceed the speed of the engine. In this condition, engine  speed is less than that of the drive system, and the helicopter  is in an autorotative state. Static balance is the tendency of an object to remain stationary  when supported from its own CG. There are two ways in  which a control surface may be out of static balance. They  are called underbalance and overbalance.  When a control surface is mounted on a balance stand, a  downward travel of the trailing edge below the horizontal  position indicates underbalance. Some manufacturers  indicate this condition with a plus (+) sign. An upward  movement of the trailing edge, above the horizontal position  indicates overbalance. This is designated by a minus (–) sign.  These signs show the need for more or less weight in the  correct area to achieve a balanced control surface, as shown  in  Figure 2-60 . The primary assembly of a type certificated aircraft is normally  performed by the manufacturer at the factory. The assembly  includes putting together the major components, such as  the fuselage, empennage, wing sections, nacelles, landing  gear, and installing the powerplant. Attached to the wing  and empennage are primary flight control surfaces including  ailerons, elevators, and rudder. Additionally, installation  of auxiliary flight control surfaces may include wing flaps,  spoilers, speed brakes, slats, and leading edge flaps.  The assembly of other aircraft outside of a manufacturer’s  facility is usually limited to smaller size and experimental  amateur-built aircraft. Typically, after a major overhaul,  repair, or alteration, the reassembly of an aircraft may  include reattaching wings to the fuselage, balancing of and  installation of flight control surfaces, installation of the  landing gear, and installation of the powerplant(s).  A tail-heavy condition (static underbalance) causes  undesirable flight performance and is not usually allowed.  Better flight operations are gained by nose-heavy static  overbalance. Most manufacturers advocate the existence of  nose-heavy control surfaces.  This section is presented for familiarization purposes only.  Explicit instructions for the balancing of control surfaces are  given in the manufacturer’s service and overhaul manuals for  the specific aircraft and must be followed closely.  Any time repairs on a control surface add weight fore or aft of  the hinge center line, the control surface must be rebalanced.  When an aircraft is repainted, the balance of the control  surfaces must be checked. Any control surface that is out  of balance is unstable and does not remain in a streamlined  position during normal flight. For example, an aileron that  is trailing edge heavy moves down when the wing deflects  upward, and up when the wing deflects downward. Such a  condition can cause unexpected and violent maneuvers of  the aircraft. In extreme cases, fluttering and buffeting may  develop to a degree that could cause the complete loss of  the aircraft.  Rebalancing a control surface concerns both static and  dynamic balance. A control surface that is statically balanced  is also dynamically balanced.  Figure 2-60.  Control surface static balance. Plus  ( + )  condition Minus  ( − )  condition Balance condition Chord line Tail-down underbalance Chord line Nose-down overbalance Chord line Level-horizontal position Rebalancing of Control Surfaces Airplane Assembly and Rigging Static Balance 2-37