UNDERSLUNG—A rotor hub that rotates below the top of the mast, as on semirigid rotor systems.
UNLOADED ROTOR—The state of a rotor when rotor force has been removed, or when the rotor is operating under a low or negative G condition.
USEFUL LOAD—The difference between the gross weight and the basic empty weight. It includes the flight crew, usable fuel, drainable oil, if applicable, and payload.
VARIATION—The angular difference between true north and magnetic north; indicated on charts by isogonic lines.
VERTICAL VIBRATION—A vibration in which the movement is up and down, or vertical, as in an out-of-track condition. 

VORTEX RING STATE—A transient condition of downward flight (descending through air after just previously being accelerated downward by the rotor) during which an appreciable portion of the main rotor system is being forced to operate at angles of attack above maximum. Blade stall starts near the hub and progresses outward as the rate of descent increases.
WEIGHT—One of the four main forces acting on a rotorcraft. Equivalent to the actual weight of the rotorcraft. It acts downward toward the center of the earth.
YAW—The movement of a rotorcraft about its vertical axis.
 

G-6

INDEX

 
 
 
normal to a hover, 9-19 normal to the surface, 9-20 

ABORTED TAKEOFF, GYROPLANE 21-1 ACCELERATE/STOP DISTANCE 21-1 AERODYNAMICS 2-1, 3-1, 16-1 autorotation, 3-8 forward flight, 3-5 general, 2-1 gyroplane, 16-1 helicopter, 3-1 hovering flight, 3-1 rearward flight, 3-8 sideward flight, 3-8 turning flight, 3-8 vertical flight, 3-4, 16-1 AERONAUTICAL DECISION MAKING (ADM) 14-1, 22-1 decision-making process, 14-3 definitions, 14-2 error chain, 14-1 factors affecting decision making, 14-5 hazardous attitudes, 14-6, 22-1 operational pitfalls, 14-8 origin, 14-2 pilot error, 14-1 risk management, 14-4 situational awareness, 14-8 stress management, 14-6 use of resources, 14-6 workload management, 14-7 AGONIC LINE 12-5 AIRCRAFT LIGHTING 13-3 AIRFOIL 2-1 angle of attack, 2-2 camber, 2-2 center of pressure, 2-1 chord line, 2-2 leading edge, 2-2 pitch angle, 2-2 relative wind, 2-2 resultant relative wind, 3-6 rotational relative wind, 3-6 span, 2-1 trailing edge, 2-2 twist, 2-1 AIRSPEED INDICATOR 12-1, 18-4 AIR TAXI 9-9 AIRWORTHINESS DIRECTIVE 6-4 ALTIMETER 12-2, 18-4 ANGLE OF ATTACK 2-2 ANTI-ICING SYSTEMS 5-11 ANTITORQUE PEDALS 4-3 ANTITORQUE SYSTEM FAILURE 11-11 ANTITORQUE SYSTEMS 1-2 tail rotor, 1-2 fenestron, 1-2 NOTAR®, 1-2 APPROACHES confined area, 10-7 crosswind, 9-20 night, 13-5