TM 1-1510-223-10 B-4         Service Ceiling The   altitude   at   which   the maximum  rate  of  climb  of 100 feet per minute can be attained for existing aircraft weight. Takeoff Weight The weight of the aircraft at lift-off from the runway. B-6.   WEIGHT AND BALANCE TERMINOLOGY. Approved Loading En- velope Those  combinations  of  air- craft  weight  and  center  of gravity which define the lim- its beyond which loading is not approved. Arm The distance from the center of  gravity  of  an  object  to  a line  about  which  moments are to be computed. Basic Empty Weight The aircraft weight with fixed ballast,  unusable  fuel,  en- gine oil, engine coolant, hy- draulic fluid, and in other re- spects as required by appli- cable regulatory standards. Center-of-Gravity A point at which the weight of an object may be considered concentrated for weight and balance purposes. CG Limits CG limits are the extremes of movement which the CG can have without making the aircraft unsafe to fly. The CG of  the  loaded  aircraft  must be within these limits at take- off, in the air, and on landing. Datum A vertical plane perpendicu- lar to the aircraft longitudinal axis from which fore and aft (usually  aft) measurements are made for weight and bal- ance purposes. Engine Oil That portion of the engine oil which  can  be  drained  from the engine. Landing Weight The weight of the aircraft at landing touchdown. Maximum Weight The  largest  weight  allowed by design, structural, perfor- mance or other limitations. Maximum Zero Fuel Weight Any weight above the value must be loaded as fuel. Moment A measure of the rotational tendency of a weight, about a specified line, mathemati- cally equal to the product of the weight and the arm. Standard Weights   corresponding   to the  aircraft  as  offered  with seating  and  interior,  avion- ics,  accessories,  fixed  bal- last   and   other   equipment specified by the manufactur- er as composing a standard aircraft. Station The    longitudinal    distance from some point to the zero datum or zero fuselage sta- tion. Takeoff Weight The weight of the aircraft at liftoff. Unusable Fuel The    fuel    remaining    after consumption of usable fuel. Usable Fuel That portion of the total fuel which  is  available  for  con- sumption  as  determined  in accordance  with  applicable regulatory standards. Useful Load The difference between the aircraft ramp weight and ba- sic empty weight. B-7.   MISCELLANEOUS ABBREVIATIONS. A/A Air to air AAA Anti-aircraft artillery AC Advisory circular, alternating current ACCEL Accelerometer, acceleration ACS Avionics control system ACT Active A/D Analog to digital ADC Air data computer ADF Automatic direction finder ADI Attitude director indicator