TM 1-1510-218-10 B-4 Service Ceiling The  altitude  at  which  the  maximum rate-of-climb   of   100   fpm   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 Envelope Those combinations of aircraft weight   and   center   of   gravity   that define     the     limits     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,   engine   oil,   engine coolant, hydraulic fluid, and in other respects  as  required  by  applicable regulatory standards. Center of Gravity (CG) A  point  at  which  the  weight  of  an object may be considered concentrated for weight and balance purposed. CG Limits CG    limits    are    the    extremes    of movement   that   the   CG   can   have without making the aircraft unsafe to fly.    The  CG  of  the  loaded  aircraft must    be     within    these    limits    at takeoff, in the air, and on landing. Datum A vertical plane perpendicular to the aircraft  longitudinal  axis  from  which fore and aft (usually aft) measurements  are made  for  weight and balance purposes. Engine Oil That  portion  of  the  engine  oil  that 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,   performance   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,  mathematically  equal to the product of the weight and the arm. Operating Weight Basic     empty     weight     plus     crew, crew’s   baggage,   publications,   and any  other  equipment  (not  listed  on Chart  C)  that   will   remain   with   the crew throughout the mission. Standard Weights corresponding to the aircraft  as  offered  with  seating  and interior,  avionics,  accessories,  fixed ballast and other equipment specified   by   the   manufacturer   as composing a standard aircraft. Station The longitudinal distance from some point   to   the   zero   datum   or   zero fuselage station. Takeoff Weight The weight of the aircraft at liftoff. Unusable Fuel The fuel remaining after consumption of usable fuel. Usable Fuel The portion of the total fuel load that is    available    for    consumption    as determined in accordance with applicable regulatory standards. Useful Load The  difference  between  the  aircraft ramp weight and basic empty weight. Zero Fuel Weight Operating    weight    plus    the     load equals zero fuel weight. B-7.  MISCELLANEOUS ABBREVIATIONS. @ At AAS Aeronautical Advisory Service AC Alternating Current ACFT Aircraft ACT Active ADC Air Data Computer ADF Automatic Direction Finder ADI Attitude Director Indicator AETM Army Engine Trend Monitoring System AFCS Automatic Flight Control System AFIS Airborne Flight Information System AGRAS Air/Ground Radiotelephone Automated Service ALT Altitude ALTSEL Altitude Select