NOTE: The weight and balance records for a particular  glider provide the empty weight and moment, as well as the  information on the arm distance.  [Figure 5-20] Some trim ballast weights are in the form of seat cushions,  with sand or lead shot sewn into the unit to provide additional  weight. This type of ballast, which is installed under the  pilot’s seat cushion, is inferior to bolted-in ballast because  seat cushions tend to shift position. Seat cushion ballast  should never be used during acrobatic or inverted flight. Sometimes trim ballast is water placed in a tail tank in the  vertical fin of the fuselage. The purpose of the fin trim ballast  tank is to adjust CG location after water is added to, or drained  from, the main wing ballast tanks. Unless the main wing  ballast tanks are precisely centered on the CG of the loaded  aircraft glider, CG location shifts when water is added to the  main ballast tanks. CG location shifts again when water is  dumped from the main ballast tanks. Adjusting the amount of  water in the fin tank compensates for CG shifts resulting from  changes in the amount of water ballast carried in the main  wing ballast tanks. Water weighs 8.35 pounds per gallon.  Because the tail tank is located far aft, it does not take much  water to have a considerable effect on CG location. For this  reason, tail tanks do not need to contain a large volume of  water. Tail tank maximum water capacity is generally less  than two gallons of water. In  Figure 5-20,  the weight of each pilot has been entered  into the correct block in the table. For the front seat pilot,  multiplying 180 pounds by +30 inches yields a moment of  +5,400 inch·pounds. For the rear seat pilot, multiplying 200  pounds by –5 inches yields a moment of –1,000 inch·pounds. The next step is to find the sum of all weights (980 pounds)  and record it. Then, find the sum of all moments (+16,400  inch·pounds) and record it. Now, find the arm (the CG  position) of the loaded glider. Divide the total moment by the  total weight to discover the CG of the loaded aircraft glider  in inches from the datum: Although some older gliders employed bags of sand or  bolted-in lead weights as performance ballast, water is used  most commonly to enhance high-speed performance in  modern sailplanes. Increasing the operating weight of the  glider increases the optimum speed to fly during wings-level  cruising flight. The resulting higher groundspeed provides  a very desirable advantage in cross-country soaring and in  sailplane racing. The final step is to determine whether total weight and CG  location values are within acceptable limits. The GFM/ POH lists the maximum gross weight as 1,100 pounds. The  operating weight of 980 pounds is less than the 1,100 pounds  maximum gross weight. The GFM/POH lists the approved  CG range as between +14.80 inches and +18.60 inches from  the datum. The operating CG is +16.73 inches from the datum  and is within these limits. The weight and balance are within  operating limits. +16,400 inch·pounds ÷ 980 pounds = +16.73 inches Water ballast tanks are located in the main wing panels. Clean  water is added through fill ports in the top of each wing.  In most gliders, the water tanks or bags can be partially or  completely filled, depending on the pilot’s choice of operating  weight. After water is added, the filler caps are replaced to  prevent water from sloshing out of the filler holes. Ballast is nonstructural weight that is added to a glider.  In soaring, ballast weight is used for two purposes.  Trim  ballast is used to adjust the location of the CG of the glider  so handling characteristics remain within acceptable limits.  Performance ballast is loaded into the glider to improve  high-speed cruise performance. Drain valves are fitted to the bottom of each tank. The valves  are controlled from inside the cockpit. The tanks can be fully  or partially drained while the glider is on the ground to reduce  the weight of the glider prior to launch, if the pilot so desires.  The ballast tanks also can be partially or completely drained  in flight—a process called dumping ballast. The long streaks  of white spray behind a speeding airborne glider are dramatic  evidence that the glider pilot is dumping water ballast, most  likely to lighten the glider prior to landing. The filler caps are  vented to allow air to enter the tanks to replace the volume of  water draining from the tanks. It is important to ensure that  the vents are working properly to prevent wing damage when  water ballast is drained or jettisoned.  [Figures 5-21  and  5-22] Removable trim ballast weights are usually made of metal and  are bolted into a ballast receptacle incorporated in the glider  structure. The manufacturer generally provides an attachment  point well forward in the glider cabin for trim ballast weights.  These weights are designed to compensate for a front seat  pilot who weighs less than the minimum permissible front  seat pilot weight. The ballast weight mounted well forward in  the glider cabin helps place the CG within permissible limits,  which allows the maximum shift in CG with the minimum  addition of weight. 5-14 Ballast Figure 5-20.  Weight and balance: front and rear seat pilot weights  and moments. Item Empty weight Front seat pilot Rear seat pilot Weight  (pounds) 600 180 200 980 total weight Arm  (inches) +20 +30 −5 +16.73 Moment  (inch·pounds) 12,000 +5,400 −1,000 +16,400 total moment