Weight and Balance Terminology

Datum-Imaginary Vertical Line

-A point all measurements are taken

-Established by manufacturer

-Located in aircraft’s Type Certification Data Sheet (TCDS) or Aircraft Specifications

-No fixed location, does not change

Example of Datum

-No fixed rule for location

-Fuel tank +60- located 60” behind the datum

-Radio -90— 90” forward of the datum

Center of Gravity (CG)

  • Point at which airplane would balance if suspended

  • How to calculate: Divide total moment by total weight of aircraft

Figure 1

Arm

-Horizontal distance from the datum

-(+) measures aft of the datum

-(-) Measures forward of the datum

-Items on the datum add weight but do not affect balance

Figure 2

Moment

  • The force that causes an aircraft to rotate in flight

  • An aircraft can rotate in three dimensions: pitch, yaw, and roll

Figure 3

  • The further an object is from the pivot point (or the longer the arm from the CG) The greater the force it exerts

  • How to calculate: multiply the weight of an item by its arm

Figure 4

Center of Lift

  • Location on the wing where lift forces concentrate

  • Typically, 1/3 the distance from leading edge of the wing

Figure 5Figure 6

Maximum Weight

-Specified in TCDS

-Includes aircraft, equipment, passengers, fuel, luggage

-Varies according to purpose of the aircraft and conditions flown

Types of Maximum Weight

-Max Ramp Weight-Heaviest weight to which an aircraft can be loaded on the ground

-Max Takeoff Weight-Heaviest weight for an aircraft at takeoff (Max Ramp Weight minus consumed fuel)

Maximum Landing Weight

-Max Landing weight-Heaviest weight for an aircraft when landing

Zero Weight

-Max Zero Fuel Weight-Heaviest weight to which an aircraft can be loaded without usable fuel (additional weight must be fuel)

Empty Weight

  • Airframe

  • Powerplant

  • Required equipment

  • Fixed ballast

  • Hydraulic fluid

  • Residual fluid

  • Full oil

  • Equipment in fixed location

Empty Weight CG Range

-The distance between allowable forward and aft empty-weight CG limits

Useful Load

-Consists of fuel, fluids, passengers, baggage, pilots, and crew (see TCDS)

-Some aircraft classified normal and utility

-Maximum Allowable Weight-Empty Weight=Useful Load

Weight and Balance Extreme Conditions

-Paper pencil check that loads aircraft tail heavy or nose heavy; check CG is still within limits

-Exception-If the fuel tank is behind the forward CG limit it retains minimum fuel

-Minimum Fuel-The amount of fuel needed for 30 min of flight

Calculating Minimum Fuel

-Piston engine aircraft use a calculation based on METO (maximum except take-off) horsepower of the engin

-For each METO hp, ½ lb of fuel used (at cruise flight 1lb/hr)

-How to calculate: Minimum fuel=Engine METO hp/2

Tare Weight

  • The weight of chocks or devices that hold an aircraft on scales

  • Subtract tare weight from the scale reading to get the aircraft’s net weight