Weight & Balance

Pilot Handbook of Aeronautical Knowledge, Balanced Center of Gravity

Introduction:

  • Weight and balance is a key factor in not only design but in the performance and stability of an aircraft under various operating conditions
  • Aircraft balance about their Center of Gravity (C.G.), whose location determines aircraft characteristics
  • The position of the center of gravity is affected by the total and the distribution of weight throughout the aircraft
  • Weight parameters ensure the wings and overall structure can support the aircraft throughout all flight envelopes
  • Before every flight, pilots must determine the weight and balance to ensure the aircraft operates within manufacturer specifications
  • Weight and balance data more than an exercise in determining where an aircraft's C.G. will fall
    • It is a safety concern that has cost the lives of several pilots and their passengers
  • Think you've got a solid understanding of weight and balance? Don't miss the weight and balance quiz below, and topic summary

Center of Gravity:

  • Forward Center of Gravity
    Pilot Handbook of Aeronautical Knowledge
    Forward Center of Gravity
  • Forward Center of Gravity
    Pilot Handbook of Aeronautical Knowledge
    Forward Center of Gravity
  • Aft Center of Gravity
    Pilot Handbook of Aeronautical Knowledge
    Aft Center of Gravity
  • The Center of Gravity is the specific point where the aircraft's mass or weight is said to center; that is, a point around which, if the aircraft could be suspended or balanced, the aircraft would remain in place at any attitude
  • It is computed during initial design and construction and is further affected by the installation of onboard equipment, aircraft loading, and other factors
  • C.G. is of major importance in an aircraft, for its position (within a designed range) has a great bearing upon longitudinal stability
  • Location of the Center of Gravity:

    • The C.G. must always be within limits; however, depending where in the allowable range the C.G. falls will affect performance [Figure 1]
      • Forward Center of Gravity:

        • Stable feeling
        • Nose Heavy
        • Longer takeoff distance (more airflow required to provide more force to lift heavy nose)
        • High stall speeds (more airflow deflection of the elevator required to maintain altitude at slower airspeeds resulting in high Angles of Attack (AoA) [Figure 2]
        • Forward Center of Gravity
          Pilot Handbook of Aeronautical Knowledge
          Forward Center of Gravity
      • Rearward/Aft Center of Gravity:

        • As the C.G. moves rearward (towards the tail), the aircraft becomes more and more dynamically unstable [Figure 3]
        • The tail will feel heavy to compensate, which requires additional nose down-force
        • Should the aircraft stall or spin, it will be much more difficult, if not impossible, to recover
        • Higher true airspeed
        • More tail down-force
        • Aft Center of Gravity
          Pilot Handbook of Aeronautical Knowledge
          Aft Center of Gravity
      • Performance impacts are due to lift/drag changes
      • Balanced Center of Gravity
        Pilot Handbook of Aeronautical Knowledge
        Balanced Center of Gravity
  • Tailwheel Center of Gravity Specifics:

    • The center of gravity for tailwheel aircraft are typically located behind the pilot
    • As discussed, the aircraft rotates about the center of gravity, meaning if directional control is lost due to erratic inputs or simply poor technique as airspeed decreases control surface effectiveness, the tail will want to come off to the side
      • This can result in a ground loop where the pilot is unable to stop the center of gravity from taking over directional control for the pilot/control surfaces
  • Center of Pressure:

    • It is important to understand that an aircraft's weight concentrates at the C.G. and the aerodynamic forces of lift occur at the Center of Pressure (CP)
    • When the CG is forward of the CP, there is a natural tendency for the aircraft to want to pitch nose-down
    • If the CP is forward of the C.G., a nose-up pitching moment is created
    • Therefore, designers fix the aft limit of the CG forward of the CP for the corresponding flight speed to retain flight equilibrium

Aircraft Weight:

  • Aircraft weight must be distributed in accordance with the design described in the aircraft's flight manual
  • Center of Gravity can also be considered the point at which all the weight of the aircraft is concentrated
  • C.G. can be expressed in several different ways:
    • Standard Empty Weight:

      • This weight consists of the airframe, engines, and all items of operating equipment that have fixed, permanently installed locations in the airplane, including fixed ballast, hydraulic fluid, unusable fuel, and full engine oil
    • Basic Empty Weight:

      • The basic empty weight (BEW) of standard airplane, optional equipment, unusable fuel (fuel that cannot be drained), and full operating fluids, including full engine oil
    • Licensed Empty Weight:

      • Like BEW, but does not count full engine oil, only undrainable oil
    • Gross Landing Weight:

      • Takeoff weight minus the fuel burned en-route
    • Ramp Weight:

      • Airplane loaded for flight before engine start
    • Zero Fuel Weight:

      • Weight of the aircraft before the addition of fuel
    • Gross Takeoff Weight:

      • Weight of the airplane just before brake release to begin the takeoff roll
    • Useful Load:

      • Weight of crew and usable fuel
    • Maximum Ramp Weight:

      • Maximum weight for ground operations
    • Maximum Takeoff Weight:

      • Maximum weight for takeoff
    • Maximum Landing Weight:

      • Maximum weight for landing based on the stress of impact on gear
    • Payload:

      • The weight of only the passengers, baggage, and cargo
  • Overweight Aircraft:

    • Most aircraft will never be too light to fly however overweight aircraft pose very serious safety threats
    • People like R&B singer Aaliyah have died when pilots neglect to complete a proper preflight
    • Limitations:
      • Longer takeoff run
      • Higher takeoff speed
      • Reduced angle and rate of climb
      • Reduced cruising speed
      • Shorter range
      • Higher stalling speed
      • Longer landing roll

Determining Weight and Balance:

  • Weight and Balance Data
    Weight and Balance Data
  • Federal Aviation Regulation 21.5 sets forth the requirement for weight and balance data ("operating limitations and information") to be furnished to the pilot
    • The presented data is under the conditions which the airplane or rotorcraft was type certificated
    • The weight and balance information falls under "operating limitations" and is a required document in determining legal airworthiness
  • Weight and balance is measured against a reference datum, an imaginary vertical plane from which all horizontal distances are measured (firewall, leading edge, etc.)
  • From that datum, an arm, which is the distance from the datum, can be measured
  • Taking a known weight and multiplying it against the arm gives a pilot what they care about, and that is the moment or measurement of the tendency of the weight to cause rotation at the fulcrum
  • Weight and balance specifics are unique to different aircraft
    • Weight and balance examples are included with the Pilot Operating Handbook (POH)
  • Many pilots, however, will learn and therefore be able to relate to the Cessna 172: [Figure 4]
    • Block 1: Determine the Basic Empty Weight of the airplane (found in POH)
    • Block 2: Determine the basic empty weight moment of the airplane (found in POH)
    • Block 3: Determine the weight of the pilot and passenger
    • Block 4: Determine the moment of the pilot and passenger (weight x arm = moment)
    • Block 5: Determine the weight of the rear passengers
    • Block 6: Determine the moment of the rear passengers (weight x arm = moment)
    • Block 7: Determine the weight of the baggage
    • Block 8: Determine the moment of the baggage (weight x arm = moment)
    • Block 9: Determine the weight of the baggage as in step 7
    • Block 10: Determine the moment of the baggage as in step 8, using the new arm
    • Block 11: Add all weights together to get the Zero Fuel Weight (Z.F.W.)
    • Block 12: Add all moments together
    • Block 13: Determine the weight of the ramp fuel
    • Block 14: Determine the moment of the ramp fuel (weight x arm = moment)
    • Block 15: Determine the ramp weight (Z.F.W. + Ramp Fuel)
    • Block 16: Determine the ramp moment (Z.F.W. moment + Ramp Fuel moment)
    • Block 17: Subtract taxi fuel used (~8 lbs)
    • Block 18: Subtract taxi fuel moment (~384)
    • Block 19: Add Z.F.W. and Ramp Weight together, then subtract Taxi Fuel to get the Gross Takeoff Weight (G.T.W.)
    • Block 20: Add Z.F.W. moment and Ramp Weight moment together, then subtract Taxi Fuel moment
    • Block 21: Estimate trip fuel weight
    • Block 22: Determine the moment of the trip fuel (weight x arm = moment)
    • Block 23: Subtract trip fuel weight from G.T.W. to get the Gross Landing Weight
    • Block 24: Subtract trip fuel moment from G.T.W. moment
    • Block 25: Divide block 12 by block 11
    • Block 26: Divide block 20 by block 19
    • Block 27: Divide block 24 by block 23
    • Block 28: Determine maneuvering speed (Va)
    • Weight and Balance Data
      Weight and Balance Data
  • Weight Shift Formula:

    • Weight Shift Formula
      Weight and Balance Handbook,
      Weight Shift Formula
    • If you shift weight after determining the aircraft's weight and balance, then verify your calculations with the weight shift formula [Figure 5]
    • Weight Shift Formula
      Weight and Balance Handbook,
      Weight Shift Formula

Case Studies:

  • National Transportation Safety Board Identification (NTSB): ANC13FA091: The NTSB determines the probable cause(s) of this accident to be:
    • The pilot's improper decision to load the airplane beyond its allowable takeoff weight and center of gravity limits, which resulted in a loss of control during the initial climb. Contributing to the accident was the external load and the downwind takeoff
  • National Transportation Safety Board Identification: ERA14LA450: The NTSB determines the probable cause(s) of this accident as follows:
    • The pilot's inadequate preflight planning, which resulted in a takeoff with the airplane's center of gravity aft of its limit and led the airplane to exceed its critical angle of attack and experience an aerodynamic stall during the initial climb. Contributing to the accident was the pilot's lack of flight experience in the aircraft make and model
  • National Transportation Safety Board Identification: ERA14CA408: The NTSB determines the probable cause(s) of this accident as follows:
    • The pilot/owner/builder's improper weight and balance calculations, which rendered the airplane uncontrollable in the pitch axis
  • National Transportation Safety Board identification: ERA14FA343: The NTSB determines the probable cause(s) of this accident as follows:
    • The pilot's failure to secure the cargo in the cargo compartment, which resulted in a weight shift that led to the center of gravity exceeding its aft limit during a go-around attempt and a subsequent aerodynamic stall. Also causal to the accident were the pilot's inadequate preflight inspection and his loading the airplane beyond the cargo compartment weight limit
  • National Transportation Safety Board Identification: CEN13IA563: The NTSB determines the probable cause(s) of this incident as follows:
    • The pilot's improper weight and balance calculations, which resulted in the airplane exceeding its weight and center-of-gravity limits and led to a loss of pitch control during takeoff, and the operator's failure to obtain required weight information and to ensure that the flight was properly loaded

Weight and Balance Knowledge Quiz:

Conclusion:

  • Understanding weight and balance allows us to determine the relationships with how heavy an aircraft is, and how the location of that weight will impact performance and handling characteristics
  • Aside from the weight and balance data that is required to be carried on board the aircraft in determining its legality to fly, am individual weight and balance is not required to be filled out before every flight
    • Pilot's should not allow the lack of regulation to allow complacency
    • It is directed in part 91.103 that "Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight"
      • It goes on to specify "information appropriate to the aircraft, relating to aircraft performance under expected values of airport elevation and runway slope, aircraft's gross weight, and wind and temperature"
  • Weight and balance is directly related to the stability of the aircraft
  • Instructions and examples can be found in the aircraft manual under section 6 for your specific aircraft
  • A few other terms which pilots may come across include:
    • Loading Graph: used to find the moment for loads in the airplane
    • Center of Gravity Moment Envelope: shows limits with proposed loading
  • Still looking for something? Continue searching:

References: