Weight and Balance 2

Equipment Required

  • Aircraft scales are the primary tools for weighing. These can include electronic scales that utilize calibrated pressure transducers or strain gauges to display weight. Common types of scales used are top-of-jack load cells and platform scales.

  • Additional equipment like a spirit level and plumb bob are crucial for ensuring the aircraft is correctly leveled before weighing. The TCDS (Type Certificate Data Sheet) specifies where to place the leveling means.

Preparation for Weighing

Before completing the weighing process, proper preparations are required:

  • The aircraft should be free of any debris, ice, or water that might affect the weight measurement.

  • Fuel should either be completely drained or fully filled in the tanks depending on the requirements (e.g., aircraft certified before 1978 need oil drainage).

  • Weight of wheel chocks, which are used to keep the aircraft stationary, should be accounted for, as this is considered tare weight that must be subtracted from the total scale reading.

  • GA (General Aviation) Aircraft: The FAA does not require them to be re-weighed after the factory weighing.

  • Commercial Aircraft: Required to be weighed every three years under CFR parts 125 and 135 regulations. Any alterations or new equipment installations that affect weight also require new weighing.

Weighing Procedure Details

  • Empty Weight: The base weight of the aircraft without any payload, which provides a reference for further calculations.

  • Longitudinal and Lateral CG: Measurements indicating the horizontal and vertical distribution of weight. Longitudinal CG pertains to the front-back balance while Lateral CG deals with side-to-side balance.

  • Maximum Weight: Includes passengers, cargo, and fuel, determining the heaviest safe weight for operation.

  • Minimum Fuel: Required for safe operations, calculated based on flight time. Usually 30 Mins

  • Respective Weight Measures:

    • Maximum Ramp Weight: Heaviest weight on the ground.

    • Takeoff Weight: Max permissible at takeoff.

    • Landing Weight: Max permissible at landing.

    • Zero Fuel Weight: Max weight excluding usable fuel, relevant for planning payload.

Aircraft Center of Gravity (CG)

Ballast (permanent or temporary) is weight that is added to move the CG (Center of Gravity) back into the forward and aft CG range.

CG Concepts

  • Center of Gravity: The point where the aircraft's weight is evenly balanced. Deviations from this point can lead to instability.

Moment Calculations

  • Weight and Moment: The moment is obtained by multiplying the weight by its arm (the distance from the datum).

Jacking the Aircraft

To ensure stability during weighing, the aircraft must be properly jacked with equal weight distribution in both wings. The process includes:

  1. Positioning a tripod jack under the nose jack fitting.

  2. Placing additional tripod jacks under the wing jack fittings.

  3. Lifting the aircraft simultaneously to avoid sideways loads.

  4. Lowering the jacks systematically after weighing is completed.

Leveling the Aircraft

Proper leveling is crucial to obtaining accurate weight readings. The airframe should be leveled using a calibrated spirit level or through tire and strut adjustments, depending on if lateral or longitudinal leveling is necessary. This procedure is vital as it helps in achieving an accurate determination of weight and CG.

Definition of Moment

  • The moment is calculated as the product of force (or weight) times distance from the datum. (Inch Pounds)

  • Moment formula: Moment = Weight × Distance (arm)

  • Distance (measured in inches) is from the datum; weights are gathered from the scales.

Empty Weight Center of Gravity (EWCG)

Definition and Calculation

  • EWCG: Empty Weight Center of Gravity; balance point for the aircraft in empty condition.

  • Calculated by dividing total moment by total weight.

Adjusting Empty Weight

Method to Adjust Weight

  • Adjust empty weight by adding/removing weights to ensure accurate readings.

    • For example, adding or removing items (wheel chocks, fire extinguisher, full fuel tanks) affects total weights

Center of Gravity Range Considerations

TCDS Information

  • TCDS (Type Certificate Data Sheet) will not list the empty weight or EWCG range; it lists CG ranges for loaded aircraft only.

  • Understand that these ranges are critical for safe flying and landing of the aircraft.

Loading and CG Limits

  • CG is measured in inches from datum, and limitations must be adhered to for safe operation.

Formulas

  • Nosewheel airplanes with datum forward of the main wheels:

    • CG = D - (F * L)

  • Nosewheel airplanes with datum aft of the main wheels:

    • CG = -(D + F * L)

  • Tailwheel airplanes with datum forward of the main wheels:

    • CG = D + (R * L) / W

  • Tailwheel airplanes with datum aft of the main wheels:

    • CG = -D + (R * L) / W

Permanent Ballast for CG Correction

Mean Aerodynamic Chord (MAC)

Definition and Importance

  • The Mean Aerodynamic Chord is critical for determining CG as a percentage of MAC.

  • Calculating CG involves determining its position relative to LEMAC (Leading Edge Mean Aerodynamic Chord) and TEMAC (Trailing Edge Mean Aerodynamic Chord).

  • Image 1: Weight and Balance Record

    • Empty Weight:

      • Defined as the weight of the aircraft with all required equipment at a fixed location.

      • Includes unusable fuel and oil (for aircraft certified after 1978).

      • Determined by placing the aircraft on scales.

    • Scale Data for Cessna 172:

      • Left Wheel: 658 lbs

      • Right Wheel: 620 lbs

      • Nose Wheel: 450 lbs

      • Total Net Weight: 1,728 lbs

    • Datum Reference:

      • Measured at Station 0.0, typically at the firewall or front face of the aircraft.

    Image 2: Empty Weight Center of Gravity (EWCG)

    • EWCG:

      • The point where the aircraft is balanced longitudinally and laterally when empty.

    • Useful Load:

      • Calculated by subtracting the empty weight from the maximum gross weight.

      • Includes usable fuel, crew, passengers, cargo, and other provisions.

    • Minimum Fuel:

      • Defined as the fuel required for 30 minutes of flight at cruise power.

      • Formula: Minimum Fuel = METO (Maximum Except Takeoff Horsepower) ÷ 2

    Image 3: Empty Weight Components

    • Residual Fuel:

      • Includes fuel trapped in lines and at the bottom of tanks (unusable fuel).

      • Must be included in the empty weight as per Type Certificate Data Sheet (TCDS).

    • 14 CFR Part 23 Regulations:

      • Aircraft certified after 1978 must include oil and hydraulic fluid at full capacity in the empty weight.

    Key Takeaways

    1. Empty Weight Importance:

      • Serves as a baseline for weight and balance calculations and must include all fixed equipment and fluids.

    2. EWCG Significance:

      • Essential for stability; ensures the aircraft's center of gravity falls within allowable limits.

    3. Residual and Minimum Fuel:

      • Both are critical for safety and must be factored into weight calculations.

    4. Compliance with Regulations:

      • Aircraft certified post-1978 must follow specific rules for including fluids in the empty weight.Maximum Landing Weight

        • The most an aircraft can weigh and safely land.

        • Aircraft often take off heavier than they can land, so they may need to jettison fuel in flight.

      • Maximum Zero Fuel Weight

        • The maximum weight of the aircraft, including all passengers, crew, and cargo, excluding usable fuel.

      • Empty Weight

        • The weight of the aircraft with all required equipment, including the airframe, powerplant, seats, fire extinguishers, oxygen bottles, fixed ballast, hydraulic fluid, and residual or unusable fuel and oil.

      • Maximum Weight

        • The maximum gross weight of the aircraft, including passengers, cargo, and fuel.

      • Maximum Ramp Weight

        • The heaviest the aircraft can be while on the ground, typically exceeding the maximum takeoff weight due to fuel burned during startup and taxi.

      • Maximum Takeoff Weight

        • The most weight an aircraft can have at the start of its takeoff roll.

        • Ramp weight is reduced by fuel consumption during startup and taxi.

      Center of Gravity (CG):

      • Definition:
        The point where the aircraft is balanced along both longitudinal and lateral axes.

      • Key Notes:

        • The center of gravity is not the physical center of the aircraft.

        • Moment arms may differ in length, and weights may vary, but moments (force × distance) are balanced.

      Additional Things to Know:

      1. Impact of Weight on Aircraft:

        • Exceeding weight limits (e.g., maximum takeoff or landing weight) can compromise safety and structural integrity.

        • Fuel burn during flight helps to reduce the total weight for landing.

      2. Importance of CG Location:

        • A miscalculated CG can lead to instability, making the aircraft difficult or dangerous to control.

        • Proper weight distribution ensures balanced moments and smooth operation.

      3. Operational Considerations:

        • Pilots and crew calculate takeoff weight, ramp weight, and landing weight carefully before each flight to meet regulatory and safety standards.

        • Loading cargo, passengers, and fuel should always respect the aircraft's structural limits.

        • The datum of the aircraft is a vertical plane from which all horizontal measurements are taken.

        • Examples:

          • Datum at the tip of the spinner.

          • Datum at the firewall.

        • The manufacturer establishes the datum location. This information, along with all weight and balance details, is listed in the aircraft's type certificate data sheet (TCDS).

        2. Station Numbers (Image 2)

        • The station number represents a specific position of a component on an aircraft.

        • It is measured in inches from the datum:

          • Fuselage Stations (FS): Points aft of the datum.

          • Forward of the datum, stations are displayed as negative.

        • Key points:

          • Water Line (WL) is a vertical reference for vertical locations.

          • Fuselage Station 350.2, WL 165.5.

        3. Buttock Line (Image 3)

        • Buttock Line (BL): A lateral reference to identify locations left or right of the aircraft centerline.

        • Used for determining symmetrical or lateral positions.

        4. Moment (Image 4)

        • Moment Definition:

          • Product of a force (or weight) multiplied by a distance (arm).

        • To calculate a moment:

          • A force (weight) and a distance (moment arm) must be known.

          • Distance is expressed in inches from the pivot point (datum