6.1 Weight & Balance Fundamentals, Scale Weighing & EWCG

6.1 Weight & Balance Fundamentals, Scale Weighing & EWCG

[!NOTE] Safety and Airworthiness Imperative: Weight and balance control is one of the most vital technical responsibilities entrusted to an FAA Inspection Authorization (IA) holder. Operating an aircraft outside its certified weight and center of gravity (CG) limits compromises aerodynamic stability, structural safety, and flight controllability. Under 14 CFR § 43.13, an IA must ensure that after any major repair, alteration, or periodic annual inspection, the aircraft's weight and balance records are accurate, up to date, and reflect actual empty weight and empty weight center of gravity (EWCG).

Title 14 of the Code of Federal Regulations (14 CFR) mandates that every certificated civil aircraft must be operated strictly within the weight and balance limitations established by the manufacturer and approved by the Federal Aviation Administration (FAA). For the holder of an Inspection Authorization (IA), weight and balance is not merely an operational pilot consideration—it is a foundational airworthiness standard. Every major alteration, structural repair, avionics upgrade, or engine replacement directly alters the mass distribution of the aircraft. Approving an aircraft for return to service without properly recalculating or physically measuring its empty weight and center of gravity violates federal safety standards and directly jeopardizes flight safety.

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Aerodynamic Principles of Weight, Balance, and Stability

Aircraft weight and balance directly controls flight performance, structural margins, and longitudinal pitch stability:

• Effects of an Out-of-Limit Forward CG: When the center of gravity is positioned too far forward, the aircraft possesses excessive longitudinal static stability, creating severe operational hazards. The nose becomes excessively heavy, requiring large up-elevator deflections to maintain level flight. This increases horizontal tail downforce, forcing the main wing to operate at a higher angle of attack and higher induced drag, which substantially increases the aircraft's stall speed. During landing flare, elevator control power may be insufficient to raise the nose at low approach airspeeds, resulting in dangerous hard landings on the nosewheel and potential structural failure of the nose gear assembly.
• Effects of an Out-of-Limit Aft CG: An aft CG condition represents one of the most lethal aerodynamic hazards in aviation. As the center of gravity moves rearward toward or past the aerodynamic center (neutral point), longitudinal static stability decreases toward neutral or becomes unstable. Elevator stick force per G is severely diminished, creating an extreme risk that the pilot will inadvertently overstress the airframe with minimal control pressure. Most critically, stall recovery becomes difficult or aerodynamically impossible: pitch trim authority is lost, elevator down-travel cannot overcome the nose-up moment, and the aircraft can easily enter an unrecoverable flat spin.
• Maximum Gross Takeoff Weight (MGTOW) Violations: Operating above certified gross weight decreases acceleration, lengthens takeoff and landing rolls, reduces climb gradient, lowers service ceiling, and compromises structural design limit load factors (+3.8g / -1.52g for normal category under 14 CFR Part 23) in turbulent air.

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Core Definitions, Coordinates, and Fundamental Formulas

Precise weight and balance calculations rely upon standardized terminology and physics definitions codified in FAA-H-8083-1B (Aircraft Weight and Balance Handbook) and AC 43.13-1B Chapter 10:

• Reference Datum: An imaginary vertical reference plane from which all horizontal distances are measured for weight and balance purposes. The datum is established by the manufacturer and specified in the Type Certificate Data Sheet (TCDS) or Aircraft Specifications. Common datum locations include the engine firewall, the tip of the propeller spinner, the nose of the fuselage, or a specified distance forward of the leading edge of the wing.
• Station: The horizontal distance, measured in inches, from the reference datum to any point along the longitudinal axis of the aircraft.
• Arm: The horizontal distance (station) from the reference datum to the center of gravity of an individual item or component. By universal convention, arms located aft of the datum are assigned a positive algebraic sign (+), while arms located forward of the datum are assigned a negative algebraic sign (-).
• Moment: The rotational turning effect produced by a weight acting at a specific arm about the datum. Calculated using the fundamental formula: $\text{Moment (in-lb)} = \text{Weight (lbs)} \times \text{Arm (inches)}$
• Center of Gravity (CG): The point about which an aircraft would balance if suspended in a level attitude. It represents the mass centroid of the entire aircraft and is calculated by dividing total moment by total weight: $\text{Center of Gravity (CG)} = \frac{\text{Total Moment}}{\text{Total Weight}}$
• Empty Weight: The weight of the airframe, engines, propellers, fixed operational equipment, unusable fuel, full operating fluids (including hydraulic fluid), and engine lubricating oil.
• Useful Load: The difference between Maximum Gross Takeoff Weight and Empty Weight: $\text{Useful Load} = \text{MGTOW} - \text{Empty Weight}$ Useful load encompasses flight crew, passengers, usable fuel, baggage, and deployable cargo.
• Payload: The load that produces revenue or accomplishes the operational mission, calculated as: $\text{Payload} = \text{Useful Load} - \text{Usable Fuel Weight} - \text{Required Flight Crew Weight}$

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Aircraft Weighing Procedures per FAA-H-8083-1B and AC 43.13-1B

When mathematical records become suspect, lost, or when major alterations render paper calculations uncertain, an aircraft must be physically weighed using calibrated platform or electronic load-cell scales. Strict adherence to standardized protocols is mandatory:

Weighing Environment and Configuration

The aircraft must be thoroughly cleaned, dried, and placed inside a closed hangar to eliminate wind gusts, drafts, and ambient thermal currents that produce fluctuating scale readings. All standard operational equipment must be installed in its approved location. Control surfaces must be locked in neutral, flaps retracted, doors closed, and brakes fully released to prevent side loads on the scales.

Fuel and Oil Configuration (CAR 3 vs. 14 CFR Part 23)

• Unusable Fuel: Aircraft empty weight includes unusable fuel. Before weighing, the fuel tanks should be drained in accordance with the manufacturer's instructions via low-point sumps until only the unusable fuel specified in the TCDS remains. If draining is impractical, the aircraft may be weighed with full fuel tanks; after weighing, the weight of usable fuel (computed at 6.0 lbs/gal for aviation gasoline or 6.7 lbs/gal for Jet-A) and its corresponding moment must be mathematically subtracted from the scale totals.
• Engine Lubricating Oil: A critical regulatory distinction exists between certification bases:
  • CAR 3 Aircraft: Under historic Civil Air Regulations Part 3, aircraft empty weight included only undrainable oil. Full operating oil was considered part of the useful load. If a CAR 3 aircraft is weighed with full oil (7.5 lbs/gal), the full oil weight and moment must be deducted to arrive at certificated empty weight.
  • 14 CFR Part 23 Aircraft: Under Part 23, empty weight includes full system oil. If weighed with an empty oil tank, the weight of full oil and its moment must be added to the scale results.

Aircraft Leveling Standards

Accurate arm measurements require the aircraft to be in its exact longitudinal and lateral level flight attitude. Weighing an aircraft unlevel introduces severe cosine measurement errors and invalidates the horizontal stations. The TCDS and manufacturer maintenance manual prescribe the exact leveling method:

1. Leveling Lugs: Built-in alignment brackets on the fuselage;
2. Plumb Bob: Suspended from a designated overhead fitting to align over an index screw on the cabin floor; or
3. Spirit Level: Placed across designated leveling points, such as cockpit seat rails, fuselage leveling screws, or the canopy sill. Leveling is achieved by adjusting main and nose gear struts, varying tire pressures, or inserting precision leveling shims under the landing gear wheels on the scales.

Tare Weight Management

Tare weight includes any auxiliary equipment on the scales that is not part of the aircraft—such as chocks, safety jacks, work stands, ground blocks, or leveling shims. Tare weight must always be subtracted from the gross scale indication to obtain net weight: $\text{Net Weight} = \text{Scale Reading} - \text{Tare Weight}$

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Scale Weighing Configuration & EWCG Formulas

Aircraft are typically weighed in a three-point configuration supported by three scales placed under the landing gear contact points:

Calculating EWCG for Tricycle (Nosewheel) Gear

When all arms are referenced directly to the datum: $\text{EWCG} = \frac{(\text{Left Main Net} \times \text{Arm}_{\text{main}}) + (\text{Right Main Net} \times \text{Arm}_{\text{main}}) + (\text{Nose Net} \times \text{Arm}_{\text{nose}})}{\text{Total Net Weight}}$

Alternatively, when using the main gear centerline as an intermediate reference with wheelbase $D$ (distance from main gear centerline to nosewheel axle) and datum at the main gear: $\text{EWCG} = \text{Arm}_{\text{main}} - \left(\frac{\text{Net Nose Weight} \times D}{\text{Total Net Weight}}\right)$

Calculating EWCG for Conventional (Tailwheel) Gear

$\text{EWCG} = \frac{(\text{Left Main Net} \times \text{Arm}_{\text{main}}) + (\text{Right Main Net} \times \text{Arm}_{\text{main}}) + (\text{Tail Net} \times \text{Arm}_{\text{tail}})}{\text{Total Net Weight}}$

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Comprehensive Worked Example: Tricycle Gear Weighing

A single-engine aircraft is weighed inside a closed hangar following an annual inspection and major structural repair. The reference datum is the engine firewall. The TCDS specifies that the main gear centerline is located at Station +60.0 inches and the nose gear axle is located at Station -10.0 inches (10 inches forward of the firewall).

Step 1: Record Scale Readings and Deduct Tare

Step 2: Compute Net Moments and EWCG

1. Main gear total weight: $810.0 + 815.0 = 1,625.0 \text{ lbs}$.
2. Main gear moment: $1,625.0 \text{ lbs} \times (+60.0 \text{ in}) = +97,500.0 \text{ in-lb}$.
3. Nose gear moment: $450.0 \text{ lbs} \times (-10.0 \text{ in}) = -4,500.0 \text{ in-lb}$.
4. Total net empty weight: $1,625.0 + 450.0 = 2,075.0 \text{ lbs}$.
5. Total net moment: $+97,500.0 + (-4,500.0) = +93,000.0 \text{ in-lb}$.
6. Empty Weight Center of Gravity: $\text{EWCG} = \frac{+93,000.0 \text{ in-lb}}{2,075.0 \text{ lbs}} = +44.82 \text{ inches aft of datum}$

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When Must the Records Change? The Negligible-Change Threshold

Not every equipment change forces a weight and balance revision. AC 43.13-1B, Chapter 10, paragraph 10-2(c) defines a negligible weight change as:

A negligible center of gravity change is a change of less than 0.05 percent MAC for fixed-wing aircraft, or 0.2 percent of the maximum allowable CG range for rotary-wing aircraft. Below those thresholds the empty weight and EWCG in the records need not be revised; above them, the change must be computed and recorded. On a 2,300-pound single, a two-pound avionics swap is therefore not negligible — a fact that catches technicians who assume "small equals ignorable."

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The TCDS EWCG Range and Adverse Check Exemption

In Section II of many general aviation Type Certificate Data Sheets, the FAA publishes an Empty Weight Center of Gravity (EWCG) Range.

• Significance: The EWCG range is an engineering design corridor established during type certification. If the newly calculated EWCG falls squarely within the published EWCG range, mathematical flight envelopes guarantee that the aircraft cannot be loaded beyond forward or aft flight CG limits under any authorized combination of passengers, baggage, and fuel.
• Relief from Adverse Checks: When an aircraft's EWCG is within this range, the IA is exempt from performing forward and aft adverse-condition loading checks. If the EWCG falls outside this corridor (or if the TCDS does not publish an EWCG range), formal forward and aft adverse checks (detailed in Section 6.2) are legally mandatory prior to return to service.

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High-Yield Exam Traps & Best Practices

• Tare Subtraction Rule: Tare weight must always be subtracted from gross scale readings. Adding tare or neglecting to subtract it is a catastrophic calculation error.
• Negative Arm Moments: Items located forward of the datum have negative arms. Multiplying a positive weight by a negative arm yields a negative moment (+W × -Arm = -Moment), which must be subtracted from the total moment.
• CAR 3 vs. Part 23 Oil Accounting: CAR 3 empty weight includes only undrainable oil, meaning full oil must be subtracted if weighed full; Part 23 empty weight includes full operating oil.
• Leveling Mandatory: Never attempt to calculate EWCG from scale readings taken while the aircraft rests in an unlevel static ramp attitude.