FAA General Written TestChapter 3 · 40 practice questions

Chapter 3: Weight and Balance

Includes 6 animated diagrams — view them live in the interactive theory reader.

Chapter: Weight and Balance

Overview

Weight and balance control is one of the most critical safety functions in aircraft maintenance. Every aircraft has certificated weight and center of gravity (CG) limits that must be respected for safe operation. The maintenance technician plays a vital role in ensuring that the aircraft's empty weight and CG are accurately documented, that any modifications or repairs are properly reflected in the weight and balance records, and that the aircraft is returned to service in compliance with its approved operating limitations. This chapter covers the fundamental principles, regulatory requirements, and practical procedures for weight and balance control as they apply to the aircraft maintenance engineer.

Key Concepts

1. Definitions and Terminology

Weight and Balance Definitions - Datum, Arm, Moment, CG Weight & Balance — Definitions and Terminology DATUM Station 0.0 Sta 100 Sta 200 Sta 300 Sta 400 Sta 500 CARGO 85 lb ARM = 120 in (Distance from datum) MOMENT (M = W × A) 10,200 in-lb (85 lb × 120 in) CG DATUM Imaginary vertical plane from which all arms are measured (AC 43.13-1B) ARM Horizontal distance from datum to the item's CG, measured in inches MOMENT Weight × Arm; the force that causes rotation about the CG (in-lb) CENTER OF GRAVITY Point where total aircraft weight is considered to be concentrated 14 CFR Part 43, AC 43.13-1B — Weight & Balance Control M = W × A   |   CG = Total Moment ÷ Total Weight

Empty Weight: The weight of the aircraft including all fixed equipment, unusable fuel, full oil, and hydraulic fluid. The empty weight is defined by the Type Certificate Data Sheet (TCDS) and includes all items that are considered part of the aircraft's standard configuration.

Maximum Ramp Weight: The maximum weight authorized for ground operations, including taxi fuel. This is a structural limit that cannot be exceeded under any circumstances.

Maximum Takeoff Weight (MTOW): The maximum weight authorized for takeoff. This is a certificated structural limit.

Maximum Landing Weight: The maximum weight authorized for landing. The CG must remain within limits for all phases of flight, including landing.

Center of Gravity (CG): The point about which the aircraft would balance if it were possible to suspend it at that point. The CG is expressed as a distance from a reference datum, measured in inches.

Datum: An imaginary vertical plane from which all horizontal distances are measured for weight and balance calculations. The datum may be located at the nose of the aircraft, forward of the nose, or at some other reference point specified in the aircraft's type certificate data sheet.

Weight and Balance CG Computation — Animated Worked Example Weight and Balance Calculations — CG Computation Worked Example Station (inches from datum) 0 20 40 60 80 100 120 140 Datum 40 lb Sta 20 25 lb Sta 55 30 lb Sta 85 CG 52.4 Step 1 — Initial CG Computation Item Weight (lb) Arm (in) Moment Wing group 40 20.0 800 Fuselage group 25 55.0 1,375 Tail group 30 85.0 2,550 TOTAL 95 — 4,725 CG = Total Moment ÷ Total Weight CG = 4,725 ÷ 95 = 49.7 in Step 2 — Add Avionics Unit (12 lb at Sta 68.0) AV-12 Item Weight (lb) Arm (in) Moment Previous empty weight 95 49.7 4,725 Avionics unit +12 68.0 +816 NEW TOTAL 107 — 5,541 New CG = 5,541 ÷ 107 = 51.8 in New CG 51.8 Moment = Weight × Arm | CG = ΣM ÷ ΣW | Per AC 43.13-1B Ch.10 FAA A&P Weight & Balance — Airframe

Moment: The product of a weight and its arm (distance from the datum). Moments are expressed in pound-inches (lb-in) and are used to calculate the CG.

Arm (Station): The horizontal distance from the datum to the center of gravity of an item. Arms are expressed in inches.

CG Limits: The forward and aft limits within which the CG must fall for safe operation. These limits are established during certification and are published in the aircraft's flight manual or type certificate data sheet.

Useful Load: The difference between the maximum takeoff weight and the empty weight. This includes pilot, passengers, cargo, and fuel.

2. The CG Envelope

CG Envelope - Weight and Balance Diagram Weight & Balance — CG Envelope Center of Gravity (% MAC) Gross Weight (lbs) 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60% 4000 3000 2000 1000 0 CG ENVELOPE (Normal Category) FWD LIMIT AFT LIMIT Fwd baggage max: 300 lbs Aft baggage max: 200 lbs STA 0 STA 100 STA 200 FUEL BAG BAG CG ⚠ OUTSIDE CG LIMITS AIRCRAFT ATTITUDE Scenario: Baggage + Fuel Loading CG Position Limits 14 CFR §23.25 / AC 43.13-1B — Airplane must be operated within approved CG envelope

The CG envelope is a graphical representation of the allowable CG positions as a function of aircraft weight. The envelope is defined by the forward and aft CG limits and the maximum and minimum weight limits. The aircraft must be operated within this envelope for all phases of flight—takeoff, cruise, and landing.

Forward and Aft CG Limits - Weight and Balance Forward and Aft CG Limits — Weight & Balance AC 43.13-1B / 14 CFR §23.23 TAKEOFF CRUISE LANDING FWD LIMIT STA 350.0 AFT LIMIT STA 450.0 CG ENVELOPE CG 380.0 CG 410.0 CG 440.0 CG 465.0 ! LOADING ERROR! FUEL BURN CG DATA (STA) Takeoff: 380.0 Cruise: 410.0 Landing: 440.0 Violation: 465.0 FWD Limit: 350.0 AFT Limit: 450.0 KEY PRINCIPLE: CG must remain within forward/aft limits for ALL phases of flight. As fuel burns, CG drifts aft — verify loading for each phase. FUEL QUANTITY: (decreasing during flight) 350 450 Normal CG travel — within limits (FAA Weight & Balance Handbook, Chapter 2)

The CG envelope is a certificated limitation. Operating the aircraft outside this envelope is a violation of the aircraft's operating limitations and is prohibited by regulation. The maintenance technician must ensure that any weight and balance data provided to the operator reflects the true empty weight and CG so that proper loading can be accomplished.

3. Weight and Balance Records

The weight and balance record is a permanent record that documents the aircraft's empty weight, empty weight CG, and the location of all equipment installed on the aircraft. This record must be updated whenever:

  • Equipment is added, removed, or relocated
  • Repairs are made that change the weight of the aircraft
  • The aircraft is reweighed
  • Any alteration is made that affects weight or balance

The record must be accurate and current. An aircraft with inaccurate or incomplete weight and balance records is not airworthy and must not be returned to service.

4. The Weighing Procedure

FAA A&P Weight and Balance - The Weighing Procedure Weight and Balance — The Weighing Procedure AC 43.13-1B Chapter 10 · Leveling, Configuration, and CG Determination LVL PTS DATUM CG MOMENT ARM SCALE 1 NOSE SCALE 2 L MAIN SCALE 3 R MAIN 325 lb 840 lb 835 lb Tare: −12 lb Tare: −15 lb Tare: −15 lb CONFIGURATION CHECKLIST Full oil ✓ Residual fuel only ✓ Std. equipment ✓ Control surfaces neutral ✓ Aircraft leveled ⚠ UNLEVEL ATTITUDE ERROR Tail-low shifts moment arms aft, causing CG error of 0.5–1.5 in ✓ CORRECT CG CALCULATION CG = Total Moment / Total Weight = 42.3 in aft of datum WEIGHT & BALANCE FORMULAS Moment = Weight × Arm CG = Total Moment ÷ Total Weight Max allowable CG error: ±0.1 in Level within 1° per AC 43.13-1B Note: Weighing must be performed in a closed hangar · Brakes released · Landing gear down Three-point weighing: nose + left main + right main

Weighing an aircraft requires careful preparation and adherence to established procedures to ensure accurate results.

Aircraft Configuration: The aircraft must be weighed in a condition that reflects its defined empty weight. This typically includes:

  • Full oil
  • Residual (unusable) fuel
  • All standard equipment installed
  • Control surfaces in the neutral position
  • Rotor blades installed (for helicopters, as they are standard equipment)

Leveling: The aircraft must be in a level attitude during weighing. This is critical because the distances from the datum to the weighing points (moment arms) are measured along the longitudinal axis. If the aircraft is not level, these distances will be incorrect, leading to an erroneous CG calculation.

The aircraft should be leveled using the manufacturer's specified leveling points, which are typically defined in the maintenance manual. These may be designated jack points, leveling lugs, or specific locations on the fuselage. Using a cabin floor or wing spar may not provide accurate leveling, particularly if the floor is worn or deformed. When leveling on a surface that has local irregularities, a longer straightedge or leveling device that spans the uneven area should be used to ensure the level indicates the true aircraft attitude.

Scales: The aircraft is weighed using calibrated scales placed under each wheel or jack point. The scales must be zeroed before weighing to ensure accurate readings. Scales that are not zeroed will produce inaccurate weight data, invalidating the entire weighing procedure.

Weighing Points: The distances from the datum to each weighing point must be measured accurately. These distances are the arms used in the CG calculation.

Temporary Equipment Removal: If any equipment that is part of the empty weight is temporarily removed for weighing (e.g., an APU removed for repair), a ballast of equal weight must be installed in its place. This ensures that the recorded empty weight reflects the aircraft's normal configuration.

5. Weight and Balance Calculations

The fundamental calculations in weight and balance are:

Total Weight: The sum of all individual weights.

Total Moment: The sum of all individual moments (weight × arm).

Center of Gravity: The total moment divided by the total weight.

CG = Total Moment / Total Weight

When equipment is added, removed, or relocated, the new CG is calculated using the formula:

New CG = (Old Moment + Change in Moment) / New Total Weight

Where:

  • Old Moment = Old Empty Weight × Old Empty Weight CG
  • Change in Moment = Added Weight × Arm (positive for added weight, negative for removed weight)

Example: An aircraft has an empty weight of 2,850 pounds and an empty weight CG of 40.5 inches. A new avionics unit weighing 12 pounds is installed at station 68.0.

  • Old Moment = 2,850 × 40.5 = 115,425 lb-in
  • New Moment = 12 × 68.0 = 816 lb-in
  • Total Moment = 115,425 + 816 = 116,241 lb-in
  • New Weight = 2,850 + 12 = 2,862 lb
  • New CG = 116,241 / 2,862 = 40.62 inches

Weight Shift: When weight is added aft of the existing CG, the CG moves aft. When weight is added forward of the existing CG, the CG moves forward. The magnitude of the shift depends on the amount of weight added and its distance from the existing CG.

Example: An aircraft with an empty weight of 5,000 pounds and an empty weight CG of 95.00 inches has a 25-pound battery installed at station 200.0.

  • Old Moment = 5,000 × 95 = 475,000 lb-in
  • New Moment = 25 × 200 = 5,000 lb-in
  • Total Moment = 475,000 + 5,000 = 480,000 lb-in
  • New Weight = 5,025 lb
  • New CG = 480,000 / 5,025 = 95.52 inches

The CG moved aft by 0.52 inches.

Three-Point Weighing: When an aircraft is weighed on three scales (nose and two mains), the CG is calculated as:

  • Total Weight = Nose Weight + Left Main Weight + Right Main Weight
  • Total Moment = (Nose Weight × Nose Arm) + (Left Main Weight × Main Arm) + (Right Main Weight × Main Arm)
  • CG = Total Moment / Total Weight

Example: Nose wheel = 450 lb, Left main = 600 lb, Right main = 625 lb. Nose arm = 35 inches, Main arm = 78 inches.

  • Total Weight = 450 + 600 + 625 = 1,675 lb
  • Total Moment = (450 × 35) + (600 × 78) + (625 × 78) = 15,750 + 46,800 + 48,750 = 111,300 lb-in
  • CG = 111,300 / 1,675 = 66.45 inches

Regulatory Requirements

14 CFR 91.9 — Civil Aircraft Flight Manual, Marking, and Placard Requirements

This regulation requires that no person may operate a civil aircraft without complying with the operating limitations specified in the approved flight manual, markings, and placards. Weight and CG limits are operating limitations. Operating an aircraft outside these limits is a violation of this regulation.

14 CFR 43.9 — Content, Form, and Disposition of Maintenance Records

This regulation requires that after any maintenance, the person performing the work must make an entry in the maintenance records. The entry must include:

  • A description of the work performed
  • The date of completion
  • The name of the person performing the work
  • If the work involves a weight and balance change, the new empty weight and CG must be recorded

Failure to update the weight and balance records after a weight change is a direct violation of this regulation.

14 CFR 43.15 — Additional Performance Rules for Inspections

This regulation requires that inspections be performed in accordance with the applicable maintenance manuals and standard practices. An annual inspection must include a weight and balance check if the aircraft's weight and balance records are not current or if there is reason to believe the records are inaccurate.

14 CFR 91.409 — Inspections

This regulation requires that no person may operate an aircraft unless an annual inspection has been performed within the preceding 12 calendar months. If the annual inspection is overdue, the aircraft is not airworthy and cannot be returned to service until the inspection is completed.

AC 43.13-1B — Acceptable Methods, Techniques, and Practices

Chapter 10 of this advisory circular provides detailed guidance on weight and balance procedures, including:

  • Weighing procedures
  • Leveling methods
  • Equipment configuration during weighing
  • Calculation methods
  • Record-keeping requirements

Common Relationships and Principles

Weight Changes and CG Movement

Weight Changes and CG Movement - Aircraft Loading Diagram Weight Changes and CG Movement — Adding/Removing Cargo FS 40 FS 80 FS 120 FS 160 FS 200 Datum CG₁ FS 95.0 CG₂ FS 112.4 CG₃ FS 87.3 +W +50 lb ΔM = +8,700 -W -40 lb ΔM = -5,320 Weight Shift Formula (AC 43.13-1B, Ch. 10) ΔCG = (W × D) ÷ Total Weight Where: W = weight moved, D = distance moved, Total Weight = aircraft gross weight Aft cargo example: ΔCG = (50 lb × 120 in) ÷ 2,300 lb = 6,000 ÷ 2,300 = +2.6 in (CG moves aft) New CG: FS 95.0 + 2.6 = FS 97.6 (within limits if ≤ FS 110.0) Forward cargo example: ΔCG = (40 lb × 100 in) ÷ 2,300 lb = 4,000 ÷ 2,300 = -1.7 in (CG moves forward) +2.6 in -1.7 in CG Limits (typical) Forward: FS 85.0 Aft: FS 110.0 Legend Original CG Aft CG (+W) Fwd CG (-W)
  • Adding weight increases the total weight and changes the CG. If weight is added forward of the existing CG, the CG moves forward. If added aft, the CG moves aft.
  • Removing weight decreases the total weight and changes the CG. If weight is removed forward of the existing CG, the CG moves aft. If removed aft, the CG moves forward.
  • Moving weight from one location to another shifts the CG in the direction of the movement.

Empty Weight CG and Loaded CG

The empty weight CG is the CG of the aircraft with no crew, passengers, cargo, or usable fuel. The loaded CG is the CG with all items loaded. If the empty weight CG is within limits but the loaded CG is out of limits, the issue is typically improper loading—too much weight in a baggage compartment, for example, can shift the CG out of limits.

Maximum Weight and Useful Load

If the empty weight increases due to accumulated repairs or equipment changes, the useful load decreases. The maximum takeoff weight is a structural limit that cannot be increased without a type certificate change. If the empty weight is too high, non-essential equipment may be removed to bring the aircraft back to its design empty weight.

CG Limits and All Phases of Flight

The CG must remain within limits for all phases of flight, including landing. A CG that is within limits at takeoff but moves out of limits after fuel burn is unacceptable. The loading must be arranged to ensure CG compliance throughout the flight.

Forward and Aft CG Limits

The forward CG limit is typically established to ensure adequate elevator authority for flare and rotation. The aft CG limit is established to ensure adequate stability and control. Operating outside either limit can result in loss of control.

Practical Considerations for the AME

When to Reweigh

The aircraft should be reweighed when:

  • The weight and balance records are missing or incomplete
  • Significant equipment changes have been made without proper documentation
  • The records are suspected of being inaccurate
  • The aircraft has undergone major repairs or alterations
  • The annual inspection reveals discrepancies in the records

Corrective Actions for Out-of-Limits Conditions

If the empty weight CG is outside the approved limits, the aircraft is not airworthy and must not be returned to service. Corrective actions include:

  • Adding ballast at an approved location to bring the CG within limits
  • Removing or relocating equipment
  • Performing approved modifications

If the maximum weight is exceeded, the excess weight must be removed. This may involve removing non-essential equipment or ballast.

Documentation

All weight and balance changes must be documented in the aircraft's permanent records. The documentation must include:

  • The date of the change
  • A description of the work performed
  • The new empty weight and CG
  • The signature and certificate number of the person performing the work

A FAA Form 337 is required for major repairs and alterations, but routine equipment changes that affect weight and balance must still be recorded in the maintenance records.

Accuracy and Precision

Weight and balance calculations must be performed with precision. Small errors in scale readings, arm measurements, or calculations can result in significant CG errors. The technician must:

  • Use calibrated scales that are zeroed before weighing
  • Measure arms accurately
  • Use the manufacturer's specified leveling points
  • Verify all calculations

Summary

Weight and balance control is a fundamental responsibility of the aircraft maintenance engineer. The technician must understand the definitions, calculations, and regulatory requirements associated with weight and balance to ensure that the aircraft is returned to service in an airworthy condition. Accurate records, proper weighing procedures, and correct calculations are essential to maintaining the safety and legality of aircraft operations.

Practice this chapter

Reinforce Weight and Balance with 40 FAA-style practice questions, matched to your weak areas.

Try Free Practice QuestionsView Pricing