6.2 Aircraft Weighing Procedures, Scale Reactions & Tare Weight

6.2 Aircraft Weighing Procedures, Scale Reactions & Tare Weight

Periodically weighing an aircraft is necessary to establish an accurate baseline Basic Empty Weight and Empty Weight Center of Gravity (EWCG). Over time, aircraft gain unrecorded weight from dirt accumulation, moisture absorption in soundproofing insulation, structural repairs, paint touch-ups, repainting, and undocumented wiring additions. Under 14 CFR Part 43, 14 CFR Part 91, and FAA-H-8083-1B, an accurate weight and balance report is a legal condition for airworthiness. Precision weighing requires strict environmental control, meticulous fluid handling, correct leveling per the Type Certificate Data Sheet (TCDS), and rigorous subtraction of tare weight.

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1. Aircraft Preparation & Environmental Control for Weighing

Errors introduced during the weighing process compromise all future weight and balance calculations. The technician must follow a strict pre-weighing checklist:

                      PRE-WEIGHING AIRFRAME CHECKLIST
  ┌────────────────────────────────────────────────────────────────────────┐
  │  [✓] Closed, Draft-Free Hangar (Doors sealed, HVAC fans OFF)           │
  │  [✓] Airframe Washed & Dried (No mud, grease, ice, or pooled water)    │
  │  [✓] Cabin Cleaned (All foreign tools, loose flight gear removed)      │
  │  [✓] Usable Fuel Drained in Level Attitude (Unusable fuel remains)     │
  │  [✓] Engine Oil Verified Full (Part 23/25) or Drained (CAR 3)          │
  │  [✓] Full Hydraulic Fluid & Other Closed Operating Systems             │
  │  [✓] Flight Controls Locked in Neutral / Flaps Retracted               │
  │  [✓] Landing Gear Down & Locked / Oleo Struts at Normal Extension      │
  │  [✓] Doors, Cowlings, & Access Inspection Panels Installed & Latched   │
  └────────────────────────────────────────────────────────────────────────┘

Environmental Conditions

• Hangar Closure: Weighing must be conducted inside a fully enclosed hangar with all hangar doors tightly shut. Heating, ventilation, and air-conditioning (HVAC) fans must be turned off. Even a gentle $5\text{ mph}$ breeze or thermal convection draft blowing across an aircraft's wing produces aerodynamic lift or downforce, shifting hundreds of pounds across the scale load cells.
• Airframe Cleanliness: The exterior must be washed clean of mud, grease, and grime, and allowed to dry completely. Water trapped inside control surface balance bays or fuselage belly bilges introduces false weight.
• Inventory Verification: All non-standard equipment (e.g., pilot headsets, charts, emergency survival kits, tool bags, removable floor mats) must be removed. All permanently installed equipment listed on the aircraft's Equipment List must be in place. If an equipment item is missing (e.g., a radio removed for bench repair), its weight and moment must be added mathematically.

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2. Fluid Management Standards During Weighing

Fluids represent a significant portion of an aircraft's total weight. The technician must ensure every fluid system complies with the aircraft's type certification basis.

                         FLUID MANAGEMENT STANDARDS
  ┌────────────────────────────────────────────────────────────────────────┐
  │                        AVIATION FUEL WEIGHTS                           │
  │  • 100LL Aviation Gasoline (Avgas):   6.0 lbs / U.S. Gallon            │
  │  • Turbine Fuel (Jet A / Jet A-1):    6.7 lbs / U.S. Gallon            │
  │  • Military Jet Fuel (JP-4):          6.5 lbs / U.S. Gallon            │
  │  • Potable / System Water:            8.35 lbs / U.S. Gallon           │
  ├────────────────────────────────────────────────────────────────────────┤
  │                        ENGINE OIL WEIGHTS                              │
  │  • Aviation Lubricating Oil:          7.5 lbs / U.S. Gallon            │
  │                                       1.875 lbs / U.S. Quart           │
  └────────────────────────────────────────────────────────────────────────┘

Fuel Draining vs. Topping Protocol

• Preferred Method (Draining Usable Fuel): Place the aircraft in the level flight attitude and drain all fuel from the fuel tank sumps and main fuel system drain valves. The fuel remaining in the lines and tank bottoms is unusable fuel, which is legally part of the Basic Empty Weight.
• Alternative Method (Full Fuel Subtraction): If draining is impractical, the tanks may be filled completely to the brim ("topped off") to a precisely known volume. The weight and moment of the usable fuel are then calculated and subtracted from the scale totals: $\text{Usable Fuel Weight} = \text{Usable Capacity (gal)} \times 6.0\text{ lb/gal (Avgas)} \quad [\text{or } 6.7\text{ lb/gal (Jet A)}]$ $\text{Usable Fuel Moment} = \text{Usable Fuel Weight} \times \text{Fuel Tank Station Arm}$
• Prohibited Practice: Partially filled tanks must NEVER be weighed, because fuel sloshing within the tanks makes accurate scale reading impossible and the exact volume/arm cannot be determined.

Engine Lubricating Oil Protocol

• 14 CFR Part 23 / 25 Aircraft: Engine oil tanks/sumps must be completely full during weighing. If weighed with oil drained, the full weight ($7.5\text{ lb/gal}$ or $1.875\text{ lb/qt}$) and moment must be added to the weight and balance report.
• Legacy CAR 3 Aircraft: Certified with undrainable oil included in empty weight. If weighed with full oil, the weight of the drainable oil ($7.5\text{ lb/gal}$) at the engine oil tank station must be subtracted from the scale results to establish the CAR 3 empty weight.

Other Aircraft Fluids

• Hydraulic Fluid: Hydraulic reservoirs and brake systems must be completely full of fluid (e.g., MIL-PRF-5606 / Skydrol) with accumulators properly pre-charged.
• Lavatory and Potable Water Systems: Lavatory waste tanks and potable water tanks must be completely drained before weighing.

3. Aircraft Leveling Protocols and TCDS Specifications

An aircraft must be in a level flight attitude when weighed. If weighed in an unlevel attitude, horizontal arms become foreshortened, components shift relative to the datum, and the calculated center of gravity will contain major errors.

                         AIRCRAFT LEVELING METHODS
                         
    Method 1: Spirit Level on Leveling Lugs / Seat Tracks
    ┌──────────────────────┐
    │      ( • ) ( • )     │  <--- Spirit level bubble centered
    └──────────────────────┘
    ════════════════════════  <--- Fuselage Leveling Bracket / Seat Track
    
    Method 2: Plumb Bob over Leveling Grid Plate
             │ (Ceiling Suspension Point)
             │
             │ (Plumb Line Cord)
             │
             ▼ (Plumb Bob Tip)
         ┌───────┐
         │ ──┼── │  <--- Plumb bob tip aligns precisely with grid crosshair
         └───────┘

Leveling Means Specified in TCDS

The Type Certificate Data Sheet (TCDS) or Aircraft Maintenance Manual (AMM) explicitly defines the mandatory leveling points for each airframe model:

1. Spirit Level on Leveling Lugs: Dedicated leveling lugs or pins machined into the fuselage structure or engine mount.
2. Fuselage Seat Track / Floor Sill: Placing a calibrated machinist's spirit level longitudinally and laterally across specified seat rail locations.
3. Plumb Bob and Datum Plate: Suspending a plumb bob from an overhead fuselage bracket and centering the bob point directly over an inscribed target crosshair on a lower floor plate.
4. Leveling Screws: Two or more leveling screws installed on the external fuselage skin used with a spirit level.

Achieving the Level Attitude

• Wheel Scale Weighing: Adjust level by deflating or inflating the nosewheel oleo strut, adjusting tire pressures, or placing calibrated ramps/shims under the landing gear wheels.
• Jack Weighing: When weighing atop jacks with load cells, adjust the height of individual mechanical screw jacks or hydraulic jacks until the spirit level or plumb bob indicates perfect longitudinal and lateral level.

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4. Weighing Equipment: Platform Scales and Electronic Load Cells

Precision scales used for aircraft weighing fall into two main categories:

1. Mechanical Beam / Platform Scales

• Platform scales use mechanical balance beams, knife-edge pivots, and counterweights.
• The aircraft is rolled up ramps onto low-profile wheel platforms.
• Operational Check: The floor beneath the platform scales must be smooth, hard, and level to prevent beam binding.

2. Electronic Strain-Gauge Load Cells

• Modern electronic load cells use bonded resistance strain gauges configured in a Wheatstone bridge circuit. When compressed by aircraft weight, the strain gauges deform microscopically, changing electrical resistance proportionally to applied load.
• Jack-Pad Weighing: Load cells are placed atop tripod jack heads, and the aircraft is lifted off the ground by its certified jack pads.
• Platform Wheel Cells: Low-profile electronic platforms placed beneath each wheel.
• Calibration Mandate: All scales and load cells must have been calibrated within their certified periodic inspection intervals (typically annually) using standards traceable to the National Institute of Standards and Technology (NIST). Electronic load cells must be energized to warm up and zero-balanced prior to loading.

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5. Tare Weight Definition and Subtraction Mathematics

Tare Weight is the weight of any extraneous equipment located on the scale platforms that is NOT part of the aircraft being weighed.

                         TARE WEIGHT ACCOUNTING
                         
  ┌────────────────────────────────────────────────────────────────────────┐
  │  Items Classified as TARE:                                             │
  │  • Wheel chocks used to prevent rolling on scale platforms             │
  │  • Jack adapters, safety locking rings, and leveling shims             │
  │  • Hoisting slings, tie-down chains, and ground cables                 │
  │  • Drip pans placed beneath engine sumps during weighing               │
  ├────────────────────────────────────────────────────────────────────────┤
  │  FUNDAMENTAL RULE OF TARE:                                             │
  │  Tare weight is ALWAYS SUBTRACTED from the gross scale reading.        │
  │  Tare is NEVER added.                                                  │
  │                                                                        │
  │  Net Scale Reaction = Gross Scale Reading - Tare Weight                │
  └────────────────────────────────────────────────────────────────────────┘

Calculating Net Scale Reactions

For every scale reaction point (Left Main, Right Main, Nose/Tail Wheel): $\text{Net Weight}_{\text{Nose}} = \text{Scale Reading}_{\text{Nose}} - \text{Tare}_{\text{Nose}}$ $\text{Net Weight}_{\text{Left Main}} = \text{Scale Reading}_{\text{Left Main}} - \text{Tare}_{\text{Left Main}}$ $\text{Net Weight}_{\text{Right Main}} = \text{Scale Reading}_{\text{Right Main}} - \text{Tare}_{\text{Right Main}}$ $\text{Total Aircraft Net Weight } (W) = \text{Net}_{\text{Nose}} + \text{Net}_{\text{Left Main}} + \text{Net}_{\text{Right Main}}$

6. Worked Numerical Examples

Example 1: Scale Reaction Data and Net Empty Weight with Multiple Wheel Tares

Scenario: A single-engine tricycle gear aircraft is placed on platform scales inside a closed hangar to determine its net weight. Wheel chocks and leveling shims are used on the scale platforms. The scale recordings are as follows:

• Nose Wheel Scale: Gross reading = $486.0\text{ lb}$; Tare weight (chocks & shim) = $14.0\text{ lb}$
• Left Main Wheel Scale: Gross reading = $825.0\text{ lb}$; Tare weight (chocks) = $6.0\text{ lb}$
• Right Main Wheel Scale: Gross reading = $818.0\text{ lb}$; Tare weight (chocks) = $6.0\text{ lb}$

Calculate the net reaction at each scale point, the total main gear reaction, and the total net weight of the aircraft.

Step-by-Step Mathematical Solution:

1. Calculate the net scale reaction for each point by subtracting tare: $\text{Net Nose Reaction } (N) = 486.0\text{ lb} - 14.0\text{ lb} = 472.0\text{ lb}$ $\text{Net Left Main Reaction } (L_M) = 825.0\text{ lb} - 6.0\text{ lb} = 819.0\text{ lb}$ $\text{Net Right Main Reaction } (R_M) = 818.0\text{ lb} - 6.0\text{ lb} = 812.0\text{ lb}$
2. Calculate the combined net main gear reaction ($M$): $M = L_M + R_M = 819.0\text{ lb} + 812.0\text{ lb} = 1,631.0\text{ lb}$
3. Calculate the total net weight of the aircraft ($W$): $W = N + M = 472.0\text{ lb} + 1,631.0\text{ lb} = 2,103.0\text{ lb}$ Summary Table:

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Example 2: Weighing with Full Fuel Deduction to Determine Basic Empty Weight

Scenario: A utility aircraft (14 CFR Part 23) is weighed with full fuel tanks because draining was not possible. The aircraft contains $50.0\text{ U.S. gallons}$ of total fuel capacity, of which $46.0\text{ gallons}$ is usable Avgas ($6.0\text{ lb/gal}$) and $4.0\text{ gallons}$ is unusable fuel. The fuel tank station is at $\text{Station } +72.0\text{ in}$. Engine oil is verified full.

• Net scale weight as weighed (including full fuel) = $2,450.0\text{ lb}$ with a total moment of $164,150.0\text{ in-lb}$.
• Calculate the true Basic Empty Weight (BEW) and the Empty Weight Center of Gravity (EWCG).

Step-by-Step Mathematical Solution:

1. Identify usable fuel quantity and weight: $\text{Usable Fuel Volume} = 46.0\text{ U.S. gallons}$ $\text{Usable Fuel Weight} = 46.0\text{ gal} \times 6.0\text{ lb/gal} = 276.0\text{ lb}$ (Note: The $4.0\text{ gallons}$ of unusable fuel must remain in the Basic Empty Weight)
2. Calculate usable fuel moment: $\text{Usable Fuel Moment} = 276.0\text{ lb} \times (+72.0\text{ in}) = +19,872.0\text{ in-lb}$
3. Subtract usable fuel weight and moment from as-weighed totals: $\text{Basic Empty Weight (BEW)} = 2,450.0\text{ lb} - 276.0\text{ lb} = 2,174.0\text{ lb}$ $\text{Basic Empty Moment} = 164,150.0\text{ in-lb} - 19,872.0\text{ in-lb} = +144,278.0\text{ in-lb}$
4. Compute the corrected EWCG: $\text{EWCG} = \frac{144,278.0\text{ in-lb}}{2,174.0\text{ lb}} = +66.365\text{ inches} \approx +66.37\text{ in}$ Conclusion: Basic Empty Weight is $2,174.0\text{ lb}$ at $\text{Station } +66.37\text{ in}$.