12.2 Aircraft Weighing Preparation, Equipment & Tare Deduction

12.2 Aircraft Weighing Preparation, Equipment & Tare Deduction

Approved-Data Control

Values and examples explain principles. Current approved maintenance data, product instructions, organisation procedures, and applicable law control actual limits, materials, intervals, methods, and acceptance.

Determining an aircraft's exact empty weight and center of gravity through physical weighing is a mandatory airworthiness process. Over months and years of operation, an aircraft's empty mass inexorably changes due to structural repairs, dirt and grime accumulation, moisture absorption in acoustic and thermal insulation blankets, system modifications, component replacements, and repainting. To ensure that flight crews and dispatchers rely on verified, accurate baseline data, EASA Part-66 certifying engineers must master the regulatory re-weighing mandates, equipment calibration standards, environmental controls, airframe leveling techniques, and mathematical tare deductions governing aircraft weighing.

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Regulatory Re-Weighing Intervals (EASA Part-CAT, Part-ML & Part-NCO)

European aviation regulations mandate periodic physical re-weighing of aircraft to prevent cumulative weight-and-balance drift:

1. Commercial Air Transport (EASA Part-CAT / CAT.POL.MAB.100)

• Fleet or individual mass control: Apply the current operational rules and approved operator programme; the applicable weighing interval is not a universal Module 7 number.
• Fleet Weighing Sample: A representative sample of aircraft from the fleet must be weighed every 4 years. The size of the sample is strictly regulated based on the total number of aircraft in the fleet type.

2. General Aviation & Light Aircraft (EASA Part-ML & Part-NCO)

• For aircraft operated under Part-ML (sailplanes, balloons, piston aircraft under 2,730 kg MTOM, light rotorcraft), physical re-weighing is conducted according to the Aircraft Maintenance Programme (AMP).
• Mandatory Non-Scheduled Re-Weighing: Re-weighing is strictly mandatory—regardless of time since the last weighing—whenever:
  • The aircraft undergoes a major structural repair, cabin interior reconfiguration, or major avionics modification.
  • The aircraft is stripped and repainted.
  • The cumulative recorded weight-and-balance alterations introduce mass uncertainty exceeding 0.5% of the Maximum Takeoff Mass (MTOM), or when the cumulative CG change exceeds 0.5% of the Mean Aerodynamic Chord (MAC).
  • The aircraft is newly manufactured or imported from a non-EASA jurisdiction.

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Weighing Equipment: Technology, Types & Calibration

Three primary categories of weighing equipment are certified for aircraft weight-and-balance determinations:

                    AIRCRAFT WEIGHING TECHNOLOGIES

   [1. Electronic Platform Scales]         [2. Jacking Point Load Cells]
   - Roll-on wheel/tire platforms          - Compression cells atop jacks
   - Strain-gauge load cells               - Aircraft lifted off the ground
   - High capacity (up to 50,000 kg)       - High accuracy; no tire side loads
   - Brakes MUST be released!              - Locking collars must be secured

1. Electronic Platform Wheel Scales

Electronic platform scales are low-profile, portable steel or aluminium pads placed directly beneath the aircraft tires. The aircraft is towed, pushed, or winched up shallow ramps onto the platforms. Each platform contains internal precision strain-gauge load cells that convert compressive structural deformation into a proportional millivolt electrical signal, digitized and displayed on a central weight indicator.

• Critical Precaution: Once the aircraft is positioned on the platforms, wheel brakes must be completely released and chocks placed lightly against tires. If brakes remain engaged, side loads from gear strut compression or landing gear track changes (scrubbing) will induce large shear forces on the load cells, generating substantial weighing errors.

2. Jacking Point Load Cells (Compression Load Cells)

Top-of-jack load cells are spherical-seat compression transducers placed directly between the head of the hydraulic maintenance jacks and the aircraft jacking pads (at the forward fuselage jacking point and main wing jacking points). The aircraft is jacked completely clear of the hangar floor.

• Advantages: Eliminates tire deflection errors, gear scrubbing, and brake binding.
• Safety Mandate: When jacking an aircraft for weighing, mechanical jacking locking collars (threaded safety nuts) must be run down continuously against the jack cylinders to prevent sudden drop in the event of hydraulic pressure loss. However, during the actual scale reading acquisition, the locking collars must be backed off slightly (by approximately 1 mm) so that 100% of the airframe mass rests directly on the load cells.

3. Mechanical Beam Scales

Legacy mechanical lever-and-poise beam scales use physical counterweights and balance knife-edges. While rarely used on modern widebody transport aircraft, they remain common for sailplanes, microlights, and light general aviation aircraft. They require perfectly level floor surfaces and zero draft.

Equipment Calibration Requirements

All weighing scales and load cells must possess a current, valid Calibration Certificate issued by an accredited metrology laboratory conforming to ISO/IEC 17025. Standard calibration validity is 12 months (or less if mandated by the scale manufacturer). If a scale has been dropped, mechanically shocked, or subjected to overload, it must be removed from service and recalibrated before use.

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Pre-Weighing Airframe Preparation Checklist

To ensure repeatable and legally binding results, the aircraft must be brought into an exact, documented configuration before taking scale measurements:

+-------------------------------------------------------------------------+
|                    PRE-WEIGHING AIRFRAME CHECKLIST                      |
|                                                                         |
|  [1. Hangar Doors Closed] --------> Zero cross-draughts / HVAC off      |
|  [2. Aircraft Clean & Dry] -------> No mud, frost, rain or rinse water   |
|  [3. Fuel Configuration] ---------> Defueled to unusable fuel level     |
|  [4. Engine Oil Servicing] -------> Filled to max capacity per AMM      |
|  [5. Hydraulic Fluid] ------------> Reservoirs full / normal levels     |
|  [6. Lavatory & Potable Water] ---> 100% drained, flushed & empty       |
|  [7. Certified Equipment List] ---> Verify life vests, fire ext, slides |
|  [8. Parking Brakes] -------------> RELEASED (for platform scales)      |
+-------------------------------------------------------------------------+

1. Hangar Environmental Conditions

• Weighing must be performed inside a completely enclosed hangar.
• All hangar doors must remain closed throughout the entire weighing procedure.
• Hangar forced-air heating, ventilation blowers, and air conditioning units must be turned off or directed away from the aircraft. Even a slight draught of 5 to 10 knots moving over an aircraft wing generates aerodynamic lift or downforce, shifting hundreds of kilograms between scales and producing completely invalid data.

2. Airframe Cleanliness & Moisture

• The aircraft exterior and landing gear bays must be thoroughly washed and allowed to dry completely. Water trapped in flight control balance bays, flap tracks, or composite honeycomb drain holes introduces significant false weight.
• The aircraft must be free of ice, snow, frost, and condensation.

3. Fluid Servicing Standards

• Fuel Tanks: The preferred configuration is defueling the aircraft down to unusable fuel. Unusable fuel (residual fuel remaining in tanks, sumps, and fuel lines after running defuel pumps) is legally defined as part of the certified empty weight. If defueling to unusable fuel is unfeasible (e.g., on widebody transport aircraft), the tanks may be topped to full or weighed with fuel, provided the exact specific gravity (density) of the fuel is measured using a calibrated hydrometer, and the fuel mass and arm are subtracted from the scale totals.
• Engine Oil: Engine oil tanks must be filled to full maximum operating capacity. If the aircraft is weighed with dry/drained engines, the AMM mandates that the certified mass and moment of full engine oil be added back mathematically into the final empty weight calculations.
• Hydraulic Systems: All hydraulic reservoirs must be serviced to normal full operating levels with actuators in their certified weighing positions (typically landing gear down and locked, flaps fully retracted, flight controls neutral).
• Potable Water & Waste Systems: Potable water tanks must be completely drained. Lavatory waste holding tanks must be thoroughly flushed, drained, and empty.

4. Equipment List Audit

Prior to weighing, the certifying engineer must walk through the cabin, cockpit, cargo bays, and avionics compartments with the official Weight and Balance Equipment List. Every piece of required emergency equipment (fire extinguishers, life rafts, escape slides, crash axes, first aid kits, emergency locator transmitters) must be present in its certified stowage bracket. All non-standard items—such as maintenance toolboxes, tech flight bags, ground covers, and spare parts—must be removed.

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Aircraft Levelling Procedures

An aircraft cannot be weighed in an arbitrary pitch or roll attitude because tilting the airframe shifts the horizontal position of every component's center of gravity relative to the landing gear contact points. The aircraft must be brought into its standard certified level flight attitude.

Methods of Levelling

1. Spirit Levels / Precision Clinometers: Placed across designated levelling lugs, pins, or levelling plates built into the airframe structure (e.g., inside the main wheel well, forward cargo bay, or pilot seat tracks) as illustrated in the AMM.
2. Plumb Bob and Datum Plate: A plumb bob is suspended from a designated ceiling bracket inside the cabin or fuselage. The aircraft is leveled until the tip of the plumb bob aligns perfectly with the center crosshair of a calibrated datum target plate on the cabin floor.
3. Electronic Inclinometers: Digital sensors mounted to airframe reference points that display longitudinal and lateral inclination in tenths of a degree.

Levelling Adjustment Techniques

• When weighing on jacking load cells, leveling is achieved simply by raising or lowering individual hydraulic jacks until all level indicators show zero.
• When weighing on wheel platform scales, leveling is achieved by inflating or deflating landing gear tires or oleo shock struts within allowable limits, or by placing precision leveling shims under the wheels.

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Tare Weight: Definition & Mathematical Deduction

Tare Weight is the weight of all extraneous equipment resting on the scale platforms that is not part of the aircraft. This includes:

• Wheel chocks used to prevent rolling.
• Jacking adapters, pads, and shims resting on scale platforms.
• Grounding cables and drip pans.

Tare weight represents an additive error on the scale readout. It must be measured and subtracted directly from the gross scale reading to determine the actual Net Weight exerted by the aircraft landing gear:

$$\text{Net Reaction Weight} = \text{Gross Scale Reading} - \text{Tare Weight}$$

Worked Tare Deduction Example

A twin-engine executive jet is weighed on three electronic wheel platform scales. Technicians place wheel chocks and protective rubber mats on each scale pad before the aircraft is winched onto the scales. Following the aircraft weighing, the aircraft is removed, and the chocks and mats are left on the scales to determine the tare weight at each weighing point.

The true net weight of the aircraft is 10,149.0 kg, not the 10,250.0 kg displayed on the gross scale readouts. Failing to deduct the 101.0 kg of tare would introduce a persistent airworthiness error into all subsequent payload and flight dispatch calculations.

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

A certifying engineer is supervising the 4-year fleet weighing check of an ATR 72 regional turboprop in a base maintenance hangar. Outside, gusty winds reach 25 knots. The hangar supervisor leaves the main hangar rolling doors cracked open by two meters to vent ground-support vehicle exhaust. Furthermore, the technicians left the main wheel emergency parking brake set to hold the aircraft on the wheel scale platforms.

The certifying engineer immediately halts the procedure. The open hangar doors allow turbulent wind eddies to enter, creating varying aerodynamic pressure differentials across the high-aspect-ratio wings that cause the digital scale readings to fluctuate erratically by ±40 kg. Furthermore, the engaged parking brakes induce internal shear stresses across the main landing gear trailing links, preventing the gear struts from settling into their natural geometry. The engineer orders the hangar doors fully sealed, shuts off auxiliary blowers, releases the parking brakes, places light chocks on the scales (recording their tare mass), and waits for the digital load cells to stabilize before recording certified readings.

Common Exam Traps

• Trap 1: Forgetting to release parking brakes on wheel platform scales. Trapped tire scrub and brake torque impart false horizontal side-loads onto the load cells, resulting in significant weight measurement errors.
• Trap 2: Adding tare weight instead of subtracting it. Tare is foreign equipment on the scale; net aircraft mass is always Gross Scale Reading minus Tare.
• Trap 3: Misinterpreting unusable fuel. Unusable fuel is NOT defueled or subtracted; it is part of the certified Basic Empty Weight. Usable fuel, however, must be drained or deducted.
• Trap 4: Neglecting scale calibration validity. Using an uncalibrated scale or one with an expired calibration sticker immediately invalidates the Certificate of Release to Service (CRS) for the weighing task.