Aircraft Weighing Procedure: Empty Mass and CG Determination

Weighing Requirements

Weighing establishes the Basic Empty Mass (BEM) and the CG position of the empty aircraft.

Aircraft Preparation

Before weighing, the aircraft must be in a specific configuration:

• Clean: Removing dirt, ice, and snow.
• Fluids: Draining usable fuel (unusable fuel remains and is part of BEM). Oil and hydraulic fluids at operating levels.
• Equipment: All fixed equipment installed (seats, galleys, avionics).
• Leveling: The aircraft must be leveled longitudinally and laterally using leveling points defined by the manufacturer.

Weighing Method

The aircraft is placed on scales at the jacking points or under the wheels.

• Reaction Forces: The scales measure the reaction force ($R$) at each point (Nose/Tail wheel, Main wheels).
  • $R_L, R_R$: Left and Right Main Wheel reactions.
  • $R_N$ or $R_T$: Nose or Tail Wheel reaction.

$ \text{Total Mass} = R_L + R_R + R_N $

Calculation of CG Position

The position of the CG is calculated using the Principle of Moments.

1. Define Datum: Usually the nose, firewall, or a specific frame forward of the nose.
2. Measure Arms: Distance from the Datum to the weighing points ($d_L, d_R, d_N$).
3. Sum of Moments: Calculate the moment for each reaction ($Force \times Distance$).
   • $\text{Moment} = \text{Reaction} \times \text{Arm}$
4. Find CG Arm: $ \text{CG Arm} = \frac{\text{Total Moment}}{\text{Total Mass}} $

Note: Distances aft of the datum are positive (+), forward are negative (-).