# Aircraft Weighing Procedure: Empty Mass and CG Determination

> Study aircraft weighing methodology to establish basic empty mass and CG position, including preparation, leveling, reaction measurements, and moment calculations.

- **Subject:** 031 — Mass and Balance
- **Section:** 031-04 — Mass and Balance Details of Aircraft
- **Topic code:** 031-04-02
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/mass-and-balance/mass-and-balance-details-of-aircraft/determination-of-aircraft-empty-mass-and-cg-position-by-weighing/

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## 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**.

![Weighing Aircraft](https://upload.wikimedia.org/wikipedia/commons/5/59/Weighing_Wittman_W-10_Tailwind._%283729290613%29.jpg)

*   **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 (-).*

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