BEM and CG Data Extraction: Using Aircraft Documentation Correctly

031-04-03Pablo Asensio Martínez2026-04-011 min

Starting Point for Calculations

For daily flight preparation, the pilot does not re-weigh the aircraft. Instead, they extract the Basic Empty Mass (BEM) and Basic Empty CG directly from the aircraft's current weighing report or the Mass and Balance Manual.

Documentation Data

The entry in the manual will typically show:

  • Serial Number/Registration: Confirming the data is for the specific aircraft.
  • Date of Weighing: When the data was established.
  • Basic Empty Mass: Value in kg or lbs.
  • Centre of Gravity: Arm (distance from datum) or Index.

Index Units (IU)

To simplify calculations and avoid large moment numbers, manuals often convert moments into Index Units.

  • Index Formula: $ \text{Index} = \frac{\text{Mass} \times (\text{Arm} - \text{Ref})}{\text{Constant C}} + \text{Constant K} $
    • This scales the numbers down to a more manageable size (e.g., 0-100 scale).

When using an Index system, the pilot:

  1. Starts with the Basic Index (corresponding to the BEM).
  2. Adds the Index change for each load item (crew, passengers, fuel) based on charts or tables.
  3. Calculates the Final Total Index.
  4. Checks if the Final Index is within the allowable Envelope.