Determining Empty Mass and Empty CG by Weighing
Why and how aircraft are weighed
Empty mass and empty CG are established by actual weighing before entry into service, after modifications, repairs or repainting that could change them, and at the interval required by the applicable regulation. The result is recorded in a weighing report and in the mass and balance documentation (aircraft logbook, loading manual), and it becomes the basis of every later load calculation.
Preparation and equipment
- Aircraft in a level, specified attitude, normally on scales under the wheels or on jacks with load cells.
- Fuel drained to unusable, oil and other fluids at specified levels, potable water and waste per the configuration.
- No persons, loose equipment according to the inventory, control surfaces neutral, brakes released, no wind or with corrections applied.
- Calibrated scales of adequate capacity, zeroed, with tare weights deducted; readings repeated and averaged.
Reading errors
Parallax when reading a dial, scales not zeroed or drifting, load not centred on the platform, aircraft not level (which changes the moment arms), temperature effects, and simple transcription errors in the three readings. A good weighing records each reading and the arm used for that weighing point.
Worked example
Datum at the nose. Nose scale 8 200 kg at 1.20 m; left main 15 400 kg at 12.60 m; right main 15 350 kg at 12.60 m. Total mass = 8 200 + 15 400 + 15 350 = 38 950 kg. Moment = (8 200 × 1.20) + (15 400 × 12.60) + (15 350 × 12.60) = 9 840 + 194 040 + 193 410 = 397 290 kg·m. Empty CG = 397 290 / 38 950 = 10.20 m aft of the datum. Converting to %MAC needs the leading edge of the MAC and its length.
Traps that cost marks
Forgetting the tare or a scale's zero offset; weighing with unusable fuel missing from the aircraft; failing to reweigh after a repaint or a heavy modification; mixing arms in inches with masses in kg; and not levelling the aircraft, which silently changes all moment arms.