Three-scale weighing after a modification
After a repaint and interior modification, a light twin is weighed in a level hangar. The aircraft is prepared with unusable fuel, full oil and coolant, no payload, and doors closed. Three platform scales are used: nose scale reads 790 kg at arm 1.15 m; left main reads 1,505 kg at arm 4.90 m; right main reads 1,495 kg at arm 4.90 m. The nose scale was found not to have been zeroed and carries a 15 kg tare; the main scales were zeroed. The previous basic empty mass was 3,740 kg at 4.18 m. The maintenance release quotes only the old figures because the new weighing sheet is still in the office. The next flight's load sheet is being prepared from the old figures. The datum and wheel arms are unchanged from the previous weighing.
1. Correct the nose-scale reading and calculate the new basic empty mass and CG.
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Correct nose reading = 790 − 15 = 775 kg. Main total = 1,505 + 1,495 = 3,000 kg. New BEM = 775 + 3,000 = 3,775 kg. Moment = 775×1.15 + 3,000×4.90 = 891.25 + 14,700 = 15,591.25 kg·m. CG = 15,591.25 / 3,775 = 4.130 m. Compared with the old 3,740 kg at 4.18 m, the empty mass has increased by 35 kg and the CG moved forward by about 0.05 m.
2. Explain why the old empty mass and CG must not be used for the next load sheet, and state the documentation action required.
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The old figures are no longer the aircraft's empty mass and CG. Using them would understate the empty mass by 35 kg and would place the CG about 5 cm farther aft than it really is. That error could hide a forward-limit problem and would make every subsequent load sheet incorrect. The new weighing result must be entered in the mass and balance report and empty mass record, the load sheet must be recalculated, and the maintenance release should reference the new figures. A second weighing or a check of the arithmetic is advisable before release.
3. List the preparation and reading errors that could invalidate this weighing, and explain how you would reduce the risk.
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Preparation must be as specified: aircraft level, doors closed, no payload, unusable fuel only, full oil and coolant, and all scales calibrated and zeroed. The nose tare is a direct reading error; other errors include an unlevel aircraft, wheels not centred, brake drag, uncalibrated scales, reading before the display is stable, wrong wheel arm, or including fuel that is not unusable. Repeat the readings and compare total mass with the previous weighing; investigate any change beyond the permitted tolerance before accepting the new empty mass and CG.