Avionics change: empty weight, CG, and whether to reweigh
The last weighing of a light twin gave an empty weight of 1,180 kg with the EWCG at 2.72 m aft of the datum. The approved envelope for weights up to 1,500 kg is 2.60–3.05 m aft of the datum; maximum take-off weight is 1,520 kg. The mean aerodynamic chord is 1.80 m long and its leading edge is 1.55 m aft of the datum. An avionics change removes 8.5 kg at 0.90 m, installs 11.2 kg at 3.85 m and adds 2.0 kg of harness at an average arm of 2.10 m. The approved weighing schedule allows empty weight and CG to be amended by calculation only if the cumulative weight change is 0.5 % of empty weight or less and the CG shift is 0.5 % MAC or less; otherwise the aircraft must be reweighed.
1. Compute the new empty weight and CG, express the CG as %MAC, and check the envelope.
Read the worked answer
Original moment: 1,180 × 2.72 = 3,209.60 kg·m
Changes:
- Remove: −8.5 × 0.90 = −7.65 kg·m
- Install: +11.2 × 3.85 = +43.12 kg·m
- Harness: +2.0 × 2.10 = +4.20 kg·m
- Net moment change = +39.67 kg·m
New weight: 1,180 − 8.5 + 11.2 + 2.0 = 1,184.7 kg New moment: 3,209.60 + 39.67 = 3,249.27 kg·m New CG: 3,249.27 ÷ 1,184.7 = 2.743 m aft of datum
%MAC: (2.743 − 1.55) ÷ 1.80 = 1.193 ÷ 1.80 = 66.3 % MAC
Envelope check: 2.743 m lies between 2.60 and 3.05 m, and 1,184.7 kg is far below 1,520 kg. Within limits.
Note the CG moved aft 23 mm even though the change was intended to be roughly neutral - the arm of the new unit is more than twice the original CG arm, so small mass changes there have disproportionate effect.
2. The customer wants the CG restored to 2.80 m. How much ballast is needed at 4.30 m, and what else must be checked?
Read the worked answer
Let the ballast be m kg at 4.30 m:
(3,249.27 + 4.30m) ÷ (1,184.7 + m) = 2.80
3,249.27 + 4.30m = 2.80 × 1,184.7 + 2.80m = 3,317.16 + 2.80m
4.30m − 2.80m = 3,317.16 − 3,249.27 = 67.89
1.50m = 67.89 → m = 45.3 kg
New weight: 1,184.7 + 45.3 = 1,230 kg, still below the 1,520 kg limit - but it eats 45 kg of useful load forever, so the operator must accept that.
Also check:
- Ballast must be a fixed, approved installation with its own arm controlled (a 'removable ballast' that the crew can shift is a load-planning item, not an empty-weight fix).
- Adding ballast to move the CG aft when the aircraft is already within limits is a handling-preference change, not an airworthiness requirement - question whether it is needed at all.
- The CG must remain inside the envelope across the whole loading range, not just empty: ballast aft can push a fully loaded, aft-loaded case out the back of the envelope.
3. May the empty weight and CG be amended by calculation, or is a reweigh required?
Read the worked answer
Weight criterion: change = 1,184.7 − 1,180 = 4.7 kg. 4.7 ÷ 1,180 = 0.40 % - passes the 0.5 % limit.
CG criterion: shift = 2.743 − 2.72 = 0.023 m = 23 mm. As a fraction of MAC: 23 mm ÷ 1,800 mm = 1.26 % MAC - this exceeds the 0.5 % MAC limit.
Therefore a reweigh is required. Both criteria must be met; passing one is not enough. This is the trap: the mass change looks trivial at 4.7 kg, but the CG consequence of a component far from the datum is what the rule is guarding against, and 1.26 % MAC is a significant slice of a 1.80 m chord.
Additional points: the reweigh must use calibrated scales on a level, still-air floor, with the aircraft in the configuration (fluids, equipment, unusable fuel) specified by the weighing schedule; the new empty weight and CG go into the weight and balance report, the aircraft flight manual schedule and the load sheet, and the old figures are superseded - a calculation amendment cannot be mixed with an old weighed EWCG.