Avionics Upgrade, Repaint Margin and a Light Forward Load
A twin-engine commuter aircraft has a maximum takeoff weight of 7,300 lb and a maximum landing weight of 7,000 lb. Its approved basic empty weight is 4,850 lb at an arm of 182.0 in aft of the datum, and the certificated CG range is 180.0 to 190.0 in. You have just completed an avionics upgrade that removed 42 lb of old equipment at arm 111.5 in and installed 58 lb of new equipment at arm 118.0 in. The baggage compartment is at arm 285.0 in and is placarded at 300 lb maximum. Ramp weight equals takeoff weight for this calculation. The chief pilot hands you two loads: a scheduled flight with 340 lb of crew at arm 120.0 in, 900 lb of fuel at arm 191.0 in, six passengers totalling 1,020 lb at arm 220.0 in and 150 lb of baggage; and a short positioning flight with the same crew, 400 lb of fuel, and no passengers or baggage.
1. Compute the new basic empty weight and empty-weight CG after the avionics upgrade, and state what the result does and does not prove.
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Keep weight and moment consistent at every step:
- Original: 4,850 lb at 182.0 in = 882,700 lb·in
- Remove 42 lb at 111.5 in: 4,683 lb·in removed, giving 4,808 lb and 878,017 lb·in
- Install 58 lb at 118.0 in: 6,844 lb·in added, giving 4,866 lb and 884,861 lb·in
- New arm = 884,861 ÷ 4,866 = 181.8 in
So the empty weight rose 16 lb and the empty CG moved 0.2 in forward. That direction surprises people - the new equipment is heavier, but the removed equipment sat further forward. Never assume 'heavier means aft'.
What it does not prove is that the aircraft is loadable. The empty CG sits inside 180.0–190.0 in, but the forward limit applies to the loaded case. The upgrade must be entered in the weight and balance report, and the loading checked again with the new figures before the aircraft flies.
2. Evaluate the scheduled flight: is it within weight and CG limits, and how much margin is really left?
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Sum weights and moments:
- BEW 4,866 at 181.8 in = 884,861
- Crew 340 at 120.0 in = 40,800
- Fuel 900 at 191.0 in = 171,900
- Passengers 1,020 at 220.0 in = 224,400
- Baggage 150 at 285.0 in = 42,750
- Total 7,276 lb, moment 1,364,711 lb·in, CG 187.6 in
Both limits pass: 7,276 lb is under the 7,300 lb takeoff limit, and 187.6 in is inside 180.0–190.0 in. The margin is the story. There are only 24 lb of weight left, so a single extra 30 lb bag puts the aircraft over MTOW. Aft CG margin is 2.4 in, and the baggage sits at the extreme aft arm, so it is the item to control.
Also run the landing case: fuel burns from 191.0 in, aft of the CG, so the CG migrates forward as fuel burns - the safe direction here. Sign the load sheet with the computed figures, not the words 'within limits'.
3. The positioning flight is refused by the crew as 'nose heavy'. Verify the claim and determine the ballast required at the baggage arm, including a sensible margin.
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Light loads are where forward CG bites:
- 4,866 + 340 + 400 = 5,606 lb
- Moment 884,861 + 40,800 + 76,400 = 1,002,061 lb·in
- CG = 178.8 in, which is 1.2 in forward of the 180.0 in limit. The crew is right.
The same aircraft that was legal at maximum load is out of limits when light, because crew and fuel sit ahead of the empty CG.
Ballast at 285.0 in: (1,002,061 + 285w) ÷ (5,606 + w) = 180.0, giving 105w = 7,019, so w = 66.9 lb. That lands exactly on the limit with zero margin, so use 75 lb and recheck: 5,681 lb, moment 1,023,436, CG 180.15 in.
Record the ballast in the log and on the load sheet, and placard the compartment. Removable ballast that nobody knows about is ballast that gets offloaded by a ramp agent - and then the aircraft flies forward of the limit.