Aft CG in a loaded six-seat single on a short wet-grass strip
You are planning a charter in a Cessna 206H from a hill strip at 2,400 ft elevation (runway 09). Loading data, all arms in inches from the datum: basic empty weight 2,180 lb at 109.0; pilot 190 lb at 35.5; front passenger 170 lb at 35.5; two rear passengers 340 lb at 112.0; baggage area 2, 100 lb at 183.0; 84 US gal usable fuel at 48.0 (6.0 lb/gal). MTOW is 3,600 lb; the aft CG limit is 95.0 in at MTOW and the forward limit is 82.5 in. Conditions: QNH 998 hPa, OAT 30 °C, wind 090/10 kt, runway 1.2% uphill, surface wet grass, TORA 1,020 m, TODA 1,220 m. The POH chart, at the calculated density altitude, 3,500 lb, still air and level dry paved surface, gives a ground roll of 610 m and a 50-ft distance of 1,060 m. POH notes: add 10% per 2% of uphill slope; add 25% for wet grass; subtract 10% per 9 kt of headwind. Planned cruise fuel flow 15 gph; the first leg is 1 h 50 min.
1. Compute the takeoff weight and CG, and the weight and CG at the destination. Is the aircraft inside the envelope throughout?
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Takeoff:
| Item | Weight (lb) | Arm (in) | Moment (lb·in) |
|---|---|---|---|
| Basic empty | 2,180 | 109.0 | 237,620 |
| Pilot | 190 | 35.5 | 6,745 |
| Front pax | 170 | 35.5 | 6,035 |
| Rear pax | 340 | 112.0 | 38,080 |
| Baggage area 2 | 100 | 183.0 | 18,300 |
| Fuel | 504 | 48.0 | 24,192 |
| Total | 3,484 | 330,972 |
CG = 330,972 / 3,484 = 95.0 in - exactly on the aft limit, weight 116 lb under MTOW.
At destination (1 h 50 min at 15 gph = 27.5 gal = 165 lb):
- Weight = 3,484 − 165 = 3,319 lb
- Moment = 330,972 − (165 × 48.0) = 330,972 − 7,920 = 323,052
- CG = 323,052 / 3,319 = 97.3 in
The fuel is carried at an arm of 48.0 in, well forward of the CG, so burning fuel moves the CG aft. Even allowing for the lower aft limit at reduced weight (the envelope narrows as weight falls), 97.3 in is outside it. The takeoff CG at the limit is therefore not acceptable - the aircraft goes out of limits in cruise, where the elevator authority and stall/spin behaviour degrade.
2. Compute the pressure altitude and density altitude, then the corrected takeoff distance over 50 ft. Does the TODA allow the departure?
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Pressure altitude = elevation + 30 ft per hPa below 1013 = 2,400 + 30 × (1013 − 998) = 2,400 + 450 = 2,850 ft
ISA temperature at 2,850 ft = 15 − 1.98 × 2.85 ≈ 9.4 °C
Density altitude = PA + 120 × (OAT − ISA) = 2,850 + 120 × (30 − 9.4) = 2,850 + 2,472 ≈ 5,320 ft
The chart value must be read at about 5,300 ft DA, not at 2,850 ft. Using the given 50-ft figure at 3,500 lb of 1,060 m:
- Uphill 1.2%: +10% per 2% → +6% → 1,060 × 1.06 = 1,124 m
- Wet grass: +25% → 1,124 × 1.25 = 1,405 m
- Headwind 10 kt: about −11% → 1,405 × 0.89 ≈ 1,250 m
TODA is 1,220 m. Even with the headwind, the 50-ft distance exceeds TODA, and without the wind allowance it exceeds it by about 185 m. The ground roll on the strip would also be near 1,405 × (610/1,060) ≈ 810 m on wet grass, leaving almost no stopping or abort margin. The departure cannot be made as loaded.
3. What single change would you make, and how would you verify that it works on both the weight-and-balance and performance sides?
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Move the load, not just the fuel.
Weight reduction alone does not fix this case. The CG problem is driven by the two rear passengers at 112 in and 100 lb of baggage at 183 in. Offloading fuel would lighten the aircraft while moving the CG further aft, making the balance worse.
Best single change: move the 100 lb of baggage from area 2 (183 in) to baggage area 1, and/or move the rear passengers forward, and reduce fuel only as far as needed for the leg plus reserves.
Check it numerically:
- Baggage moved from 183 in to about 150 in removes 100 × 33 = 3,300 lb·in → takeoff CG falls by 3,300/3,484 ≈ 0.95 in, to about 94.0 in, giving real margin instead of zero.
- Re-run the destination case with that new loading and confirm the CG stays forward of the aft limit after the fuel burn.
- Recompute the takeoff distance at the reduced weight (the chart is read at the actual weight, not at 3,500 lb) and confirm 50-ft distance is below 1,220 m with at least a 15–20% margin on wet grass, or wait for the surface to dry and/or a cooler part of the day.
- Recheck the abort/ground-roll margin against TORA 1,020 m.
If the checks still fail, the departure is a no-go: a wet-grass, uphill, high-density-altitude strip with an at-limit aft CG has no single correction that rescues it, and the correct answer is to wait or split the load.