Hot, High, Short, Tailwind, and 35 kg Overweight
Cessna 206H, MTOW 1,633 kg, at a 400 ft-elevation grass strip at 15:00 local. Runway 15 is 900 m long, but a NOTAM closes the first 120 m, leaving 780 m available; the surface is dry grass with a 1 percent upslope. OAT 32°C, QNH 1,009 hPa, wind 190/10 kt. Ramp weight is 1,668 kg with three passengers, 60 kg in the rear baggage hold, and 240 L of fuel. The POH chart (zero wind, level, dry grass) at 2,500 ft pressure altitude and 32°C at 1,600 kg gives 640 m to 50 ft; the manual's wind correction adds 21 percent for a 10 kt tailwind. A 15 m tree line stands 60 m past the departure end. CG at 1,668 kg is 1.02 m aft of datum; the envelope at that weight is 0.95 m to 1.20 m. The rear baggage hold arm is 3.40 m aft of datum.
1. Compute the density altitude and the wind-corrected takeoff distance, and compare it with what is actually available.
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Density altitude
Pressure altitude ≈ 400 ft (QNH 1,009 is close enough to standard to ignore for this purpose). ISA temperature at 400 ft = 15 − (1.98 × 0.4) ≈ 14.2°C. Deviation = 32 − 14.2 = +17.8°C. DA ≈ 400 + (118 × 17.8) ≈ 2,500 ft. The chart figure quoted is therefore already at the right density altitude: 640 m to 50 ft, zero wind, level, dry grass, 1,600 kg.
Corrections
- Tailwind 10 kt: +21 percent → 640 × 1.21 = 774 m.
- Upslope 1 percent: typically 5–10 percent penalty on the ground roll; this pushes the figure above 800 m.
- The aircraft is 35 kg above MTOW, which adds a few more percent.
Comparison. 774 m (before slope) against 780 m available leaves 6 m - about one aircraft length. Add the 1 percent upslope and the figure exceeds the runway. Add the 15 m tree line 60 m past the end and the 50 ft screen is not achievable before the trees.
Conclusion: as loaded and at 15:00, the takeoff performance is not merely marginal - it fails once slope and obstacle are included. The 640 m chart number is not the answer; it is the starting point.
2. The pilot proposes to offload 40 kg of baggage from the rear hold to fix the overweight condition. Evaluate that plan.
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Weight
1,668 − 40 = 1,628 kg, which is 5 kg under MTOW. Weight problem solved.
Balance
Removing mass aft of the CG moves the CG forward. Moment change = 40 kg × 3.40 m = 136 kg·m, taken off the aft side.
New CG = [(1,668 × 1.02) − 136] / 1,628 = (1,701.4 − 136) / 1,628 = 1,565.4 / 1,628 = 0.961 m
That is still inside the 0.95 m forward limit, but only by 11 mm - roughly a sandwich in the wrong place. Recheck the limit at 1,628 kg, because forward limits are weight-dependent and typically move aft as weight decreases. At a limit of 0.96–0.97 m the aircraft would be out of the envelope forward, which on takeoff means heavy elevator forces, a long ground roll, poor rotation, and a higher stall speed in the landing configuration later.
Better plan
- Offload or redistribute from a position closer to the CG: a passenger, or baggage moved to the forward cabin area, so weight comes off without a large CG shift.
- Or reduce fuel rather than rear baggage - but check the fuel is actually needed for the leg plus reserve before using it as ballast.
The lesson: every mass change must be run through both weight and balance, not just the weight column.
3. It is 15:00. Give a decision and a specific alternative that would make the departure legal and safe, with numbers.
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Decision: do not depart as loaded at 15:00. Three independent limits are breached or barely met - weight, CG margin, and obstacle-limited takeoff distance - and they interact: the aft-heavy loading raises rotation and liftoff speed, which lengthens the ground roll exactly where the runway is shortest.
A workable alternative - wait for temperature to fall
Depart at 17:30 with OAT 28°C:
- ISA deviation = 28 − 14.2 = +13.8°C.
- DA ≈ 400 + (118 × 13.8) ≈ 2,030 ft.
- Chart value at 2,030 ft and 28°C at 1,600 kg ≈ 570 m.
- Tailwind correction +21 percent → 570 × 1.21 ≈ 690 m.
- Available 780 m → about 90 m (13 percent) margin, before slope. With the 1 percent upslope this is tight but defensible in a 206 only if the aircraft is at or below 1,600 kg and the tree line is cleared by a comfortable margin - otherwise use the opposite direction.
Two further improvements
- Depart Runway 33 into wind. That removes the 21 percent penalty entirely and eliminates the displaced-threshold restriction in that direction, at the cost of a 1 percent downslope (a much smaller penalty).
- Reduce to 1,600 kg or below properly, using a forward offload so the CG sits mid-envelope rather than at the forward limit.
A no-go is not a failed flight; a takeoff that needs the last 6 m of a grass strip in a 206 is a test flight with passengers aboard.