Rotation on a Short Runway with a Forward CG
You are flying a light twin (MTOW 2,100 kg, wing span 11.8 m) from a 1,150 m sealed runway at 1,200 ft pressure altitude, ISA +12°C. The load sheet places the CG 4 cm forward of the mid-point of the permitted envelope, and the aircraft is 60 kg below MTOW. Rotation speed is 78 KIAS; at this weight and density altitude the accelerate-stop distance is 980 m and the take-off distance to 50 ft is 1,090 m. On the previous sector the pilot noted that the aircraft needed noticeably more back-stick than usual to unstick, and the trim wheel was set to the take-off mark. There is a 6 kt tailwind component reported by the tower, though the windsock shows light and variable. The runway has a 0.8% upslope in the first 400 m.
1. Assess whether the take-off should proceed, identifying which performance margins are actually being eroded and by how much.
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The published figures already assume zero wind and level runway, so every correction must be applied on top. The 6 kt tailwind adds roughly 10% to both the ground roll and the 50 ft distance; at this weight that pushes the 1,090 m figure toward 1,200 m, which exceeds the 1,150 m available even before slope is considered. The upslope adds a further increment to the ground roll (of the order of 5–10% for a 0.8% gradient at this density altitude). The accelerate-stop distance of 980 m also grows with tailwind, because the airspeed at which the decision is made is reached later along the runway. The forward CG does not change the book distances directly, but it raises the stick force and pitch attitude needed at rotation, lengthening the ground roll and increasing the risk of over-rotation or a late unstick. Net judgement: the aeroplane cannot meet the 50 ft screen with the reported tailwind; the take-off should be rejected or delayed until the wind is confirmed headwind or calm and the actual density altitude is re-checked.
2. Explain why the heavy back-stick on the previous sector is a stability-and-control warning rather than merely a handling quirk, and what it implies for the go/no-go decision.
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A forward CG increases the nose-down moment that the tailplane must counter, so more download on the tail and more elevator deflection are required to rotate. That has three consequences. First, the elevator may reach its limit before the required attitude is achieved, particularly at low speed where dynamic pressure is lowest, so rotation speed effectively rises. Second, the trim setting marked for take-off assumes a CG near the middle of the envelope; with the CG 4 cm forward the marked setting under-trims the aircraft, adding to the pilot's workload at the most critical moment. Third, a forward CG raises the stick-force gradient and reduces the static margin in pitch in the sense that the aircraft is more reluctant to raise the nose, which lengthens the ground roll and eats into the already thin margin. The correct response is to re-run the load sheet, confirm the CG is inside the forward limit for the actual zero-fuel and take-off weights, and if the margin is small, move load aft within limits rather than simply pulling harder.
3. If the flight is delayed and the temperature rises to ISA +20°C, describe how you would re-evaluate the take-off without a full performance computer.
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Density altitude is the controlling variable. At 1,200 ft pressure altitude, ISA +20°C is 5°C above the ISA temperature for that level (ISA at 1,200 ft is about 13°C, so actual is roughly 33°C), which corresponds to a density altitude some 2,000–2,500 ft higher than the pressure altitude. Using the AFM's density-altitude or temperature-correction chart, read the take-off distance at the actual weight and apply the wind and slope corrections in the order the manual specifies. Without a computer, the safe method is to use the chart conservatively: read the distance at the next higher weight increment and the next higher temperature band, then add the tailwind and slope increments. If the resulting figure plus a 15% safety factor exceeds the available runway, the take-off is not authorised. The forward CG does not enter the chart but must be re-checked against the forward limit at the new weight, since fuel burn during taxi may have shifted it further forward.