Wet Runway and the Second-Segment Obstacle
A twin-engine narrow-body jet with a structural MTOW of 69 000 kg is planned out of a runway where TORA, TODA and ASDA are all 2 400 m. Elevation is 1 350 ft, QNH 1009, OAT 33 °C. Planned take-off mass is 61 800 kg. From the flight manual: dry accelerate-stop distance at 61 800 kg is 2 150 m; a wet runway increases accelerate-stop distance by 15 percent; take-off distance required increases by 60 m per 1 000 kg of mass; lowering V1 from 148 kt to 140 kt shortens accelerate-stop distance by 90 m but lengthens the take-off distance to 35 ft by 120 m. Dry take-off distance required to 35 ft is 2 180 m; on a wet runway this is unchanged (no aquaplaning assumed). The second-segment gross gradient at 61 800 kg and 30 °C is 3.2 percent; net gradient is gross minus 0.9 percent; it decreases 0.10 percent per °C above 30 °C and improves 0.15 percent per 1 000 kg below 61 800 kg. The net flight path must clear, by the standard 35 ft (10.7 m), an obstacle 60 m above the runway end located 3 000 m from the point where 35 ft is reached.
1. Is the take-off at 61 800 kg legal on the wet runway with V1 at 148 kt? Show the field-length reasoning and what the crew must change.
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Accelerate-stop check. Dry accelerate-stop distance 2 150 m; wet factor 1.15 gives 2 472 m against an ASDA of 2 400 m. The take-off is not legal at V1 148 kt: there is insufficient runway to stop from an engine failure at V1 on the wet surface.
Corrective action - reduce V1. Lowering V1 to 140 kt gives a dry accelerate-stop distance of 2 150 − 90 = 2 060 m, and wet 2 060 × 1.15 = 2 369 m, inside 2 400 m with 31 m to spare.
But V1 is a two-sided decision. A lower V1 makes stopping easier and continuing harder. The take-off distance must be rechecked: 2 180 + 120 = 2 300 m to 35 ft, inside the 2 400 m TODA with 100 m to spare. So the field length is satisfied, but only just, and the margin is now small enough that any change - a tailwind, a later line-up, another 1 000 kg - invalidates it.
What the crew must do. Recompute with the actual wind and temperature at the time of take-off, not with the flight-plan figures; brief the reduced V1 and the fact that the stop decision now comes earlier and at lower speed; and confirm that no other limit (tyre speed, brake energy) is exceeded by the higher V1 rejected case. V1 must never be raised to make the second segment work - that is the classic way to fail the stop case.
2. Does the net flight path clear the obstacle? Show the calculation and the remedy.
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Gradient required. The net flight path starts at 35 ft (10.7 m) and must clear the obstacle by 35 ft: required rise = 60 + 10.7 = 70.7 m over 3 000 m = 2.36 percent.
Gradient available. Gross 3.2 percent; net = 3.2 − 0.9 = 2.3 percent. OAT is 33 °C against the 30 °C of the chart, i.e. 3 °C above, costing 3 × 0.10 = 0.30 percent. Net available = 2.3 − 0.3 = 2.0 percent.
Result. 2.0 percent available against 2.36 percent required: the net flight path is about 11 m short at the obstacle. The take-off is not legal on this departure at 61 800 kg, even though the runway lengths now work.
Remedies, in order of preference.
- Reduce mass: at 0.15 percent per 1 000 kg, a 0.36 percent deficit needs about 2 400 kg, so roughly 59 400 kg. This also improves the field length.
- Use an alternative departure procedure (a turning departure that avoids the obstacle, or a displaced start point) if one is published and the aircraft can fly it.
- Wait for a lower OAT, which recovers 0.1 percent per °C.
Note the trap: the mass reduction is driven by the obstacle, not the runway, and a crew that only fixed the wet-runway V1 problem would depart illegally. Note also that ISA at 1 350 ft is about 13 °C, so 33 °C is roughly ISA +20 - performance is degraded on both counts.
3. The crew considers an intersection departure to save taxi fuel. How does that change the analysis and what is the governing principle?
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It changes three things, all in the wrong direction.
- TORA/TODA/ASDA shrink. The declared distances for the intersection are shorter than 2 400 m, so the 2 369 m wet accelerate-stop distance and the 2 300 m take-off distance may no longer fit - the field-length margin that was already only 31 m and 100 m would disappear.
- The obstacle moves closer in flight-path terms. The net flight path is measured from where 35 ft is reached, and with a shorter TODA the aircraft reaches 35 ft earlier along the physical runway, so the horizontal distance to the obstacle is reduced and the gradient required rises. A 300 m loss of take-off run at 3 000 m turns a 2.36 percent requirement into about 2.6 percent.
- The screen height may not be achievable within the remaining TODA at all, which is a field-length failure, not just an obstacle failure.
Governing principle. Every take-off must satisfy the field-length, obstacle and climb requirements simultaneously and in the configuration actually used, with the declared distances for the actual entry point. An intersection departure is a new performance calculation, not a fuel-saving tweak; the correct answer here is to use the full length, and to accept the taxi burn.