Wet, uphill and a gusting tailwind
A twin-jet is scheduled to depart with a take-off mass of 72 400 kg. Aerodrome elevation 380 ft, QNH 1004 hPa, OAT 32 °C, average upslope on runway 14 of 0.8%. Declared distances: runway 14 TORA 2 450 m, TODA 2 600 m, ASDA 2 650 m; runway 32 TORA/TODA/ASDA 2 450 m. Reported wind 300°/18 kt gusting 28 kt. The runway is wet with patches of standing water. The operator's limits are 10 kt tailwind and 15 kt crosswind on a runway with standing water. From the AFM, at these temperature and pressure conditions, the field-length-limited take-off mass is 76 800 kg dry, 73 400 kg wet and 70 900 kg with standing water. The AFM also requires V1 to be reduced by 5 kt when standing water is present. The aircraft has no automatic thrust restoration.
1. Compute the wind components for runways 14 and 32 and decide which runway may be used.
Read the worked answer
Runway 14 (140°), wind 300°/18 gusting 28:
- Angle between wind and runway = 160°, so the tailwind component = 18 × cos 20° = 16.9 kt and the crosswind = 18 × sin 20° = 6.2 kt (9.6 kt in the gust).
- 16.9 kt of tailwind exceeds the 10 kt operator limit before the gust is even considered. Runway 14 is not usable.
Runway 32 (320°):
- Angle 20° → headwind 16.9 kt (26.3 kt gust), crosswind 6.2 kt (9.6 kt gust). Both are inside the 15 kt standing-water crosswind limit.
Consequence: the departure must be replanned onto runway 32. Note that runway 32 has TORA/TODA/ASDA all 2 450 m - the 150 m clearway and 200 m stopway belong to the runway 14 direction only and cannot be used. Recompute take-off data on the correct declared distances; do not simply reverse the runway 14 figures.
2. With take-off mass 72 400 kg and standing water on the runway, is the take-off legal from runway 32? Quantify the reduction needed and list realistic options.
Read the worked answer
The standing-water field-length-limited mass is 70 900 kg, so 72 400 kg is 1 500 kg over. The take-off is not legal.
Cross-check the other cases: dry 76 800 kg and wet 73 400 kg both exceed 72 400 kg, so standing water is the binding case. One patch is sufficient - the whole runway does not need to be flooded for the contaminated figures to apply.
Options, in order of preference:
- Reduce mass by 1 500 kg - offload about 18 passengers or 1 500 kg of freight. Cutting fuel is not available: sector fuel is committed and landing weight was already the limiting case.
- Wait for the water to drain and obtain a fresh runway report of wet (not contaminated). The limit then rises to 73 400 kg, a 900 kg margin. This requires a new report and a full new take-off calculation.
- Depart from another aerodrome or make a technical stop.
Thrust reduction by assumed temperature or derate is not permitted on a contaminated runway, so no performance can be recovered that way.
3. The AFM requires V1 to be reduced by 5 kt on standing water. What does that protect, and what is the risk if the crew keeps the original V1?
Read the worked answer
V1 is the speed by which, after an engine failure, the aircraft can still be brought to a stop within the remaining ASDA.
On standing water the braking coefficient falls sharply - hydroplaning can remove most of the tyre-to-surface friction - so the accelerate-stop distance for a given V1 grows and can exceed the 2 450 m ASDA of runway 32. Reducing V1 by 5 kt moves the decision point earlier and restores the stop margin.
If V1 is left unchanged:
- the accelerate-stop distance may exceed ASDA, so the aircraft cannot be stopped on the runway after a failure before V1;
- the crew is effectively committed to continue at a speed at which the stop case is no longer assured.
After reducing V1, verify that it is still at or above VMCG and that the resulting Vr and V2 are achievable. If not, the take-off mass must be reduced further - the V1 reduction cannot be bought by accepting an illegal VMCG margin.