Standing water, tailwind and a displaced threshold
Night, continuous rain. An aircraft is planned to land at a single-runway aerodrome: runway 09/27, 2,400 m, but LDA for runway 27 is 2,200 m because of a displaced threshold. Elevation 400 ft. Wind reported 090/12. Only runway 27 has a straight-in approach; circling to runway 09 is prohibited at night. The AFM landing distance at the planned landing mass, flap 30, no wind, dry runway, with all mandatory factors already applied, is 1,450 m. The operator's chart applies one surface factor - wet runway 1.15, standing water 1.35 - and adds 20% per 10 kt of tailwind. Braking action is reported medium and there is standing water in the touchdown zone. Fuel overhead allows one approach plus a 25-minute diversion to an alternate with a 2,600 m dry runway, ILS into wind and 8 km visibility, with the final reserve intact.
1. Compute the landing distance required on runway 27 and state whether the landing is permitted.
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The reported surface condition is standing water, so the standing-water factor applies, not the wet factor; the tailwind is 12 kt, which is over one 10 kt increment.
- Reference distance: 1,450 m
- Surface factor (standing water): 1,450 × 1.35 = 1,957.5 m
- Wind: 090/12 against runway 27 (heading 270) gives a 12 kt tailwind → 20% per 10 kt, so at least one full increment of 20%: 1,957.5 × 1.20 = 2,349 m
LDA is 2,200 m. The required distance exceeds the available distance by about 150 m, so the landing on runway 27 is not permitted on performance grounds. Note also that a 12 kt tailwind is normally outside the operator's tailwind limit for landing (commonly 10 kt), which is a second, independent reason the approach should not be flown.
2. The commander proposes using flap 40 (AFM figure 1,380 m) and argues that 'standing water is just very wet, so the 1.15 factor should be used'. Evaluate that reasoning.
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Test both parts of the argument.
With flap 40 and the correct factor: 1,380 × 1.35 = 1,863 m; × 1.20 = 2,235.6 m - still 35 m beyond the 2,200 m LDA. So the configuration change alone does not make the landing legal.
With flap 40 and the wrong factor: 1,380 × 1.15 × 1.20 = 1,904 m, which 'passes'. This is exactly the trap. The factors in the operator's chart are not interchangeable and not cumulative: each corresponds to a measured stopping-performance penalty. Standing water produces dynamic aquaplaning risk and loss of tyre-to-surface friction well beyond that of a merely wet runway, plus spray and possible loss of directional control; braking action 'medium' already signals degraded friction. Choosing the lower factor because the answer becomes acceptable is a motivated error, not a performance calculation.
Also, the chart's tailwind increments are discrete: a 12 kt tailwind cannot be treated as one increment in the hope that the extra 2 kt is negligible. Rounding in the favourable direction is never permitted.
3. What should the commander actually do, and how should it be briefed?
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Divert. The margins all point the same way:
- No legal landing distance. Even the best configuration for the available runway is short by about 35 m with no safety margin; the current configuration is short by 149 m.
- No headroom to fix it by burning fuel. Landing distance scales with mass; to bring 2,349 m down to 2,200 m needs roughly a 6% reduction in the reference distance, i.e. several tonnes of fuel. The aircraft overhead has only one approach plus diversion fuel with the final reserve intact - that fuel is not available to be burned.
- Tailwind at or beyond the limit on a contaminated runway at night, when the only straight-in approach is to the into-wind-wrong runway.
- The alternate is clearly suitable (2,600 m, dry, into wind, ILS, 8 km) and is within the planned diversion fuel.
Brief: cancel the approach before final if possible rather than after a go-around in rain; advise ATC of the diversion and reason; advise the company and arrange handling/fuel; recompute the landing performance at the alternate with the actual landing mass and note that residual ice or water on the airframe of a wet aircraft will add to the landing distance.