Takeoff Performance on a Contaminated Runway
You are planning a takeoff in a twin-engine turboprop from a 1,600 m runway at 900 m elevation, OAT 28 °C, QNH 1013. The runway is wet with standing water in patches after rain. AFM figures for the actual weight give a dry takeoff distance of 1,050 m and a wet-runway factor of 1.15. The aircraft has a published V1 of 105 kt, VR 110 kt, V2 115 kt for these conditions. The first 300 m of the runway are uphill by 0.8%.
1. Calculate the wet takeoff distance and decide whether the takeoff is legal, stating the assumptions you use.
Read the worked answer
Wet distance = dry distance × wet factor = 1,050 × 1.15 = 1,207.5 m.
That is the distance to reach a given height on a wet surface; but the regulatory question is the takeoff distance available, which is the runway length corrected for slope and any displaced threshold or clearway credit. The 0.8% upslope over the first 300 m adds roughly 2.4 m of equivalent distance per 300 m segment in the conservative direction, so add about 3 m, giving roughly 1,210 m. Against 1,600 m of runway, the takeoff is comfortably legal with about 390 m margin.
The assumptions matter: the wet factor applies to the accelerate-stop and takeoff distances in the AFM, the slope correction is applied to the runway available, and no clearway is assumed. The examiner wants to see that you apply the factor to the correct distance and compare it with the corrected runway, not with the bare runway length.
2. If the standing water were reported as contamination requiring a further 15% penalty, what would change and what would you do?
Read the worked answer
A further 15% gives 1,207.5 × 1.15 = 1,388.6 m, plus slope about 3 m, so roughly 1,392 m against 1,600 m. Still legal, but the margin falls to about 208 m, and the accelerate-stop distance is now the limiting case, not the takeoff distance, because on a contaminated runway the stopping performance degrades faster than the acceleration performance.
Actions: recompute the accelerate-stop distance with the contaminated-runway factor and confirm it is within the runway; if it is not, reduce weight (offload fuel or payload) or wait for the water to drain. Also check that V1 is still achievable and that the crosswind is within the contaminated-runway limit. The judgement point is that on a contaminated runway you must verify both the takeoff and the accelerate-stop cases, and that weight reduction is the standard remedy when the margin is thin.