Wet and Contaminated Runways - ATPL 032 — Performance

Dry, wet, contaminated

Dry — no visible moisture on the surface.

Wet — the surface carries moisture or water up to 3 mm deep: reflective, but not flooded.

Contaminated — more than 25 per cent of the surface within the used length and width lies under more than 3 mm of water, slush or loose snow, or under ice or compacted snow of any depth.

The contaminant list is its own LO: water patches, frost, dry snow, wet snow, slush, ice, compacted or rolled snow, and frozen ruts or ridges each behave differently under the wheels, from the drag of the loose kinds to the near-zero friction of the frozen ones, and the frozen ruts add a steering hazard on top.

Hydroplaning

On a wet enough surface the tyre can ride its own bow wave and leave the runway without leaving the ground. Three kinds share the name: dynamic hydroplaning, where standing water lifts the tyre bodily; viscous, where a thin film on a smooth surface (painted markings, rubber deposits) lubricates the contact at much lower speeds; and reverted rubber, where a locked, skidding tyre boils the film beneath it and rides its own steam. The dynamic kind has famous arithmetic: with the tyre pressure in psi, the onset speed is about 9·√P for a rotating wheel and 7.7·√P for a non-rotating one, and the lower, non-rotating figure is the limiting one for a wet-runway operation, because it is the touchdown case: wheels arriving unspun on a wet surface can hydroplane at a speed the rolling take-off would have shrugged off.

Three pointers complete the 07 block, all to the same shelf: any wind limitations for contaminated operations live in Part B of the Operations Manual under Limitations, the associated take-off and landing procedures under Normal Procedures, and the contaminated-runway performance data under Performance. The exam asks only where to look, and the answer is Operations Manual Part B three times over.

Scheduling on the slippery surface

The wet take-off is the dry method with two deliberate changes. The stop is protected by a wet V1, lower than the dry V1, conceding the decision earlier while the braking is poor; and the continued take-off is allowed a 15 ft screen instead of 35, buying back part of the distance the lower V1 costs the go case. Reverse thrust, refused credit on the dry runway, is credited in the wet figures. The consequence of the wet V1 is exam material in itself: the reduced screen means less margin over the ground at the end of the take-off, a trade the regulator accepts to keep the stop honest.

Contamination goes further: the performance is computed from dedicated contaminated-runway data, because the surface now fights the aeroplane twice. Displacement drag is the cost of shoving the contaminant out of the tyre's path; impingement drag is the spray striking gear, flaps and airframe. Both grow with speed and then fade once the tyres plane over the layer, and because the acceleration they steal is hard to predict, crews are taught to monitor the acceleration against expected marks and reject early if it dies. The wider hazards and their management follow the same logic: degraded and unpredictable stopping, directional control at the mercy of patchy friction, spray into engines and gear bays, and the remedies of reduced mass, the lower V1, stricter wind limits and, where the data allow nothing, not going at all.

One optimisation earns a preview here: a derated take-off on a contaminated runway can be a genuine benefit, because certifying the lower thrust as a new rating lowers VMCG, and a lower VMCG permits the lower V1 the slippery stop wants. The implication rides along: less thrust means longer distances, so the derate only helps while everything still fits the declared lengths. The full flex-versus-derate story is the next lesson's opening act.

Dry, wet, contaminated

Check you understood

Hydroplaning

Scheduling on the slippery surface