Wet and contaminated · two penalties
Separate reduced braking from the extra drag of water, snow and slush.
What does the runway condition report change?
A dry runway is free of visible moisture and contamination in the intended area. A wet runway has visible dampness or water up to and including 3 mm. Standing water above 3 mm, slush, snow, ice and other conditions are assessed using the applicable contaminated-runway definitions and reporting system. Contaminant type, depth, coverage and temperature matter; a runway-condition code is not a universal take-off friction coefficient.
Read the surface, not just a colour
A dry runway is free of visible moisture and contamination in the intended area. A wet runway has visible dampness or water up to and including 3 mm. Standing water above 3 mm, slush, snow, ice and other conditions are assessed using the applicable contaminated-runway definitions and reporting system. Contaminant type, depth, coverage and temperature matter; a runway-condition code is not a universal take-off friction coefficient.
Distinguish wet patches, frost, dry/wet snow, compacted snow, ice and frozen ruts. Some contaminants create displacement drag as the wheels push material aside and impingement drag when spray strikes the aircraft. These penalties can reduce acceleration while the same surface reduces braking. A good-looking low-speed acceleration is not proof of the high-speed continuation case.
Hydroplaning has several mechanisms
Dynamic hydroplaning occurs when water pressure supports the tyre. A commonly taught estimate for a rotating tyre is about 9√p kt, with tyre pressure p in psi; the non-rotating-wheel estimate is about 7.7√p kt and is lower. These are approximate onset guides, not guaranteed safe thresholds. Hydroplaning can persist below the onset speed.
Viscous hydroplaning involves a thin fluid film on a very smooth surface and can occur at much lower speeds. Reverted-rubber hydroplaning involves a locked/skidding hot tyre, steam and rubber changes. Anti-skid, correct touchdown and braking technique, appropriate tyre condition and avoiding unsuitable contaminated conditions reduce risk; no single dynamic formula rules out the other mechanisms. Operator Part B contains the applicable wind limitations, normal procedures and performance instructions.
Why can both go and stop deteriorate together?
Wet/contaminated take-off calculations need their approved distance, friction and speed assumptions. Wet V1 is the decision speed scheduled for the wet case; it is often lower than the dry value to control stopping demand. That can move the failure earlier in the continuation case. Do not keep the old dry go distance while changing only the stop case.
The dry solution does not carry over
Wet/contaminated take-off calculations need their approved distance, friction and speed assumptions. Wet V1 is the decision speed scheduled for the wet case; it is often lower than the dry value to control stopping demand. That can move the failure earlier in the continuation case. Do not keep the old dry go distance while changing only the stop case.
CS-25 wet take-off distance uses its specific 15-ft construction and is compared with the dry requirement; the aircraft must still achieve the prescribed V2 by 35 ft. Contaminated-runway procedures can impose additional speed, thrust, wind or system restrictions. A generic percentage increase on dry distance cannot reproduce all these effects.
Use the complete approved option
Displacement and impingement drag increase thrust required during the roll; reduced friction worsens rejection and directional control. Monitor acceleration using the aircraft/operator’s approved method. An improvised speed-at-distance rule cannot replace the approved performance calculation or rejection criteria.
Where specifically approved, a fixed derate may lower the applicable minimum-control-speed basis and help an otherwise constrained contaminated-runway case. It also reduces thrust and climb capability and can restrict subsequent thrust restoration. Assumed-temperature reduced thrust and fixed derate have different eligibility: do not infer that permission for one allows the other on contamination. Delay, clear the runway, reduce mass or use an approved alternative when the case cannot be substantiated.
Can you make the performance decision?
Four original questions. Work from the stated conditions and review the reasoning.