gjhome, re contaminated data (#3). If there is still some confusion, remember that contaminated performance is obtained from a different process than dry/wet.
- Wet operations are based on factored dry data (1.67 factor); basic dry data comes from manufacturers tests.
- In wet conditions it is assumed that the additional 15% margin (1.92 factor) provides equivalent safety to dry, this is not always true.
- A wet runway is considered to be contaminated if the water depth is greater than 3mm (think about how you determine that). Conversly less than 3mm of slush is ‘wet’, but how do crews determine what ‘slush’ is? (Near 0 deg C, transient conditions, SG 0.85)
- Contaminated operations now require the use of specific data, this is obtained either by testing or by calculation; see CS 25.1591 (page 1-G-6) and particularly AMC 1591 (page 2-G-25).
The certification requirements for contaminated data make many additional assumptions which operators should be aware of: e.g
- Operators are expected to make careful and conservative judgments in selecting the appropriate performance data to use for operations on contaminated runways. Appropriate data is not defined, but I assume that it refers to that in an approved AFM.
- … it is not possible to produce performance data that will precisely correlate with each specific operation on a contaminated surface.
- Note that aquaplaning speed is defined by 9*_/P for calculation purposes, but other research sources indicate 7*_/P is the more important value (tyre spin up speed vs spin down speed).
- Contaminated data is probably (usually is) based on the use of reverse thrust – a major difference from dry/wet operations.
- Operation on runways contaminated with water, slush, snow, ice or other contaminants implies uncertainties with regard to runway friction and contaminant drag and therefore to the achievable performance and control of the aeroplane during take-off, since the actual conditions may not completely match the assumptions on which the performance information is based. Where possible, every effort should be made to ensure that the runway surface is cleared of any significant contamination.
and …
The provision of performance information for contaminated runways should not be taken as implying that ground handling characteristics on these surfaces will be as good as can be achieved on dry or wet runways, in particular following engine failure, in crosswinds or when using reverse thrust.
Regulatory passing the buck to the operator?
There is an interesting technical review of wet takeoff performance in the presentations
in FLIGHT OPS - aerodynamics & performance section
See “Wet Runway (Physics - Certfication & Application)” N.B asphalt / concrete comparisons, page 11-.
Also see “Understanding Range Of V1”