BBK, re factors, I agree.
See the
Boeing example in fig 2 and compare the distances for the dry and good braking action when using max manual brakes. Comparing other braking levels may be impossible due to the use and interaction of reverse and autobrake.
The root of the FAA’s concern appears to be that the ‘good’ wet factor is insufficient for all ‘good’ wet runways. This may be due to the operational assessment of ‘good’ being incorrectly reported or that the assumed level of friction used to calculate the wet data is too high.
There is some support for the latter view in
AC25-32 standardised advisory landing data, where for future advisory dry distances, (the baseline friction for wet calculations) should be reduced to 90% of the certificated dry value.
don, you are correct, but my ‘suggestion’ was to invoke thought about the accuracy of determining the type of runway surface and the intensity of rainfall, irrespective of touchdown point, speed, etc.
Runway surfaces change with time, rainfall reports may be inconsistent, or not agree with piloted interpreted WXR returns. Thus the suggestion is for additional ‘conservatism’ to that already required in the SAFO to account for the variability or unknown aspects in determining the landing distance.
The SAFO provides the theory, pilots have to put this into practice.
Thus ‘no such thing as a good wet runway’; i.e. ‘medium’ braking action could be the baseline for a conservative assessment of the landing distance on any wet runway.
P.S. technically it is not the precipitation (rainfall) rate which determines the runway braking action, but the amount of water already on it – perhaps the most important of the unknowns.