Braking system efficiency - Loss of braking - wet runway

[tribo]

Determine estimated aircraft tire effective braking coefficient µ eff by using the following equations:

µ eff = 0.2 µ max + 0.7 µ max2 (µ max less or equal to 0.7) (4a)
µ eff = 0.7 µ max for µ max (µ max greater than 0.7) (4b)

These relationships between aircraft tire maximum braking and effective braking friction coefficient are based on the assumption that the total aircraft braking-system (tires, brakes, hydraulics, gear and antiskid efficiency can be generated by a single curve 4(a) and (b)

The above is quoted from the report: NASA Technical Paper 2917 - Evaluation of Two Transport Aircraft and Several Ground Test Vehicle Friction Measurements Obtained for Various Runway Surface Types and Conditions - A Summary of Test Results From Joint FAA/NASA Runway Friction Program, February 1990

Aircraft used B727 and B737

How relevant are these equations for todays braking systems (more than 20 years later)?
What level of uncertainty do the assumption tires, brakes, hydraulics, gear and antiskid efficiency can be generated by a single curve represent?

[safetypee]

Current certification aspects are in CS 25.109 where there are vaguely similar equations.
Some of the flight test assumptions are in AMC 25.109 para 3.5. “The tyres and brakes should not be new, but need not be in the fully worn condition. They should be in a condition considered representative of typical in-service operations”.
Anti-skid efficiency is discussed in para 3, (92% max), and the runway surface in para 3.3, which should be smooth.

Porous runways are discussed in 109 (d) 2

There are references to ESDU 71026 for braking coefficient of friction:

IHS ESDU 71026

IHS ESDU 95015

Also, see www.spaceagecontrol.com/AC25-7A.pdf references to 25.109