tribo
The definitions are a reasonable certification/regulation summary, but they need to be associated with the operating situation, harmonized with ATC descriptions, and as far as possible ‘error proofed’.
Normal / excellent would (could?) only be associated with a dry runway, thus an ATC statement such as “Runway dry, braking normal” conveys both the description of the runway conditions and the braking action that provides a link to the data that the crew could use (dry landing data).
Similarly, “Runway wet, braking good” provides the description and the link to wet landing data, which should be used. However this is complicated where operators are allowed to use dry data on a wet grooved runway; here the actual stopping distance may lie between “normal” and “good”. Furthermore this might have to accommodate additional descriptors i.e. Damp, Water patches (AIC 61 / 99), and there could be further complications with “Runway wet with water patches” as the braking action could be good, medium/fair, or even poor.
From this discussion and the SW 737 at Midway thread – Boeing data (statement above), we might conclude that there should not be any ‘medium / fair’ operation, instead as soon as a runway is ‘contaminated’ the braking action should only be stated as ‘poor’ (use of most conservative data) so as to guard against aquaplaning. Operations should be prohibited in ‘nil’.
The ‘error proofing’ has to cope with helpful ATC who might use non standard or enhanced descriptions (human nature), and crew’s who misinterpret information in their choice of landing data (risk assessment).
The choice in the data is between every day operations using factored data that has well proven margins (the norm in memory), and a contaminated operation with relatively lower safety factors or none at all (rarely encountered or practiced).
Factored data is normally used on runways that have a narrow range of braking conditions, normal / excellent (wet/dry), good (wet), that appear easy to assess accurately both by ATC and pilots.
The ‘lesser’ factored (contaminated) data covers a wider range of conditions, which are more difficult to assess (requirement for Mu meter); thus, there is opportunity for error in assessing the runway conditions that may contribute to the pilot using inappropriate data.
An associated issue may be that pilots do not understand the differing levels of risk associated with ‘factored’ contaminated data, and that regulators may have assumed too much about the pilots knowledge/judgment of risk and of the validity of the data. Hence the Boeing / Airbus documents, and the Flight Safety Foundation’s presentation raising the industry’s awareness to these problems.
Overall, I would simplify the definitions by removing ‘excellent’ and ‘fair’ and link the remainder with descriptions of the runway surface. Further simplifications where there could be a range or potentially ambiguous combinations of descriptors, would limit the operation by the use of more reliably factored data or the most conservative data, i.e. only use ‘poor’ landing data on a ‘wet with water patches’ or ‘contaminated’ runway. Delete medium from practical operations.