alf5071h
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’.
The definitons needs to be harmonized with those used by the ground staff who gather the information and describe the operative surface. ATC, when transmitting the information must use harmonized and standardised terms. '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).
Dry
Ground/ATC - Runway dry
Crew - Use dry landing data
Default Aircraft Braking Coefficient = 0.4
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.
Wet
Ground/ATC - Runway wet
Crew - Use wet landing data
Default Aircraft Braking Coefficient from 'equations'
Airplanes certified to Amendment 25-92 of Part 25 (effective March 20, 1998) or Change 15 of JAR-25 (effective October 1, 2000) must have wet runway takeoff performance data provided in their Airplane Flight Manuals (AFM). These data are considered to be normal operating limitations for the airplane in the same manner that dry runway data are. The FAA and JAA certfication requirements for how these performance data are developed are the same. Thus, the airplane type certification regulations for wet runway takeoff performance are harmonized.
Don Stimson, FAA at the 2003 Boeing Performance and Flight Operations Engineering Conference
The harmonized regulation 25.109 (c) and (d) has two sets of 'equations'. One set applies to grooved runways.
In the hormonizing prosess the term 'Damp' are no longer in use as damp is considered wet. (A damp runway is a wet runway)
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’.
I do not agree. The terms Good, Medium, ..... applies to runway contaminated with snow, slush and ice. They have been used for decades. The aircraft operators need a 'scale' for relating operations on contaminated/slippery runways. There are documents bridging this need. Now we have to look at the work done by EASA. At these links,
http://www.easa.eu.int/doc/Rulemakin...PA_14_2004.pdf
http://www.easa.eu.int/doc/Rulemakin...RD_14_2004.pdf
you will find documents related to operations on contaminated runways. These documents identify surface conditions and combine them with default friction values (Aircraft Braking coefficient).
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.
'Error proofing'
Using terms and definitions from the EASA documents we speak the same language as used in certification. For many surface conditions we can link to and refine the Good, Medium, ..... scale. Thus harmonize between the operational terms and the certification/regulation terms. There are however difficulties related to the term ' Specially prepared winter runway' which we have to handle. More on that later.
Please have a look at the EASA documents.