Draft AC 139.C-06 v1.0 - Skid resistance of aerodrome pavements and Draft AC 139.C-07 v1.0 - Strength rating of aerodrome pavements (Part 139 consequential ACs)

The revised Part 139 Manual of Standards (MOS) was made on 05 September 2019. In support of the new rules CASA committed to provide additional guidance material in the form of 'advisory circulars' (AC) to assist aerodrome operators in design, operation, and maintenance of certified aerodromes.

AC 139.C-06 'Skid resistance of aerodrome pavements' and AC 139.C-07 'Strength rating of aerodrome pavements' were subsequently drafted to explain regulatory or standard requirements associated with runway pavements, as well as to provide context for the legislation and its application. It should be noted that the content of an AC is intended to provide good practice and guidance and is not mandatory. CASA acknowledges, in some instances, there may be alternative methods that provide for an acceptable means of compliance, and as such, it is the aerodrome operator's responsibility for determining the best way in which they will meet their regulatory responsibilities.

Between 12 August and 10 September 2020 aerodrome operators, aerodrome consultants and internal CASA personnel, were invited to review the proposed guidance and provide comment on the suitability of the material in support of the revised MOS.

The consultation has closed, and a summary of feedback is provided below. CASA would like to thank industry for providing this feedback.

A total of nine submissions were received— six from consultants and three from aerodrome operators.

Eight respondents consented to their comments being made public, one requested their submission to remain confidential.

Summary of feedback

In addition to recommended changes that were largely editorial in nature, a number of important matters were raised that required further consideration and clarification, these included:

• requirement to meet surface texture or surface friction characteristics
• terminology (consistency and definitions) as well as inclusion of references to support content
• functionality of aircraft tyres in water dispersal
• phenomenon of aquaplaning
• surface testing means, locations, frequencies, and the principle of 'averaging'
• expected timeframes for adopting a new pavement classification system
• inclusion of information to support assessment of the bearing strength of unrated pavements
• inclusion of risk-based considerations for pavement concessions.

All comments provided by industry have been carefully considered and where appropriate, incorporated into each AC.

AC 139.C-06 was amended, based on apparent differences in interpretation, to make clear the requirement for surface texture or surface friction testing. Runway grooving is not an acceptable means of compliance in its own right, as they are prone to deterioration over time and even closing up. Grooved runways must meet friction requirements and sealed runways that are not grooved must meet either the minimum standards for surface friction or surface texture.

Sand patch testing has been clarified as the only approved method for measuring surface friction. Should an aerodrome operator seek to utilise an alternate method, CASA approval will be required. The principles associated with establishing an average surface texture depth were further explained. While one test per location is agreed, 'average' refers to within a test location and not across them. The intent is that given the criticality of these areas low texture wheel paths cannot be 'fixed' by averaging with high texture runway edges. Additional guidance was also provided with regard to performing continuous friction measuring equipment (CFME) surveys.

Regarding the life expectancy of sealed pavements, no airport should be planning its maintenance based on 15 years. CASA recommends aerodrome operators should, in addition to complying with the minimum legislative requirements, make their own assessments for the need to conduct additional measured assessments based on the type of aircraft movements and frequency of operations at their aerodrome.

While there was reported confusion by interchangeability of terms skid resistance and friction, it is important to understand that skid resistance equals surface texture and friction, working in combination. Each of the terms are clearly defined in the definitions table and the terms are used very specifically to indicate different issues.

Contrary to feedback received, aircraft tyres do have a tread pattern. Although limited, the circumferential grooves assist in providing escape paths for water.

The phenomenon of aquaplaning was expanded to include explanations on dynamic hydroplaning, reverted rubber, and viscous hydroplaning, as requested, and as a means to assist understanding issues that may require resolution.

To address the question of accuracy, it has been demonstrated that for a dense grade mix, the size of the large stones has very little effect on the macrotexture. This is because the gaps between the larger stones are still filled by the finer particles. Therefore, the resulting texture is almost always 0.4-0.6 mm.

Given the small number of block paving surfaces and, in the absence of full-length concrete runway surfaces in Australia, specific comments on these surfaces were not considered warranted. CASA will consider their inclusion in future revisions.

Additional references were added throughout the document as requested. Aggregate quality assurance was considered beyond the scope of this document, and preparing NOTAMs is addressed in other guidance material.

AC 139.C-07 is about pavement strength rating and is not intended to educate designers. Amendments were made to make clear that the new strength rating system (ACR-PCR) has not yet been adopted by Australia. A target date for implementation (November 2024), has been advised, and CASA will provide ongoing industry education and support well in advance of the transition.

Content was included to support use of APSDS (Airport Pavement Structural Design System) software (commercially developed in Australia) which provides another methodology for the design and construction of aerodrome pavements.

K-values for pavement design are 'indicative' values which come from published literature which has now been referenced.

Equivalence factors relative to the thickness of types of pavement materials have also been referenced as requested, and an example as to how equivalence is applied has been included.

Content from the now redundant AC (139-25(0)) - Strength Rating of Aerodrome Pavements, has been incorporated based on the request for guidance on assessment of bearing strength of unrated pavements.

Further explanation, including an example and referencing, has been provided to explain normalised values with respect to flexible and rigid pavement thicknesses.

Additional guidance has also been included in support of a risk-based decision-making process when considering a pavement concession.

To avoid the potential for erroneous information, ACN values are not included in CASA guidance, this information should be obtained from the airline or aircraft manufacturer as appropriate.

AC 139.C-06 and AC 139.C-07 will be published concurrently with this summary of consultation.

CASA will monitor the effectiveness of the new Part 139 of CASR and the MOS with a view for further revision based on industry feedback and safety outcomes. A post-implementation review is planned to occur after the end of the transition period of 13 November 2022.