AATMP-13 runway friction and braking conditions

What it is

AATMP-13 is the NATO Allied Air Traffic Management Publication (AATMP) that standardizes how participating nations measure runway surface friction, the grip a runway pavement still offers a braking aircraft. Its stated purpose is so that nations can "maintain airfields in a timely and effective manner" and "carry out aircraft operations safely." It does not bind a supplier or a company directly: nations record their agreement to use it in STANAG 3634, and it reaches an airfield through a nation's own implementation of that agreement, or a tasking or contract that names it, not by existing on its own.

Two friction levels, kept deliberately separate

The document draws a line between two measurements that share a name but not a purpose. Maintenance Friction Levels come from standardized testing, run with a self-wetting continuous friction measuring device fitted with a smooth tread tyre, and are used mainly by engineers to plan and fund pavement maintenance. Operating Friction Levels come from testing under real ambient conditions and are compared against agreed thresholds to advise Operators, and through them aircrew, on braking action in the conditions of the moment. AATMP-13 states plainly that correlation between the two "has not yet been officially established," and that it only standardizes the first: setting Operating Friction Levels stays "the responsibility of individual participating nations," informed by recommendations in two civil documents rather than fixed by this publication.

What the testing regime actually requires

A nation may substitute a decelerometer vehicle or a locked-wheel spot measuring device for the reference continuous device, but only once it holds adequate correlation data against a recognised continuous device and has run enough tests to validate the analysis; it must then make other participating nations aware whenever it uses the substitute device, and provide the supporting correlation evidence if another nation asks for it. Testing itself is expected when a runway is first constructed, after resurfacing, and periodically after that, with each nation setting its own interval based on factors such as aircraft type, usage frequency, climate and pavement type.

Three variables are standardized so results are comparable between nations: the minimum number and layout of survey runs, the method used to calculate the friction values, and the size and layout of areas of low friction that count as significant. Paragraph 10 scales the minimum number of standard survey runs with runway width - six runs for a 30 m, 45 m or 60 m wide runway, rising to eight for a 90 m runway - run parallel to the centreline and covering its full length, with more recommended where formation take-offs are routine. The average of all survey runs is the Maintenance Friction Level, though paragraph 11 notes that a significant low-reading section of runway (broadly, around 100 m) can trigger rectification even when the overall average is acceptable.

The decision rule when a reading is low

Annex A sets two thresholds for each of nine device types, at two standard test speeds: a Maintenance Planning Level, defined as the friction level below which corrective maintenance action should be initiated, and a lower Minimum Friction Level, defined as the level below which information that the runway may be slippery when wet should be made available. Paragraph 12 carries the two thresholds at different weight. Below the Maintenance Planning Level, corrective maintenance action should be initiated - a recommendation. Below the Minimum Friction Level, corrective maintenance action shall be taken, and the runway's slippery-when-wet status must be promulgated by NOTAM - both mandatory. Paragraph 13's further set of factors, including pavement drainage, surface age and contamination, prevailing winds and operational requirements, is meant to inform the remedial action chosen, not just the trigger for taking one.

Who implements it, and how

AATMP-13 addresses "participating nations" and, in practice, the Operating Authorities and Operators that run a NATO airfield day to day. A dedicated annex is written for a further role: the NATO Led Service Provider implementing the STANAG "at existing or planned airfields as well as during deployed operations." That annex sets out a general checklist before implementation - checking the publication's suitability for the type of operation, its currency, the related documents, and the ratification and reservation status of the nations involved - followed by a determination of whether existing airfield facilities and procedures already comply. It also lists specific safety considerations, consequences and mitigations covering a low pavement friction coefficient, incorrect friction measurement, inappropriate remedial action, and the training of airfield pavement maintenance personnel. None of this replaces the full safety surveys already required under STANAG 4720's ATM Safety Management System; the document is explicit that those "shall still be carried out" regardless.

Standards it references

• STANAG 4720Background