AEP-56 airfield pavement condition index surveys

What it is

AEP-56, the NATO Standard Method for Airfield Pavement Condition Index (PCI) Surveys, is an Allied Engineering Publication that gives NATO nations a single, repeatable way to score how deteriorated an airfield's runways, taxiways and aprons are. The score itself, the Pavement Condition Index, runs from 0 (a failed pavement) to 100 (excellent), and it is produced by walking the pavement, recording specific visible defects, and running the results through a fixed calculation, not by opinion or general impression. Edition B, Version 1 was promulgated in August 2026 and supersedes the prior edition, which nations are instructed to destroy. The agreement of NATO nations to use AEP-56 is recorded in STANAG 7181, and nations may ratify with reservations: Croatia's records that its air force contracts the survey to a civilian service rather than running it in-house, and Norway's substitutes a trained contractor on a five-year cycle for the base method's implied approach.

The document is addressed to the people who actually do the work. Annex F is written for "NATO Led Service Providers" applying the method "at existing or planned airfields as well as during deployed operations," and Chapter 5 describes airfield managers and pavement engineers who consult with aircraft operators, maintainers and manufacturers to decide what a given PCI figure should mean for their airfield. It is not addressed to a certifying body, and it creates none.

The survey method: branches, sections and sample units

A pavement network is broken down in three stages before anyone inspects anything (Chapter 2). It is divided into branches by use, a runway, a taxiway, an apron, then into sections of uniform construction, traffic and condition history, then into standard-size sample units: roughly 465 m² for asphalt pavement or 20 slabs for concrete pavement. Which sample units actually get inspected is a statistical choice: survey every unit, calculate the number needed for a 95% confidence level using the formula in clause 2.1, or use simpler fixed percentages the document reports as common practice, 50% of runway keel, 25% of the rest of the runway, 33% of taxiways, 25% of aprons. Units with an unrepresentative distress, a utility cut or an oil spill, are inspected separately as "additional samples" so they do not distort the statistical result for the section as a whole.

Rating distress and calculating the PCI

Each chosen sample unit is walked and its distress recorded against a defined catalogue: seventeen distress types for asphalt pavement, numbered 41 to 57 (Annex A), covering structural failure such as alligator cracking and rutting, surface defects such as bleeding, raveling and weathering, and dimensional distortion such as corrugation, depression and swell; sixteen distress types for concrete pavement, numbered 61 to 76 (Annex B), covering slab cracking and breakup, joint seal damage and spalling, and settlement or pumping. Most distress types carry three severity levels, low, medium, high, defined by a measurable criterion, a crack width in millimetres, a rut depth, a percentage of missing aggregate, rather than a subjective call, and each has its own rule for how it is measured (area, length or slab count).

Turning that data into a PCI figure follows a fixed procedure (Chapter 4). Each distress and severity combination becomes a density percentage, looked up against a deduct-value curve (Annex C for asphalt, Annex D for concrete), then reduced through an iterative correction process until the highest corrected deduct value is found. The PCI is 100 minus that value. Section, branch and network PCI figures are all area-weighted averages of the level beneath them (clause 4.3), so one poor sample unit does not stand for a whole network without that weighting being applied. AEP-56 states plainly that "this STANAG does not require the use of a particular automated PCI calculations software" (clause 4.4): manual calculation is a recognised practice, and each nation chooses its own tool, the commercial PAVER package being one named example.

Two things about the catalogue are worth knowing before working with it. First, three separate condition-rating scales sit on top of the same PCI number, a seven-band scale, a five-band scale the document attributes to the French, and a three-band simplified scale, a system the document notes "was adopted by and is also described in ASTM D5340." Second, the distress catalogue carries a second, ASTM-style numbering convention in braces alongside its primary 41-76 numbers, and the document warns this convention "produces multiple distresses with the same numerical identifier," so working from the bracketed numbers alone risks confusing two different defects.

Using the PCI to manage risk and maintenance

AEP-56 does not tell a reader what PCI is acceptable. It supplies the measurement, and Chapter 5 shows how individual nations build their own thresholds on top of it. The United States sets different minimum PCI values for primary, secondary and tertiary pavement, restricts gross aircraft load "to 75% of the calculated rated load" once a pavement's PCI falls below 40, and requires a named command-level sign-off before cargo aircraft can operate below that figure; the Netherlands uses a simpler warning-and-minimum PCI pair by pavement type. The PCI can also feed a FOD (foreign object damage) index built from a subset of the same distress data, and some nations combine it further with a structural ratio and a skid-resistance measure into a wider engineering assessment rating. None of those thresholds are AEP-56 requirements; they are national practice the document reports as illustration of how the score gets used once it exists.

What it does not cover

The PCI "does not measure structural capacity, nor provide a direct measurement of skid resistance or roughness" (clause 1.4): it is a surface-condition score, not a structural or friction assessment, and an organisation that needs those needs separate methods. AEP-56 also says plainly that it "does not address all of the safety problems associated with its use," leaving safety and regulatory practice to the user, which is what the safety considerations in Annex F exist to flag rather than resolve. And the method sets no minimum acceptable PCI of its own: every action level and load restriction described in Chapter 5 belongs to the nation reporting it, not to AEP-56.

Standards it references

• STANAG 4720Background