Pavement Strength: ACN-PCN and What a Survey Can Tell You

A pavement can look excellent and still be the wrong pavement

Two different questions get asked about the same runway, and they get confused constantly. How is the surface holding up? And how much load will the structure underneath carry, how many times, before something below the wearing course gives up?

Those are different properties. They are measured with different instruments, and a good answer to one tells you very little about the other.

A runway resurfaced last summer can present an almost flawless surface over a base that was never built for the aircraft now scheduled onto it. A twenty year old concrete apron can look tired, joints spalled and sealant long gone, and still carry everything on the schedule with margin, because the slab and the subgrade under it were designed for something heavier than anything that parks there. Surface condition and pavement strength can move in opposite directions, and regularly do.

This post is about the second question. The first has its own treatment in Pavement Condition Index Explained , and the boundary between the two is the thing worth getting right.

What a PCN actually reports

The Pavement Classification Number is a published statement of bearing strength, written as a five-part code, aimed at one decision: whether a given aircraft can use a given pavement without a weight restriction.

Read it one part at a time. The number is the strength rating. The letter after it says whether the structure is rigid or flexible, which is a question about whether there is a load-distributing concrete slab in it, not about what the top surface is made of. A concrete runway with a bituminous overlay on top is still reported as rigid. The third letter is the subgrade category. The fourth is the highest tyre pressure the surfacing will take. The fifth says how the number was arrived at, and it is the part most people skip.

One note on that tyre pressure row, because it trips people up. Older references still quote 1.5, 1.0 and 0.5 MPa for categories X, Y and Z. The current categories are 1.75, 1.25 and 0.50 MPa. If a source gives you the older set, it predates a revision and the rest of it may be stale too.

A reporting system, not a design method, and not a condition assessment

This is the part that gets lost. EASA’s implementation guidance puts it flatly: the method is meant only for the publication of pavement strength data in the aeronautical information publication, and it is not intended as a pavement design or pavement evaluation procedure, nor does it restrict the methodology used to design or evaluate a pavement structure.

So there are three separate activities and one of them is not the others. Design produces a structure. Evaluation works out what that structure can take. Reporting compresses the result into a code an airline can compare against a published aircraft rating. The comparison rule is simple: if the aircraft rating is at or below the pavement rating, the aircraft may operate without weight restriction, subject to the tyre pressure limit. Unrestricted here does not mean unlimited, because pavement life is finite and every movement spends some of it.

Nothing in that chain is a condition assessment. A pavement with a healthy published number can be shedding aggregate, and a pavement with a poor one can be immaculate.

Two ways to arrive at the number

The last letter of the code is the honest one. It tells you how much analysis is behind everything to its left.

Technical evaluation, code T. A specific study of the pavement and the aircraft it is meant to serve. The evaluator collects the layer thicknesses, the elastic modulus and Poisson’s ratio of every layer, and the subgrade modulus. Then the traffic: aircraft type, number of departures, operating weight, and a lateral wander standard deviation that differs by movement area, because aircraft scatter more across a runway than along a taxiway and barely at all on an apron. From that comes a cumulative damage factor, the ratio of applied coverages to coverages to failure, summed across the traffic mix by Miner’s rule and computed on the subgrade failure mode. The reported number is the rating of a critical aircraft at the weight that would consume exactly the available fatigue life.

Using aircraft experience, code U. No structural model. The evaluator starts from the heaviest aircraft regularly using the pavement, then checks what the pavement is doing about it. ICAO’s guidance is explicit about that second step: the condition of the pavement in relation to cracking, distortion or wear, together with the maintenance it has needed, is of first importance. A pavement in good condition is taken to be carrying its traffic satisfactorily. Indications of advancing distress show it is being overloaded. The guidance goes further and tells the evaluator where to look, comparing behaviour in wheel paths against behaviour between them, busy taxiways against quiet ones, braking zones, and known weak spots such as low points of grade or old pipe crossings.

The two can give different answers for the same pavement, and neither is lying. A technical evaluation on a pavement with lost construction records assumes conservative moduli and produces a pessimistic number. A using-aircraft evaluation on a young pavement produces an optimistic one, because damage has been accumulating for three years and will not appear at the surface for another six. When an airport’s published number moves sharply after a re-evaluation, the method letter usually moved with it.

The method is changing

ICAO adopted a successor with Amendment 15 to Annex 14 Volume I in 2020. The Aircraft Classification Rating and Pavement Classification Rating method, ACR-PCR, became applicable on 28 November 2024 after a four year transition, replacing ACN-PCN. The technical guidance is in ICAO Doc 9157 Part 3, Pavements, 3rd edition, 2022. In the United States, FAA AC 150/5335-5D now covers PCR reporting through FAARFIELD, and the PCN-era edition 150/5335-5C is cancelled. EASA has deferred applicability in its member states to a later date while the method is transposed into the EU regulatory framework, so which code an airport publishes right now depends on whose rules it is under. Check your own authority rather than assuming.

Three things about that change matter operationally. The new method is no longer built around a single critical aircraft chosen by judgement, it works from the whole intended traffic mix with each aircraft’s real lateral offset from the centreline. Subgrade category moves to one elastic modulus scale covering rigid and flexible pavements alike, instead of k-values for one and CBR for the other. And there is no mathematical correlation between the two systems and no formula to convert a PCN into a PCR. A spreadsheet that claims to do it is producing fiction.

What has not changed is the structural work underneath. A technical evaluation still needs layer thicknesses and a subgrade modulus. A using-aircraft evaluation still rests on observed pavement behaviour. The reporting label changed, the physics did not.

What a survey can establish about strength, and what it cannot

Here is where a drone scan sits, and the honest version is narrow.

A camera measures the surface. Everything a published rating depends on structurally lives below it. That boundary does not move with better sensors, more overlap or a lower flight altitude.

But some surface distress is evidence about the structure, and the using-aircraft method names it as an input. Alligator cracking in the wheel path is fatigue of the bound layers under repeated load rather than ageing of the binder. Rutting that presents as a broad basin, with the asphalt cracked where it flexed to follow, points at the base or the subgrade rather than at the mix. Faulting and pumping at a joint says load transfer has gone and fines are moving underneath. And the spatial pattern carries information on its own: distress concentrated in wheel paths and absent between them is a load signature, while distress spread evenly across the surface is usually environmental.

None of that measures capacity. It is evidence about capacity, which is a different thing and still worth having.

The same boundary applies to the condition index. The Pavement Condition Index is a surface-distress rating, and its own standard says it cannot measure structural capacity. A high index is not a strength claim, and a low one is not proof of structural failure.

Using both kinds of information together

The division of labour is clean once the boundary is drawn. The survey tells you where to look and when something has changed. The structural testing tells you what it means and what to publish.

Three cases where that actually changes a decision. First, new load-related distress in a wheel path on a pavement whose published number was arrived at by using-aircraft experience. The evidence the code letter rests on has changed, so the code should be re-examined. Second, an asphalt overlay being planned on condition grounds. An overlay changes the reported strength, and the coring the structural evaluation needs is cheaper done alongside the overlay design than a year later. Third, adding a heavier type to the schedule. The formal check is the aircraft rating against the pavement rating, but the condition survey is what tells you whether the existing traffic is already consuming more of the structure than the model assumed.

There is also a timing gap worth naming. A published rating is valid for the traffic it was computed against and should be re-evaluated when traffic changes significantly or after major rehabilitation. Between those events, nothing in the reporting system is watching the pavement. Our runway surface scan fills that interval, and it produces the first four rows of the table above and none of the last five. We are clear about which is which in the report, because a survey that implies a strength finding it did not measure is worse than no survey.

A published rating is a statement about a structure. A survey is a description of a surface. Confusing them costs money in both directions, either coring a pavement whose cracking turns out to be entirely thermal, or putting a heavier aircraft onto a pavement whose excellent surface sits on a base that ran out of life two fleet changes ago.