Blog Post

How to evaluate pavement strength using ACN-PCN

787 Guide • May 26, 2021

So how do you decode a PCN code string like 74/R/A/X/T? Would that pavement strong enough for a Boeing 787?

This blog post is a shortened version of the Pavement Strength chapter in the 787 Guide app. See links below to get the app!

You could probably work an entire career in a major carrier without ever coming across having to do an ACN-PCN assessment. As far as I remember, it was not taught in ATP theory (at least not back in the 90s when I did it). Therefore, you won't be alone finding all this somewhat vague.

What does ACN-PCN even stand for? Aircraft Classification Number and Pavement Classification Number were introduced by ICAO in the early 1980s to simplify assessing the bearing strength of a pavement. It may come off as complicated at first, but it is actually an elegant system that improved upon a wild plethora of earlier methods. 

Note that pavement strength evaluation is mostly for normal operations. When selecting an airport in an emergency, bearing strength is perhaps not a primary consideration. Any piece of runway is better than no runway, and being on the ground is usually better than not, if the situation is bad. But there are corner cases, and when the checklist says “land at nearest suitable airport” there could be situations where you want to take pavement strength into that “suitable” decision. 

Finding the PCN code
Let’s look at Uzbekistan. Being in the middle between Europe and East Asia  it has a few enroute alternate airports of interest. In the Jeppesen manual we have Tashkent, Samarkand and Termez; all three on the same page, here replicated next to each other for clarity.

While Tashkent is certainly the major airport, they all look quite acceptable as enroute alternates. Long runways, airport of entry, and so on. There are certainly other aspects to discuss here, but let's focus on the PCN.


Decoding the PCN code
Tashkent 08L/26R runway has a PCN code that reads 74/R/A/X/T. The ACN-PCN system works by comparing the PCN value, in this case 74, with the aircraft ACN value. But the aircraft ACN value depends not only on the aircraft weight, but also on pavement type and the pavement sub grade strength. We need to know these two in order to find the ACN value.


The letters following the number 74 contains this information.


Entering the PCN in the ACN-PCN calculator supplied with 787 Guide Pro and then tapping View PDF Report, we see the following decoder:

The first letter, R, indicates that this runway has a rigid pavement, most likely concrete. The second letter, A, indicates that the subgrade, the foundation underneath the concrete, is of high strength. These are the two details we need in order to find our ACN, but it should also be checked that the maximum tire pressure for this pavement is more than that of the 787. In this case it is. The code shows X for high (254 psi) and our tire pressure shouldn't be more than 230 psi. The last letter, T, simply indicates how the data in the PCN code was calculated. For most airports that can accept a 787, you'll see T.


Finding the ACN
In the table below the decoder, you'll see the table for ACN at Maximum Taxi Weight. An official version of this table can be found in the Jeppesen manual or equivalent route manual.


We use the column for Rigid Pavements and the sub column for Subgrade A. The ACN in this case is 65.


Comparing the PCN and ACN
We're done! Comparing the two values is the easy part: 65 is less than 74 so pavement strength will not be an issue in Tashkent. This wasn't too surprising, Tashkent is a major airport that sees a lot of heavy traffic. That's what I meant in the opening paragraph, that if you only fly to major airports you will rarely ever have to think about pavement strength.


Lose some weight, honey
But what about Samarkand then? Beautiful city with lots of history. Worth a visit. Can we go visit with a fully loaded 787? This would be the typical case scenario where ACN-PCN evaluation might come up: your airline has been asked to fly a charter flight to a smaller airport that's not in the regular network.


Let's decode the PCN code string and find out:

Oh, that won't work. PCN is 48 and ACN is 76. Notice how it went up from 65 to 76 because this pavement has subgrade strength B.



Now, remember that this ACN is for Maximum Taxi Weight, so for a lighter weight it might be okay. How do we find out? The ACN table in the Jeppesen manual also shows the values for the aircraft empty weight. Linear interpolation between this weight and the maximum weight is an acceptable calculation. Alternatively, go to the "787 Airplane Characteristics for Airport Planning" document found on the Boeing website and use the charts.


For now, let's just plug the PCN into the 787 Guide Pro ACN-PCN calculator and run the evalutation with a lower weight.

Looks like 175,000 kgs gives us an ACN of 46, which would be fine. With that hefty limit on the takeoff and landing weight, operation into Samarkand would be viable with regards to pavement strength. Someone else will have to calculate if it makes financial sense...


The full Pavement Strength chapter is found in the 787 Guide app, available for both iOS and Android. The ACN-PCN calculator is available to 787 Guide Pro subscribers in the iOS app.


Download the app on the iOS App Store here!


Download the app on the Android Google Play Store here!


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