How to evaluate pavement strength using ACN-PCN
So how do you decode a PCN code string like 74/R/A/X/T? Would that pavement strong enough for a Boeing 787?
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!
