Haroon,
An over-simplification of the ACN system, useful because it does explain it well, is that on a strong runway, the ACN is simply twice the wheel load in tonnes. So your 777, with 12 main gear wheels, at 300 tonnes MTOW, has 25 tonnes per single wheel which is an ACN of 25 x 2 = 50. And a linear relation of weight to ACN is fine.
The runway [pavement system] is modelled as a linear elastic layer system. That means that if the wheel load is 25 tonnes and the response is 50, then a wheel load of 28 tonnes means a response of 56.
Ok – it is not quite as simple as that. The load on the main gear wheels is a bit less because some load is taken by the nose gear. Then if you have a weaker subgrade (more so as you move weaker and down to Very low CBR = 3%), the runway pavement gets pretty thick and you find that the loading from some of the other wheels in the gear start to load the pavement under your wheel, so their loads have to get added to this wheel load. You end up with an “equivalent single wheel load” which adds all these contributions together, and which is why for weak subgrades, the ACN is now pretty much double what it was for strong subgrades, say 100. No need to understand that in detail, because for a given subgrade CBR, the relation of weight to ACN is linear and you can use the tables to calculate it.
For your question about Jeppesen, the explanation is that the ACN is just a straight-line graph with ACN vs aircraft weight. And it is a linear relation. The graph doesn’t change for a given aircraft and undercarriage layout. It can be interpolated. It can be extrapolated. Since aircraft weights change with time and variant (but provided that the gear layout stays the same), the graph still applies and you can look up or calculate the ACN. Most published tables tend to be out of date as aircraft weights creep up with time. Here is my example of the 777 graph.
ACN chart 2009
In your Jeppesen table with 160,481 kgs mass, and Medium CBR 10 subgrade, they gave an ACN of 25. Checking with the same graph (and software) for the 777-300, 157,850 kgs mass, and Medium CBR 10 subgrade gives an ACN of 24, which is the same as your example. Either numbers are valid for interpolation (or extrapolation). Shucks, the plane could have a gold plated interior and an operating mass empty of 200,000kgs and its ACN at 200,000 kgs mass and Medium CBR 10 subgrade would then be 33. That is still valid for interpolation when used with the MTOW numbers. Let me run the software and throw some more numbers out to check, for medium CBR 10 subgrade: 250t mass = ACN 45, 280t = 53. Empty mass 150,000kg = ACN 23 (as you correctly calculated).
Umm - I also find that working out the interpolation from the tables is not that easy so I either use the many graphs available at
Airport Engineering or I use airport pavement design software.