ACN Calculation

[AWH]

I notice that all references (that I have looked at) on determining ACN values either explain the process conceptually or refer to software. Can anyone provide me with or refer me to a step by step example showing the ACN calculation? Once I have worked through this, I should be more comfortable reading it from a table or using software to calculate it.

[john_tullamarine]

Have a read of this search (http://www.pprune.org/search.php?searchid=7756445) for starters.

Overrun may have time to offer you some comments if your luck holds.

[OverRun]

AWH (and JT); I'll reply tomorrow in detail.

For now, let me share my comment to my airport pavement engineering students - "no-one can easily and intellectually understand ACN and PCN, so I suggest you work through the tedious and turgid process and then keep a worked example to copy each time you need it".

If you ever get comfortable with it, then please let us all know how it works.

Having said that, I have derived a couple of ACNs from first principles lately - for the Shorts Belfast (the manufacturer's chart was wrong), An124 (COMFAA was [unusually] in error) and for the ATR42.

I reckon that COMFAA is the greatest asset to any pavement engineer ever, and ACN and PCN have become much more accessible as a result.

[galaxy flyer]

OverRun

To jump in here, can you recommend a source of ACN, by type. The only source I have been able to find is a PDF file from Transport Canada. Any better ones.

GF

[mutt]

Terminal section of the Jeppesen charts......

Mutt

[mkdar]

check this one out
Aircraft Classification Number - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Aircraft_Classification_Number)

[OverRun]

To add to my earlier comments, let me quote from Bruce Rodway, a very senior and respected Australian airport engineer, who was writing comments for a draft Australian CASA publication on ACN/PCN. Academically I’m concerned about plagiarism, and since Bruce took the time to write these words, I am going to properly attribute them.

In my experience I have found that, in general, ICAO’s ACN-PCN system is very poorly understood. Many people who make serious efforts to gain an understanding by reading ICAO documents, technical papers and other explanatory documents provided by various authorities often become frustrated, and are mislead and confused both by the explanations that are provided and by the conflicting and incorrect advice that is offered. In particular, the selection of a PCN remains a mystery to many airport operators. It seems that the system itself is hard to explain and hard to understand.

The ACN is a number expressing the relative effect of an aircraft on a pavement for a specified subgrade strength. The ACN of an aircraft is arbitrarily defined as twice the Derived Single Wheel Load, expressed in thousands of kilograms. So simplistically, an aircraft weighing 10 tonnes with a single tyre on each gear leg, and with 100% of the weight on the two main gear wheels, has 5 tonnes per tyre. The DSWL is 5 tonnes, and the ACN is twice this which is an ACN of 10.

To quote Bruce again:
ACNs must be calculated using a prescribed technical method. In the case of flexible pavements, the method utilises the US Army Corps of Engineers pavement thickness design method, often referred to as method S77-1. An aircraft’s ACN is calculated from its weight, its wheel layout, its tyre pressure, and the ICAO strength category of the subgrade soil beneath the runway it intends to land on. To standardize the ACN calculation and to remove operational frequency from the relative rating scale, the ACN-PCN method specifies that ACN values be calculated for 10,000 coverages. The ACN of each aircraft is clearly defined. Unlike the selection of a PCN, no judgement is involved in determining an ACN.

The actual physical meaning of ACN is often considered to be difficult to grasp. The ACN is defined in terms of an aircraft’s Derived Single Wheel Load (DSWL), which is the load on a single wheel, inflated to 1.25 MPa, that would cause pavement damage equal to that caused by the aircraft’s actual multiwheel gear (dual, dual-tandem, tridem etc) at its actual gear load and its actual tyre inflation pressure. The ACN of an aircraft is arbitrarily defined as twice the DSWL, expressed in thousands of kilograms. For example, if the DSWL of an aircraft has been calculated to be 14,000 kilograms, the aircraft’s ACN is 28. No special significance should be attached to the factor of two. It is just an arbitrary scaling factor, chosen to give a convenient range of ACN numbers. The smallest to the largest current aircraft have ACNs that range from about 2 to 140. Bigger aircraft usually have bigger ACNs, but not always. If the bigger aircraft has more wheels to spread its weight than the smaller aircraft its ACN could be lower. For example, a 116 tonne Boeing 757 has lower ACNs than a 79 tonne Boeing 737.

Each aircraft has an ACN (Aircraft Classification Number). It is a number that expresses the pavement damage caused by an aircraft relative to that caused by other aircraft. In this context, “damage” does not imply any sudden unsafe breaking of the pavement surface. In the case of flexible pavements surfaced with asphalt or sprayed seals it refers to the gradual accumulation of the small, permanent surface deformations that occurs along wheelpaths when aircraft wheels pass along the runway. After 10 to 20 years the surface may become too rough and rutted for use by aircraft and its life is over. Typically the runway is then resurfaced with asphalt to restore surface flatness and its life begins again. In other words “damage” refers to the gradual consumption of the runway’s design life.

Aircraft that have the same ACN do equal damage (ie consume pavement life at the same rate.). However, damage is not simply proportional to ACN. For example, an aircraft with an ACN 10% greater than that of another aircraft does not do only 10% more damage to the pavement. It might do approximately two to four times the damage done by the lighter aircraft. If its ACN is 30% greater than that of the smaller aircraft it could cause more than 10 times the damage. The actual relative damage can be calculated and depends upon the thickness of the pavement and the CBR of the underlying subgrade. In other words it is not possible to say that one aircraft is always X times more damaging than another aircraft. The value of X will be different for each runway.

[OverRun]

galaxy flier,

Mutt has given the Jepp source. The TC source you mention is useful and widely used.

This website has weight/ACN charts and includes some less common aircraft Airport Engineering (http://profemery.info/aviation.html#ACN%20CHARTS)

Cheers
OverRun

[AWH]

OverRun
Thank you for the information provided, it really puts everything in perspective. I would like to work through the process and create the example as recommended.
I 'understand' the last step. The tick it seems is to arrive at the ESWL incorporating the alpha factors etc. As I understand this is based on the CBR cover curve approach. I am still searching, but if you could point me to or provide me with the info to derive ACN from first principles, I would appreciate it.

[AWH]

Okay, method S77-1 is key. Refering to the "CBR cover curve approach" I am obviously considering flexible pavements for this round.

[OverRun]

AWH,

Detailed discussion on the EWSL and the CBR cover curve can be found in suitable great detail with worked examples in the (older) textbooks.

The ICAO pavements manual (1983 is the latest version: ICAO Aerodrome Design. Manual, Part 3, Pavements, 2nd Edition, 1983) has this, as does the pavement engineer's bible - Yodar & Witczak [Yoder & Witczak; Principles of Pavement Design 2nd Ed. Wiley 1975].

The older version of the FAA pavement design manual (up to and included AC150/5320-6D) also has some discussion but does not explain EWSL or CBR cover curve very well.

ICAO/Yodar are very much pre-computer, and thus not available electronically. There might (?) be a link to the ICAO manual in the "useful website and document references" thread on Tech Log.

This is opening the door to fall very deep into the arcane and unintelligible aspects of the topic. Be sure your water wings are well pumped up :)

[Sir George Cayley]

Interestingly, ICAO have just issued a notice of proposed rule change that amends the criteria for ACN/PCN calcs. It affect Annex 14.

I'm not clever enough to give a link to it but if someone can I think it's very informative.

A big discussion within ICAO's Pavement Sub-Group has been the factor applied to the metric for 6 wheel bogeys (read 773) the Alpha factor. I think Boeing won.

If an airport wants to pin its pavements' resilience to load then a falling weight deflectometer might help. Just don't stand too close unless you want very flat wide feet:{

There's the 10% overload allowance too btw.

Sir George Cayley

[AWH]

OverRun: Thank you for this information, I am glad you point me to Yodar that was one of the sources on my list - now I know it is worth getting hold of. I was also able to find the link to electronic ICAO docs but can't find Part 3 (doc9157)...
http://dcaa.slv.dk:8000/icaodocs/ (http://dcaa.slv.dk:8000/icaodocs/)
I know of the udated aplha factors published in 2007. Sir George, thank you for informing us of the latest notice, I will follow this up. Use of FWDs will definitly form part of 'figuring out' the PCN side of the ACN-PCN concept - which I believe is a slightly larger can of worms :confused:

[OverRun]

AWH - if you are in pavement business, Yoder is a great textbook to buy and keep for decades. I suggest working through the examples in the first couple of chapters (and the single and two layer examples, but not the three layer), as a precursor to the runway design stuff.

The ICAO 9157 must unfortunately only be available as hardcopy (written in the pre-computer days), but can be purchased from the ICAO shop. I got excited when I checked my library and thought I found an electronic copy, but it was only a couple of pages of it that I had scanned earlier.

A link to a paper discussing the background to the new alpha factors is here: http://profemery.info/alpha.pdf

Sir George - the FWD (and its big brother HWD) are indeed a useful way of collecting data to evaluate the pavement strength. They introduce a whole new layer of complexity in analysis though and produce mind-boggling amounts of data with no clear or immediate answer. One way of accessing and using the data simply is to apply benchmarks, which can be done by any airport engineer with a simple spreadsheet. There is a paper here which discusses this and shows the pretty pictures in Figures 3 and 4:
http://profemery.info/papers/Airport_deflection_AMS_2009.pdf

Cheers
OverRun