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Old 24th Feb 2004, 16:32   #1 (permalink)
Join Date: Dec 2003
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Runway Strength

Can someone explain the system of stating runway load limits (PCN??) and is it possible to relate these to A/C types?

I’m often asked to assess obscure a/ports & strips and have previously based the above on “what have you landed here previously?” (probably the most accurate…)

(Be as complex as you need to be & practical advice welcome)


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Old 24th Feb 2004, 20:55   #2 (permalink)
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Hi Mini,

It's really quite simple to determine if one of your aircraft can use a particular strip.

Each Aircraft has an Aircraft Classification Number (ACN).

Each Runway has a Pavement Classification Number (PCN).

If the ACN is less than or equal to the PCN, then the Runway is strong enough for that particular aircraft.

If the ACN is greater than the PCN, then that aircraft cannot safely use that runway.

The PCN is published for every runway, and you should have no difficulty in getting hold of ACN's

Simple really

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Old 24th Feb 2004, 21:05   #3 (permalink)
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I think you will find that the PCN for a given runway is for continuous use.

It is usually possible to land an aircraft with a higher ACN than the PCN given provided that the airport authority gives permission on an occasional or one off basis.
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Old 24th Feb 2004, 21:10   #4 (permalink)
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Do you use Jeppesen's for assessing airfield usability?
If you do in the Airport Directory Section there is a work through guide for PCN and ACN.


PCN = Pavement Classification Number. For example look up BFS. RWY 07/25 is given as PCN71/R/B/X/T

There is a de-code on page 5 of the airport directory.
R = Rigid pavement
B = Medium (Sub grade strength)
X = Medium (Tyre pressure category)
T = Technical (The method used for assessment)

Pages 7 thru 22 of this section contain tables relating to ACN (Aircraft Classification Number).

Look up the Aircraft Type. In my case BAe146-200 with standard pressure tyres (Page 11)

There are two weights given. One is empty the other is Max Apron Mass. Assuming I wish to operate through the airfield with a full aircraft I would work on the larger figure.

Moving across the table you will come to two sections.
ACN relative to Rigid pavement types and ACN relative to Flexible types. Refering to my PCN for BFS above 'R' stands for rigid so this is the box I would use. 'B' is the sub grade for medium so again this is the cloumn I would look down. It gives an ACN of 21.7 for this aircraft at this weight.

As this does not exceed the PCN of 71 I am well within limits for this airfield.

When airfeilds start getting into Isolated wheel loads and the like, it gets much more complicated.

If you want to know anything else you can always PM me,

Hope this helps.

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Old 25th Feb 2004, 10:29   #5 (permalink)
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What type of aircraft are you talking about? Does it have a PCN published for it? If so and as splitter has pointed out, its easy to find the ACN for an airport and compare both.

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Old 26th Feb 2004, 00:52   #6 (permalink)
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YS, exactly what I am looking for, Jeppesens probably exists in someones drawer at HQ, but I've never seen it. I presume I can get a copy on-line?

Mutt, very varied, depends on the job & place. Usually Antonovs or IL76. As these have soft field L/G I,d imagine that their ACN's will be fairly low.

JW411, good point that. I would have to clarify in advance where liability would lie and the extent of that liability should a "heavy" landing cause damage.

CIPWM, thanks a lot, makes it simple.

Guys, this is a lot of good information - thanks to all.

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Old 7th Mar 2004, 16:32   #7 (permalink)
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Pavement Strengths

Way back in my day - 50s - 70s we had LCNs

Load Classification Numbers.

Flight manuals had graphs against AUW and LCNs for airfields were published some place.

When did LCN have a name change ?

Bit like the change from Dead Reckoning to Ded (for deduced) Reckoning.
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Old 15th Apr 2004, 14:01   #8 (permalink)
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When did LCN have a name change ?
It didn't, just got replaced by a newer system.


The ACN/PCN system (Section 7.9) as referenced in ICAO Annex 14, "Aerodromes," First Edition, July 1990, provides a standardized international airplane/pavement rating system replacing the various S, T, TT, LCN, AUW, ISWL, etc., rating systems used throughout the world.
I'm familiar with the ACN/PCN system, the LCG system, and to some extent the LCN system.

But I'm really confused where an airfield lists Maximum AUW (all up weight in x1000Kgs) dependant upon type of undercarriage assembly. (This applies to most of the airfields in the USA)

The only reference I have decodes the following codes:

S = Single Wheel
T = Twin Wheel
B = Bogie
DB = Double Bogie

But I'm coming across codes such as AUW100S/190D/360DT/550TRT/850DDT

Can anyone decode the following for me?


Rgds SB777

p.s. which of the codes above would a B777-200 come under?

Last edited by SpeedBird B777; 17th Apr 2004 at 13:12.
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Old 16th Apr 2004, 12:35   #9 (permalink)
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I managed to get an answer to some of this over on Airliners.net

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Old 17th Apr 2004, 12:10   #10 (permalink)
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Ok, I've been searching round the FAA web site all day still looking for an "official decode".

This page http://www.naco.faa.gov/content/naco...s/legendAD.pdf has the remark:

"Runway Weight Bearing Capacity/or PCN Pavement Classification Number is shown as a codified expression.
Refer to the appropriate Supplement/Directory for the applicable codes e.g.
Rwy 14-32 S75, T185, ST175, TT325."
The only problem is I don't have access to the "appropriate Supplement/Directory" http://www.naco.faa.gov/index.asp?xm...y/af_directory

Anyone got a "NACO Airport/Facility Directory" they could look this decode up in for me?
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Old 17th Apr 2004, 14:01   #11 (permalink)
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I'll take a look in more detail tomorrow at your questions. In the meantime, a part of what you are looking for is at the following:


Simply work out your aircraft weight, look up the ACN in the graph, and check it is lower than the published PCN for the runway. If it is a bit higher, as JW411 said, there are procedures for handing that. Note the B777 graphs because they are like the An-22.

The B777 is a triple tandem (which should be TT, but could be called TRT). Caused a real problem when it got invented because the american pavement engineer's theory couldn't handle THREE axles. Anyway I'll come back tomorrow, and answer Speedbird's questions too.

Mini - in the meantime, do you have any examples of the words that you are encountering in assessing the limits at various airfields? Is it ACN or PCN or MAUW or what? Many countries have their own words, even if they mean the same thing. A bit confusing sometimes. Also, do you have an example of an actual aircraft type/operation and some of its potential takeoff weights. Also its empty weight and zero fuel weight. If you do, then I can give a more complex answer to illustrate how the bits fit together.
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Old 17th Apr 2004, 23:05   #12 (permalink)
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OverRun et al,

First of all thanks for all the feed back on this.

If I can clarify the situation a little:

When I'm asked to do an assessment, it is more often than not to try to get an overall picture of the capacity of the airport/airfield in the event that we may need to use it, i.e. its not that we're scheduling an imminent arrival.

The aircraft selected in the event that we do need to use it will be based on the assessment. We have had an instance in the past (before my time...) where an a/c has sat on the ramp for three days because the required unloading equipment wasn't available.

I can't give specific examples of local terminology, but I'm trying to get an overall view on how this is assessed and how I might extract relevant data from officialdom.

Sorry if this is a little vague but I'm not a pro! I was tossed into this at the deep end...

Cheers to all
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Old 18th Apr 2004, 04:21   #13 (permalink)
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Decoding the undercarriage assembly. As well as decoding the various authorities words, I've expressed it in terms of the simple approach of tyres per main gear leg [helps me understand it better. '']

The most common civil aircraft undercarriage configurations are:
S = single wheel = one tyre per main gear leg = DC-3
D = dual = T = twin wheel = double = two tyres per main gear leg = Boeing 737
DT = dual tandem = dual in tandem = B = bogie = TT = twin tandem = 4 tyres per main gear leg = Boeing 767
DDT = double dual tandem = DB = double bogie = 4 tyres per main gear leg, and two main gear legs per side = Boeing 747

The more exotic configurations are (usually military):
ST = Single-tandem = C-130
SBTT = Single-Belly Twin Tandem = KC-10 or DC10-40
TT = Twin-tandem type (includes quadricycle landing gear) = B-52
TRT = Triple-tandem landing gear = C-17
TDT = Twin delta-tandem landing gear C-5

and of course the Boeing 777. I may have been wrong in my comments about TT and TRT above. Boeing themselves call this a tridem gear, and designate this as TD = tridem dual. But you won't find it mentioned as that very often.

Decoding the 'airfield listed maximum AUW dependant upon type of undercarriage assembly' is next. It does apply to most of the airfields in the USA, which means that it is usually MAUW in '000 lbs except for those airports that express it in '000 kgs. Confusing at times.

The interpretation of these ratings is that any aircraft having that designated type of main landing gear can operate up to the allowable gross weight (in 1,000 pounds) indicated by the numerical value of the rating. For example, the rating D145 means that each of the A319, A320, DC9, MD80, F27, F28, BAe146, B727, or B737 aircraft can operate on the rated pavement at gross weights up to 145,000 pounds, regardless of the individual airplane characteristics such as wheel spacing or tire pressure. The common and overriding factor is that these aircraft all have dual-wheel main landing gear arrangements.

The All Up Weight (AUW) method of assessing allowable gross weights is somewhat nebulous in that it is a method that allows the operation of many different aircraft on the pavement without regard to its pavement loading characteristics. This method does not consider factors such as number of wheels on a main gear, differences in wheel spacing, tire pressure, or percent of weight on the main gear. The ACN/PCN method takes much better account of these factors, and is more common outside the USA.

So now we can start to decode the airfields. Do it in two passes – the first is the literal pass, and the second is to check if it makes sense and someone hasn't got pounds and kilograms mixed up:

Rwy 14-32 S75, T185, ST175, TT325.

The first pass of decode gives us:
S = single = 75,000 lbs
T = twin = 185,000 lbs
ST = single tandem = 175,000 lbs
TT = twin tandem = 325,000 lbs.

Now check that against real aircraft to see that it is in lbs and not kgs, and that it makes sense:

S = single = 75,000 lbs. Ignore this – it helps the engineer think in terms of ESWL which is beyond the scope of this discussion.
T = twin = 185,000 lbs. Boeing 737-800 HGM has max takeoff weight (MAUW) of 172,500 lbs – that's pretty close, and I know the A321 is a bit heavier which makes this limit credible.
ST = single tandem = 175,000 lbs. Lockheed L-100 is 155,000 lbs MTOW, and the military allow an overload in their C130s, which makes this limit credible.
TT = twin tandem = 325,000 lbs. Boeing 767-200 is 300,000 lbs MTOW for the regular and 387,000 lbs for the ER version, which makes this limit credible.
I get a sense that the runway structure here is a bit thin, and the larger variants of the 767 are pushing the limit. The 767-400 ER would probably be over the limit, as would the B747.

Next decode:

The first pass of decode gives us:
S = single = 100,000 lbs
D = dual = T = twin = 190,000 lbs
DT = dual tandem = TT = twin tandem = 360,000 lbs.
TRT = Triple-tandem landing gear = 550,000 lbs
DDT = double dual tandem = 850,000 lbs.

Now check that against real aircraft to see that it is in lbs and not kgs, and that it makes sense:

S Ignore this as before.
T = twin = 190,000 lbs. Boeing 737-800 HGM (MAUW) of 172,500 lbs; A321 is a bit heavier which makes this limit credible.
DT = dual tandem = 360,000 lbs. Boeing 767-200 is 300,000 lbs MTOW for the regular and 387,000 lbs for the ER version, which makes this limit credible.
TRT = triple-tandem = 550,000 lbs. C-17 is 585,000 lbs MTOW. The Boeing 777 is 506,000-660,000 lbs MTOW. I simply don't know which they are referring too. Is it TRT or TD or TT or what? If there is a National Guard or Air Force base at the airport, then it could be the C-17. If the airlines are operating 777s, then it could be the 777.
DDT = double dual tandem = 850,000 lbs. Boeing 747-400 is around 850,000 lbs MTOW depending on the version, which makes this limit credible.

What are the limits at this runway mean in terms of ACN/PCN? Well I checked the ACN charts for typical aircraft for each category to see what ACN would be equivalent. This is approximate since different aircraft in a single category can have slightly different ACNs, as mentioned above.

Type '000 lbs ACN
D 190 50
DT 360 48
TRT 550 46
DDT 850 62

So I guess this runway would have a PCN of 48-50. The rating for the DDT is higher at 62, which could mean that the pavement can be higher rated at PCN 62,or the runway really is PCN 50ish, and the higher weight limit may have been set for a strategic reason.


When you're faced with having to stab in the dark a bit, then there is another method called the "Using aircraft method of determining PCN".

This procedure can be used when there is limited knowledge of the existing traffic and runway characteristics. It is also useful when engineering analysis is neither possible nor desired. Accuracy of ratings based on "Using aircraft" is by nature less than that for a Technical evaluation, but PCNs can be assessed more quickly and with minimal cost. However, airport authorities should be more flexible in the application of a Using aircraft PCN in that the rating has not been rigorously determined. There are only two basic steps required to arrive at a "Using aircraft PCN":

1. Determine the airplane with the highest ACN in the traffic mix currently using the runway. This is the critical airplane.

2. Assign the ACN of the critical airplane as the PCN.

Then check your aircraft ACN is below that PCN. It can be hopelessly over-conservative, but hopefully not too optimistic.
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Old 18th Apr 2004, 04:36   #14 (permalink)
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i believe you are aware that the PCN only needs to be satisfied IF the acft operates there on a regular basis. however, the airfield CAN still be used for a one-off emergency landing.

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Old 18th Apr 2004, 10:05   #15 (permalink)
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As SuperRanger and JW411 have indicated above -

You can exceed, with the airport's permission, on a reasonably regular basis, the allowable PCN by up to 10% without problems. So for a PCN of 50, an ACN of 50+10%=55 would usually be allowed. That could be an extra 15,000 kgs for a Boeing 777. No problems say once a week.

You might be able to exceed, with the airport's permission, on an occasionable basis, the allowable PCN by up to 25% with few problems. So for a PCN of 50, an ACN of 50+25%=62.5 might sometimes be allowed. No problems for something like a one-off series of 3-5 charters. There was a great post by JW411 about dinners on Greek Islands that can help this frequency increase:

If you are pushing it, and you exceed, as a once-off, the allowable PCN by 40%, you'll walk away, but there may be some problemxs with ruts in the runway. That is usually no problem for the aircraft, but is for the airport. Hint - the real pros do this at night. If the plane gets off again successfully, the airport doesn't find out until the next morning's runway inspection, by which time you are long gone. I am told that if you feel the aircraft start to bog, give it lots of omph and it usually crawls out, up and away.

How far can you go? Well I reckon the folowing photo was at an overload of 100%, and the 747-400 didn't sink in until it slowed down to 20 knots.


And they all walked away, which counts for a lot.
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Old 18th Apr 2004, 11:28   #16 (permalink)
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OverRun, Thank you very much for taking the time to try and explain all of this for me.

The airfield I was using as an example was ROCKFORD (RFD) not because I have any intention of going there, but just because it includes the code TRT.

I guess if an airfield doesn't list TD and only lists DT and DDT I'm stuck having to use the DT figure, or would it be reasonable/possible to interpolate between the two.

The ACN/PCN system is beginning to look so simple now!

Thanks for the point about the figures sometimes being in lbs rather than Kgs. The Aerodrome Directory I have access to (Aerad) states all its figures in Kgs, where as its becoming apparent that some of there figures are in Kgs but about 75% of the airfields listed (in the USA) are in fact in lbs! Doh!

A couple of other questions for you

1) I sometimes still see Isolated Single Wheel figures quoted.
e.g Rome (LIRF) ISWL45000.

Do I just multiply this by the number of main wheels?

2) The maximum certified tyre pressure for the B777-200 is 215 psi. So under the ACN/PCN system W and X codes are fine, but a few airfields have Y (Low, Max 145psi) listed even though the PCN number seems quite acceptable.
e.g. Tel Aviv (LLBG) PCN 84/F/B/Y/U

Would I be likely to cause damage to the runway here, or is it just like JW411's dinners on Greek Islands?

Rgds SB777.

Last edited by SpeedBird B777; 19th Apr 2004 at 11:01.
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Old 19th Apr 2004, 03:06   #17 (permalink)
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Rockford RFD

TRIPLE-TANDEM TRT550 RY 07/25 & TRT590 RY 01/19.

Following my posts above on this topic, rather than interpolation, we can apply the two basic steps required for the "Using aircraft PCN" to find out what the Boeing 777 limits are:

1. Determine the airplane with the highest ACN in the traffic mix currently using the runway. This is the critical airplane.

Using TRT550 as being the critical plane, and guessing this is a C-17 at 550,000 lbs. Then from the C-17 ACN chart (newly minted at
a C-17A at 550,000 lbs (which divided by 2.2 = 250,000 kgs) has an ACN for an assumed subgrade of rating B of 55.

2. Assign the ACN of the critical airplane as the PCN.

Assign the ACN 55 as being the runway PCN = 55

3. From the Boeing 777 ACN chart (same source as the C-17 chart), for the same assumption of subgrade of rating B, the ACN of 55 is an aircraft weight of 226,000 kgs.

So in summary, if the RFD runway strength rating of TRT550 was actually meant as the Boeing 777, then the load limit is 550,000 lbs (or 250,000 kgs). If it was meant as the C-17, then the Boeing 777 limit is 226,000 kgs (which is 497,200 lbs). Conservatism suggests the lower limit until the airport has confirmed otherwise.

If the airport only published DT and DDT, I would still use the "Using aircraft PCN" method rather than just interpolating between DT and DDT, because that gives a rational and acceptable basis for judgement. I would then check with the airport concerned.

SpeedBird B777, your two other points:

Rome (LIRF) ISWL45000. Look I started the calculation, but the risk of confusing ISWL and ESWL is too great, and from the computer printout in front of me, the same aircraft with a SWL of 45,521 can also be rated as an SWL of 273,125 for the same load. Multiplying SWL by the number of main wheels will either be OK or a ton of grief. Sorry, but you'll have to ask the airport what they mean.

Airfields with low tyre pressure such as Y (Low, Max 145psi) listed, even though the PCN number seems quite acceptable, are doing this because the surfacing is weak although the pavement below is strong. In an emergency, you can land there. Otherwise get a pavement concession from the airport. Damage to the surfacing is possible when the aircraft is turning. The sort of damage is loose stones, which the two large Trent (Dyson look-alikes) love to suck up. This is one Greek dinner that you could directly wear the consequence of, as the fan disintegrates at takeoff power. The concession, if granted, will usually by on the basis of previous experience and/or an inspection after each landing and takeoff.
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Old 19th Apr 2004, 10:48   #18 (permalink)
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Thanks once again for all your help OverRun.

It's been fascinating following your "Using aircraft PCN" calculations to convert the codes used in the USA, but rather goes beyond anything I could do in flight.

Interesting point about the low tyre pressures, particularly as we were operating to this airfield on a daily basis not so long ago.

Brgds SB777.
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