Simply put, you shouldn't be placing that pretty airplane of yours on any surface not strong enough to support it. Just because you were able to taxi it there, doesn't mean it is going to stay there. One of my office mates at the Pentagon once watched helplessly as his empty C-5A Galaxy sunk more than a foot into the tarmac after the K-Loaders put nearly a hundred thousand pounds of cargo on it. You could see the same thing after the fuel truck pulls away.
It would be a simple matter except for the fact not everyone uses the same terminology, not everyone makes the necessary pavement measurements, and sometimes those measurements don't apply to any place other than the runway.
Always remember that the airport manager is concerned about his PCN versus your ACN primarily in terms of how it impacts the life of his pavement. You need to be concerned in terms of how is it going to damage your airplane. The best question you can ask is "have you parked aircraft of my type on your ramp recently?"
Here then is some guidance from the ICAO, the FAA, and the G450 Aircraft Operating Manual, as well as a few other more esoteric sources as noted in the list of references below. Examples are shown in green. I've added a few comments of my own in blue.
[Advisory Circular 150-5335-5, ¶ 3.] In 1977 the ICAO established a Study Group to develop a single international method of reporting pavement strengths. . . . The study group developed the Aircraft Classification Number - Pavement Classification Number (ACN-PCN) method. Using this method, it is possible to express the effect of individual aircraft on different pavements by a single unique number which varies according to pavement type and subgrade strength, without specifying pavement thickness. This number is the Aircraft Classification Number (ACN). Conversely, the load carrying capacity of a pavement can be expressed by a single unique number, without specifying a particular aircraft. This number is the Pavement Classification Number (PCN).
[Advisory Circular 150-5335-5, ¶ 3.] The system is structured so that a pavement with a particular PCN value can support without weight restrictions, an aircraft which has an ACN value equal to or less than the pavement's PCN value.
[ICAO Annex 14, Vol I, pg. 1-2] Aircraft classification number (ACN). A number expressing the relative effect of an aircraft on a pavement for a specified standard subgrade category.
[ICAO Annex 14, Vol I, pg. 1-6] Pavement classification number (PCN). A number expressing the bearing strength of a pavement for unrestricted operations.
If the ACN is less than or equal to the PCN, the pavement can support the aircraft without weight restrictions.
[ICAO Annex 14, Vol I, ¶2.6.6] Information on pavement type for ACN-PCN determination, subgrade strength category, maximum allowable tire pressure category and evaluation method shall be reported using the following codes:
[ICAO Annex 14, Vol I, ¶3.13.3] Recommendation.— Each part of an apron should be capable of withstanding the traffic of the aircraft it is intended to serve, due consideration being given to the fact that some portions of the apron will be subjected to a higher density of traffic and, as a result of slow moving or stationary aircraft, to higher stresses than a runway.
Caution: The published PCN normally covers only the runway and will occasionally be listed for taxiways. (The ICAO Recommendation quoted here is not enforced.) It will almost never be given for parking ramps. The surest way to ensure the ramp is stressed for your aircraft is to ask if they normally park aircraft of your weight class in these spots. When in doubt, move the aircraft or ask for metal plates to place under the wheels. It is almost never advisable to fully fuel the aircraft days prior to departure unless you are certain of the ramp's load handling ability.
[ICAO Annex 14, Vol I, ¶19.1.1] Overloading of pavements can result either from loads too large, or from a substantially increased application rate, or both. Loads larger than the defined (design or evaluation) load shorten the design life, whilst smaller loads extend it. With the exception of massive overloading, pavements in their structural behavior are not subject to a particular limiting load above which they suddenly or catastrophically fail. Behavior is such that a pavement can sustain a definable load for an expected number of repetitions during its design life. As a result, occasional minor overloading is acceptable, when expedient, with only limited loss in pavement life expectancy and relatively small acceleration of pavement deterioration. For those operations in which magnitude of overload and/or the frequency of use do not justify a detailed analysis, the following criteria are suggested:
Figure: G450 ACN, from G450 Performance Handbook, pg. PC-8.
Consult the ACN Charts in aircraft manuals. In the case of a Gulfstream G450, the appropriate charts are found in the G450 Performance Handbook:
Note there are four such charts:
You select the chart corresponding to the pavement type and your tire pressure. Then you match the other PCN elements to find the correct line to use.
In the worst case scenario, a rigid pavement and a maximum weight aircraft with tires at maximum recommended tire pressure, a G450 will never have an ACN higher than 27.
You can significantly lower that number by adjusting tire pressure to fit the load, but you will greatly increase the wear of the tires. I know some pilots do this without problems but I do not. I long ago made the decision that the cost of tire failure is just too high and I treat the tires with a great deal of respect. You should get to know your tire leak rates and never attempt a flight when their pressure is too low. See G450 Systems / Landing Gear Tires for more about tire wear and tire pressures.
The PCN for a given runway might be listed in the information for the airport in the Jeppesen Airport Directory.
"Might" but "might not." Some airports simply do not publish their PCN but will let you know if you call. There are also airports where nobody seems to know and you will have to resort to finding out what aircraft are based there or are often seen there.
Nantes/Atlantique Apt of Entry
In this example for Nantes, France, runway 3/21 is 9514’ long, the surface is MACADAM, and:
F = Flexible Pavement
C = Low Subgrade Strength
W = High Tire Pressure Category -No Pressure Limit
T = Data Based on Technical Evaluation of Pavement
For our example aircraft, the G450 cannot be loaded or set up in a manner to exceed the PCN for the runway in Nantes, France. You would still need to make sure the taxiways and aprons are of sufficient strength.
Figure: G450 ESWL, from G450 Performance Handbook, pg. PC-12.
[EuroControl ATM Lexicon] ESWL - The theoretical load which, if acting on a single tire, with a contact area equal to that of one tire of the assembly, will produce the same effect on the movement area as the multiple wheel assembly.
The ESWL doesn't really mean much to pilots. It was devised in the 1940s by the U.S. Army Corps of Engineers and the Federal Aviation Administration (FAA) as a way to adapt what is called the CBR (California Bearing Ratio) thickness design measure for flexible airport pavements. The equivalent single-wheel load (ESWL) concept relates multiple-wheel gear loads to an equivalent single-wheel load for substitution into the CBR equation.
(For more on how ESWL is determined, see the DOT/FAA/AR-06/7 Alpha Factor Determination Using Data Collected at the National Airport Pavement Test Facility.)
While some sources, such as the Jeppesen Airport Directory, say the ESWL is an equivalent of the SIWL, Single Isolated Wheel Load, it isn't true. The SIWL times the number of main wheels equals the maximum gross weight of the aircraft. If that were the case, a G450 would have an SIWL of 75,000 / 4 = 18,750 pounds. As the Performance Handbook extract shows, a G450 has a much higher ESWL, 27,439 pounds. Why?
The main gear tires on each strut are closely spaced so the stress on the pavement under those two tires is not uniform. The EuroControl ATM Lexicon formulas account for this spacing.
I've never seen an airport post ESWL requirements. But the number is needed for a system you will see in many airports using a British system . . .
Figure: Load Classification Group from Jeppesen Airport Directory / Legend and Explanation, §11.
[Jeppesen Airport Directory / Legend and Explanation, §11] At some airports the bearing strength of runway pavement is defined by Load Classification Number (LCN) / Load Classification Group (LCG). The LCN / LCG has to be determined for a given aircraft and compared with the specific runway LCN / LCG. Normally the LCN / LCG of an aircraft should not be above that of the runway on which a landing is contemplated. Pre-arranged exceptions may be allowed by airport authorities. The aircraft LCN / LCG can be determined as follows:
Airports reporting their runway strength in the LCG system are primarily found in the following countries: Mongolia, Myanmar (Burma), Nigeria, South Africa, Turkey, United Kingdom, and Zimbabwe.
The British LCG/LCN rating system is based on the original LCN system which was developed by ICAO in 1965, but makes no distinction between asphalt (flexible) and concrete (rigid) pavement. Since these two surfaces react to loads differently, LCG type LCNs are not considered to be a highly precise measure of pavement strength particularly for flexible pavements.
The LCN/LCG system is going the way of the dinosaur and you won't even see it at most U.K. airports, which have mostly adopted the current ICAO standard of ACN/PCN. But there are still a few, as with Benson (EGUB):
[Jeppesen Airport Directory - Legend and Explanation, §4.e.] Runway Bearing Strength:
In this example for NIORT, FRANCE, the weight limits for runway 07/25 are expressed in thousands of pounds for each main gear for different wheel configurations:
Since all published pavement load limits presume that the MLG supports 95% of the aircraft gross weight, and Gulfstream aircraft MLG support 91% of the aircraft weight, the maximum aircraft gross weights in the above example would be:
In the U.S. Airport Directory, you may see ACN/PCN but you are also likely to see limits posted for type of landing gear and total gross weight. While the previous method, Runway Limits Per Wheel, also specifies landing gear type, it then divides the weight per landing gear. The FAA directory specifies total gross weight. For example:
KBED Runway 11/29
Many U.S. airports simply do not publish their runway limitations in the airport directory, but with a little detective work you can find them in some cases. At Nashua, NH (KASH) for example, the limit is given on their web site as 80,000 lbs and yet they have a G-V based there. The airport has made an exception for that airplane. In some cases they airport manager may not know the limit but may tell you they have had aircraft of your type operating there for years.
First: Read the fine print in the introduction
IN OTHER WORDS -CONTACT THE AIRPORT AUTHORITY
G-IV / 450 normal tire pressure is 189 psi.
G-IV /450 ACN never exceeds 24 on a flexible pavement or 27 on a rigid pavement, therefore, if the PCN is greater than 27, you are good to go. If the PCN is less than 27, check the QRH. (The PCN can be as low as 10, depending on gross weight and other factors.)
G-IV ESWL at maximum gross weight is 27,439 pounds.
G-V /550 normal tire pressure is 198 psi.
G-V /550 ACN never exceeds 29 on a flexible pavement or 34 on a rigid pavement, therefore, if the PCN is greater than 34, you are good to go. If the PCN is less than 34, check the QRH. (The PCN can be as low as 11, depending on gross weight and other factors.)
G-V / 550 ESWL at maximum gross weight is 32,904 pounds.
There are quite a few airports we operate in/out of that publish either single wheel weight limits or both single and dual wheel wheel weight limits- (generally it's the small GA airports we're concerned with)....I know if there only is a single wheel weight limit number to go into the books to find your Equivalent Single Wheel number since we are dual wheel...the question is- if there is a dual wheel weight limit number published - that I cannot comply with- can I always just use the formula to figure out my ESW number in order to be in compliance with the published single wheel weight limit number - or if there is a dual wheel number published - must I adhere to that weight limit no matter what?
There are competing issues here:
- You don't want your airplane sinking into the pavement, but you want to go there.
- The airport doesn't want their pavement cracking over time because airplanes are too heavy, but they want your business.
How I approach the entire ACN/PCN issue is to use the numbers and charts to see if what I want to do is possible. If they are good, then I press on to the next step of airport validation. If they are not good, which appears to be your case, or they just cannot be found, then I go for further research.
The most effective thing to do, I believe, is to call the airport manager. You can often get the number from the airport's website, airnav.com, or call the FBO and ask them for it. Explain to the airport manager what you are flying, how heavy you expect to be, etc. If the airport manager says they frequently get aircraft of your type and not to worry, call the FBO and ask if their visiting aircraft of your type get a lot of fuel for an idea of how heavy they depart.
If, on the other hand, the airport manager says he's never seen an aircraft of your type but they get lots of big airplanes so come on down, you still aren't done. Some larger aircraft actually have lower ACN's than smaller aircraft because of the distance between gear, size of the tires, etc. In this case, once again, call the FBO and ask what types of airplanes they get. I once went to an airport where they regularly got B-737s and a few phone calls later I found out those aircraft can have an ACN as low as 23 so I planned on being no heavier than that.
The GV I flew was based out of Nashua, NH and the airport published a weight limit well below our max takeoff weight. The posted limit, they told me, was for the life of the pavement and that as their top paying customer they wanted our business and assured us the pavement could take our weight, don't worry about it. We didn't worry about it.
But sometimes you have to worry about it. We flew that GV to one of the Greek islands where the runway had a high enough PCN but the ramps did not. We arranged to have large metal plates brought in and we taxied onto the plates. You've probably seen the photo of another GV sunken into a ramp's pavement. Ouch.
So, bottom line, there are a lot of variables and you need to call the airport and ask. The PCN isn't a limitation that says you can't do something. But getting it wrong is expensive so doing your homework is a must. Besides, getting it on the record that you talked to the airport manager can help you if something does go wrong.
Advisory Circular 150/5335-5, Standardized Method of Reporting Airport Pavement Strength - PCN, 6/15/83, U.S. Department of Transportation
DOT/FAA/AR-06/7 Alpha Factor Determination Using Data Collected at the National Airport Pavement Test Facility, Office of Aviation Research and Development, Washington, DC 20591, March 2006
ICAO Annex 14, International Standards and Recommended Practices, Annex 14 to the Convention on International Civil Aviation, Aerodromes, Vol I, July 2009
G450 Performance Handbook, GAC-AC-G450-OPS-0003, Revision 20, November 30, 2011
Gulfstream Aug 2006 Presentation - Runway Pavement Loading, Leo McStravick, Flight Operations, August 2006
Jeppesen Airport Directory / Legend and Explanation