ACN Versus PCN
N222LX at Malcolm KcKinnon Airport (St. Simmons, GA) — note: I have several shots of this incident but don't know where they came from. If you do, please "Contact Eddie" below, I would like to use the photos for an upcoming magazine article.
- Development of a Standardized Method — In 1977 the ICAO started the process of standardizing what had become quite a mess. As a result, ACN/PCN will serve you well in most of the world.
- Aircraft Classification Number - The Science — Your aircraft manufacturer should provide you with charts that give you your ACN for a variety of conditions. Knowing the science behind the number, however, makes it easier to decide when you are okay even when the PCN provided by the airport is questionable (or missing).
- Aircraft Classification Number - A Few Examples — Even if all you want is the number, chasing through the charts can be a trying experience. Here are a few examples.
- Pavement Classification Number - The Science — Major airports should publish their Pavement Classification Numbers but often you will find other metrics that leave you guessing. If you know a little bit about the science of PCN determination, you will be better armed to tackle those other methods.
- Reading a PCN — Reading a PCN isn't too tough as long as your aircraft manuals have charts for rigid and flexbile pavement and each of these charts have the four categories of subgrades. Where it can get complicated is figuring the tire pressure and evaluation method.
- Pavement Classification Number - A Few Examples — Your best source of PCN information is from your airport directory. Failing that, you can call the airport directly. If your PCN exceeds you ACN, you are good to go. The ICAO recommends that each part of an apron (taxiway or ramp) "should" be capable of withstanding the traffic of the aircraft it is intended to serve. Here again you should call the airport and ask.
- Alternate Methods — More and more of the world is jumping on the ACN/PCN bandwagon and you hardly ever see older, dated methods. Hardly ever, but they still exist. Here are a few.
- Exceedance / Lack of Data — If you venture away from the airports your aircraft normally visits, sooner or later, you are going to be confronted with a situation where your planned ACN exceeds the published PCN, the weight of the aircraft exceeds an alternate metric, or there just isn't any information available. What are you going to do?
Too much information you say? Well you can cut to the chase and look for a step-by-step method on how to handle the pavement loading question. See: Plan B, below.
Development of a Standardized Method
[Advisory Circular 150-5335-5C, ¶ 1.1] Annex 14 to the Convention of International Civil Aviation, Aerodromes, contains a standard that requires member states to publish information on the strengths of all public airport pavements in its own Aeronautical Information Publication.
[Advisory Circular 150-5335-5C, ¶ 1..] In 1977, ICAO established a Study Group to develop a single international method of reporting pavement strengths. The study group developed, and ICAO adopted, the Aircraft Classification Number - Pavement Classification Number (ACN-PCN) method. Using this method, it is possible to express the effect of an individual aircraft on different pavements with a single unique number that varies according to aircraft weight and configuration (e.g. tire pressure, gear geometry, etc.), pavement type, and subgrade strength. 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 or detailed information about the pavement structure. This number is the Pavement Classification Number (PCN).
[Advisory Circular 150-5335-5C, ¶ 1.2.3] The ACN-PCN system is structured so a pavement with a particular PCN value can support an aircraft that has an ACN value equal to or less than the pavement's PCN value. This is possible because ACN and PCN values are computed using the same technical basis.
[Advisory Circular 150-5335-5C, ¶ 1.3] The use of the standardized method of reporting pavement strength applies only to pavements with bearing strengths of 12,500 pounds (5 700 kg) or greater.
Aircraft Classification Number
You are going to get your Aircraft Classification Number (ACN) from your aircraft flight manual or other documents provided by your manufacturer. What follows are the nuts and bolts that go into that number. While you can certainly survive without this information, knowing it will help you evaluate a surface when the information provided by the airport is less than complete.
When discussing pavement hardness, a standard used by many is the California Bearing Ratio:
[ICAO Doc 9157, Part 3, Glossary]
- Preferred Term: CBR
- Secondary Term: California Bearing Ratio
- Definition: The bearing ratio of soil determined by comparing the penetration load of the soil to that of a standard material (see ASTM D1883). The method covers evaluation of the relative quality of subgrade soils but is applicable to sub-base and some base course materials.
[Advisory Circular 150-5335-5C, ¶ 1.2.1] ACN is a number that expresses the relative effect of an aircraft at a given configuration on a pavement structure for a specified standard subgrade strength.
Figure: ACN flow chart, from ICAO Doc 9157, Part 3, page 3-3.
- From the aircraft manufacturer
- From ICAO Doc 9157, Part 3, Appendix 2
- From ICAO Annex 14, Attachment B, Table B-1
- From ICAO Doc 9157, Part 3, figure 1-4
- From ICAO Doc 9157, Part 3, figure 1-5
For the purposes of a pilot trying to decide if the pavement can support the aircraft, the math behind the computation of the number isn't really important. What does matter, however, are the items in the flow chart that the pilot can control or needs to understand when comparing one aircraft to another. A Boeing 737-800, for example, can have a higher ACN than a Boeing 747-400, depending on loading. You cannot assume a smaller (or lighter) aircraft has a lower ACN. Therefore, from left to right on the flow chart . . .
Aircraft mass and center of gravity position
[ICAO Doc 9157, Part 3, ¶22.214.171.124 f)1)] The maximum ACN of an aircraft is calculated at the mass and c.g. that produces the highest main gear loading on the pavement, usually the maximum ramp mass and corresponding aft c.g.
Figure: Auxiliary leg versus main leg, from ICAO Doc 9157, Part 3, Appendix 1, ¶1.3.
[ICAO Doc 9157, Part 3, ¶3.2.4] The aircraft mass is transmitted to the pavement through the undercarriage of the aircraft. The number of wheels, their spacing, tire pressure and size determine the distribution of aircraft load to the pavement. In general, the pavement must be strong enough to support the loads applied by the individual wheels, not only at the surface and the subgrade but also at intermediate levels. For the closely spaced wheels of dual and dual-tandem legs and even for adjacent legs of aircraft with complex undercarriages the effects of distributed loads from adjacent wheels overlap at the subgrade (and intermediate) level. In such cases, the effective pressures are those combined from two or more wheels and must be attenuated sufficiently by the pavement structure. Since the distribution of load by a pavement structure is over a much narrower area on a high strength subgrade than on a low strength subgrade, the combining effects of adjacent wheels is much less for pavements on high strength than on low strength subgrades. This is the reason why the relative effects of two aircraft types are not the same for pavements of equivalent design strength, and this is the basis for reporting pavement bearing strength by subgrade strength category. Within a subgrade strength category the relative effects of two aircraft types on pavements can be uniquely stated with good accuracy.
With an aft center of gravity it is not uncommon to have a weight distribution that is 90 percent or higher on the main landing gear.
Figure: Wheel arrangements, from ICAO Doc 9157, Part 3, Appendix 1, ¶1.6.
The arrangement of the wheels dramatically affects ACN values. Smaller aircraft, say in the Gulfstream range, will have dual wheels close enough to be considered a single wheel on some pavements. Larger aircraft, say in the Boeing 747 range, will have dual tandem trucks that are large enough to be considered four separate wheels.
[ICAO Doc 9157, Part 3, ¶126.96.36.199 c)] The results of pavement research and re-evaluation of old test results reaffirm that except for unusual pavement construction (i.e., flexible pavements with a thin asphaltic concrete cover or weak upper layers), tire pressure effects are secondary to load and wheel spacing, and may therefore be categorized in four groups for reporting purposes as:
(Updated by: ICAO Annex 14, Amendment 11, ¶2.6)
- Unlimited - No pressure limit
- High - Pressure limited to 1.75 MPa (254 psi)
- Medium - Pressure limited to 1.25 MPa (181 psi)
- Low -Pressure limited to 0.50 MPa (73 psi)
This isn't written very well, but given the "standard" tire pressure used in their calculations is 1.25 MPa (181 psi), we can assume anything from 182 to 254 is "high" pressure. That places most jet tires in the high category. (The G450 standard tire pressure is 189 psi.)
Figure: Theoretical pavements, from ICAO Doc 9157, Part 3, figure 1-2.
[ICAO Doc 9157, Part 3, ¶3.2.3] The terms "rigid" and "flexible" have come into use for identification of the two principal types of pavements. The terms attempt to characterize the response of each type to loading. The primary element of a rigid pavement is a layer or slab of Portland cement concrete (PCC), plain or reinforced in any of several ways. It is often underlain by a granular layer which contributes to the structure both directly and by facilitating the drainage of water.
- Flexible pavement:
- [ICAO Doc 9157, Part 3, Glossary] A pavement structure that maintains intimate contact with and distributes loads to the subgrade and depends on aggregate interlock, particle friction, and cohesion for stability.
- [ICAO Doc 9157, Part 3, ¶3.2.3] A flexible pavement yields more under surface loading merely accomplishing a widening of the loaded area and consequent reduction of pressure layer by layer. At each level from the surface to subgrade, the layers must have strength sufficient to tolerate the pressures at their level. The pavement thus depends on its thickness over the subgrade for reduction of the surface pressure to a value which the subgrade can accept. A flexible pavement must also have thickness of structure above each layer to reduce the pressure to a level acceptable by the layer. In addition, the wearing course must be sufficient in strength to accept without distress tire pressures of using aircraft.
- Rigid pavement:
- [ICAO Doc 9157, Part 3, Glossary] A pavement structure that distributes loads to the subgrade having as its surface course a Portland cement concrete slab of relatively high bending resistance.
- [ICAO Doc 9157, Part 3, ¶3.2.3] A rigid pavement responds "stiffly" to surface loads and distributes the loads by bending or beam action to wide areas of the subgrade. The strength of the pavement depends on the thickness and strength of the PCC and any underlying layers above the subgrade. The pavement must be adequate to distribute surface loads so that the pressure on the subgrade does not exceed its evaluated strength. A flexible pavement consists of a series of layers increasing in strength from the subgrade to the surface layer. A series such as select material, lower sub-base, sub-base, base and wearing course is commonly used. However, the lower layers may not be present in a particular pavement. The pavements meant for heavy aircraft usually have a bituminous bound wearing course.
[ICAO Doc 9157, Part 3, ¶3.2.2] The subgrade is the layer of material immediately below the pavement structure which is prepared during construction to support the loads transmitted by the pavement. It is prepared by stripping vegetation, levelling or bringing to planned grade by cut and fill operations, and compacting to the needed density. Strength of the subgrade is a significant element and this must be characterized for evaluation or design of a pavement facility or for each section of a facility evaluated or designed separately. Soil strength and therefore subgrade strength is very dependent on soil moisture and must be evaluated for the condition it is expected to attain insitue beneath the pavement structure. Except in cases with high water tables, unusual drainage, or extremely porous or cracked pavement conditions soil moisture will tend to stabilize under wide pavements to something above 90 per cent of full saturation.
[ICAO Doc 9157, Part 3, ¶188.8.131.52 a)] The subgrade strength categories are identified as high, medium, low and ultra low and assigned the following numerical values:
- High strength; CBR 15 (rigid pavements), above CBR 13 (flexible pavements).
- Medium strength; CBR 10 (rigid pavements), CBR 8 to 13 (flexible pavements).
- Low strength; CBR 6 (rigid pavements), CBR 4 to 8 (flexible pavements).
- Ultra low strength; CBR 3 rigid pavements), CBR below 4 (flexible pavements.
Derived single wheel chart
[ICAO Doc 9157, Part 3, ¶184.108.40.206 d)] The concept of a mathematically derived single wheel load has been employed in the ACN-PCN method as a means to define the landing gear/pavement interaction without specifying pavement thickness as an ACN parameter. This is done by equating the thickness given by the mathematical model for an aircraft landing gear to the thickness for a single wheel at a standard tire pressure of 1.25 MPa (181 psi). The single wheel load s o obtained is then used without further reference to thickness; this is so because the essential significance is attached to the fact of having equal thicknesses, implying "same applied stress to the pavement", rather than the magnitude of the thickness. The foregoing is in accord with the objective of the ACN-PCN method to evaluate the relative loading effect of an aircraft on a pavement.
[ICAO Doc 9157, Part 3, ¶220.127.116.11 e)] The ACN of an aircraft is numerically defined as two times the derived single wheel load, where the derived single wheel load is expressed in thousands of kilograms. As noted previously, the single wheel tire pressure is standardized at 1.25 MPa. Additionally, the derived single wheel load is a function of the subgrade strength. The aircraft classification number (ACN) is defined only for the four subgrade categories (i.e., high, medium, low, and ultra low strength). The "two" (2) factor in the numerical definition of the ACN is used to achieve a suitable ACN vs. gross mass scale so that whole number ACNs may be used with reasonable accuracy.
Aircraft Classification Number - A Few Examples
ACN Example: Dassault Falcon 2000
Figure: F2000 ACN, from Dassault Falcon 2000 Airplane Characteristics for Airport Planning, pg. 11.
The Falcon 2000 manuals include separate ACN charts for flexible and rigid pavements. The A, B, C, and D are explained in the Performance Manual.
- (A) - High
- (B) - Medium
- (C) - Low
- (D) - Ultralow
Notice how the numbers get higher as the subgrade gets lower. The lesson here is that the hardness of the pavement under the airplane has an impact on the airplane's ACN. A lower subgrade raises the ACN, meaning you need to find a pavement with a higher PCN.
ACN Example: Gulfstream G450
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:
- Rigid pavement, 189 psi tire pressure
- Flexible pavement, 189 psi tire pressure
- Rigid pavement, tire pressure to match load
- Flexible pavement, tire pressure to match load
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 Landing Gear Tires for more about tire wear and tire pressures.
Pavement Classification Number - The Science
[Advisory Circular 150-5335-5C, ¶ 1.2.2] PCN is a number that expresses the load-carrying capacity of a pavement for unrestricted operations.
[ICAO Doc 9157, Part 3, ¶3.3.1] The pavement classification number (PCN) is an index rating (1/500th) of the mass which an evaluation shows can be borne by the pavement when applied by a standard (1.25 MPa tire pressure) single-wheel. The PCN rating established for a pavement indicates that the pavement is capable of supporting aircraft having an ACN (aircraft classification number) of equal or lower magnitude. The ACN for comparison to the PCN must be the aircraft ACN established for the particular pavement type and subgrade category of the rated pavement as well as for the particular aircraft mass and characteristics.
[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:
- Pavement Type:
- (R) - rigid
- (F) - flexible
- Subgrade Strength:
- (A) - High
- (B) - Medium
- (C) - Low
- (D) - Ultralow
- Tire Pressure:
- (W) Unlimited - No pressure limit
- (X) High - Pressure limited to 1.75 MPa (254 psi)
- (Y) Medium - Pressure limited to 1.25 MPa (181 psi)
- (Z) Low -Pressure limited to 0.50 MPa (73 psi)
- Evaluation Method:
- (T) - Technical
- (U) - Aircraft experience
[ICAO Doc 9157, Part 3, ¶3.3.2] A rigid pavement is that employing a Portland cement concrete (PCC) slab whether plain, reinforced, or prestressed and with or without intermediate layers between the slab and subgrade.
[ICAO Doc 9157, Part 3, ¶3.3.2] A flexible pavement is that consisting of a series of layers increasing in strength from the subgrade to the wearing surface. Composite pavements resulting from a PCC overlay on a flexible pavement or an asphaltic concrete overlay on a rigid pavement or those incorporating chemically (cement) stabilized layers of particularly good integrity require care in classification.
[ICAO Doc 9157, Part 3, ¶3.3.3] The subgrade of the reported pavement belongs must be established and reported. Normally subgrade strength will have been evaluated in connection with original design of a pavement or later rehabilitation or strengthening. Where this information is not available the subgrade strength should be determined as part of pavement evaluation. Subgrade strength evaluation should be based on testing wherever possible. Where evaluation based on testing is not feasible a representative subgrade strength category must be selected based on soil characteristics, soil classification, local experience, or judgement.
[Advisory Circular 150-5335-5C, ¶ 18.104.22.168] Aircraft tire pressure will have little effect on pavements with Portland cement concrete (concrete) surfaces. Rigid pavements are inherently strong enough to resist tire pressures higher than currently used by commercial aircraft and can usually be rated as code W.
[Advisory Circular 150-5335-5C, ¶ 22.214.171.124] Tire pressures may be restricted on asphaltic concrete (asphalt), depending on the quality of the asphalt mixture and climatic conditions. Tire pressure effects on an asphalt layer relate to the stability of the mix in resisting shearing or densification. A poorly constructed asphalt pavement can be subject to rutting due to consolidation under load. The principal concern in resisting tire pressure effects is with stability or shear resistance of lower quality mixtures. A properly prepared and placed mixture that conforms to FAA specification Item P-401 can withstand substantial tire pressure in excess of 218 psi (1.5 Mpa). Item P-401, Hot Mix Asphalt (HMA) Pavements, is provided in the current version of AC 150/5370-10, Standards for Specifying Construction of Airports. Improperly prepared and placed mixtures can show distress under tire pressures of 100 psi (0.7 MPa) or less. Although these effects are independent of the asphalt layer thickness, pavements with well-placed asphalt of 4 to 5 inches (10.2 to 12.7 cm) in thickness can generally be rated with code X or W, while thinner pavement of poorer quality asphalt should not be rated above code Y.
[ICAO Doc 9157, Part 3, ¶126.96.36.199 c)] The ACN-PCN method also envisages the reporting of the following information in respect of each pavement: [. . .] maximum tire pressure allowable;
(Updated by: ICAO Annex 14, Amendment 11, ¶2.6)
Many manuals still specify an older set of categories: High, Medium, Low, and Very Low. These have been replaced as shown.
While it isn't explicit throughout the manuals, it appears that so long as your tire pressure does not exceed the tire pressure code, the particular PCN code is okay. If, for example, your aircraft has high tire pressure, then a PCN code of W or X is okay but a code of Y or Z would disqualify your aircraft.
[ICAO Doc 9157, Part 3, ¶3.3.5] Commonly, evaluation is an inversion of a design method. Design begins with the aircraft loading to be sustained and the subgrade strength resulting from preparation of the local soil, then provides the necessary thicknesses and quality of materials for the needed pavement structure. Evaluation inverts this process. It begins with the existing subgrade strength, finds thickness and quality of each component of the pavement structure, and uses a design procedure pattern to determine the aircraft loading which the pavement can support. Where available the design, testing, and construction record data for the subgrade and components of the pavement structure can often be used to make the evaluation. Or, test pits can be opened to determine the thicknesses of layers, their strengths, and subgrade strength For the purpose of evaluation. A technical evaluation also can be made based on measurement of the response of pavement to load. Deflexion of a pavement under static plate or tire load can be used to predict its behaviour. Also there are various devices for applying dynamic loads to a pavement, observing its response, and using this to predict its behaviour.
[ICAO Doc 9157, Part 3, ¶3.3.5] When for economic or other reasons a technical evaluation is not feasible, evaluation can be based on experience with "using aircraft." A pavement satisfactorily supporting aircraft using it can accept other aircraft if they are no more demanding than the using aircraft. This can be the basis for an evaluation.
[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.
Reading a PCN
PCN decoder, from Eddie's notes.
The pavement type will be provided in the PCN and is used to select the correct chart from aircraft manuals.
The subbrade strength will be provided in the PCN and is typically presented by four different lines or curves on each chart.
The airport is obligated to report the highest value possible while the aircraft pressure is pretty much what it is. So what do you do when they don't match? Higher tire pressure narrows the tire's contact patch which means more of the weight is concentrated on a narrower area. Lower tire pressures are easier on the pavement because the weight is distributed over a wider area. If the PCN indicates a "W" there is no limit to tire pressure and you are all set. If the PCN indicates a "X" you are okay as long as your pressure does not exceed 254 psi.
There isn't much you with with evaluation other than to note it. The technical evaluation comes from engineering studies; the aircraft experience is just a log book of the aircraft that have used the pavement without trouble and infering that similar or smaller aircraft will be okay. You can obviously have more confidence in a technical evaluation.
Pavement Classification Number - A Few Examples
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
- 90′ LFRS NTE +01:00* N47 09.4 W001 36.5
- ATS 0228002570/71; Fax 0228002569. Aeroclub 0240751316. Apt Operator Fax 0228002529.
- 03/21 9514′ MACADAM. PCN 49/F/C/W/T. LDA 21 8825′. TODA 21 9711′. HIRL. HIALS 03.
- H24. Customs: 0600-1800, O/T for sked flights, for nonsked flights 2hr PNR before ETA.
- F-3, Jet A-1.
- Fire 9 1800-0400 level 7 or O/R.
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 = Unlimited Tire Pressure Category -No Pressure Limit
T = Data Based on Technical Evaluation of Pavement
For our example aircraft, the G450 will always have an ACN below the PCN for the runway in Nantes, France so you are okay as far as the runway is concerned. You would still need to make sure the taxiways and aprons are of sufficient strength.
The Valley (Clayton J. Lloyd Intl) Apt of Entry
- 127′ TQPF AXA -04:00 N18 12.3 W063 03.2
- 264-497-2513, 264-497-5483, 264-497-0310. Apt Manager (264) 497-3510.
- 10/28 5456' ASPHALT. PCN 22/F/A/W/T. TORA 10 4964'. TORA 28 4964' LDA 10 4472' LDA 28 4472'. TODA 10 4964'. TODA 28 4964'. ASDA 10 4964'. ASDA 28 4964. RL.
- Customs: 1100-0200.
- 100 octane, Jet A-1.
- ABN. Fire 5.
In this example for Anguilla, runway 10/28 is 5456' long, the surface is ASPHALT and:
PCN = 22
F = Flexible Pavement
A = High Subgrade Strength
W = Unlimited Tire Pressure Category -No Pressure Limit
T = Data Based on Technical Evaluation of Pavement
For our example aircraft, the G450 might have a problem at higher gross weights and you could find yourself with no problem getting in but a problem loading enough fuel without damaging the ramp. It would be worth a call to the airport manager to ask (a) do you often get aircraft of this type parked on your ramp? (b) can the runway and ramp take a higher ACN value aircraft? (c) if it is safe to do so, is there a financial penalty involved with exceeding the PCN?
Single Isolated Wheel Load
[Jeppesen Airport Directory, Legend and Explanation, 29 Jan 2010] Single Isolated Wheel Load times number of main wheels = allowable aircraft weight.
SIWL appears to be a Jeppesen convention for eventually figuring the British LCN/LCG numbers.
More about this: LCN/LCG.
Equivalent Single Wheel Loading (ESWL)
[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 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.)
Figure: G450 ESWL, from G450 Performance Handbook, pg. PC-12.
[Jeppesen Airport Directory, Legend and Explanation, 29 Jan 2010]
Equivalent Single Wheel Load, a calculated value for multiwheel legs. The resultant value is considered to be the same as SIWL for determining LCN.
While some sources, such as the Jeppesen Airport Directory, infer that 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.
See LCN/LCG, below.
Landing gear specific limits
[ICAO Doc 9157, Part 3, ¶188.8.131.52] The United States Federal Aviation Administration method of designing and reporting airport pavement strength is in terms of gross aircraft weight for each type of landing gear. This permits the evaluation of a pavement with regard to its ability to support the various types and weights of aircraft. Comparison between the pavement strength (reported as gross weight for aircraft equipped with single wheel, dual wheel, and dual-tandem wheel undercarriages) and the actual gross weight of a specific aircraft will establish the pavement's ability to accommodate the aircraft. In 1978 the United States Federal Aviation Administration adopted the California Bearing Ratio (CBR) method of flexible pavement design, edge loading assumption for the design of rigid pavements and the Unified Soil Classification System.
[ICAO Doc 9157, Part 3, ¶184.108.40.206] The gear type and configuration dictate how the aircraft weight is distributed to the pavement and determine pavement response to aircraft loadings. Lt would have been impractical to develop design curves for each type of aircraft. However, since the thickness of both rigid and flexible pavements is dependent upon the gear dimensions and the type of gear, separate design curves would be necessary unless some valid assumptions could be made to reduce the number of variables. Examination of gear configuration, tire contact areas, and tire pressure in common use indicated that these follow a definite trend related to aircraft gross weight. Reasonable assumptions could therefore be made and design curves constructed from the assumed data. These assumed data are as follows:
- Single gear aircraft. No special assumptions needed.
- Dual gear aircraft. A study of the spacing between dual wheels for these aircraft indicated that a dimension of 20 in (0.51 m) between the centreline of the tires appeared reasonable for the lighter aircraft and a dimension of 34 in (0.86 m) between the centreline of the tires appeared reasonable for the heavier aircraft.
- Dual tandem gear aircraft. The study indicated a dual wheel spacing of 20 in (0.51 m) and a tandem spacing of 45 in (1.14 m) for lighter aircraft, and a dual wheel spacing of 30 in (0.76 m) and a tandem spacing of 55 in (1.40 m) for the heavier aircraft are appropriate design values.
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
- Dimensions: 7011 x 150 ft. / 2137 x 46 m
- Surface: asphalt/grooved, in good condition
- Weight bearing capacity:
- Single wheel: 78.0
- Double wheel: 100.0
- Double tandem: 190.0
- Runway edge lights: high intensity
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 GV 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.
Load Classification Number (LCN) / Load Classification Group (LCG) British Military System
[ICAO Doc 9157, Part 3, ¶4.3]
- It is the United Kingdom practice to design for unlimited operational use by a given aircraft taking into account the loading resulting from interaction of adjacent landing gear wheel assemblies where applicable. The aircraft is designated "the design aircraft'' for the pavement. The support strength classification of the pavement is represented by the design aircraft's pavement classification number identifying its level of loading severity. All other aircraft ranked by the United Kingdom standards as less severe may anticipate unlimited use of the pavement though the final decision rests with the aerodrome authority.
- The Reference Construction Classification (RCC) system has been developed from the British Load Classification Number (LCN) and Load Classification Group (LCG) systems. Pavements are identified as dividing broadly into rigid or flexible construction and analysed accordingly.
- It is the United Kingdom practice to follow the ICAO ACN/PCN reporting method for aircraft pavements. The critical aircraft is identified as the one which imposes a severity of loading condition closest to the maximum permitted on a given pavement for unlimited operational use. Using the critical aircrafts ACN individual aerodrome authorities decide on the PCN to be published for the pavement concerned.
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:
- Obtain Single Isolated Wheel Load (SIWL / ESWL) for the aircraft from Aircraft Operations Manual and locate this figure in pounds or tons, on the left scale of the chart.
- Locate tire pressure on the scale to the right.
- Connect the points found in 1 and 2 with a straight line. Where this line crosses the center scale read your aircraft LCN / LCG.
- This LCN / LCG should not be above the published runway LCN / LCG.
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):
- 203′ EGUB BEX Mil. 00:00* N51 37.0 W001 05.7
- ARO Fax (01491) 838747. ATS (01491) 827017, 827018.
- 01/19 5981′ ASPH/CONC. LCG IV. TODA 01 6309′. TODA 19 6099′. RL. HIALS.
- Rwy 01 Right-Hand Circuit.
- Strictly 24hr PPR. Customs: By operational requirements.
- F-3, Jet A-1+.
- IBN. Fire 6 Fire Cat 4 outside ops hr.
Runway Weight Limits Per Wheel
Jeppesen appears to use a hybrid method that is fairly self explanatory, though they do not give a source for their methodology.
[Jeppesen Airway Manual / Airport Directory / Airport Data / Legend and Explanation, 31 Oct 2014, §4.e.] Runway Bearing Strength:
- S or SW — (Allowable aircraft weight) for single wheel per leg configuration
- T or DW — (allowable aircraft weight) for tandem or dual wheel per leg configuration.
- TT or DDW — (allowable aircraft weight) for twin tandem or double dual wheel per leg configuration.
- TDT — Runway weight bearing capacity for aircraft with twin delta tandem landing gear.
- DDT — Runway weight bearing capacity for aircraft with double dual tandem type landing gear.
- AUW — All Up Weight (without regard to wheel configuration).
- S/L — (load per leg) for single wheel per leg configuration.
- T/L — (load per leg) for twin or tandem wheel per leg configuration.
- TT/L — (load per leg) for bogie or twin tandem wheel per leg configuration.
- 201′ LFBN NIT +01:00* N46 18.8 W000 23.7
- Aeroclub des Deux Sevres 0549282941; Mobile de Niort 0777880327, des Deux Sevres 0679507975. Apt Operator 0549243722; Mobile 0615923763, 0686278601; Fax 0549241802.
- 07/25 2231′ GRASS. LDA 25 1903′. Rwy 25 Right-Hand Circuit.
- 07/25 5850′ MACADAM. T/L 33, TT/L 60, S/L 22. RL. Rwy 07 Right-Hand Circuit.
- AFIS 11 MAY-29 SEP Mon-Fri 0900-1300LT & 1500-1900LT, Sat Sun 1000-1330LT & 1500-1900LT. 30 SEP-10 MAY Mon-Fri 0900-1200LT & 1400-1730LT. O/T PPR preceding workday before 1700LT. Customs: O/R via ARO.
- F-3, Jet A-1.
- Fire 1.
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:
- S/L 22 = 22,000 lbs for a single wheel per leg (MLG)
- T/L 33 = 33,000 lbs for a twin or tandem wheel leg (MLG)
- TT/L 60 = 60,000 lbs for a twin tandem wheel leg (MLG)
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:
- S/L 22 = 44,000 lbs + 4% or 1,760 lbs = 45,760 lbs
- T/L 33 = 66,000 lbs + 4% or 2,640 lbs = 68,640 lbs
- TT/L 60 = 120,000 lbs + 4% or 4,800 lbs = 124,800 lbs
When Things Are Not Obvious
First, read the fine print in the introduction of two of your best sources:
- FAA AIRPORT DIRECTORY: "Runway strength data shown in this publication is derived from available information and is a realistic estimate of capability at an average level of activity. It is not intended as a maximum allowable weight or as an operating limitation. Many airport pavements are capable of supporting limited operations with gross weights in excess of published figures. Permissible operating weight, insofar as runway strengths are concerned, are a matter of agreement between the owner and user."
- JEPPESEN: "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." and "The appropriate authority may establish criteria to regulate the use of a pavement by aircraft with an ACN higher than the PCN reported for that pavement."
[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:
- For flexible pavements, occasional movements by aircraft with ACN not exceeding 10 per cent above the reported PCN should not adversely affect the pavement;
- For rigid or composite pavements, in which a rigid pavement layer provides a primary element of the structure, occasional movements by aircraft with ACN not exceeding 5 per cent above the reported PCN should not adversely affect the pavement;
- If the pavement structure is unknown, the 5 per cent limitation should apply; and
- The annual number of overload movements should not exceed approximately 5 per cent of the total annual aircraft movements.
On the one hand, there have been several cases of corporate aircraft sinking into the ramp at airports where the PCN was questionable. On the other hand, the PCN system is designed for the long term usage of the pavement and just because the ACN exceeds the PCN doesn't mean the pavement can't handle the airplane. The decision could go either way.
See Plan B below.
No information is posted
Smaller airports may not post any data at all. If they are a public use airport somebody probably has the data. Even private airports will need something for insurance reasons.
In either case, you should see Plan B below.
Comparing one aircraft to another
You will quite often hear an airport manager say "we park airliners here all the time, come on down!" The wheel spacing on a large aircraft can mean it puts less stress on the pavement than your fully loaded GV and you really have to be careful when comparing one aircraft to another.
These are a few of the aircraft, for illustrative purposes, assuming standard tire pressures and high (A) subgrades. The range shown is for operating mass empty to maximum ramp weight.
|Aircraft Type||Weight Range (lbs)||ACN (Rigid, Subgrade A)||ACN (Flexible, Subgrade A)||Source|
|Airbus A300-B2||192,371 - 304,014||19 - 34||20 -35||Jeppesen|
|Boeing 707-320B||142,800 - 328,000||14 - 39||15 - 39||Jeppesen|
|Boeing 737-100||57,200 - 97,800||12 - 23||11 - 21||Jeppesen|
|Boeing 737-800||91,300 - 174,00||23 - 49||20 - 43||Jeppesen|
|Boeing BBJ3||100,000 - 188,200||26 - 56||23 - 48||Jeppesen|
|Boeing 747-400B||364,000 - 878,200||18 - 53||18 - 53||Jeppesen|
|Boeing 757-300||142,400 - 273,500||15 - 36||15 - 33||Jeppesen|
|Embraer ERJ 175||47,399 - 83,026||11 - 22||10 - 20||Jeppesen|
|Falcon 2000EX||- 41,500||8 - 12.5||6.5 - 10.5||Dassault|
|Global Express BD-700||45,000 - 99,750||12 - 29||10 - 25||BD-700|
|Gulfstream G450||45,000 - 75,000||13 - 24||11 - 21||G450|
|Gulfstream G550||55,000 - 90,900||17 - 32||13 - 25||G550|
Bombardier BD-700 Quick Reference Handbook, page PERF-04-3
Gulfstream G450 Performance Handbook, pages PC-6 to PC-9
Gulfstream G550 Performance Handbook, pages PC-10 to PC-12
Dassault Falcon 2000EX Performance Manual, Supplement 2, pages 4 to 5
Jeppesen Airway Manual / Airport Directory / Airport Data / ACN Tables
As can be seen, landing gear geometry often has more to do with an aircraft impact on the pavement than weight. A fully loaded Boeing BBJ3, for example, has a higher ACN than a fully loaded Boeing 747-400B on rigid pavement. Fully loaded G450s, G550s, or BD-700s have higher ACNs than older Boeing 737-100s.
Plan B — How to approach the pavement loading issue
You can methodically approach aircraft pavement loading so that most of the time the answers are cut and dried and you can move on to your next flight planning task. You would typically pick the airport by checking for runway length and width, reasoning that if those two criteria are not met there is no use considering that airport any further. You next step should be to apply the same "go / no-go" criteria to aircraft pavement loading.
- Check the airport directory — your Jeppesen (or similar) manuals should list a runway PCN or one of the alternate metrics listed above, such as ESWL, Landing Gear-Specific Weights, LCN/LCG, or Runway Weight Limits Per Wheel.
- Determine aircraft classification number — this is typically done with aircraft manuals, usually in a performance handbook or the performance section of the flight manual. If the aircraft weight is below the applicable metric, the runway itself is confirmed. You still need to check the taxiways and ramps.
- Confirm taxiways and ramps — If you have not heard of your aircraft type using the airport's taxiways and ramps, you should call the airport manager, the fixed base operator, or any other source of information. You might also consider typing the airport's name and your aircraft type into YouTube and you might see videos of proof the ramp will take the aircraft.
- Call the airport manager — If you can't get the confirmation you need, call the airport manager. If he or she does not have recent knowledge of your aircraft type at the airport, you need to evaluate the types of aircraft versus yours. You can also ask for how recently the runway, taxiway, and ramp areas had been tested and the results of those tests. Keep in mind that the airport manager may have reasons to encourage you to come (revenue, etc.) but may not understand that a GV might put more stress on the pavement than a Boeing 737.
You need to check the PCN in order to determine the ACN; the type of pavement and subgrade are needed to properly compute the aircraft's ACN.
Figure: Boeing 737-100 versus Gulfstream G550 ACN on rigid pavement, from Eddie's notes.
You should always keep in mind that most aviation professionals — FBO managers, mechanics, even pilots — don't really understand the stresses an aircraft places on pavement depend more on the distance between each wheel than on the weight of the aircraft itself. In the drawing, the B-737 weighs 8% more than the G550 and yet puts 40% less stress on the pavement.
- Gulfstream aircraft have a built in margin of 4%. Most published limits presume the MLG carries 95% of aircraft weight. Gulfstream MLGS support 91% of aircraft weight. (Remember that the FAA airport / facilities directory weights are total gross weight.)
- Manage fuel loads into / out of low strength runways.
- Reduce tire inflation pressure to the minimum for aircraft gross weight - this reduces the ACN number.
GIV / 450 Specifics
GIV / 450 normal tire pressure is 189 psi.
GIV /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.)
GIV ESWL at maximum gross weight is 27,439 pounds.
GV / G550 Specifics
GV /G550 normal tire pressure is 198 psi.
GV /G550 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.)
GV / G550 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 150/5335-5C, Standardized Method of Reporting Airport Pavement Strength, 8/14/2014, U.S. Department of Transportation
Bombardier BD-700 Quick Reference Handbook, Rev 80, June 03, 2014.
Dassault Falcon 2000 Airplane Characteristics for Airport Planning, Pub 9245, Rev 5 - June 2012.
Dassault Falcon 2000EX Performance Manual, DGT95353, March 01, 2013.
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
Gulfstream G450 Performance Handbook, GAC-AC-G450-OPS-0003, Revision 20, November 30, 2011
Gulfstream G550 Performance Handbook, GAC-AC-G550-OPS-0003A, Revision 27, July 28, 2008
McStravick, Leo, Gulfstream Aug 2006 Presentation - Runway Pavement Loading, Flight Operations, August 2006
ICAO Annex 14 - Aerodromes - Vol I - Aerodrome Design and Operations, International Standards and Recommended Practices, Annex 14 to the Convention on International Civil Aviation, Vol I, 6th edition July 2013
ICAO Doc 9157 - Aerodrome Design Manual - Part 3 - Pavements, International Civil Aviation Organization, Second Edition, 1983
Jeppesen Airways Manual