Additionally, the derived single-wheel load is a function of the sub-grade strength. Four subgrade categories are defined high, medium, low, and ultra low for each pavement type flexible or rigid. The ACN of an airplane is a function of not only its weight but also the design parameters of its landing gear such as the distances between the wheels of a multiple-wheel landing gear assembly.
Manufacturers are required to calculate the ACN for a new aircraft as it comes into service and publish the results in the flight manual. The tables give ACN values for two weights, one at the maximum total weight authorized and the other at the operating weight when empty. When discussing pavement hardness, a standard used by many is the California Bearing Ratio:. 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. You cannot assume a smaller or lighter aircraft has a lower ACN. Therefore, from left to right on the flow chart. 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.
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 range, will have dual tandem trucks that are large enough to be considered four separate wheels. This isn't written very well, but given the "standard" tire pressure used in their calculations is 1. That places most jet tires in the high category. The G standard tire pressure is psi.
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. It is prepared by stripping vegetation, leveling 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. 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.
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. As noted previously, the single wheel tire pressure is standardized at 1. 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. The Falcon manuals include separate ACN charts for flexible and rigid pavements. 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. Consult the ACN Charts in aircraft manuals. In the case of a Gulfstream G, the appropriate charts are found in the G Performance Handbook:. 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 G will never have an ACN higher than 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 G Landing Gear Tires for more about tire wear and tire pressures. 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. 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.
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. 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. 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 can withstand substantial tire pressure in excess of psi 1. Improperly prepared and placed mixtures can show distress under tire pressures of psi 0. Although these effects are independent of the asphalt layer thickness, pavements with well-placed asphalt of 4 to 5 inches 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.
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. The internal program calculations and equations of the ICAO programs were also followed and incorporated in the new. The FAA also has a large amount of guidance material available on their website.
The ranges of subgrade strength covered by these standard subgrade categories designated as A, B, C and D are shown below.
Based on recent findings of full scale pavement tests, ICAO agreed to the following revisions concerning the alpha factor values: a. Aircraft classification number The aircraft classification number ACN is a number expressing the relative effect of an aircraft on the runway pavement for a specified standard subgrade category, using a method defined by the International Civil Aviation Organization ICAO.
The ACN is defined for only four subgrade categories high, medium, low, and ultra low. This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files. Beech Starship Beech King Air , Series. Beech King Air , C, , C.
Bombardier Canadair CL, Bombardier Challenger Bombardier CRJ Series. Bombardier Dash 8 Q, Q Series. Bombardier Dash 8 Q Series. Bombardier Dash 8 Q
0コメント