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Controlling Aircraft Loads

Airport pavement operations involves determining and controlling the aircraft loadings that can operate safely on the pavement without causing structural damage to either the aircraft or the pavement.

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landing gear touchdown

Airport pavements will deteriorate rapidly if they are subjected to aircraft loadings that exceed the structural design strength of the pavement. To prevent pavements from being overloaded and failing structurally, their strengths must be evaluated and reported (the PMS "Evaluation" activity) and aircraft loads should not be allowed to exceed the reported strengths.

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Critical Aircraft Loads

It is commonly thought that the most critical loading on an airfield pavement occurs when an aircraft touches down on a runway surface - but this is not the case. As a general rule, less than 50% of the weight of an aircraft impacts on the runway at touchdown - most of the aircraft weight actually remains airborne. Also, aircraft are considerably lighter on touchdown having lost a great deal of their weight due to the burning of fuel in flight. During the takeoff run, an aircraft gradually "transfers weight" from the landing gear to the wings as it accelerates along the runway towards rotation speed.

Air France B747

The most critical loading is static and occurs when the aircraft is parked on the apron prior to departure. At this time, the aircraft is at its maximum weight being full with both payload and fuel. The aircraft loading remains critical as it travels over the taxiway route to the runway end and gradually lessens as it accelerates through the takeoff run.

Pavement surface stability is also critical under heavy aircraft loads, especially when an aircraft turns on an asphalt surface - as when moving from a taxiway onto a runway threshold or when performing a 180 degree turn at a runway end to prepare for takeoff. High tire pressures can easily tear or shove an unstable asphalt surface.

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Aircraft Load Control Systems

Two engineering systems are used at Canadian airports for the control of aircraft loadings on airside surfaces. The first is the unique Canadian Aircraft/Pavement Load Rating (ALR/PLR) system which was first introduced at airports in Canada during the late 1970's. The second is the system developed by the International Civil Aviation Organization (ICAO) which is known as the Aircraft/Pavement Classification Number (ACN/PCN) system introduced for worldwide use in the mid-1980's.

Both the ALR/PLR and the ACN/PCN systems are simple to use. Each aircraft is assigned a number that expresses the structural loading effect of the aircraft on a pavement for a specified pavement type and standard subgrade category. Each airport operating authority reports site pavement strengths using the same numbering system. The pavement is capable of accommodating unrestricted operations by an aircraft provided the aircraft load number is less than or equal to the pavement strength number. Maximum tire pressure limitations may also be applied to some pavements which may further restrict certain aircraft operations.

Both load control systems are based on the static application of aircraft loads to the pavement surface making them somewhat conservative in nature. The correlation between the two systems is high and aircraft load assessments performed using either system will yield almost identical results.

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The Canadian Aircraft/Pavement Load Rating (ALR/PLR) System

ALR TableUnder the Canadian ALR/PLR system, the range of present day aircraft loadings has been divided into 12 groups according to their pavement strength requirements. The table at left lists typical aircraft in each of the 12 groups when the aircraft are at their maximum operating weights. All aircraft with the same ALR will have similar loading characteristics and hence similar pavement thickness requirements.

plr chart

Each pavement facility at an airport is assigned a Pavement Load Rating (PLR) on a scale of 1 to 12 to indicate its strength with a rating of 12 being a strong pavement and and a rating of 1 being a very weak pavement. PLR's can be summarized on a PLR Chart which is a small site plan of the airside pavements (a typical chart is shown at the left). The PLR chart indicates the suitability of a pavement area for unrestricted operations by any aircraft that has an ALR and tire pressure not exceeding the limits shown on the chart.

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The ICAO Aircraft/Pavement Classification Number (ACN/PCN) System

ACN Table

In order that pavement strengths can be reported and understood internationally, ICAO developed a very similar ACN/PCN system. Under the international ACN/PCN system, each aircraft has been assigned an ACN (Aircraft Classification Number) that indicates design thickness requirements for the aircraft on a more expanded scale that ranges from an ACN of 5 for light aircraft to an ACN of 120 or more for heavy aircraft. Some typical ACN values for aircraft are shown in the table above.

To comply with ICAO requirements, Canadian International airports report pavement strengths in a dual format which includes standard international PCN codes. Typical PCN codes are shown on the right hand side of the PLR Chart above. A PCN code has five parts - the first part is the actual PCN number, the second part is the pavement type ("F" for "flexible" or "R" for "Rigid"), and the third part indicates the subgrade strength category ("A" for high and "D" for low strength). The fourth part lists any tire pressure limitation which may apply - a "W" indicates that there is no tire pressure limit. The fifth part indicates the evaluation method used to determine the pavement strength - "T" if derived from an engineering study or "U" if based on satisfactory aircraft usage.

The PCN number indicates the suitability of a pavement area for unrestricted operations by any aircraft that has an ACN and tire pressure not exceeding the limits shown on the chart for the stated pavement type and subgrade strength category.

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Allowing Aircraft/Pavement Overloads

Aircraft with an ALR/ACN greater than the PLR/PCN reported for a pavement may still be allowed to use the pavement subject to the approval of the airport operating authority. However, the airport authority should fully understand the implications of allowing overload operations in terms of the accelerated structural deterioration and the reduction in pavement service life which may occur.

The overloading of pavements can result from either loads that are too large, a substantially increased load application rate, or both. With the exception of massive overloading, pavements in their structural behavior are not subject to a particular limiting load above which they will suddenly or catastrophically fail. Behavior is such that a pavement can sustain a definable load for an expected number of repetitions during its service life. As a result, occasional minor overloading is acceptable, when expedient, with only a limited loss in pavement life expectancy and relatively small acceleration of pavement deterioration.

For flexible pavements, occasional movements by aircraft with ACN values no more than 10 percent above the reported PCN should not adversely affect the pavement. For rigid type pavements, the ACN should not exceed the reported PCN by more than 5 percent. Overloads beyond these limits should be based on the results of a detailed engineering study that compares the individual aircraft load to the structural capability of the pavement.

Overload movements should not normally be permitted on pavements that are exhibiting signs of distress or failure and should be avoided during periods when the subgrade may be weakened by thaw following frost penetration or water infiltration. When overloads are allowed, the pavement should be inspected regularly by the airport authority to ensure that unacceptable structural damage is not taking place.

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More Information on Aircraft/Pavement Operations

Full details on aircraft/pavement load control systems, procedures and overload guidelines for airfield pavements can be found in the appropriate Pavement Technical Documentation.

PLR and PCN strength data for runways at major Canadian airports can be found by following the link to Pavement Strength Data.

ALR and ACN values for most commercial aircraft operating in Canada today can be found and downloaded from the Aircraft Data link.

Computer software for calculating aircraft ALR and ACN values can be found by following the link to Computer Software.

 


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