Airports, airfields and pavements – from beginning to present day
Obviously, all airfield pavements are designed to absorb aircraft loads, to withstand the environmental factors and to provide the safety of taxiing, take-off and landing of aircraft. On each stage of its development, the Airport’s technical parameters adequately corresponded to the development of aviation technology, a variety of aircraft types and missions, possibilities of the state economy.
At the dawn of aviation and in the early years of its development, at the beginning of the last century and in the 1920-30s, all the airports had unpaved airfields without any complicated structures and equipment. Currently, the Airport is a system of complex structures with a total area of up to five hundred thousand square meters including artificial pavements, service and technical areas, communications, radio and lighting equipment.
Modern Airport common view:
The main part of any airport is the Runway . Airport pavement design has undergone dramatic evolutionary changes in the last century: runways from unpaved areas a couple of hundred meters long eventually turned into runways with flexible multi-layer or hard concrete pavements 2500-4000 meters long, requiring large financing for construction and operation. Since the mid-40s, the main types of artificial pavements for runways, taxiways, aprons, aircraft parking lots are built from hard (mainly reinforced) of asphalt (flexible) concrete. From this period to the present day the airfield pavement design is based on the variations of asphalt-concrete for rigid pavements or artificial multi-layer materials for flexible ones. Now all the design methods and principles are regulated by FAA/EASA advisory circulars .
Rigid airfield pavement design (typical) is shown in the figure below:
Flexible airfield pavement:
Airfield pavements rupture and repair
It was found that any irregularities or damages in the runway highly affect on the flight crew operation and technical condition of the aircraft – particularly, landing gear structures.
The main factors causing the destruction of runway pavements are the impact of loads during aircraft takeoffs and landings as well as natural and climate effects.
The following loads are transferred to the pavement during the operation:
1. mechanical stress;
2. force and temperature effects of a gas jet from civil and military aircraft jet engines.
Natural factors affecting runway pavements:
1. daily and seasonal temperature differences;
2. air humidity;
3. precipitation (frequency, amount and seasonal distribution);
4. soil freezing;
5. winds (strength, direction and duration);
6. icing (ice layer on the runway pavement).
Damages from natural and climate effects occur after relatively long periods of time, at once with it, the mechanical damages from aircraft landings, runs and takeoffs are more significant after a shorter time and require several limitations on airplanes, regular inspections and repairs.
Overloads especially in combination with incorrect de-icing and cleaning and/or temperature effects produce pavement cracks. Cracks can gradually increase length and depth, as well as a branch in different directions. Due to such a critical effect of overload on airfield pavement, it is recommended to pay extra attention to the Airfield condition and perform additional inspections. Empirical formula to define frequency to evaluate grade of airfield pavement is the following:
u – total number of landing/takeoff operations by all aircraft per day/week/month
U i – max allowable number of landing/takeoff operations for particular airplane number i, between 2 unplanned inspections, in dependence from overload coefficient
p i – relative number (Weight coefficient) of all operations for particular airplane number i.
Generally, reasons to evaluate the airfield pavement are the following:
- detection of damage and defects during operation or construction;
- before future reconstruction of the airfield;
- the pavement is to be strengthened for regular use by heavier aircraft;
- resumption of interrupted construction;
- damage caused by natural or technological disasters;
- standard operation periods expiration.
To evaluate the airfield pavement (and also for successful design) it is required to establish the following:
1. Aircraft types permanently based on the aerodrome, their take-off and landing operations intensity, max take-off and landing weights (MTOW, MLW) distribution for each type of aircraft;
2. Chassis diagrams and parameters (number and relative position of the wheels, the distance between the wheels, pressure in the pneumatics) of the main landing gears (MLG) of all aircraft types permanently based on the airfield;
3. Types, the intensity of operations and distribution of MTOW and MLW of other aircraft operating the airfield;
4. Airfield areas that are subject to the most significant operational impact (runway main starting section, most used taxiways and main taxiway section, main parking lots).
Based on the information above, pavement is assessed for its bearing capability, residual life, operational loads impact on the damages and their growth.
Airfield pavement defects and damages are detected by visual inspection with the use of measuring instruments and making photos of the damages. Length of longitudinal and transverse cracks in the concrete plates, transverse dimensions of shells, surface layer peelings and delamination are measured with steel tape measurers. The depth of shells and potholes in the asphalt concrete pavement is measured with steel and wooden rulers.
Identification of profile coating fractures, the unacceptable difference in adjacent plates slope, coating sections with subsidence, and swelling are carried out by surface geometric leveling with the use of ordinary or precision optical levels.
Optical Level operation
Identification of pavements latent defects (gaps between the structural layers, subsidence of the base due to erosion, etc.), which is unable to determine by visual inspection, is carried by such instrumental methods as ultrasonic inspection, thermal imaging, radioisotope analysis, etc.
Laboratory tests of pavement samples allow obtaining the actual values of such physical parameters like strength, thermal resistance, density, etc.
Falling Weight Deflectometer is used primarily on airfield pavements for evaluating the elastic stiffness of various pavement layers which in turn can be used to indicate material condition, and for determining the load transfer efficiency at joints in rigid pavements. This is one of the nondestructive test methods to evaluate the airfield pavement.
Falling Weight Deflectometer
Obtained data should be collected and formalized in the Evaluation Report. This report serves as the basis for making the decision about pavement renovation or total rebuilding with full design procedure briefly described below.
Main stages of Airfield Pavement Design
Since the Runway is a major element of the airport, all design stages, instruments and investigations are described for the design of the Runway pavement.
Runways and taxiways should be placed relative to other major operating elements (terminal building, cargo areas, aprons air traffic services and parking, etc.) to provide the airport configuration maximum overall efficiency. In establishing a new runway layout, the factor influencing runway location and orientation should be considered first. Also, it is mandatory to consider the aircraft types operating the runway, meteorological conditions, surrounding environment and volume of the air traffic expected on each runway. Main factors to consider are listed below:
1. Location of neighboring airports.
2. Obstruction and topography.
3. Soil investigation
4. Built-up areas and noise.
5. Air traffic control technique.
6. Wind direction and visibility.
7. Capacity (air traffic volume).
Airport pavements design requirements are thoroughly regulated by, in particular, FAA Advisory Circulars 150/5320-6, 150/5335-5, and 150/5370-10 .
According to FAA requirements, mechanistic-empirical pavement design procedures are implemented in FAARFIELD and COMFAA computer programs. These programs use layered elastic and three-dimensional finite element-based design procedures for new and overlay designs of flexible and rigid pavements respectively. The program allows selecting in semi-automatic mode the correct pavement type and structure based on the weight-dimension parameters of the Aircraft planned to operate the designed runway according to ICAO requirements.
A-380 Landing Gear parameters
Antonov An-124 Footprint modeled in COMFAA Software
Nowadays, the software is able to perform the automated design procedures taking into account the following factors: Stabilized Base Course, Base or Subbase Contamination, Drainage Layer, Subgrade Compaction, Swelling Soils, Pavement Life.
Typical Input structure for COMFAA and FAARFIELD is shown below:
FAARFIELD/COMFAA Programs overview