About the National Airport Pavement Test Vehicle (NAPTV)
The NAPTV simulates a controlled aircraft wander which is programmable and can operate in manual or fully automatic modes. During traffic tests, the NAPTV typically travels at 2.5 to 3 miles per hour but can reach up to 15 miles per hour. The NAPTV is approximately 1.3 million pounds and is driven by 16 – 50 horsepower AC torque motors. The NAPTV is stopped by a regenerative braking system before engaging pneumatic brakes at a full stop.
During traffic tests the NAPTV is operating in full automatic mode. The NAPTV Operator defines the starting and stopping points for the NAPTV. They also define where the start and end points are for each test section. Wheel loads are also defined for each test section. The NAPTV Operator can program which load modules and what load will be applied to each test section.
Carriages and Load Modules
The NAPTV is comprised of two carriages that can accommodate up to five load modules each. In the current configuration, there are 2 - single wheel load modules and 8 – dual wheel load modules. This allows for configurations up to 18 wheels with loads up to 75,000 pounds (333.75kN) per wheel.
The north carriage is designated “Carriage 1”, the south carriage is designated “Carriage 2”. The east-most load module on each carriage is numbered “1”; the west-most load module on each of the carriages is numbered “5”. Therefore, load module 1-1 is on the north carriage and faces the east end of the test facility. Similarly, load module 2-5 is on the south carriage and faces the west end of the test facility. Inner wheels are designated as “A” and are closest to the test facility centerline, where the outer wheels are designated as “B”. The north carriage currently has a single wheel load module installed at location 1-5. The south carriage currently has a single wheel module installed at location 2-1. All dual wheel load modules except the single wheel load module installed at location 2-1 have 52 inch radial aircraft tires installed. The single wheel load module installed at location 2-1 has a 49 inch bias ply aircraft tire. Each of the load modules can be operated independently to simulate any desirable configuration with the installed load modules. Aircraft tires are inflated with compressed nitrogen at pressures based upon the load application.
Each of the load modules has its own digital position/force feedback hydraulic control system accomplished by a motion controller to hold position (inches) or load (pounds) on the test pavement. The digital motion controllers have feedback from module position, hydraulic pressures, and test pavement elevation measurements that are used to extend, hold a load, or retract the load module.
The NAPTV has safety interlocks built into the system that monitor load module load, position, and tire pressure. If any of the thresholds are reached for these parameters, the NAPTV will safely abort all operations and shut down.
Dual wheel spacing on each of the modules can be adjusted with axle replacements. Tandem spacing of the modules can be set by using different lateral mounting positions on the carriage. Simulation of a single wheel load requires the installation of a single wheel module.
During construction cycles 1 and 3, only 2-wheel modules were installed. Specific load configurations are published under each construction/traffic testing cycle.
The NAPTV simulates realistic aircraft wander by varying the lateral position of the carriages to simulate a normal distribution of aircraft during traffic testing. The wander pattern used for the majority of traffic testing consists of 66 NAPTV passes, arranged in 9 wander positions (or tracks). The wander positions and sequences were chosen to simulate a normal distribution of aircraft traffic with a standard deviation of 30.5 inches (representing the current design condition for airport taxiways). Normally, the same wander is used on both carriages and it is changed at the west end of the test pavement. Specific wander patterns are published under each construction/traffic testing cycle.
Each of the NAPTV carriages uses an absolute positioning system. An absolute positioning system retains its zero or home position until it’s physically reset by the operator. The carriages are zeroed from the test facility centerline. Wander positions for Carriage 1 have a range that can be from 0 to -20 feet, and for Carriage 2 it is from 0 to +20 feet. The wander location is based on the center position of the load module. Therefore on a dual wheel load module at wander position -20 feet means the center of the outer wheel is at -22 feet, 3 inches. The inner wheel would be at -17 feet, 9 inches. Each of the carriages are moved by a 2000 rpm motor that goes through a 1600:1 reduction gearbox on both ends. The carriages can be moved manually or automatically. They move at approximately 0.169 miles per hour top speed. Position error at any wander position is less than 0.5 of an inch.