Airport wind sheer sensor hit by e-toll interference

Airport wind shear sensor zapped by e-toll interference

The electronic tolling equipment at the mainline toll plaza of the Dulles Greenway was intermittently interfering with transmission of wind shear alert data at Washington Dulles airport for a couple of weeks in the spring, according to an FAA official. After the interference with the airport’s Low Level Wind Shear Alert System (LLWSAS) was discovered and the Dulles Greenway and the equipment manufacturer Mark IV Industries were informed of the problem, they tuned the antenna-reader equipment at the toll plaza, successfully suppressing the interference. No recurrence of the problem has occurred since the spring according to the FAA.

However in early October the wind shear alert system at the airport again had problems receiving data from sensors on the Greenway side of the airport and a supervisor, not unreasonably in view of the earlier problem, accused the toll road equipment of once more generating interference. The FAA called in the FCC but this time they found no interference attributable to the e-toll system. The FAA then traced the October problem to a bad piece of RF cable at the wind sensor producing losses in data transmissions.

We got a tip-off about an interference incident in April but could not get any confirmation. We also heard about the airport supervisor’s complaints about e-toll interference in October. He wouldn’t say anything but FAA public affairs in Queens NY investigated and gave us this account.

7km of the 21km toll road are built on leased land within the airport boundaries and the main toll plaza with transceivers in its roof gantry above the toll collection lanes is about 1km from the end of the main north-south runway on the northeast fringe of the airport. There are LLWSASs like those at Dulles at 120 airports around the U.S. They depend on gathering data from ground level wind sensors placed around the airport property and transmitting their data to a central point by radio. Sudden changes in wind velocity at a single sensor can indicate possible wind sheer. A more advanced Terminal Doppler Radar system that ‘looks’ directly for windshear events up in the atmosphere is being introduced to supplement the LLWSASs at about 50 U.S. airports, and one of these is due to go into Dulles airport in 1997. Wind sheer is blamed for a number of terrible air crashes in which the plane becomes unflyable and plunges into the ground. The only known safety measure is to inform pilots of wind sheer location so they can stay away from it. (Contact FAA Arlene Salac 718 553 3010)