What you should know about radomes
By Greg Napert
Radome repair and inspection today are becoming more critical as aircraft navigation is more precise and the rules governing navigation are changing. New weather detection technology, as well as reduced separation make it necessary that radomes manufactured, remain transparent to radar.
"In an ideal world, the best radome would be no radome," says Ron Bauer at Norton Performance Plastics. "Of course, that isn't practical," he explains, "as the equipment would be damaged without a radome. And on the other end of the spectrum, a metal radome would stand up to erosion and impact damage, yet that isn't practical either, as a metal radome is not compatible with radar."
Bauer says that it is unfortunate today that as good as radar systems are, aircraft manufacturers don't always understand the need for radome performance.
"Most of the new radomes that come out of the OEMs today are a Class C," he says. "The radomes work fine with conventional weather radomes, but when it comes to Doppler windshear detection and other types of radar, the OEM radomes are just not good enough.
"Then the airlines go and install thousands of dollars in windshear detection equipment, and it doesn't work the way it's suppose to."
The wiser airlines understand this, and some order the new aircraft minus the radome. They then order the radomes that they need from places like Norton or NORDAM-Texas and install these Class A radomes as part of their radar system.
Third party suppliers such as Norton and NORDAM, competitors in the radome manufacturing and repair business both manufacture radomes. NORDAM-Texas focuses on the repair of all aviation radomes in all categories, while its manufacturing is mainly for transport category aircraft. Norton manufacturers and repairs a wide range of radomes available for all sizes of aircraft.
For performance purposes, it is critical to maintain these radomes in their original condition. Each repair can degrade the performance of the radome if it isn't done correctly.
"It's a matter of understanding the basic principles behind radome operation," explains Bauer.
He explains, "The physics of the radome are fairly simple when you look at them. The transmission wave of the radar is a sine wave that can be measured exactly., So in order to tune the radome, you simply have to manufacture its wall to a particular thickness; one that won't interfere with the radar sine wave. Too thick and it won't work — too thin and it won't work. The materials are not nearly as critical to the radome's transparency to radar as the thickness of the wall and the orientation of the materials used.
"This is why it is also critical to maintain repaired areas to exactly the thickness of the original radome. Any variation of more than .005 inch from the original thickness can begin to impact the transmissivity of the radome."
Transmission efficiencies are measured in terms of the average one-way signal that the radar returns under laboratory conditions and the minimum signal it will return. There are classified by letter A through E with A being the best and E being the worst. An "A" radome has a rating of 90/85. This means it will conduct an average of 90 percent of the signal beamed in one direction with a minimum of 85 percent at any one location on the radome. The ratings for all categories, according RTCA Document DO213, are as follows:
A — 90/85
B — 87/82
C — 84/78
D — 80/75
E — 70/55
Another important characteristic of the radome is its ability to bleed off static electricity. This is done through the use of anti-static/anti-erosion coatings and through the lightning diverter strips extending from the frontal area of the radome to the airframe. "These diverter strips, says Bauer, continuously collect static build-up from the surface of the radome and conduct it to the airframe without severe sparking or arcing."
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