The most effective method is to use a thermoscanning device. Moisture can be detected via Infrared Inspection just after an aircraft lands. If there is water ingress in the radome it will freeze and be easily identified with our Top Of The Line, Highest Resolution FLIR T-620 Infrared Camera Manufactured (307,200 pixels – 640×480 resolution). The thermoscan is an infrared camera inspection that has been used in the past was to detect variances in surface temperature. The procedure involves heating the radome in an oven, removing the radome and allowing it to cool, and watching the radome with the camera to confirm moisture.
What you should know about radomes
By Greg Napert
Maintenance may require testing
From a maintenance perspective, radome repair facilities, such as NORDAM-Texas, see a great deal of problem radomes that have been repaired incorrectly or repaired so many times, they no longer meet performance standards.
Marc Overton, sales manager at NORDAM-Texas says, “We joke around here that the radome is kind of the “Rodney Dangerfield” of the airplane because it “Doesn’t get any respect.”
For years and years, maintenance personnel have perceived the radome as simply an aerodynamic housing that goes on the front of the aircraft. As a result, they often wrongly perceive that all repairs that qualify from a structural aspect are acceptable. Technicians need to realize that the radome needs special attention and that industry needs and requirements are changing.
For example, according to NORDAM, Boeing’s 737NG structural repair manual basically states that any radome repair performed for an aircraft which utilizes predictive windshear, must be tested for transmissivity after any repairs. If you don’t intend to operate with predictive windshear, you are not required to test.
Overton explains that many maintenance facilities don’t have a radar test range where they can verify the performance of the radome. So let’s say you have a radome that’s brand new and becomes damaged in the hangar. The maintenance facility may be able to repair a radome which is acceptable from a structural aspect, but, the question becomes — How does the repair affect the radar’s performance?
“One of the biggest problems with radomes is moisture ingression,” says Overton. Moisture inside the radome can significantly impact the performance of the radar. “Somewhere around 80 to 85 percent of all radomes we repair can be attributed to moisture ingression.”
Water collects into the honeycomb cells or reservoirs in a conventional honeycomb core, and as the moisture goes through freeze/thaw cycle, the ice expands and breaks through the surrounding cell walls. The honeycomb compartments become damaged. This cycle continues until the damage grows and the performance of the radome degrades.
“The actual result of this freeze/thaw cycle is what we call a “soft spot,” which consists of delamination and core disintegration — where the cell wall loses its support. It’s very difficult for radar transmission to penetrate the delamination and the moisture,” he says.
Overton adds, “The diverter strip attachment holes, holes made by static discharge, and erosion on the radome can all contribute to rapid ingression of moisture. Moisture is like a mouse in a house — you never know how it gets in there, but it does.”
Moisture in the radome can falsely indicate storm cells on the radar scope. So you see something there when there really isn’t. Or conversely, the radar might be mis-directed so that the pilot is missing a storm that is right in front of him.
Incoming Inspection Procedures
According to NORDAM-Texas, the number one procedure for incoming inspections is to examine the radome for moisture and delamination. There are a number of ways to check for moisture starting with inspecting for any obvious visual damage. When viewed from the back of the radome, severe moisture is typically visible as discoloration of the radome. To verify that these discolorations are due to moisture, a handheld moisture detector is used which indicates the level of moisture.
The radome is also scanned with a moisture detector in the areas where the moisture is not detected with the naked eye.
It’s possible to get false moisture indications near the diverter strips or near other hardware. You will know it’s a false indication because you remove the hardware or diverter strip and the indication disappears.
Overton says, “You will occasionally also get a moisture reading from antistatic paint. If the antistatic paint is too conductive, it may provide a reading for the moisture detector.” The most effective method is thermal imaging as moisture will be detected via infrared inspection.