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."
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 looses 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.
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."
Another option is to use a thermoscanning device. The thermoscan is nothing more than an infrared camera that is used 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.
To be good at moisture detection you really have to combine a number of tools, as well as experience in knowing when and where to look.
Choose your repairs carefully
Most repairs to radomes can be done in such a way that they have little or no impact to the performance of the radar.
Some repairs, however, do impact transmissivity of the radar. The key is to know by how much.
According to Overton, "Impact damage or soft spot repairs to conventional radomes with honeycomb cores often require replacement of the core material."
Core replacement is made using honeycomb material or using a flexible honeycomb core called flexcore. NORDAM-Texas says that you ideally want to avoid using potting compounds as they can be detrimental to radar. A dry splice, or foaming adhesives, is the best bet to help maintain transmissivity.
Potting compound will affect the transmissivity of the radome. The result is a loss in the overall efficiency of the radome. With each subsequent repair to the radome, the performance continues with a downward trend. With conventional repairs using honeycomb, flexcore or fluted core technology, there will be some impact on the performance. You can minimize the impact, however, by minimizing the number of repairs by the educated use of repair materials.
Radomes may be able to withstand multiple repairs and still remain in their class. If the radome maintains borderline transmissitivity within its class, one repair may drop the radome to a lower class — which may be unacceptable to the operator.
Typical boot installation procedure(Photos by PM Research, 1999) February 1999
The following procedure is edited material taken from PM Research's Shop Manual. It is representative of the installation process only. For complete details on boot installation, PM Research provides a manual and video available by calling (716) 593-3169. 3M also has installation manuals and videos available by calling (651) 733-9288.
1. Be sure the nose cone is completely refurbished and the paint has had sufficient time to cure. If stripes are applied to an area the mask will cover, sand the edges of the stripes and use rubbing compound to restore the luster.
2. Follow the recommended procedure using solvent to remove any wax or release agent, then wipe the residue clean with a distilled water, alcohol, and soap mixture.
3. Check to be sure you have the proper mask for the aircraft. When you dry fit the mask to the aircraft, keep in mind that the mask is manufactured with a good deal of additional material on it and the rear of the mask may not appear to fit properly. This area will be trimmed during the installation process. The mask only has to cover the radome toward the back of the aircraft to at least a point where a yardstick rests on the radome when held against the radome and tipped at a 45 degree angle, or by placing the 90 degree angle of a carpenter's square across both sides of the radome with the center of the 90 degree angle on the center line of the radome. The protective mask need only cover to the point where both legs of the square touch the radome.
4. Remove the flexible mask from the shipping support by cutting the mask with a new razor blade.
5. Make a template to the desired finish size out of the support that is shipped with the mask. Cut and sand the template smooth. It will be used to center and mark the final location of the mask. (There are various ways to align the boot. 3M recommends marking the center of the radome with a pencil, positioning the mask in the desired location on the radome and marking the mask to match the radome mark, and then aligning the mark during final installation).
6. Dry fit the boot to the radome and when you are happy with the alignment, mark the top center or 12 o'clock position by cutting a small slit at the top of the mask in the area that you will trim away.
7. Peel a small amount of liner from the mask and spray the adhesive liberally with the water/alcohol/soap solution. Continue peeling and spraying the liner in small increments, until the liner is removed and the adhesive side of the mask is thoroughly sprayed.
8. Liberally spray the aircraft radome with the water/alcohol/ soap solution.
9. Position the mask on the radome, lubricate the surface of the mask with soap solution, and begin to work the air and bubbles from the liner from front to back.
10. Use a squeegee to continue to work the water and air to the back of the boot until there is no air or water beneath the mask. Always work from the center of the mask toward the back of the aircraft being careful not to get any area stuck down while moisture remains in front of it. Also, use only moderate pressure on the squeegee. Too much pressure can cause a condition where hundreds of small bubbles appear under the mask, resembling beer foam.
11. Wipe off excess fluid with paper towels.
12. Place template over mask and mark the desirable trim line. Then tape along the trim line with 1/4-inch masking tape.
13.Using the edge of the tape as a guide, trim the mask with a new industrial knife or razor blade.
14. Remove the tape, peel away the excess material, and wipe away the pen line that was used for trimming.
14. Final squeegee to assure all wetting solution is purged from beneath the mask. Pay special attention to areas around stripes, working all moisture out of the ridges.
15. If any blisters are discovered, it is allowable to use a pin to prick a small hole in the blister. Then use a squeegee to work out any entrapped air or moisture. The mask will lay down and stick.
Common Problems Encountered When Installing Masks:(From PM Research) February 1999
Problem 1: Applied protective mask and it looked fine when they left for the night. The next day, they noticed small blisters or bubbles under mask.
Cause: Paint is still outgassing.
Solution: Be sure paint is totally cured before applying the mask. The bubbles can be eliminated by pricking them with a pin or needle, and squeegeeing them down again.
Problem 2: After application of mask, customer noticed "beer foam" appearance under mask.
Cause: Mask was overworked and squeegeed too hard.
Solution: We suggested they apply a new mask, and use moderate pressure on squeegee.
Problem 3: Customer stated that protective mask would not stick. Mask was returned to P.M. and was found to be ok.
Cause: Either wax or oil, or silicone wax was on the aircraft which prevented adhesion. Additionally, there may be a teflon finish on the aircraft, the spray bottle or solution may have been contaminated. Another possibility may be the paper towels you're using have an adhesive that is acting as a release agent.
Solution: Be sure your equipment is not contaminated. Also, note that silicone wax, rtv and other silicone agents can spread over a wide area and act as a release agent for paint, as well as for adhesive. They seem to keep coming back up to the surface no matter how hard you try to clean them off. Try to keep these materials out of your shop.
Problem 4: The adhesive is gumming up or balling up under the mask.
Cause: Improper mixture of wetting solution (too much alcohol) or wrong alcohol. Do not use deice alcohol. Use the drug store variety or "rubbing alcohol" that is 70 percent by volume.
Solution: Mix wetting solution according to shop manual.
Problem 5: Blisters appear while mask is being applied.
Cause: Wetting solution trapped underneath is caused by sealing down the back before the wetting solution is completely squeegeed from the front of the mask.
Solution: Always squeegee from front to back only. Start at the center and work all moisture toward the back of the mask (Back of the mask is toward the back of the aircraft). If you are beyond this point and blisters already exist, prick each blister in the center of the blister with a pin and squeegee toward prick hole to expel moisture. The mask will lay down and stick.
Problem 6: Mask puckers up in back.
Cause: Extra material is supplied on most masks.
Solution: Trim the mask off farther toward the front. Note the mask should cover the radome toward the back of the aircraft at least to a point where a yardstick rests on the radome when held against the radome and tipped at a 45 degree angle.
Problem 7: Mask has bug stains on it.
Preventative action: After application, the mask should be waxed with a good, non-silicone paste wax. Periodic cleaning and waxing will keep your mask looking good.
Problem 8: Mask has rust stains underneath it
Preventative action: Use only distilled water in mixing wetting solution. Tap water can contain many contaminants, including rust particles.