Radome Repairs

Beat up by rain and alternately baked and frozen, fixing one isn't a job for the local body shop.

If your airplane is radar equipped, you probably give the radome little more than a passing glance on each pre-flight. After all, its nothing but a simple nose cone that should last forever.

Chips, dents and scars? No problem. Its just fiberglass. Come your annual, the local body shop can whip up a quick repair and the thing is as good as new in no time.

Yet nothing could be further from the truth. A radome is far more than a simple nose cone but a carefully designed electromagnetic window costing up to $10,000 or more on some corporate aircraft.

When flawed, a radome can degrade the efficiency of radar and lure an airplane into destructive weather hazards. It has happened.

Radar manufacturers respect radome engineering: More than 50 percent of radar performance complaints are traced to radome flaws, which the radar manufacturers really have nothing to do with. And appearances can be deceiving. Because they look okay cosmetically and don’t seem very complicated to begin with, radomes are rarely blamed for radar performance problems they often cause.

The typical radome is a honeycomb core, layered fore and aft with resin impregnated fiberglass facings. Its meant to be a durable and reliable shield for the delicate electronics behind the dome.

A better radome is faced with quartz laminates (Quartzel) and is more impact and erosion resistant, lighter and impermeable to water. Like Supermans Krypton, the modern fabric Kevlar has also found its way into radome construction and seems to be the perfect material, being strong and incredibly light. As the structural fiber within a resin system, it looks good and performs we’ll when new, if designed properly. However, many are not.

Kevlar radomes installed on some corporate jets have proven quite hygroscopic (water absorbing) and the material has a rather large coefficient of thermal expansion which varies according to the fiber axis. This means that Kevlar radomes tend to expand and contract with altitude temperature changes, which induces micro-cracking. Water gets into those cracks and the structure itself absorbs water like a dehydrated camel.

Of all the options, fiberglass, fiberglass-quartz and Kevlar , avoid Kevlar-faced radomes. Fiberglass is good and quartz is superior. Operators who have reported radome troubles tend to endorse quartz as being reliable, erosion resistant, long lasting and efficient.

Whatever the material, breaks and gaps in the laminates or resin system invite water access to the core.

Outer layer impact (bird strikes, hail impact, rain erosion or hangar rash) can damage protective paint and primer, delaminate the layers or crush the core. There’s really no such thing as minor damage to a radome. Whatever alters its engineering precision has deteriorated radar performance.

How They Work
Understanding how radomes work makes it easier to see why minor damage is so critical. Ideally, all energy would penetrate a radome at zero degrees incidence. But since the radome is curved, thats not possible.

Angles other than zero add thickness, however minute, encouraging refraction, reflection or distortion of the radar pulse. Energy deflected downward from the radomes upper surface, for example, can cause an otherwise efficient radar beam to detect ground returns, which look like a weather return to the unsuspecting pilot.

One foreign regional airline nearly grounded its entire fleet because of what it thought was faulty radar. Not surprisingly, the problem was traced to faulty radomes, which were drastically reducing the radars range and causing them to ground paint. Once good radomes were installed, the manufacturers radars and reputation were salvaged and pilots noticed significant range improvements.

The perfect radome is electrically invisible and doesnt exist as far as the radar is concerned. However, all radomes cause energy losses, expressed by transmissivity value or transmission efficiency; the percentage of energy remaining after its absorbed, reflected and refracted by the radome compared to the energy available if the radome werent there at all.

Transmission efficiency of 90 percent is considered the minimum standard. But thats for a one-way trip. In other words, a superb radome has a 20 percent round-trip penalty. So if you have 75 percent efficiency, it sounds just fine but is actually pathetic performance, with a 50 percent round-trip penalty. The radar would be essentially useless.

Critical to transmission efficiency is tuning radomes to the radar frequency. With the exception of two airlines, airborne radars use X-Band at 9.4 gHz, which yields a wavelength of 3.2 cm or 1.26 inches. Wavelength is critical to how radar sees the water droplets which define weather and its also critical to radome engineering.

Radomes are thickness-tuned to approximately 1/4 the wave length or .315 inch. Physically, theyre built of five precise layers. Weve mentioned the inner epoxy/fiberglass facing, the honeycomb core and an outer protective facing. The remaining two layers are primer and paint. But not just any primer or paint.

The paint scheme averages .008 inch in thickness, including filler, primer, anti-static and top-coat layers. Adding repair paint of only .005 inch thickness seems an obvious enhancement but it probably degrades the electromagnetic purity of the radome. Its like smearing Vaseline on your glasses. You simply cannot add layers to a radome indiscriminately, including unapproved hard plastic caps or rubber erosion boots.

Daily Abuse
Radomes are abused in flight by rain, sand and grit, high velocity airflow and hail. The paint and primer often erodes, peels or cracks. Its discouraging to see aircraft in lines awaiting takeoff, stopped so closely in trail that jet exhaust or propwash is sand blasting the fragile surface of the radome.

Moisture is the major barrier to radar transmission and is the cancer of radomes. The pressures of climb, descent and high speed act to drive moisture into the radome via any wound, no matter how minor. With altitude-related freezing and thawing, even micro-cracking allows moisture to seep in and freeze, enlarging the wet area over each flight cycle.

Moisture content is so critical that any professional repair starts with a moisture content test. A moisture-infected radome must be stripped, dried at 125 degrees F and a new inner skin created.

Well-intentioned mechanics often restore radome appearance with a can of spray paint: covering cracks, replacing paint, sealing in the moisture and concurrently, re-engineering the radomes efficiency.

Badly botched repairs are often far too thick, since its impossible to control resin application accurately without the correct equipment and materials, even though such repairs will probably pass cosmetic muster.

Paint selections (even when applied correctly) should be limited to approved neoprene, polyesters or polyurethane; Rustoleum and most metallics arent recommended. Remember, a 10 percent detriment in performance is a 6 percent detection loss in short range, according to some engineers. A 10 percent improvement shows impressive gains in performance at longer ranges, in some cases improving the range from 70 miles to 120 to 140 miles.

Where to Go
Amazingly, some airframe manufacturers don’t plan production in concert with radome producers. Norton Performance Plastics Corporation is the international leader in radome design, production and repair. According to Ben Mackenzie, Nortons Director of Technology and Engineering and a commercial IFR pilot himself, most of the companys business comes from frustrated manufacturers trying to resolve the radar complaints of frustrated customers.

Radome testing can be as inexpensive as $375. A radome is generally tested by Norton the day of arrival and repaired within three weeks. Loaner, rental and leased radomes are available through Norton to keep grounded aircraft in the air.

New radomes can range from about $3000 in exchange programs to as much as $51,000 for a giant Airbus dome in a pristine new condition. Radar manufacturers say that 30 percent of all complaints originate in radome flaws and Nortons Mackenzie says most radar installations are far more reliable and trouble free than many operators suspect.

Can the pilot check radome efficiency? Field tests are reasonable but not precise. Turn on the radar and paint a radar image of a hangar, mountain or even tilt down to acquire ground returns. Make careful note of what you see or even consider taking a quick Polaroid photo.

Decrease the gain control, desensitizing the radar until the object just disappears. Put the radar in standby, remove the radome and turn the radar on again. Anything you see now is what you did not see through the radome.

Its a visual image of radome losses. Obviously, youre not testing all angles and areas of the radome nor the faint signal returns of storms more than 100 miles away, but youre getting a good fix on potential radome flaws.

Take the radome off and hold it in the sunlight. Since its translucent, look for consistent thickness, black spots, water streaks along the inside edge and any apparent cracks or punctures. Debonds appear as lighter areas around the edges of the honeycomb cells.

Another quick field test is the so-called coin test, which you can easily do every so often on pre-flights. Using the thin edge of a quarter, tap the radome at various points on its surface. The sound should be a sharp click; a duller thunk indicates that some of the laminations may have debonded or that water has found its way into the honeycomb core.

When aloft, the average well-maintained radar should ground paint out to the square-root of its AGL altitude. At 5000 feet, thats about 70 miles; at 10,000 feet, its 100 miles. Attempted ground paint beyond 100 miles is not practical. The impact azimuth is too slight and it simply skips into the atmosphere with no return. While youre at it, see if ground paint is consistent from side-to-side.

Do all these tests before you reach for the telephone and call the radar manufacturer. Time and time again, says Mackenzie, apparent flaws in the radar are really the fault of the radome and they can be easily diagnosed.

Bad Repairs
Field repairs can be as hazardous as any embedded thunderstorm. A twin-engine airplane under repair fell forward off scales and the radome struck a tug. Since the radome is fiberglass (like Chevy Corvettes), who might fix it better than a local fiberglass repair shop?

The repair was cosmetically beautiful. The owner didnt complain. Airborne and confronted by a squall line, the wide gap in the line (dead ahead) looked both safe and inviting to the owner-operator. An accident investigator determined that the aircraft fell in pieces after flying directly into a level 6 thunderstorm. The repair had simply blinded all radar capability forward of the airplane.

Radomes, or actually the metal antennas within, are splendid lightning attractors, even though the dome itself is practically transparent to lightning. Static buildups are also electrical routes. Dramatic discharge flashes happen from static accumulation alone. Any puncture, even microscopic, of the radome from any electrical attack is another water route aided by pressure changes and freeze/thaw cycles.

Lightning strikes pierce the radome wall. Massive heating can blow off the inner skin of the core, dramatically weakening the structure and possibly allowing it to collapse inward. A lightning strike followed by erratic airspeed can be an indication of radome damage with shape change or even radome loss (assuming the pitot system to be downwind of the radome).

Many radomes have lightning diverter strips bonded on to the surface of the dome and grounded to the airframe. The strips cause streamering which returns much of the energy back toward the lightning leader which initiated the strike. The strips are grounded to the airframe, which allows the residual charge to be diffused over the expanse of the airframe.

Like the radome, diverter strips need to be checked for degraded efficiency. This can only be done by a qualified shop, but given the consequences of a lightning damaged radome, its worth the minor expense of having it done.

One indication of degraded or open lightning diverter strips is a buzz or crackle on the comm radios when flying in rain. This is caused by p-static building up on the radome thats unable to find its way harmlessly to ground. What you hear is the static arcing to ground across an open or damaged strip.

Mackenzie tells us that just about any radome damage-including major lightning hits-can be repaired, although he occasionally sees damage that is uneconomical to fix.

Plan to have new radomes tested (unless purchased from a source like Norton) and insist that new aircraft purchases come with radomes meeting the RTCA standards for the class required by the radar manufacturer. Check and test any radome thats been repaired in the field. Its a minor investment for a major return in safety.

For radome study, check out the FAAs Advisory Circular 43-14, Maintenance of Weather Radar Radomes. It has additional information on radomes not covered in this article.

Also With This Article
Click here to view the Radome Checklist.
Click here to view “Protecting Against Rain Erosion.

-by Dave Gwinn
Dave Gwinn is a TWA Captain and an internationally recognized authority in weather radar. He conducts regular seminars in radar technique and can be contacted at 913-831-3338.