A scheduled maintenance job of a customer’s helicopter reveals a “battered and bruised” radome — one that obviously has seen better days. Radomes perform a critical role in today’s aircraft, so any type of damage — from small tears to large punctures — can have a significant impact on the safety of a helicopter. That said, a radome requires ongoing maintenance to ensure proper transmission of the weather radar signal, long before it reaches the stage of complete failure.
Importance of Maintenance
Designed to be transparent to radar or radio waves at a particular frequency range, the radome also protects a helicopter’s antenna surface from elements while withstanding airload.
Like most components on a helicopter, radome maintenance is critical in keeping the helicopter’s level on track for safety and performance. As with all aspects of the aircraft, it is less costly to keep a part maintained rather than to let it degrade to a point where a full overhaul is necessary.
“Radomes tend to be seen much like another fairing on an aircraft, and from the outside aero surface, that is its purpose,” says Leigh Sargent, president and founder of Applied Composites Engineering (ACE), a company that has extensive experience with repairing and testing radomes for both the business jet and commercial aviation industries. “But unlike any other fairing, it needs to transmit radar, typically X band for weather, and any degradation in its ability to do so will affect the radar’s operation.”
As Sargent explains, a damaged radome is analogous to wearing scratched-up prescription eyeglasses. One day you will trip over, or in the case of the radome, not seeing clearly the extent of weather — but, in the case of radomes, the consequences can be far worse.
“Unlike eyeglasses where you can see the degradation, in many cases it is not obvious on a radome,” says Sargent. Small cuts or miniscule punctures can wreak havoc on the radome’s long-term structure and performance.
The single largest effect on radar degradation is caused by water ingression into the honeycomb core. Continuing to use the eyeglasses analogy, Sargent equates this to eyeglasses that are fogged up, as the radar cannot penetrate the water and increased reflection can take place.
“The pilot may be seeing weather which does not even exist or the reverse, which rapidly becomes a safety issue,” says Sargent. “This issue is often caused by the painted surface being penetrated, which can be through very small flaws such as micro cracking, erosion, paint chip, or static pops. Unfortunately, when this happens it can migrate and grow rapidly as the water penetrates further into the radome, particularly if the aircraft is consistently going from moist to freeze environments.”
As Mike Manning, vice president at Dallas Aeronautical Services in Cedar Hill, TX, explains, additional common damage to a radome can occur from a weather event such as hail or strong rain on the climb, or lightning strikes. Impact damage to the radome can occur on the ramp, during maintenance or during towing.
“We call this ‘random acts of God or stupidity,’” says Manning. “If the outer skin of a radome is compromised and is not noticed right away, moisture will enter the radome during each flight and the non-metallic core soaks it up like a sponge. What could have been an easy fix turns into a major repair.”
What To Look For
Unfortunately, manufacturers require no repetitive inspections for radomes. That’s why industry experts recommend that helicopter maintenance professionals thoroughly evaluate the conditions of the radome each time general maintenance and inspections are performed on the helicopter.
When inspecting a radome, mechanics should look for signs of lightning strikes or heavy static discharge pops. The general visual condition of the protective clear boot on the tip of the radome also should be inspected closely.
“Lightning strikes and static discharge pops appear as small burns in the paint,” says Manning. “Paint erosion also can be a sign of hail impact.”
Water ingression caused by micro cracking in the composite is a common flaw. “Trapped moisture or a repair done incorrectly will change the way the system performs and undermines the information given to the pilot during flight,” says Manning.
In fact, the ability of water at high speeds to find micro flaws in a surface and erode it is far worse than the occasional dust on landing and takeoff, which is normally impacted at lower speeds.
“A high-pressure washer also can induce major damage to the radome if placed too close to a painted surface,” says Sargent. “Any flaws in the final coat on the radome can diminish its ability and thus the longevity to protect the structure of the radome.”
As part of its ongoing research, ACE undertook testing at the rain erosion test facility at Wright Patterson Air Force base in Dayton, OH, testing several coatings and paint types to determine the best and worse types of for radomes.
Saint-Gobain Aerospace, a Ravenna, OH-based company specializing in the design development and manufacture of high-performance, high-quality radomes, stresses that radomes with excessive amounts of filler/primer surfaces and/or multiple coats of paint (anything in excess of 12-15 mils total thickness) will experience loss of efficiency resulting from the excessive thickness that de-tunes the radome.
According to Saint-Gobain, too thick of a coating can reduce the efficiency of a 90-percent radome down to 50-60 percent. That’s why, when refinishing a radome, maintenance professionals should not paint over existing coatings as the cumulative thickness of the old and new finishes will reduce the radome’s efficiency.
So what is the best methodology for evaluating the condition of a radome? Initially, tap tests are the traditional method of finding problems with a radome. Using a quarter, a mechanic taps along the radome, listening for a different sound such as lower note with a dead sound, as compared to the immediate surrounding area.
“Unless you know the radome, the mechanic needs to be careful that he is not tapping over an area where the change in sound is due to a different composite laminate and not damage,” says Sargent.
In addition, a visual inspection should be made whereby mechanics look for paint degradation, dents, static pops, needle-sized holes, and obvious cracking or impact damage. The mechanic should also thumb push the surface and see if there are any areas that deflect or are soft, which normally reveals a disbond from the core.
Underestimating or ignoring visual damage is a common mistake that mechanics or maintenance personnel make. Manning stresses that if you see anything that looks questionable, it is vital that you perform a tap test and listen for disbonded or delaminated areas around those areas that look concerning.
While some radome repairs can be performed in the field, others need to occur in an experienced composite shop. “In the field, small wet laid repairs can be undertaken for a single skin repair, usually the inside skin,” says Sargent. “These are typically limited to a few square inches as transmissivity testing is not available and with smaller repairs, less than a few square inches, even if done incorrectly, there is little effect on radar.”
Also in the field, Sargent says minor paint touch ups and radome boot replacements, as well as anti-erosion treatments, can be performed. Anything more extensive will affect the anti-static coatings below the topcoats and may alter the static discharge of the radome as well as the transmissivity of the radome itself.
Transmissivity is Key
Fiber orientation, overlaps, resin type and quantity all play into signal transmissivity — one of the most important design components of radomes. If one or more of these is not taken into consideration, or the mechanic lacks the adequate skill in preparing and applying the repair, the transmissivity of the radome can be affected adversely. “We often see poor repairs undertaken where fillers are used that totally blank out transmissivity in the area of the repair,” says Sargent.
Paint is an enormous factor in radome maintenance, not only for transmissivity but also in protection of the radome structure. An anti-static paint is one of the layers applied onto the radome that impacts transmissivity.
“There is a fine line between its ability to dissipate static build up yet allow radar through,” says Sargent. “It takes a high degree of skill from the painter and inspection equipment for paint thickness and the degree of conductivity obtained. Too little and static pops become prevalent, which rapidly degrades the radome. Too much and you cannot pass transmissivity testing.”
To further protect the radome, a clear urethane boot can be applied right on the nose of the radome. “This is by far the best method in protecting the leading portion of the radome, which is the most susceptible, but does increase the likelihood of static pops,” says Sargent.
Manning agrees that transmissivity of repairs is critical as it relates to radome inspection and repair work. “The repaired areas will have to match the transmissivity signal properties of the surrounding areas,” says Manning. “If it doesn’t match, then the repair is a failure. Repairs have to be performed carefully and this is where experience comes in.
“We ask a lot of our radomes. They have to be light, aerodynamic and maintain the right signal pass-thru. Cosmetically, they are the first thing that anyone on the ground sees and they are the first thing that hits the elements during flight. Repetitive inspection of radomes helps you keep your aircraft ready to fly and helps catch the damage early when is easier, faster and cheaper to repair.”