Maintenance and testing tips to ensure optimum radar performance
By Jon Tripp
It’s patronizing to say that shortcuts and poor maintenance habits in the aviation industry can have deadly consequences. However, all too often, such practices are common in regard to the repair and maintenance of nose radomes. Whether intentionally ignored for expediency or merely misunderstood due to the evolution of radomes, OEM specifications are often not strictly followed. This practice must be stopped or weather radar performance may be significantly impacted
The function of the nose radome has dramatically changed over the past two decades. While once it served primarily as a protective shell and aerodynamic fairing for a radar system, the nose radome has evolved into a protective shell that also serves as a window for electromagnetic radiation generated and received by the weather radar. The advent of predictive wind shear radar systems has upgraded the radome structure to functional electrical component status in addition to its protective properties.
The term "radome," derived from the words radar and dome, was originally interpreted as a dome-shaped, radar-transparent structure installed on an aircraft; but is now a general term for any radar enclosure whether it is airborne or ground-based. The information in this article pertains to aircraft-mounted, dome-shaped radar enclosures of the fluted core and honeycomb core radomes currently used on modern commercial jet transport aircraft. However, the information can also be applied to general aviation private planes and/or ground-based radar enclosures of various configurations.
Maintenance of the radome is required to preserve the electrical transmission properties of the unit to design levels. Diminished wind shear detection capabilities of the weather radar system is certain if poor maintenance or inadequate or inappropriate repairs are performed on the radome structure.
The FAA requires aircraft equipped with predictive wind shear radomes to maintain performance at a class "C" level or better. Boeing (which now includes Douglas Heritage products), Douglas and Airbus qualify commercial aircraft radomes through laboratory testing to a class "C" level. As an extra margin of safety, radar system manufacturers are required to demonstrate radar performance to acceptable standards of wind shear detection and performance using a radome with a classification one level lower than the radar system is certified to.
Radomes are classified according to their respective transmission efficiency for the "window," or the part of the radome that is illuminated by the radar antenna in a continuous azimuth or elevation scan. The average transmission efficiencies are rated as follows, from highest to lowest:
Class "A" radomes have 90% efficiency
Class "B" radomes have 87% efficiency
Class "C" radomes have 84% efficiency
Class "D" radomes have 80% efficiency
Class "E" radomes have 70% efficiency
The core selection of the radome reflects the design philosophies of the respective OEM and each core material affects radar performance differently through the core’s inherent properties. Of course, each OEM or manufacturer believes their products and designs are superior to their competitors’ designs. Each has their advantages as well as drawbacks when used on an aerial platform, and each has varying degrees of radar transmission capability, or transmissivity, which is defined as "the act or process of signal transmission by radio waves." Peak radar efficiency and resolution require a clear, distortion-free antenna view through the radome window for transmissivity and reflection (return energy to the radar), with minimal defraction (bending of the radar energy as it passes through the radome).
Fluted core radomes
Douglas Heritage Aircraft radomes use the fluted core design. The fluted core was chosen for four primary reasons:
1. Ease of manufacture
2. The ability to prevent moisture entrapment
3. High damage tolerance
4. Excellent radar transmission capability
NOMEX® honeycomb core radomes
Boeing Heritage and Airbus aircraft radomes use the NOMEX honeycomb core design and was selected for ease of manufacture. This design also exhibits good damage tolerance and excellent radar transmission capabilities.
Supplemental Type Certificate (STC) holders also manufacture and market radomes for the commercial market with cores made from Divinycell™ foam and NOMEX honeycomb. Just as with the OEMs, each STC holder advertises that their units have high damage tolerance and superior transmissivity capability.
Damage and repair issues
Damage and repairs to the radome skin and core can dramatically affect radar performance. OEMs provide criteria for electrical testing of repaired radomes that is currently specified in the specific aircraft models’ structural repair manual (SRM), component maintenance manual (CMM) and/or overhaul manual (OHM).
Impact damage to the radome, such as small punctures or cracks in the skin from bird strikes, small hail, lightning strikes, or rain erosion can permit moisture ingression into the core. Moisture will promote delamination of the core/skin bond and degrade electrical performance of the radome. Some radome OEMs use infrared thermography in addition to electrical testing to determine if moisture is present in the radome core.
Doppler and pulse radars are very sensitive to spurious signals that might bounce off the inside of radomes and are particularly sensitive to side lobe reflections. Repairs and damage to the radome directly impact the radar efficiency and resolution, which depend on a clear, non-distorted and reflection-free antenna view through the radome.
Testing after repairs
Repaired radomes should be electrically tested to verify that the repair meets the recommended Radio Technical Commission for Aeronautics (RTCA) Document DO-123 criteria for electrical performance level for class "C" radomes. For the purpose of electrical evaluation (transmissivity), repairs including paint or topcoat are defined as follows:
A. Repairs made within 10 inches of the radome trimline (the radome trailing edge), even though they require the same high quality as the remainder of the radome, do not require electrical testing.
B. Repairs which extend into the core and exceed four inches in the largest dimension after repairs.
C. Repairs affecting more than the outer ply of the glass cloth, which exceed six inches in the largest dimension after repair, and do not extend into the core.
D. Repairs affecting the outer ply only and exceeding eight inches in the largest dimension after repair.
E. Multiple small repairs which have a total repaired area that falls into the category of a single major repair of the same type as defined in B, C, and D.
F. Multiple repairs done in the same area.
G. Repairs done in the vicinity of the old repair that overlap or are tangent to the previous repair.
Be careful when painting
Aside from the actual repair of a damaged radome, the topcoat and protective primer must also be restored in a manner that is not only consistent with the OEM’s finish specification, but also falls within the range of acceptable transmissivity for the radome type and class.
Proper restoration of radome topcoat due to paint peeling, erosion or repair should be accomplished per SRM/CMM/OHM requirements for the respective manufacturer and applicable OEM process standards using only OEM-specified materials and repair design requirements, or OEM-approved alternate materials. Note: Three additional coats of paint or six months flying can degrade radome performance by one class. Operators need to be aware that the use of repair materials other than those specified in the applicable OEM repair media, or use of a core material other than that of the original design when performing repairs, can significantly degrade weather radar performance.
FAA Advisory Circular 43-14 specifically states that all repairs to nose radomes, no matter how minor, should return the radome to its original or properly altered conditions, both electrically and structurally. When a repair adds or replaces skin plies, electrical testing must be performed (unless otherwise specified differently in each respective OEM repair media) with the techniques and procedures of Aircraft Technical Committee, Report No. ARTC-4 (Electrical Test Procedure for Radomes and Radome Materials) paragraph 4.1. Unless electrical testing of radomes is accomplished after repairs have been performed per the above criteria, the operator will not be able to determine the total reduction in weather radar capability. Only electrical testing with specialized test equipment can ensure proper transmissivity, reflection and defraction properties after repairs have been performed.
The capabilities of modern aircraft require properly functioning weather radar systems. In order to maintain the critical peak performance of systems that rely on a nose radome, OEM specifications and instructions must be strictly followed. In addition, a consistent testing system must be put in place and executed in order to ensure repairs and maintenance practices are successful.
More Radome Tips
Radomes have the unique job of having to withstand enormous forces associated with being at the leading edge of the aircraft while at the same time having to be "invisible" to the radar. Issues that need to be taken into consideration when repairing radomes is the location of the repair, the size of the repair, and the amount of previous repairs if any. Manufacturers’ repair guidelines should be followed at all times.
Several products are available to help protect radomes from erosion damage including a protective mask developed by PM Research of Wellsville, NY. It is a pre-formed polyurethane film 0.012 inches thick that is applied over the radome in order to provide additional protection. Just as with other maintenance performed on the radome, the installer should ensure that performance of the radar system is not detrimentally affected.
Ron Bauer with Saint-Gobain Performance Plastics (formerly Furon Co. and Norton Co.) states "Even putting on too much paint can affect performance. Paint coatings that are too thick can drop performance by as much as two classes. If paint personnel are not aware of this, they could be affecting the aircraft’s radar performance without even knowing it."
Radomes should be inspected externally on a regular basis. Obvious defects like bird strikes or lightning strikes warrant further detailed inspection. Since few aircraft manufacturers have scheduled inspection intervals whereby the radome is removed for inspection, any time the radome is removed, it is good practice to perform a thorough inspection on the assembly. Tap-tests can be performed to determine any structural flaws. In addition, the assembly can be inspected from the inside for any evidence of defects like lightning strikes or water intrusion. All components like latches or lightning strips should be checked for security.
Typical Test Equipment Setup
The test equipment and test setup discussed here are provided for reference only. Actual test equipment and test fixtures used for electrical testing may vary, depending on manufacturer’s recommendations.
As shown in Figure 1, a fixture is required to mount a C band or X band receiving antenna.
Figure 2 shows a fixture for the transmitting antenna and radome shell. It is important to maintain the approximate same physical relation between the radome and the system antenna when performing electrical testing. This is done to replicate the relationship of the radome and antenna as if it were installed on an aircraft. The radome mounting fixture must be capable of rotating the radome from 270 degrees through 0 degrees through 90 degrees azimuth at an elevation of -15 degrees, 0 degrees and +15 degrees with respect to the fixed transmitting and receiving antennas. In addition, the radome fixture needs to include a recording device that will indicate angular displacement of the radome on the test recording.
It is critical to select a test site, which is free of objects that are capable of interfering with the radiation pattern between the transmitting and receiving antennas. Objects that could interfere with the radar transmission could give false or misleading values during the test, not accurately representing the true "window" of the radome.
Figure 1 – Key
1. Antenna System Reflector and Feed
2. Crystal Mixer
3. Test Fixture
4. Low Loss Coaxial Cable
5. Matching Network (Part of System Antenna Receiver)
Figure 2 – Key
1. Signal Generator
4. RF Cable
5. System Antenna, Reflector, and Feed