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)
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