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.
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