Page 44 - Twin & Turbine May 2017

TLC For Your Radome
The nose on your airplane does more than keep the wind from blowing through.
by Archie Trammel
As I taxied in to the ramp at the Greenville South Carolina Airport, the flight department manager waved me in and helped tie down. I had arrived to conduct a training session for his pilots on convective meteorology and airborne radar operation. While helping me tote my projectors
and slides to the meeting room, he pointed to one of his de Havillands and said, “Maybe while you’re here you can tell us why the radar in that airplane is so weak. We’ve had it checked out but still a faded looking paint and ghost images.”
From 50 feet away I could see the problem. The airplane had been flown through light hail, leaving pock marks in the paint on the radome. Instead of stripping all the damage off and refinishing it properly, someone had simply sanded out damaged areas and added a coat of cover-up paint. I suggested the radome be sent off to a certified radome repair facility for a complete refinish. A couple of months later I saw the aircraft on the ramp at Windsor Locks. Its nose was now pearly white, in contrast to the off-white color on the rest of the aircraft.
It’s imperative that aircraft operators understand the nose of their aircraft is actually the lens through which radar signals must pass out and extremely faint radar reflections must come back through for the radar to function properly. It’s your widow to hazards ahead. For that reason, the radome structure must have a precise thickness – so precise it’s critical to the thickness of a single coat of paint. A radome must never be painted with an even number of coats, only an odd number. Why? Because the overall thickness of the completed radome with paint must be exactly an odd number of the frequency of your radar. If you think this is all malarkey, ask your radar tech to let you read the shop’s copy of RTCA/DO-213. In it you will find 38 pages on the engineering, proving, manufacturing and repair of radomes. It was assembled by 68 of the world’s premier radar scientists, engineers, manufacturing specialists, airline experts, FAA Inspectors and radome testing professionals ever assembled. Included was radar engineers Roy Robertson (Collins), Dick Hayes (Honeywell), Dyral Kuntman (Bendix). None are better qualified.
In the document you will find that radome minimum performance standards are exhaustively spelled out. Even an allowable degradation in transmissivity performance due to age, repairs and general wear and tear is specified by grades. The grades are A down through E. Somewhere in your
radar manual from the manufacturer (or check your avionics shop’s “Installation and Repair Manual” for your radar) you’ll likely find that the radar manufacturer recommends a Class A radome. To put that in perspective, should transmissivity of your eyes degrade to E level your driver’s license will be revoked.
Can you believe there are aircraft flying today – privately owned, corporation operated, in airline fleets – with Class E radomes?
In appendix B of the DOC are five pages of instructions on how to repair and refinish a radome. The short of it is, a radome should go to a proper radome repair and testing facility about once each five years, plus anytime the aircraft is repainted, or following a radome repair. Only those facilities that have a tested and computerized “anechoic” (I prefer to call them “acoustic”) chamber should be used. There are only three in the United States:
42 • TWIN & TURBINE
May 2017