TLC For Your Radome

TLC For Your Radome




The nose on your airplane does more than keep the wind from blowing through.

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: California Radomes, Santa Clara, California; Nordam in Tulsa, Oklahoma; and Saint-Gobain, formally Norton, in Ravenna, Ohio. It’s possible other facilities in the world have personnel and equipment necessary for proper radome manufacturing and repair, but if so they aren’t widely known.

Field Tests and Repairs

But suppose you operate out of a jungle strip in darkest Africa, or some remote south sea island? Are field tests and repairs possible? Yes, to a limited degree. You begin tests with a simple coin and a “tap” test. A quarter size silver coin will do. With the coin you tap all over the exterior of the radome. Be certain to tap carefully around any openings in the radome, such as those around openings for landing light installations. You’re listening for a musical like “ping”. If you hear a dull clunk anyplace on the dome you’ll have to go plead with your banker for a loan. That “plunk” is telling you there’s a delamination in the radome layers, or water has soaked into the fiberglass, or whatever the radome is made of.

Incidentally, wise pilots also conduct a ping, ping inspection of the radome on every walk around, in case water got into it on the ramp.

With skill and patience it’s sometimes possible to repair a delamination with a fiberglass repair kit; and sometimes possible to dry water out with a hair blower. But without proper testing you can’t be certain your fix was a fix. And a too hot hair blower can blow the entire effort with a meltdown.

Assuming you’re lucky and the radome passes the “tap” inspection, next you must check it for transmissivity. This one will require a couple of hours. First, remove radome fasteners, leaving just 2 or 3 to hold it in place. Then power the aircraft up and taxi out to a remote area of the airport. Park with the nose pointed at some prominent building on the far side of the airport, at least a mile or more away, or some building or object beyond the airport boundary. That’s “the target.” Turn your radar on, select the shortest displayed range, full-up tilt. Then carefully decrease tilt until an echo, the target, appears. Adjust tilt to achieve the strongest possible echo from it. Then reduce the CAL control, (probably misnamed “GAIN”) slowly until the target echo is just barely visible on the radar. Then, without changing any radar control, shut down, remove the radome, get back in the airplane and see if that echo has changed. If the target echo is now stronger – even the slightest bit – you have a radome transmissivity problem. Reposition the aircraft left and right and check the target out on the left and right sides of the radome. If that target is brighter – left, right or centered – bad news.

What to do? If the difference, radome on or radome off, is very small, you can take a chance and continue to fly behind it safely. If you are a risk-taker, allow for the slight transmissivity lost you know is there. Advice? Assume echoes with yellow in them actually contain red. Assume an echo showing red contains hail of some size.

But suppose the difference with radome on/off is pronounced? Is a correction in Timbuktu possible? No. Problem is, to restore the radome’s performance will require the services of technicians with extensive experience in stripping radomes, plus the correct primers and paint for radomes. In some cases, lightning diverter strips will need to be replaced. And, finally, don’t forget about that computerized anechoic chamber for testing.

Here, briefly, is what is required. First, a liquid stripper may not be used. The old finish must be carefully, carefully, sanded off without compromising the base structure. Then a special radome undercoat applied, followed by an odd number of finish coats. After the paint is cured, into that chamber it must go for testing by an engineering inspector.

Continuing with an inspection while the radome is off, check it for pin holes. How? On a bright sunny day point it up toward the sun, stick your head inside and turn it this way and that, searching for any tiny, tiny bright holes. If you find any, mark the location so they can be plugged from the outside with a very small dab of fiberglass or other base material. Very, very small dab. (Incidentally, never apply paint or anything such as an anti-static paint, to the inside of a radome.

Hopefully, your radome will pass all the above self-tests. If not, below are phone numbers for you to contact. Ask first about an exchange for a refurbished and tested one.

California Radomes

Nordam

Saint-Gobain

408-562-1919

918-476-8338

330-298-4105

For those curious about what DOC-213 says in reference to radome repairs in Appendix B, I will be pleased to send you a PDF file copy. You can e-mail me at archie@radar4pilots.com.

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