Avionics Technology: The Pointy End 'Finishes' First

A look at radome maintenance


Yes, the title is intended to be a pun and it does highlight a genuine issue in supporting aircraft of today.

A radome is a covering whose primary purpose is to protect an antenna often associated with ultra high frequency devices operating in the GHz range such as weather radar and satellite telephone systems. It is a part of the airframe and must possess certain physical as well as electrical properties. It should be strong enough to withstand the effects of elements encountered in flight and yet be contoured to minimize drag.

These properties vary with the shape, design speed, and size of the aircraft on which it is to be installed. Electrically, it should permit the passage of transmitted signals and any returns with minimum distortion and absorption. A certain electrical thickness has to be achieved which is related to operating frequency, and the type of material and construction used. The electrical thickness is specified for a certain range of operating frequencies. Radar efficiency depends upon a clear, nondistorted, reflection free antenna view through the radome.

Consequently, precision in construction will provide optimum performance.

Types of radomes
There are two general types of radomes. The “thin wall” and “sandwich” types.

Thin wall devices are considered to be thin relative to the wavelength of the radar. They are useful when the frequency is low enough to permit a skin thickness which will satisfy the structural requirements. Sandwich types consist of two or more plastic skins separated by a dielectric core. The core may consist of honeycomb plastic sections, hollow flutes, or foam plastic.

The dielectric and separation of the skins will depend upon the wavelength of the radar frequencies.

Static electricity
Unfortunately, it is necessary to manufacture certain antenna fairings from di-electric materials making them a prime source of static generation.

As the speed of the aircraft increases, so does the friction of the air passing over the surface which increases the charge. Aircraft cruising at low speeds are less likely to be affected. Shape is another factor, therefore small pointed radomes tend to build charges faster than large blunt ones.

In low humidity conditions the static electricity on a radome surface can build up resulting in spark discharging causing radio interference over a wide band of frequencies. Surface damage often appears as chipping of the paint and small pits or pinholes burned in the surface. These punctures rapidly increase in size as carbon deposits resulting from the charring encourage further strikes. If unchecked, additional damage will occur from water penetrating the surface into the core.

Lightning diverter strips extending from the frontal area of the radome to the airframe should be part of every radome installation on high-speed aircraft. These diverter strips continuously collect static buildup from the surface of the radome and conduct it to the airframe without severe sparking or arcing.

Paint and coatings
The primary function of paint on the aircraft radome is to protect from the harsh environments associated with temperature extremes, high-speed impact with abrasive particles, sunlight, and high voltage surface charges. If the coating gets damaged the protection is defeated.

Most aircraft primers and topcoats are suitable for use and include: polyester, polyurethane, alkyd-enamel acrylic, and epoxy.

Any paint finish that has a pearlescent or “metal flake” appearance should be avoided, since many of these paints contain very small slivers of aluminum or bronze metal called leafing pigments which can have a significant effect on the radar signal.

Anti-static coatings contain graphite or carbon particles to make them semi-conductive so that static charges do not build up on the radome. These coatings should be used with discretion since a heavy application can result in attenuation of the radar signal. Also ensure that the type being considered is suitable for radome use since some anti-static coatings are more conductive than others.

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