Addition to what the others said: When radios were first installed, the spark plugs at the time had no shielding of their own. They had to put what looks like a long metal can around them for radio frequency shielding. The eyebrows were needed to accomodate the long can. I've only seen these cans on a plane once in the last 20 years.
Modern equipment can get the same job done without the long can so some people are removing the eyebrows. -- Ed Burkhead East Peoria, Ill. N3802H, 415-D
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