AC 20-111 DATE 10/10/80
ADVISORY CIRCULAR
DEPARTMENT OF TRANSPORTATION Federal Aviation Administration Washington, D.C.
Subject: CXM-1lNICATI0N INI'ERFERENCE CAIBED BY lNINTENTIONA.L KEYED
MICROPHONES
1. PURPOSE. 'lhis c:rlvisory circular alerts the aviation a:mnunity to the potential hazards created by unintentional keying of microphones resulting in radio transmissions fran airborne, nobile, and ground based radio transmitters and gives guidance oo alleviating ensuing hazards.,,,
2. CANCELIATION. Advisory Circular /JC 20-89 dated March 22, 1974, is canceled.
3. BACKGROUND. In 1970 the Federal Aviation Mninistration (FAA) ooted a need to eliminate unintentional radio enissions and proposed that the Federal c:amnunications Ccmnission (FCC) eliminate the present exemption of aircraft stations fran Section 87.75(d), 'tthich requires a radiation indicator. 'Ihe FCC issued a rotice ana received much a::iverse ccmnent. FAA then requested FCC to withdraw the prop::>sal pending further investigation of the magnitude of the problem. In 1974 Advisory Circular l'h. 20-89 was published for the p.irpose of p.iblicizing the problem of cnmnunication interference caused by unintentional radio transmissions and availability of warning devices. Since 1978 air traffic oontrol facilities have reported several systems errors 'tthich resulted fran unintentional continuously keyed microphones causing air traffic oontrollers' inability to o:::mnunicate with airborne aircraft.
4. DISCUSSION.
a. Unintentional anissions fran radio transmitters in the aviation services are caused by microphone buttons sticking in the "on" {X)Sition after being keyed or activated for transmissions, operators sitting on microphones and depressing the 11 µish-to-talk 11 buttons, microphones 'Wedged in seats, equipment malfunction and water present in microphone circuits of mobile and ground based transmitters. These conditions can, aoo have, occurred without the knowledge of the operator.
Initiated by: AWS-330
10/10/80AC 20-lll
b. Air traffic oontrol facilities rronitor a large rrumber of transmissions on several frequencies. Interruption of service or interference en these frequencies presents a safety hazard and levies an econanic burden oo other operators due to the inevitable delays the frequency blockages create.
c. Cne means of reducing the problems v:ould be to install a visual nnnitor on aviation service transmitters. Such devices are on the market today and are reasonably economical to purchase and install. They consist of. a rerrote warning light men r-f energy is radiated. As a service tool they also indicate men transmitted p::,wer is v;eakened or men rrodulation circuits malfunction.
d. Benefits gained by the installation of a visual rronitor are:
(1) Reduce operating costs due to equipment malfunction by redu:ing repair and downtime.
(2) Prevent premature failure of equipment since rrost aviation service transmitters are rot designed for continuous service operation.
(3) Verify transmitter operation or malfunction during preflight and inflight situa.tions.
5 • GUIDANCE.
a. This advisory circular should be :p:>sted in a conspicoous location in all FM flight service station pilot briefing areas, airport operations offices, airlines operations offices and offices responsible for the administration and operation of airport rrobile vehicles and traffic control areas , including line naintenance buildings. A sticker type placard should be posted in a conspicuous place oo every radio equipped airport vehicle to read.
in essence, "IS YOUR MICROPHONE BUITON SIUCK?"
b. Operators planning installation of new camn.nications equipnent should consider the economic advantages of incorporating a visual rronitoring system of transmitter radiation as an additional feature to this new a:iuipment.
c. Operators of aircraft and airport rrobile and ground based transmitters should provide a means for the proper storage of the microphone men the microphone is rot in use.
d. Pilots and operators of airport rrobile vehicles vho suspect difficul ties with their microphones should unplug them imnediately rather than leave them connected mtil a serviceable replacement c.an be installed.
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e. Cperators who might have other ideas as to h::>w best to ensure aJainst frequency blockages caused by unintentional keyed mikes should ronvey their views to the local FAA facility for dissemination to other interested persons.
Director Airworthiness Par 5 3