Boeing has received reports of stuck microphone (“Mic”) transmit switches. Although rare, a stuck microphone transmit switch (sometimes called a “hot mic”) creates a possible hazard to all others using the radio frequency because the continuous radio transmission blocks the frequency and prevents normal communication.
A stuck microphone transmit switch may be suspected if normal voice “traffic” transmissions are no longer heard on the radio and the frequency suddenly becomes quiet. The crewmember with the stuck microphone transmit switch is unable to receive external radio transmissions on the radio selected. Boeing has not received a report of a continuous radio transmission due to an internal radio failure.
When the stuck microphone switch is transmitting on a radio, radio communication is possible by a different crewmember using:
Various airplane models have a RADIO TRANSMIT EICAS advisory message. This message indicates a VHF (or HF) radio has transmitted for 30 seconds or more. This EICAS message is independent of the radio manufacturer or model and will remain displayed until the affected radio is no longer transmitting. In the case of a stuck microphone transmit switch, the message will remain displayed until the stuck microphone switch is no longer connected to any of the radios.
The Collins VHF-900 series and Honeywell RTA-44D VHF radios for all 777 and some 747, 757 and 767 airplane models have an automatic transmission inhibit feature enabled on the ground only that will inhibit the transmitter after 30-35 seconds of continuous transmission.
If a stuck microphone transmit switch is suspected, all flightcrew should immediately select the Flight Interphone (or Interphone) transmit position on their respective audio select panels. The crewmember with the stuck mic transmit switch will be heard transmitting continuously, although in the stuck handmicrophone PTT switch case, the PTT signal may be active with no actual voice output. Once the affected audio select panel is identified and is in the Flight Interphone (or Interphone) transmit position, other crewmembers may resume normal communications on all radios.
The affected audio select panel should remain in the Flight Interphone (or Interphone) transmit position until the microphone switch is no longer stuck in the transmit position.
Boeing Flight Operations Technical Bulletin 67, 24th September 2001