Abstract: A device and system for preventing channel blockage on aircraft voice communication frequencies caused either by the pilot beginning a transmission while someone else is transmitting on the same channel or by a stuck microphone condition. The device includes an arrangement so that one does not interrupt other transmitters having the same device. The present device enforces a reciprocity by use of a subaudible tone which is transmitted. If the tone is detected, the device will not interrupt such a transmission. However, if someone else is not transmitting the subaudible tone, the device can ignore their transmission and operate as though the particule radio channel is available for use. An anti-blocking switch extends between the push-to-talk line as connected to the transceiver and the push-to-talk switch. The single anti-blocking switch may be used to inhibit transmission corresponding to a particular channel being occupied and corresponding to a stuck microphone.

Abstract: Systems for preventing channel blockage on aircraft voice communication frequencies caused either by a pilot beginning a transmission while someone else is transmitting on the same channel (i.e., "stepping on" or "blocking" the first transmission), or by a "stuck microphone" condition resulting from undesired activation of the microphone switch circuit. In accordance with the invention, a "step-off" device (SOD) and a "stuck microphone reliever" (SMR) are combined in a single device, termed an "anti-blocking device" (ABD). The entire device may be provided in retrofit form and draws all its operating power from the radio push-to-talk switch line, with no internal modification to the radio required. The step-off device portion of the anti-blocking device senses when a selected radio frequency is in use, and when in use, prevents transmission even when the microphone switch circuit is operated.