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Radio Couplers - When & Why

Radio couplers perform two functions in the autopilot system. First, they are an interface for an HSI system. Second they receive the radio information, where it is processed and mixed with the heading/course information. This composite signal is then passed to the amplifier for autopilot command.

The following couplers are used with our standard DG (52D54M): the 1C388, 1C388M and 1C388P. The 1C388 is the older version coupler made before the use of HSIs in general aviation. Although this unit is compatible with newer CIIB and CIII autopilots, it is not recommended. Improvements in circuit design of the 1C388M have made the older couplers unreliable. The 1C388M and 1C388P are identical units inside and out except for the face plate. The 1C388P was made especially for Piper Aircraft for use in the Autocontrol and Altimatic autopilot series and is no longer in production.

The next model of couplers are the ones used in ARINC (synchro) based systems. These models are the 1C388C, 1C388MC and the 1C388PC couplers. These couplers only accept heading signal (not course) from the HSI. As with the couplers mentioned earlier in regards to age and production status, the same applies to these models: the 1C388C and 1C388PC are no longer in production.

The next model is the 1C388-2 used with our NSD series of HSIs. It will also work with certain models of HSIs manufactured by Sigma Tek. For specific models, contact the HSI manufacturer. The 1C388-2 has separate input lines for heading and course datum information. Operation of this coupler differs from the standard coupler, in that when intercepting a VOR or LORAN signal, the course needle will be set to the radial or bearing for the autopilot to properly track. In the reverse mode the 1C388-2 will automatically reverse radio and course sensing to the autopilot for proper LOC/BC function.

The last coupler we offer is the 1C388-3 used with ARINC HSIs that have heading and course datum outputs. This coupler is also recommended when a King KI525A along with the KA52 adapter is used. We do not recommend the use of the 1C388M and the KA57 adapter when the KI525Ais installed.