Vision for Avionics Interconnects
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After a long dry spell, avionics for general aviation aircraft have undergone a renaissance in the late 90s with sophisticated digital technologies and designs.  The new products are primarily digital in nature, and are ranging from traditional radios to high sophisticated integrated flight and navigation management systems, fuel management, and engine monitoring and management systems previously costing far beyond the range of a GA or even many corporate aircraft owners' capabilities.  The demand is being driven by lower equipment prices, stimulated demand from the homebuilder/kitplane market, and the general revitalization of general aviation at large.

The new avionics products themselves are becoming more integrated within the avionics box, and recently a trend has emerged to pack more functions into a single box (e.g. the Garmin GNS430 NAV/COM/GPS/MAP, or various engine monitoring systems available for experimental aircraft).  What has not seen any progress is the requirements of wiring and installation, and interconnections and integrated display between the various functions of the combined avionics stack.  Highly sophisticated and multi-function digital boxes are still being strung together with a rat's nest of hand-constructed analog audio and signal connections.

These traditional interconnects have a number of disadvantages:

  • Very expensive and complex to build wiring harnesses
  • Each separate connection is a point of failure
  • Difficult to troubleshoot and maintain
  • Lots of opportunity for RFI

Imagine a new generation of avionics components that are designed for ease of connection as well as ease of integration of their functionality and readouts into one or more rollup displays, such as EFIS display screens and enunciator panels.  The principles of these new avionics are:

  • All interconnects are digital, including digital input sensing signals, output signals, and audio signals.  The advantages of a digital bus are simple two wire interconnections, greater noise resistance, and the potential to share and display data arbitrarily using data exchange standards.
  • All devices use standardized data exchange formats, so that cross-vendor interoperability is possible and a single multi-function display/user interface device can act as the display and controller for all of the avionics and sensing devices in the aircraft.
  • Audio signals are available to all devices without complex interconnects.

At its simplest level, this proposal would offer much simpler hookup of avionics devices.  Almost every avionics box could be interconnected with five or four connections:

  • Antenna in or out
  • Data (2)
  • Power and Ground

Assuming avionics devices would have their own front panel like today's traditional avionics boxes, the data bus would handle communication of audio signals between transceivers, audio switch panels, and headset jacks (usually 5 shielded wires for both in and out), connections between navigation receivers and display units (at least 5 shielded wires for a simple CDI display).

In a typical GA aircraft panel the biggest savings will come through simplification of the audio wiring.  Typically a GA aircraft has these audio devices:

  • Nav1
  • Nav2
  • Com1
  • Com2
  • Marker Beacons
  • ADF (less common in the future)
  • DME (less common in the future)
  • Speaker
  • Handheld Mic
  • Intercom wiring:  4 or 6 place mic and phone headset jacks, plus Push-to-talk switches for pilot and copilot
  • Auxiliary music inputs
  • Warning horn inputs (e.g. stall warning, landing gear up warning)

Usually all of these signals come together at the audio panel, resulting in upwards of 40 wire connections for a 4 place full-IFR system.  The wiring diagram looks something like this:

Analog-style audio wiring diagram

Instead of this rat's nest of wires, imagine a single common two-wire interconnect between all of the audio generating devices.  In this world all audio is transmitted in a standardized digital format, with each device handling A/D and D/A signal conversion internally using readily available chips developed for consumer electronics devices such as CD players.  Instead of being a complex switching and analog mixing device, the digital audio panel has a two-wire connection to the common digital data bus and is effectively a software-only device with all switching and mixing handled in the audio panel software.

The headset jacks do become slightly more complicated in the digital world, in that the jack now becomes a conversion interface between the digital data bus and the traditional analog microphone and headset jack.  The disadvantage of this is increased cost, however the advantage is much simpler wiring (two wire common connection to all jack boxes) and the potential for individual volume controls, muting at the jack converter box.

The digital picture would look like this (depending on network topology, it could either support daisy chains or a star configuration).


This may not seem like a big reduction on the surface.  Because there's a standard transmission format and electrical characteristics, though, the physical interconnects would also be standardized, much like the plugs used on computer equipment are standardized to be plug-and-play compatible (e.g. USB plugs).  Instead of hand-building wiring bundles, interconnects would be installed simply by buying a pre-built, integrally shielded connector cable with molded-in connections, available in fixed lengths such as 1', 2', 3', 4', 8', 12', etc.  Installation really would become a plug-and-play activity with no soldering, bundle construction, or tedious wiring diagrams.

A more aggressive approach would be to have the interconnects go away entirely and rely on wireless intra-plane data transmission.  This makes installation as simple as just mounting the box and hooking up power.  This strikes me as too aggressive to begin with for the physical standard, because the local wireless technology is not mature in the consumer market, and aircraft tend to be fairly noisy with radio-frequency signals between radios, transponders, generated RFI from the ignition systems, radar signals, etc.

Audio information would be transmitted in digital format on the data bus using a standardized format.  Digital audio transmission is a well-understood problem thanks to consumer digital audio devices such as Compact Disk and MP3 players, and the bandwidth is easily handled.  A/D and D/A conversion chips are relatively cheap and each device on the audio bus will handle its own conversion.  Mixing of the signals can occur digitally, which allows for very simple audio panels - everything is in software.