Arch rivals in the air transport marketplace, Boeing and Airbus are in complete agreement on one matter: When fiber optic technology is mature enough to replace the copper wiring in aircraft, its accompanying connectors will meet a single standard specification. Regardless of the aircraft model in which it is installed and the black box to which it is attached, each fiber optic connector should be defined by a single standard.
The aircraft manufacturers so strongly favor a single connector standard that they have agreed to share their results on tests of fiber optic materials. "They say there is no confidentiality here," says Dan Martinec at ARINC Inc. Martinec is the secretary of the Airlines Electronic Engineering Committee (AEEC), New Installation Concepts (NIC) subcommittee, which is discussing the preparation of a standard to establish specs for fiber optic materials and connectors used for 1000BaseT (1,000-MHz) ultra high-speed data buses. He admits that it may be premature to begin developing the standard specification, since an application for the 1000BaseT bus has yet to be defined. However, when the time is right, the airline members of the AEEC probably will direct the NIC subcommittee to ensure only one fiber optic solution will be specified. Responding to the airlines’ need for a single solution, the aircraft manufacturers and connector makers, too, have agreed to lend their experience in the preparation of a fiber optic connector standard, according to Martinec.
One is Better Than Two
Why are Boeing and Airbus so adamant about a single standard specification for fiber optic connectors? They want to avoid rival specifications, as is the case with copper wire connectors for today’s 100BaseT Ethernet buses, Martinec explains. Specification development for these connectors has been an ongoing battle that presumably culminated at the AEEC general session in Orlando last month. Unlike the case with the fiber optic cable and connectors, Boeing and Airbus had developed independent and incompatible, copper wire solutions before the AEEC standardization activity had commenced. Last December, the AEEC directed the NIC subcommittee to select the "best" of two copper cable and connector solutions–quadrax and twinax. However, members of the NIC subcommittee were expected to report to the AEEC member airlines that the battle resulted in a tie.
"The problem is, there is no best one," says Martinec. "So when we drew up the draft supplement to ARINC 600 [covering standards for racking, boxes and connectors], we included both definitions."
Though this dual-standard decision doesn’t make the job of affixing 100BaseT Ethernet interfaces on black boxes easy, the compromise is understandable. Both quadrax and twinax appear to have unique but equally beneficial qualities. The quadrax connector has four pins connected to a single cable, which provides two-way Ethernet data communications. The twinax connector (example, shown above) has two pins, providing one-way communications. Two twinax connectors and two wires are required for two-way communications.
Since both the quadrax and twinax contacts fit in the same size holes in the back of a box, the former connector type has the obvious advantage of reduced weight and volume. Twinax connectors require twice the weight and space of the quadrax connector, and twinax wiring accounts for about 50 percent more weight, says Martinec.
Airbus favors the quadrax solution and plans to make it the physical backbone for all Ethernet buses in its super jumbo A380. "With such a huge airplane and with so many data buses, they need to find all the real estate they can in the EE [electronic equipment] bay," says Martinec, explaining Airbus’ decision.
An open issue with quadrax, however, is that the reliability and maintainability "has not been proven," he adds. Quadrax connectors have small, size 24 pins that join thin 24-gauge wire. These are more likely to break than the larger, size 22 connector and thicker 22-gauge wire in the twinax solution. Nevertheless, Airbus has subjected the quadrax connectors and wire to a battery of tests–in an extreme temperature range and for crimp strength, contact resistance and dielectric withstanding voltage, among other conditions–and determined that they meet all the manufacturer’s requirements.
Maintainability also is an issue with quadrax. Replacing a quadrax connector requires "extreme care," the ARINC official explains. "It has to be lined up perfectly, and the wires have to be properly preloaded. And placing four wires into a contact is more difficult than placing two wires."
Works in the Triple-Seven
Twinax has opposite characteristics from quadrax. While it produces more weight and occupies more space, twinax promises to be more reliable and easier to maintain. "Twinax has been on the Boeing 777 in high-speed bus cabin applications for some time, with no real problems," says Martinec. He adds that Boeing is a twinax proponent. If the U.S. manufacturer were to replace twinax connectors and wire, it would prefer a third choice: fiber optic, he says.
So what happens now that the NIC subcommittee has recommended that AEEC allow both quadrax and twinax connector and wire solutions? "Now AEEC likely will direct its other subcommittees to make the choice of one or the other for each box with a 100BaseT Ethernet interface," Martinec replies. "If weight and space are not critical factors, then twinax may be the choice. But if, say, 90 percent of the systems are to be in Airbus aircraft, then [the subcommittee members] may want to go with quadrax."
Connector and wire specifications already have been established for one item: the airborne network file server. It calls for the quadrax solution.
So how can one box with a 100BaseT Ethernet interface be adapted to various types of airplanes? Adaptors can be fabricated, Martinec replies. They are called "Ys" because the adaptor will either join two wires into one connector or one wire into two connectors, forming a "Y" in either case. It is a workable solution–but one that obviously is not preferable to establishing a single standard specification.
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