About 30 years ago, the U.S. military thought that the 1-megbit-per-second Mil-Std-1553 data bus would suit its data communications needs for the long term. While still used — even in new airplanes — for highly deterministic, low-data-rate applications, 1553 is “too slow … [for] the amount of data that is now being pushed around the aircraft from sensors, cameras and avionics,” said Robert Moore, a senior principal engineer with TE Connectivity’s Aerospace, Defense & Marine business unit.
The U.S. Air Force installed CAT 5e, 100-megabit-per-second Ethernet cables for some applications about five years ago, said Grant Lawton, an application engineer for military and aerospace with W.L. Gore. Now the service is replacing some of that cabling with CAT 6a, 10-gigabit-per-second cables. Waiting in the wings is CAT 8, 40-gigabit-per-second Ethernet, driven by military requirements.
While military and civil aviation share standards with the commercial world of homes and office buildings, aviation has special requirements — the military even more so. “As we fly aircraft for 35 years [or more], we are keenly aware of the need for standardizing common components that not only meet the specified data rates, but that are robust, supportable and capable of meeting our mission requirements,” said Oliviu Muja, wiring systems principal technologist with the Wiring Systems Branch at the Naval Air Systems Command (NAVAIR).
NAVAIR representatives have worked closely with colleagues from commercial aviation within the Society of Automotive Engineers (SAE) AS6070 high-speed data cable groups to define and capture the requirements for aerospace cable, he said. So far, “slant sheets” — or subordinate specs — have been published for CAT 5e, CAT 6 and CAT 6a, among others. These slant sheets cover areas such as physical requirements, data rates, basic architecture, conductor size, materials for dielectric, shielding, and jacketing, temperature ranges, bend radii, color-coding and marking ability, Muja said.
While naval aircraft use a combination of legacy and newer-technology buses, Ethernet is becoming more common, he said. “Most aircraft, if they don’t employ it, will employ it” in the future.
NAVAIR focuses on reliability, supportability and cost, as well as throughput. If, based on mission requirements, you select next-generation speeds, that may mean specialized materials, cables and connectors, which would translate into higher costs, longer lead times and harder integrations, Muja explained. You could find yourself locked into one vendor, who could change the product at any time without notice, raise the price or just simply cease production, added Bill Bassett, head of NAVAIR’s Wiring Systems Branch.
But consensus standards implemented in commercial off-the-shelf technologies are intended to mitigate these risks. Use of these promotes reliability, affordability and accessibility to parts that meet requirements, Muja said.
W.L. Gore already offers cables qualified to the requirements of AS6070/5 and /6. Its RCN9034-24 and RCN9047-26 Aerospace Ethernet Cables are qualified to both specs, Lawton said. TE plans to qualify an existing product to slant 6. (Slant 5 and slant 6 correspond to CAT 6 and CAT 6a, respectively.)
For avionics data-bus needs such as the flight control system, it probably won’t be necessary to go beyond 1 gigabit per second any time soon, said Adrian Milne, global products manager for civil air products with W.L. Gore. But special applications will drive demand for higher data rates on both military and commercial aircraft.
“Personally, I don’t see 40 gigabit Ethernet being served on an aircraft with copper,” Moore said. Aircraft typically have numerous mated connector pairs, which affect signal integrity and strength as the cables traverse connector pairs. “I believe that 10 gigabit Ethernet [copper cable] … will be the limit, especially on military aircraft,” he said.
It’s not a question of whether the military might need more speed. The “only question” is whether 40-gigabit-per-second Ethernet will be implemented on copper or fiber, he said. “Fiber can handle 40 gigabits-per-second with less loss and longer distances.” It is also immune to electromagnetic interference effects.
Others aren’t so sure. “The higher speeds are harder to pull off [with copper], but it is doable, and there are at least five connector companies that make aerospace contacts that can support … [40-gigabit-per-second] data speeds,” Lawton, said.
“When designers specified … 100-megabit-per-second cables, [they thought] that there was no way they would ever use up all that bandwidth,” Lawton said. Then came the success of in-flight entertainment and personal electronic devices streaming wireless data, in the commercial airline industry. “On the military side we see that technologies for electro-optical/infrared systems, radar and distributed computing are evermore capable and are indeed starting to use all the bandwidth that can be had.”
In short, copper cables in land and air vehicles are doing data speeds never considered feasible outside of fiber optics as of five to 10 years ago, Lawton says. In the last five years or so the sheer data requirements have increased tremendously, Milne adds. “The pace of change is faster than it’s ever been before,” and the market is consuming all the bandwidth it can get.
Ethernet, the lingua franca of avionics data communications, has long burst out of its circa 3-megabit-per-second starting blocks. Military and commercial aircraft are implementing applications using 10 gigabits-per-second.
Aviation relies heavily on commercial standards, and both commercial and aerospace cables share features such as the number of data pairs and electrical performance characteristics, Moore said. But aerospace cables have additional requirements. The material, for example, needs to be “resistant to 20 different fluids that the cable may come into contact with,” he said. And cables need to survive temperatures ranging from negative 55 to 200 degrees C.
Aerospace cables use stranded versus solid conductors to survive high levels of vibration. The stranded conductors, implemented in multiple, smaller strands, also allow for increased durability and reliability in an environment where the wire will be flexed a lot, said Don Slutz, senior product manager at Carlisle Interconnect Technologies. But stranded conductors take a toll, Moore pointed out. He estimates that aerospace-grade stranded conductor constructions will run about 17% shorter distances, compared to the solid conductor construction used in commercial-grade cables. Numerous other requirements relate to matters such as smoke and toxicity.
Certain materials, alloys and conductors that are required in aerospace restrict transmission distances compared with commercial applications, added John Dunn, technical director for wire and cable with Carlisle.
What’s more, “where a commercial shielded Ethernet cable would typically have a metallized wrap and drain wire for the shielding, aerospace cables will have one or more woven shields to minimize EMI [effects],” Moore said.
Going from 100 megabits-per-second to 10 gigabits-per-second in just a few years is amazing, especially since you are not replacing that with a lot heavier cable, Milne said. The weight increase is only about twofold because you go from four conductors to eight conductors.
The military could use higher-speed cables to distribute sensor signals that have been collected and digitized from radar, electronic warfare and electro-optical/infrared surveillance sensors. The faster the sampling rate can be, the more precise the signal data can be, and the more rapid the response can be.
Unmanned air vehicles that are used as sensor platforms could also benefit from faster data cables, Lawton said. He cites a 40-gigabit-per-second Ethernet application that is being developed for a large unmanned platform.
Sensor fusion and high-definition video also drive military bandwidth, Slutz said. “Aircraft are loaded with sensors that collect a tremendous amount of data that they have to move through their bus structure” in order to process it or move it to a recording device for later download.
Avionics flight test systems could use higher bit rates, as well, said Troy Troshynsi, director of marketing and product development with Avionics Interface Technologies, a unit of Teradyne. “Even if the avionics network is operating at 1-gigabit Ethernet, … you have to monitor data at several points in this network, and you need to multiplex that monitored data into a single flight test data recorder.” So for that data stream to the recorder, you have to have a higher-speed network interface, he said.
On the commercial side, in-flight movies are the absolute minimum expectation on flights of any length. In theory, 100-megabit-per-second Ethernet could support distributing 20 streaming movies to the seat backs, Milne said. There are other infrastructure factors, but if you go to an airplane with 300 seats, 100-megabit-per-second Ethernet doesn’t cut it.
The commercial aviation industry is looking at 10 gigabits-per-second for IFE system backbones and things like remote media loading, where you’ve got to move content quickly before the plane takes off, he says. W.L. Gore sells 10-gigabit-per-second Ethernet cable, mainly into in-flight entertainment or cabin data systems and media loading systems.
At the same time, as passenger seats become much smaller and thinner, cables have to run through narrower channels, Milne said. And business-class seats have a lot of motors and moving parts on them, so that the cables may go through multiple cycles of flexing.
Milne also anticipates use of high-speed Ethernet cable in wiring up commercial aircraft interface devices, which distribute data such as aircraft position, to supplementary systems such as satcoms or electronic flight bags.
While the fiber adoption rate has increased dramatically across both military and civil aviation, Milne doesn’t expect copper cable to become redundant any time soon. Field serviceability is a big issue for fiber.
“Copper cabling has continuously surpassed the supposed limitations that were predicted to be its demise,” according to officials at Harbour Industries. This latest push toward 40-gigabits-per-second is just one example. “Also, often ignored in this debate is the move to wireless technology. Wireless still requires a lot of wires, but a reduction in the raw footage of cable on an aircraft may reduce the overall benefit of fiber’s weight-per-foot advantage,” according to Harbour.
Nevertheless copper cables are also a challenge. NAVAIR is working with manufacturers to come up with a high-speed, robust Ethernet spliced solution to take to the standards body, Muja said. Unless you can splice it, you have to unconnect the cable and run a whole new cable, Bassett said.
W.L. Gore has worked with NAVAIR and ITT “to promote the concept of using a slim-profile, impedance-controlled contact with cord-mount accommodation as a splice when a cable gets damaged,” Lawton said. The idea would be to use the “insert” of a size-five connector — the portion of the connector inside the protective shell — so that the wires that have been terminated on each side of the break can be reconnected smoothly into the connector insert contacts in a controlled and reliable manner.
Another issue with fiber is the transceiver electronics. The implementation of optical transceivers with photosensitive devices is not as robust, Slutz said. And broken fiber is hard to put back together.
There is also the question of how you physically protect the glass, Milne said. How do you protect it from vibration and bending, and how can you get it into the aircraft without breaking or damaging it?
Although fiber is easier to deal with for high speeds, there are complications, including “limited bend radius, … and the delicate interconnections … are troublesome in harsh environments with a lot of dust and [foreign object debris],” Troshynsi said.
Connectors face the same challenges as cables, and there are so many variations and flavors, Muja said. “Reliability and maintainability are where we’re falling short.”
“We tried to standardize on a connector that gives solid CAT 6a capability and performance, while still meeting all the robust[ness] requirements. We published Mil-Dtl-32546, which defines the connector.” The spec, along with its detail sheets, has been out about 18 months, he said.
“The beauty of the [new] connector is that it you can use it with multiple protocols,” Muja said. And it is maintainable with the standard tools that avionics technicians use. Before this spec, there was “no dedicated, high-speed connector at all.”
The current standard aerospace connector that some manufacturers have used to route high-speed Ethernet signals is Mil-Dtl-38999, he said. The problem, however, is that when it comes to signal transmission, “the connection is so lossy that it simply doesn’t function.” Mil-Dtl-32456, on the other hand, “is designed specifically for high-speed applications.”
TE Connectivity and others are working to introduce products that meet the new spec. TE’s FAS-X connector is being qualified to Mil-Dtl-32546, Moore said. “The basis of the electrical performance that the connector was designed to meet was for 10-gig Ethernet,” he said. “It has been found that it can be used for other protocols, but again, the basis for the design and performance was Ethernet.” AVS