Complex avionics and microwave instrumentation systems serve critical roles aboard Boeing Army Rotorcraft Systems' CH-47 and MH-47 Chinooks, MV-22 and CV-22 Osprey tiltrotor aircraft, and RAH-66 Comanche armed reconnaissance helicopters. The company's Ridley Park, PA facility produces completed fuselages for these aircraft destined to fly for US forces including Marines, Air Force, and Army units around the globe. In addition to thousands of components, each chip set incorporates scores of coaxial cables for avionics including communications, navigation, and aircraft-protection (AP) systems. These miles of cables and their connectors must provide fail-safe performance, so testing these cable assemblies is particularly critical to ensuring overall system reliability.
Like all other equipment and systems aboard the helicopters, the coaxial cables, their connectors/terminations, and their antennas must be thoroughly tested to assure conformance to applicable specifications for continuity, insertion loss, VSWR, and other reliability and performance parameters. These include--but are not limited to--phase delay and distance-to-fault (when fault diagnostics are required) measurements to point out cable discontinuities. While just one of many thousands of tasks associated with producing a battle-ready aircraft, these testing procedures are traditionally time-consuming and tedious. Should a completed aircraft experience failures associated with the coaxial cables after it has been delivered to a military customer, more expensive troubleshooting is required by engineers and technicians. An optimized measurement system can help weed out any such potential failures in the coaxial cables.
To improve the time-consuming manual test procedures employed by Boeing on these aircraft, Kathy Kocher, a Boeing test engineer for more than 14 years, sought to upgrade the company's existing manual test equipment with automated functional test procedures (FTPs). Kocher and associates at Boeing worked with Joe D'Ignazio at Eastern Instrumentation (Philadelphia, PA), a technical representative for In-Phase Technologies Inc., (Clarksburg, NJ). In-Phase Technologies is well known in the high-frequency industry as a designer and developer of custom ATE systems for analog, digital, RF, microwave, and lightwave measurements.
The efforts of Kocher and her team resulted in a custom automated coaxial-cable test system that eliminated virtually all of the problems associated with the previous manual test procedures. Kocher, who has been involved with coaxial cable testing aboard Boeing helicopters for the past seven years, notes, "The first system was put to work on our Chinooks which are completed here on site. The V-22 fuselages are completed here as well, but after testing they are shipped to another manufacturing facility for final assembly."
Of the company's manual cable testing procedures, Kocher says, "In a good day, a technician could test about five cables aboard a V-22, for example." Even with that relatively low productivity, Kocher admits, "We were usually disappointed with the test results."
Herman Richardson, who represents the Manufacturing Engineer Test System (METS) Group for both the Ospreys and Chinooks, mentions the old way of working: "In the past, by the time our helicopters got to the customer and they started plugging in all the black boxes, there would be failures for many reasons." Richardson, who also serves as a production and instructor pilot for the CH-47 Chinook, is mainly responsible for customer training and instructing military flight crews on the aircrafts' avionics systems after the factory testing and customer acceptance phases. But checking the cables and connectors was never a priority.
Once a completed aircraft has been factory tested, a customer's flight crews bring the systems up to functional status. "When they turn on the electronics, they are looking for certain things to happen," according to Richardson. While these procedures do not constitute "acceptance testing," Richardson calls them the "portion of the delivery where the customer uses the equipment for the purpose for which it is intended." He adds, "Once everything is powered up, it is expected to operate as per the operator's manual." He admits that if a system failure occurs, "There's no reason to think that the coax cable is bad, or that electrical loss is emanating from a poor connection or defective connector or antenna."
Richardson notes that even when customers tested coaxial cables in the field and determined its conformance to specifications, there was still no way to know whether tests were performed properly. "Even when they detect a good signal, when the cable is connected to its antenna and its receiver the performance loss is reduced by some degree to the point that, in a lot of cases, the system is not usable," he says. All of this resulted in substantial waste of time, money, and effort.
As a US Army pilot prior to joining Boeing, Richardson was familiar with the Chinook's avionics systems. After joining Boeing, he was determined to improve the way that these systems and their cables and connectors were being tested. His opportunity came as part of a joint effort with Kocher and key Boeing people, including Richard Hartford, a manufacturing engineering test technician, and Kirk Thompson, a Boeing engineer on the V-22 program.
Typically, there are between 20 and 40 individual coax cable runs in each aircraft. Testing them took weeks to accomplish, sometimes delaying production and ultimate delivery of shipsets to the company's completion centers in other parts of the country. Because the new system is automated, relates Thompson, he "only gets involved now if there's a problem or when a new requirement for a functional test procedure (FTP) comes up, based on new or modified equipment/cables aboard a helicopter." According to Thompson, the new system is so easy to use that "the FTPs tell you step by step what to do, so that someone who is unfamiliar with it from another part of Boeing can open up the FTP, review the step by step test procedure instructions, and move along with the testing." He adds that "we have fewer errors and using the FTP provides an ideal set of guidelines." He noted that programming the system for new cables is straightforward: "We can set parameters for the cable and our video screens will indicate pass/fail modes or display substantially more information about cable performance, depending upon application."
Boeing's people, the In-Phase group, and D'Ignazio worked together to develop the new automated coaxial cable test system (Fig. 1) as well as the FTPs that essentially eliminated previous problems. The automated test system (ATS) not only performs qualitative functions (basically go/no-go testing), but also provides complete statistical data and documentation with regard to precise performance parameters such as insertion loss, frequency roll off, and the distance to a fault (see sidebar). The system also provides an internal confidence test routine, an internal path-loss calibration routine, and pictograms for instructing test operators in connecting cables, detectors, and standards. For distance to fault incidences, the system prompts for propagation factor for the various cable dielectrics. During cable measurements, the system may also be configured for statistical process control functions by archiving all test data into a permanent database.
According the Kocher, "Our new automated test systems are substantially more robust, and eliminate all of the previous variability problems as a result of operator discretion." As evidence, she offers, "In the past few years there have been virtually no field failures of any coax cable runs that have been through the new ATS."
Kocher also said that when the new ATS was first used on the Chinook helicopter it was initially used for testing Nav/Com equipment. Following success with those systems, additional avionics and other Aircraft Protection Systems (APS) were tested as well (Fig. 2). She added that, "Before we received the new ATS, we had to have a much more experienced operator to use the manual systems."
Hartford notes that using the old method on the Osprey took about ten days to "get through all the cables we needed to test, and the failure rate was pretty high." With the new automated system the "failure rate dropped off to practically nothing. It has reduced our entire test time down to about four to five days--about half of the previous time with substantially better results."