For all its second-generation targeting sensors, integrated avionics and centralized computing power, the U.S. Army’s RAH-66 scout/attack helicopter must still be easy to deploy and maintain. The fast, stealthy Comanche will go to war quickly and stay in the fight with the help of a powerful portable (intelligent) maintenance aid (PMA–pronounced PiMA). The PMA will troubleshoot faults accurately, and it will help build a database to better support the entire Army aviation fleet. The operating concept was demonstrated successfully last December to clear the way for Comanche engineering manufacturing and development.
The Army now plans to buy 1,213 Comanches to revitalize its cavalry squadrons and light attack battalions. The first unit equipped is expected to field eight aircraft in 2006.
PMA production hardware will not be selected until around 2004. However, the ruggedized notebook computer stowed aboard every RAH-66 will contain the electronic logbook, interactive electronic technical manuals (IETMs), and diagnostic engine now in development.
Charles Reading, supportability division chief at the Army Comanche program manager’s office, explains, "The diagnostic engine is kind of a very smart crew chief with a lot of experience to help our young maintainers."
Comanche maintainers in austere operating locations will use the PMA to sustain a higher operational tempo than possible with today’s helicopters. "We will be able to determine immediately what is wrong with the aircraft and how to get the aircraft flying again much quicker," Reading asserts.
The RAH-66 operational requirements document calls for wartime operational availability equal to or greater than 75% with an operational tempo of six hours per day. More than any aircraft before, the densely-packed and highly integrated "system of systems" has been designed to look after itself.
Sikorsky Aircraft PMA leader Jim Magson points out, "It’s hard to give the PMA the total credit for reduced maintenance. The Comanche design has the entire aircraft do the work of diagnostics. PMA is the display for the aircraft that shows it all."
Boeing manager for integrated test equipment Steve DeSantis explains, "We’re spending a lot of time optimizing on-board diagnostics. The only air vehicle that has approached it this way is the 777, and that program did not develop external diagnostic elements to the degree that the Comanche intends to employ."
Boeing and Sikorsky won the competition to build the Comanche in April 1991. Within the industry team, Sikorsky Aircraft has primary responsibility for PMA hardware and software. Boeing has the lead for the RAH-66 IETMs, and for the portable instrumentation package (PIP) that resolves troubleshooting ambiguities "in the shadow of the aircraft." Together, PMA and PIP will support a deployable war fighter in a simpler, more mobile maintenance organization.
Maintainers and manpower integration (MANPRINT) have been keys to the Comanche since Army leaders first envisioned a replacement for their light scout, attack and utility helicopters around 1983. To replace a larger fleet and cut operating costs, the more reliable, available, and maintainable light helicopter (LH) had to fly more than simpler aircraft, yet require less support.
Three Down to Two
When the planned utility version was abandoned in 1988, the LH program focused on a sophisticated night/adverse weather, armed reconnaissance helicopter supported by only two levels of maintenance. Maintainers in field units remove and replace avionics modules and electromechanical line replaceable units (LRUs) with the help of built-in test (BIT) functions and built-in test equipment (BITE). Depots in the operating theater or back in the U.S. repair modules do more extensive overhauls.
The Army learned important supportability lessons when it fielded the big, complicated AH-64A Apache attack helicopter in 1986. The Apache requires a three-level maintenance organization with field, intermediate and depot assets. The aviation intermediate maintenance (AVIM) level includes the enormous truck trailers used to troubleshoot and repair the Apache avionics.
The two-level maintenance organization of the Comanche fits the fast-moving forces on the "non-linear" battlefields Army leaders expect in the future. Tomorrow’s more rapidly-deployable units will need short logistics tails free of specialized repair equipment. Their armed scouts will have to deploy overseas quickly without the cumbersome AVIM.
"The real plus to the Army is a smaller logistic footprint," says Reading. "We won’t have to haul equipment with us."
Apache avionics are built around LRUs with BIT functions to tell which boxes need replacement. The early fault detection/location system ("Fiddles") in the AH-64A isolates faults only down to the box. Faults within LRUs can be traced to circuit cards only with the more elaborate resources of the AVIM.
The Comanche mission equipment package (MEP) is built around the SEM-E (standard electronic module-Size E). About 25% of the real estate in each 6.38-by-5.88-by-0.58-inch (16.2-by-14.9-by-1.5cm) building block is devoted to built-in testing. Comanche BIT isolates faults down to the module, and modules are removed in the field and returned to the depot or factory for integrated circuit-level repairs.
With today’s aircraft diagnostics and test equipment, ambiguous fault indications mean the false-removal rate at which good LRUs are pulled by mistake is about 50%. Maintainers resort to "swapatronics" to resolve ambiguous faults. They exchange known-good components with the suspected faulty members of an ambiguity group and retest to determine the faulty part.
Comanche field maintainers must be able to pull faulty modules from MEP bay racks and replace them with a very high level of confidence. The Comanche specification calls for false removals no greater than 3%. To achieve such high diagnostic confidence in the field requires a new window into complex systems, one that insures that good components are not removed from the aircraft and subsequently returned to the suppliers for test and checkout.
Ride the Bus
The highly integrated, fault-tolerant avionics suite of the RAH-66 ties targeting, navigation, communications and other subsystems to twin mission computer clusters (MCCs). These are "supercomputers the size of shoeboxes." Unlike today’s federated avionics, the Comanche mission equipment package centralizes computing resources in the two MCCs with their shared mass storage unit (MSU).
The MSU records self-test fault codes from microprocessor-based avionics controllers all over the aircraft. It also recalls the flight regime at the time of the fault.
"If we get a failure that happened in a climbing left turn at 400 feet and 20ï¿½ F, we want to duplicate the fault by putting the modules under the same stress," Reading explains. "The problem today is, we can’t put them under the same stress."
Fault codes will be collected through the Mil-Std 1553B databus. The RAH-66 uses 50 MHz high-speed fiber-optic buses to link MEP sensors to the MCCs, and the MCCs with one another. A separate fiber optic video distribution bus carries targeting imagery to the mission computer clusters. However, aircraft systems including laser gyros, radar altimeters and engine sensors interface with the 1-MHz 1553B bus.
A Hamilton Standard air vehicle interface control system collects inputs and "conditions" the data from relatively low-speed analog sensors on aircraft systems for the 1553B bus.
Comanche engine data is no different from air vehicle and MEP data. The Chandler Evans full authority digital electronic control on each LHTEC T800 engine puts diagnostic and life data on the 1553B bus.
Crew Chief’s Notebook
Once the Comanche lands, a maintainer removes the PMA from the fuselage MEP bay and retrieves the aircraft fault log from the mass storage unit. The data is retrieved either via data transfer from a removable card within the MSU or by using the Mil-Std 1553B data bus connection on the aircraft. PMA software automatically fills in the blanks in the electronic logbook, entering all discrepancies since last startup on a display that has the format and "feel" of a paper logbook. Discrepancies in windshield wipers and other mechanical parts not directly on the databus can be entered manually by pilots through the cockpit display units (CDUs) or by maintainers through the PMA touch display or keyboard.
Steve Desantis at Boeing observes, "Other than manual logbook entries, the aircraft is completely integrated. There isn’t too much we can’t look at."
With the logbook filled, the PMA can be disconnected from the aircraft. The maintainer chooses which discrepancy to address first, and the PMA sends him or her on three possible repair paths. If the fault code is unambiguous (a single, clear problem), the PMA takes the maintainer to the appropriate remove, repair, or replace section of the interactive electronic technical manual.
The paperless IETM is an Oracle-based Class 5 relational database that presents information from different sections as needed without sequential page-turning. It tracks parts removed and replaced, and reminds maintainers to close access panels.
The Comanche PMA should contain all the RAH-66 technical manuals on a 6-gigabyte hard drive. By comparison, today’s Black Hawk utility helicopter manuals fill 25 to 30 paper binders.
"In the paper world, it’s very hard to maintain revisions with the logistics associated with hard copies," Sikorsky Aircraft diagnostics leader Gerry Boyd explains. "The electronic manuals can fit on a CD-ROM, and the IETM can be mailed to the user community or downloaded directly via the World Wide Web."
Less obvious faults with a stable "ambiguity group," including two or three possible culprits, call on the diagnostic engine to launch the most likely repair/replace procedure. The engine formulates an educated deduction like that made by an experienced human maintainer, and it has an extremely high probability of making the correct choice.
To resolve more complicated multiple ambiguities, the diagnostic engine launches a manual troubleshooting sequence, and the PMA is connected to the portable instrumentation package. The PMA diagnostic engine recommends procedures in an order determined by relative failure rate and the amount of time required to get to the test- point in the aircraft. "It’s possible to sequence troubleshooting to be more efficient using diagnostic algorithms and a computer, rather than the traditional troubleshooting trees found in maintenance and repair manuals," Boyd says.
The 45-pound (20.4-kg) VXI-based PIP is connected to the aircraft to run automatic tests on specific components. It uses the PMA to connect the components in the ambiguity group, then does the minimum number of safe-to-turn-on tests to assure the maintainer made the correct connection to the system under test.
After a successful safe-to-turn-on test, the PIP then executes the minimum number of automated tests necessary to break the failure ambiguity group. Boeing has built two demonstration PIPs so far.
The PMA will download far more data than the maintainer needs. Reports from individual Comanches and potentially other aircraft with the PMA interface will be collected in the Army’s Global Combat Support System. There, it will provide fleet-wide trend data and prognostics to better define spares inventories and other support parameters.
Sikorsky Aircraft used a first-generation, developmental maintenance aid (DMA) to support the Comanche from its first flight in 1996. The 40-pound (18.1-kg) Science Applications International Corp. (SAIC) workstation had no IETMs or PIP interface and used a UNIX-based operating system. Last year, it was upgraded to a Windows NT-based notebook system. Two DMA demonstrations performed in early December 1999 for a Milestone II engineering manufacturing development decision were hosted on a ruggedized, magnesium-cased Panasonic CF-25 notebook computer.
One demonstration at the Sikorsky flight test center at West Palm Beach, Fla., had the notebook computer download fault codes from the subsystem power unit on the prototype Comanche. The other test tied the developmental maintenance aid into flight control hardware at Boeing’s Philadelphia, Pa., facility. In both cases, the notebook computer pulled 25 fault codes from real Comanche systems, displayed maintainer information, and found the appropriate repair-and-replace procedure automatically.
A production PMA will probably be smaller, lighter and more rugged than the off-the-shelf demonstration hardware. Commercial computers are evolving rapidly, and the PMA may be hosted on an Army-standard notebook used for different purposes on different platforms.
"Hardware is the least of our problems," Reading says. "We’ll take the latest we can get at the time."
The PMA may use head-mounted displays to free maintainers from paper schematics. With a virtual view of the Comanche to assist with battle damage repairs, maintainers may visualize the trajectory of penetrating rounds and collateral damage inside the real helicopter.
Comanche deputy program manager Darrell Harrison notes, "The hardware at this point is unimportant. What is important is the software and the interface between the aircraft and the portable maintenance aid."
Software work is now underway on the PMA-PIP interface, on troubleshooting and repair verification routines, and on other elements of the PMA system. The work draws on the best available commercial technology, as well as code developed for the Comanche program. "In some cases, the software we’re developing uses off-the-shelf components such as the browser technology for our technical manuals," Jim Magson at Sikorsky observes.
Beyond the Army’s immediate goals for a more deployable, available armed scout, the Comanche program with its extensive BIT and the portable maintenance aid will have important design implications for the entire aerospace industry. Sikorsky Aircraft already is adapting PMA concepts to the new S-92 Helibus for commercial and military helicopter operators.
The Comanche is a smart, new fighting machine that knows what ails it and how to fix it. That insight can have dramatic payoffs for a whole range of sophisticated platforms.