Since their emergence more than 20 years ago, health and usage monitoring systems (HUMS) have expanded from oil and gas rotorcraft to military helicopters, fixed-wing aircraft, drones and business jets. HUMS systems not only monitor the health of vibrating and spinning parts but record the operational context of events and provide data for trend analysis and condition-based maintenance (CBM). HUMS technology is also edging toward connectivity features and real-time cueing.
Basic rotary-wing HUMS systems collect and record vibration data from sensors on critical areas such as the rotors, engines, gearboxes and drive shafts. They use simple thresholds based upon a few basic characteristics to determine whether the aircraft needs maintenance or can fly, said Kip Freeman, business director, rotorcraft and vehicle health systems, at Goodrich, a major HUMS supplier.
But vibration analysis, alone, provides an insufficient snapshot of aircraft condition, Freeman says. Although vibration-only systems can detect a gross defect a few flight hours before a component is about to break and thus contribute to safety, Freeman says, “full-capability” systems provide exceedance monitoring, regime recognition and advanced diagnostics.
Vibration-only systems, however, can actually reduce readiness and flight dispatch reliability, Freeman maintains. Vibration signals that are processed into “condition indicators” tend to have a significant amount of “scatter.” A vibration-only system that knows nothing about how the aircraft is being flown will be collecting data under so many different conditions on the ground, at hover and in forward flight making it hard to set appropriate state-of-health thresholds, Freeman says. That’s why flight regime data and sophisticated analytics are needed to drive better maintenance decisions.
“The HUMS system provides an ongoing stream of data that can be used to determine the health of a system and its underlying components,” said Peter Fuchs, senior manager, analytics and technology for Sikorsky Aerospace Services. “With better information about component health, we can keep parts on wing longer, and identify those that should come off before they disrupt operations.”
Because HUMS data can help avoid unnecessary overhauls and unscheduled maintenance, it increases aircraft availability and reduces operator costs. The U.S. Army, which currently operates 2,563 HUMS-equipped helicopters, provides a wealth of examples.
HUMS technology is helping to move Army aviation towards condition-based maintenance, which focuses on aircraft data rather than strictly on flight time. CBM exploits the ability to monitor the health of components and change them when necessary rather than at fixed intervals. It’s about “driving unscheduled maintenance towards scheduled maintenance, [based on] your knowledge of what the aircraft is doing,” explains Cathy Ferrie, director of the Xworx research and development unit of Bell Helicopter. But it’s also about reducing scheduled maintenance that’s not necessary, Freeman adds.
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| The U.S. Army’s 160th Special Operations Aviation Regiment (SOAR) uses ground-based HUMS software programs help analyze the vibration, tachometer and flight regime data from the Mil-Std-1553 buses on Black Hawks and Chinooks. |
“We’ll always have some time-based maintenance all over the aircraft,” says Steve Blasey, deputy director of maintenance and logistics for the Army’s 160th Special Operations Aviation Regiment (SOAR). But CBM allows greater maintenance flexibility and reduces maintenance costs while maintaining flight safety. The 160th SOAR uses Honeywell HUMS on its MH-47G Chinooks and MH-60M Black Hawks. It is working with the manufacturer on a smaller box for the MH-6 Little Birds.
Sometimes the data can drive changes to the maintenance manual although such actions are the responsibility of the Army Engineering Directorate (AED), which looks at vehicle health fleet-wide. One change involved the timing of oil cooler drive shaft bearing replacements on the regiment’s Black Hawks, Blasey recalls. “The bearing’s life was extended as long as the component is monitored by a HUMS solution.” As a result, the part gained several hundred hours of flight time.
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| Sikorsky said its Fleet Management Operations Center in Trumbull, Conn., “acts as the focal point of our overall fleet analytics effort.” The center, established in 2006, identifies ways to reduce aircraft operating cost and minimize downtime. |
The 160th’s line mechanics look at their own aircraft flight data, as well, and can replace parts when necessary. If a mechanic sees high vibration on a bearing, for example, and he knows that the aircraft is getting ready for a long mission, he can change the bearing, Blasey says. “That increases the safety of the aircraft.”
The regiment has demonstrated an approximately 75 percent reduction in the flight time required to validate rotor track and balance adjustments on its Chinooks. The average maintenance flight time for rotor balancing has decreased from 8 to 10 hours to 2 to 3 hours since HUMS systems were installed in 2007. Multiply the decrease by the Chinook’s $7,000-per-hour flight cost, and there’s a sizable maintenance saving over the long haul. And more hours are spent on missions, rather than in the hangar.
Evidence also suggests that HUMS has contributed to increased overall structural health. “The total number of work orders related to structural cracking seems to have gone down,” Blasey says. “We believe that’s associated with a reduced vibratory signature of the aircraft,” he says, alluding mainly to the Chinooks. The thinking is that, because HUMS allows the rotors to be balanced to tighter tolerances and because the systems are monitored more regularly than had been possible in the past, structural wear and tear has been reduced.
Digital Data
The 160th SOAR’s HUMS systems gather not only vibration and tachometer data, but also flight regime data from the Mil-Std-1553 buses in the Black Hawks and Chinooks. This includes information on engine health, power settings, attitude, altitude and turn rates, among other things.
Today HUMS is really a maintenance tool, says Chris deLong, Honeywell’s senior manager for technical sales. But HUMS systems are collecting so much additional data via 1553 or ARINC 429 data buses that they allow the reconstruction of the flight regime of an aircraft when an event occurred. This not only deepens analysis but also opens opportunities for data services. Of course, regime awareness can also be achieved — at a greater level of granularity — on analog aircraft fitted with hundreds of sensors.
Digital architectures help reduce the weight and cost of HUMS systems while allowing the systems to dramatically increase the amount of data that is monitored and recorded, Freeman says. However, digital systems can limit the type of data available because data on the bus might be pre-filtered information, as opposed to the raw sensor data.
An example is the monitoring of metal chips in a gearbox, a sign of bearing degradation. On older analog aircraft Goodrich could read data from the chip detector sensors and see “every little spurious chip going past in the oil,” Freeman says. Maintainers used this methodology in addition to vibration-based health indicators.
But with digital aircraft, “the HUMS may only monitor the heavily filtered signal that goes to the cockpit,” Freeman says. Digital systems still give enough warning but there’s less granularity because you’re not tapping directly into the sensors.
Goodrich collects digital data from data bus-equipped aircraft such as the MH-60R/S Seahawks and the UH-60M Black Hawks. On older analog aircraft the company uses on-board data concentrators that tap into analog signals and convert them to a digital format.
Cost/Benefit
Although the latest technology is expensive, the Army probably wouldn’t want to be without it. According to Honeywell, in 2008, Maj. Gen. James Myles, then commander of the Aviation and Missile Command, attributed more than $112 million in savings to HUMS on two aircraft types during a two-year period:
â–¶ $24 million: 51 T700 engines not replaced on UH-60/AH-64 for overspeed/overtorque;
â–¶ $30 million: Addition of UH-60/AH-64 flight hours available due to increased readiness/availability and maintenance man-hours saved;
â–¶ $49 million: Three Class A AH-64 accidents avoided;
â–¶ $6.6 million: Components not replaced and returned to service and overhaul savings due to early failure detection by the HUMS system; and
â–¶ $2.8 million: AH-64 tail gearbox scheduled maintenance savings/avoidance.
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Goodrich said its Vigor HUMS, which is standard on Sikorsky’s S-76D, provides medium-sized helicopters with full HUMS capability in a lightweight package.
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Honeywell further cites a study of HUMS on AH-64 Apaches that found the technology reduced mission aborts by 30 percent, maintenance test flights by 20 percent and scheduled maintenance by 5 percent to 10 percent. There are about 725 Apaches in the Army fleet and Honeywell is contracted to equip them all, deLong says.
Goodrich points to data from the Army Black Hawk program office to the effect that HUMS has allowed them to increase fleet readiness by 5 percent to 10 percent with a 17 percent decrease in total maintenance. The maintenance savings came from a 52 percent decrease in unscheduled maintenance and a 1 percent decrease in scheduled maintenance, Freeman says. The statistics are based on data from the Goodrich HUMS. The company has equipped at least 1,300 of the Army’s approximately 1,900 Black Hawks, he estimates.
Additionally, HUMS can sometimes save lives. After an AH-64 Apache accident from which the crew walked away, Honeywell analyzed HUMS data and found three additional aircraft that would eventually have crashed had steps not been taken. The problem was isolated to a bearing in the tail rotor, deLong says.
The ultimate goal of HUMS technology, however, is to provide ample notice of impending issues. Goodrich aims at alerting operators of problems 60 to 100 flight hours in advance of possible incidents, Freeman says. “While we do have cases where operators have told us our system helped to avoid an incident, those are and should be rare and due to mitigating circumstances such as battle damage or unknown types of failures of parts that are not monitored but happen to be detected just ahead of a failure.”
Some operators want a maintenance downlink. The 160th SOAR, for example, wants to accelerate status updates and maintenance response by means of wireless data downloads during taxi, before power shutdown.
Honeywell has demonstrated and flight tested an interface between the SkyConnect Tracker II fleet tracking system, which uses Iridium, and Honeywell’s HUMS products. This way, “when an exceedance or an event occurs, the HUMS can send a discrete signal to the maintenance guy on the ground,” deLong says.
Honeywell also is developing an interface, using the new Tracker III system, that will receive and then transmit to the ground a 429 data word from the HUMS. The data word will provide additional granularity as to what the HUMS event was, deLong says. This product will be available in the third quarter 2012.
The next big step for HUMS may be skin sensing. “Everyone’s clamoring for structural health monitoring,” says Freeman. The question is how to do it so it is small, lightweight and accurate a big challenge. Aside from the economic aspects, today’s skin sensors are not yet mature enough to detect cracks and feed HUMS system for actionable purposes, Freeman says.
Bell Helicopter, along with the rest of the HUMS community, is seeking a solution. But the airframer may have an edge, as it studied the issue, among other potential maintenance technologies, under the Army Operations Support and Sustainment Technology program.
Bell also is looking at integrating sensors as a result of its focus on the most weight- and cost-efficient maintenance processes,” says Ferrie.
Sikorsky’s Fuchs said a significant thrust of his company’s efforts have been beyond traditional HUMS systems into more comprehensive analytics. “We’re working to draw on many elements of field data, not just HUMS, to enhance safety, increase an aircraft’s operational availability and to reduce operating costs. Our ultimate goal is to provide predictive support,” he said. Sikorsky has developed a suite of software tools designed to gather, condition and analyze data, allowing the company to compare data across an entire fleet to isolate issues that might be common to several aircraft. “The goal is to provide an aircraft operator an opportunity to perform necessary maintenance before unplanned component failures disrupt flight operations. We wan to turn unplanned maintenance into planned maintenance.”