HUMS: Not Just for Heavy Iron Anymore

Cover Photo:  by Phyllis Utter/HAI

Improved technology brings HUMS to light, medium markets.

What if an operator could tell—just by looking at a computer screen—that a particular bearing on an aircraft was showing signs of premature wear? How would that change how maintenance is planned and conducted?

Heavy civil and military helicopter operations have had that capability for decades through onboard HUMS (health and usage monitoring systems), but adoption of that technology by operators of smaller aircraft had stalled. But that’s changing, as the next generation of HUMS equipment is evolving to meet the needs of the light and medium rotorcraft markets.

Early Challenges for HUMS

“The core of HUMS is probably component-vibration monitoring and maybe engine monitoring,” explains Nick Mayhew, industry cochair of the US Helicopter Safety Team (USHST). “But some systems include rotor track and balance (RTB), and other systems build in both RTB and flight-data monitoring [FDM]—specifically, the ability to pull flight parameters off the aircraft.”

Mayhew calls HUMS “a great concept” but concedes that the original products had challenges. “In the early days, the technological building blocks just weren’t there yet. On the shipboard side, sensors, cables, and computers were heavy and didn’t have the horsepower to do acquisitions fast enough. Wi-Fi and cellular were in their infancy, so you had to get the data off manually. The cloud wasn’t around, so the aircraft data could only be accessed from a local PC. And the user interfaces were terrible, so you needed experts in signal processing for interpretation.”

First-generation HUMS were also questioned, fairly or unfairly, after some high-profile accidents, according to Andrew Swayze, head of strategy and marketing for GPMS, a HUMS developer and manufacturer in Cornwall, Vermont. Specifically, Swayze cites two accidents—one in 2009 and one in 2016—involving two Airbus Super Pumas in which the HUMS apparently didn’t clearly identify component breakdown. At the same time, he notes, early HUMS were famous for false positives.

“As a result of these issues and the fact that some systems weighed in excess of 120 lb. and could cost several hundred thousand dollars, adoption was limited to large helicopters in military and offshore energy segments,” Swayze says. He points to a 2013 RotorHub article titled “Bad Vibrations” in which an engineer is quoted as saying, “It would be fair to say that HUMS has not lived up to early expectations.”

After roughly 25 years since HUMS was introduced, GPMS estimates that approximately 85% of commercial helicopters in service still aren’t HUMS equipped.

HUMS Technology Matures

By the 2000s, the technology that HUMS depended on began to change, coupled with the proliferation of smart connected devices and shifting customer expectations.

“The technology is now available to enable quick and reliable remote download,” says Mayhew. “This also offers opportunities to incorporate the collection and harvesting of FDM with HUMS information.”

“We saw an opportunity to modernize HUMS, address its earlier shortcomings, and make the technology accessible to all operators,” says Eric Bechhoefer, PhD, founder of and chief engineer at GPMS. “The initial HUMS systems were designed for heavy aircraft and for operators that had staff dedicated to data interpretation. But today’s systems are lightweight and designed so that the average maintainer can use them to solve everyday problems.”

Bechhoefer and his team put their HUMS system, Foresight MX, through extensive hardware and software development. To address false positives and improve detection, sensor processing and threshold settings were greatly modified. “Our health algorithm typically improves the signal-to-noise ratio by 3 to 4.5 dB. Its fault-finding capability is about 5 to 10 times more sensitive than in other systems,” he notes. “In practice, this means we can identify individual components showing wear, see faults at an earlier stage, track their propagation, and then layer in estimates of remaining useful life to simplify logistics planning and reduce unplanned aircraft downtime.”

Foresight MX was certified on the Bell 407GXi in 2019, under an amendment to the prior 407GX certification. This August, it is slated to be certified on another light utility helicopter, the Airbus AS350 AStar.

Bell confirms that it’s moving HUMS technology into its lighter aircraft. “Bell is the only OEM to offer a HUMS for a light single-engine helicopter,” says Brian Tucker, manager for Connected Fleets at Bell. “Currently, we’re working to make HUMS a part of the type design for the larger Bell 525 when that aircraft is certified.”

Asked about the advantages that the new-­generation HUMS will offer, Tucker cites the automated alerts to help users know immediately—whether in flight or upon landing—if something needs attention. “Foresight’s prediction of remaining useful component life is truly unique and helps operators plan for maintenance,” he says.

Tucker also notes Foresight’s bused smart-­sensor architecture. This, he explains, puts the data acquisition and processing into the sensor itself, so all data can be acquired simultaneously. “While older systems might acquire data once per hour, the HUMS on the Bell 407GXi acquires data 20 times per hour. In addition, the wiring is simplified,” he says. “The data bus wires string among the sensors like Christmas tree lights, unlike analog systems with dedicated wiring for each sensor. This results in lower weight—just 8.8 lb.”

Using HUMS to Solve Problems

Prior to using Foresight, Jeff Byrne, a helicopter pilot and aircraft maintenance engineer for Municipal Enterprises Ltd. in Halifax, Nova Scotia, had no experience with HUMS, but he’s now a fan.

“The Foresight system is too easy,” he says. “When you land, it uploads the aircraft data automatically, and within two seconds the new information is there on your tablet screen. You look at the component dashboard and if you see all greens, you know you are good to go.” The system’s troubleshooting and diagnostics features, says Byrne, take the guesswork out of formerly complex diagnostic procedures, such as addressing rotor vibration.

David Poe, who heads a crew maintaining a Bell 407GX at Hill Air Corp. in Dallas, Texas, agrees. “RTB traditionally requires that operators attach specialized equipment, take dedicated flights to manually acquire readings, make adjustments, then refly the aircraft to validate the solution, often multiple times,” he explains. “Foresight is designed to change this paradigm. The system is ‘always on’ and takes RTB acquisitions automatically, so that a predictive solution is there every time the aircraft lands.”

In late 2019, Poe received a high-vibration alert via Foresight’s email notification feature, which indicated that the helicopter’s main rotors were showing a vibration velocity of 0.4 inches per second (IPS) in vertical forward flight, an orange-level measurement that displayed as “Attention Needed” on the Foresight dashboard. Poe implemented the recommended solution, and the average vibration velocity for hover went from 0.35 IPS to 0.06 IPS, and for 120 knots, from 0.4 to 0.03 IPS.

The advantages of modern connectivity make HUMS a driver for improved efficiency in maintenance, explains Reuben Mann, head of marketing for SKYTRAC Systems, an aviation data hardware and software provider headquartered in Kelowna, British Columbia. His company is collaborating with helicopter OEM Leonardo and CHC Helicopter of Irvine, Texas, to certify SKYTRAC’s Real-Time HUMS (RTH) System as a retrofit install on the medium-twin AW139 helicopter—with line-fit coming soon. Work is also in progress to certify RTH on the Leonardo AW169 and AW189.

“Innovative product features for our HUMS solution include real-time in-flight exceedance notifications of HUMS parameters that monitor critical helicopter components, including the drivetrain, engines, and gearboxes. These can then be transmitted directly to the ground when exceedance limits are reached,” says Mann. “This gives maintenance crews an alert at the detection of anomalies in vibration patterns before the aircraft lands. When frequency vibrations are detected that deviate from the rest of the fleet, maintenance crews can look into the severity of the alert, order replacement parts, or swap out the damaged component at the earliest possible date.”

Mann says that email and text message alerts, combined with the data that is automatically downloaded after each flight, help operators plan for parts and repairs ahead of time. The flow of real-time data from the aircraft reduces turnaround times and unscheduled maintenance downtimes while also improving flight safety.

HUMS and FDM

According to GPMS’s Bechhoefer, integrating the functionalities of HUMS and FDM is key to gaining greater value for both systems.

“FDM and HUMS have always been closely related,” he explains. “You might say that HUMS is information about how your aircraft is operating. FDM is information about how your aircraft is being operated. The first focuses on the machine; the second, typically, on the pilot, but the two are highly interrelated.” For example, Bechhoefer says, if a pilot has a hot start, that will be captured by FDM and the exceedance recorded by HUMS. He points to a specific case involving a GPMS customer.

“The pilot called, hundreds of miles from base, complaining of low power and rotor vibration,” he says. “The maintainer was able to look at the aircraft in Foresight and confirm—in the ‘Mechanical Diagnostics’ view—the drop in engine performance and confirm—in the ‘RTB’ view—that the rotor was at 0.3 IPS. But he was then able to use FDM information to see that the outside air temperature was -10°C for the trip.”

As Bechhoefer explains, the maintenance technician was able to attribute the loss of engine performance to the fact that the pilot was using bleed air. He concluded that the RTB imbalance was specific to an elastomer that would recover as the temperature rose. “That, in fact, saved the maintainer a surprise trip to diagnose the problem on the aircraft.”

Other vendors also see the value of a single-unit, integrated HUMS/FDM system. In fact, Honeywell Aerospace’s new RECON incorporates both helicopter vibration-health monitoring and FDM, according to Pat Nuanez, Honeywell’s senior offering manager.

“RECON could, and should, be a central FDM system. We’re currently monitoring many assets within the helicopter that are used to provide valuable maintenance data,” says Nuanez.

Decreased downtime, lower costs, and improved safety due to the greater predictability of part failure or items needing maintenance are the advertised benefits of RECON, which is targeted to the medium and heavy helicopter operator. The data can be downloaded to a PC or a quick-access recorder—an airborne flight recorder designed to provide quick and easy access to raw flight data via USB, cellular network connections, or standard flash-memory cards. Data analysis can be done on a PC or web tool.

“Modularity is a key innovation to RECON. Using modularity, we can significantly reduce the need for long cables throughout the aircraft,” Nuanez explains. “RECON uses power over Ethernet (POE), so a single Cat 5 cable is used to communicate between the modules. Each module within the RECON system has its own dual core processor and memory storage, giving RECON the power and capability to meet today’s needs and the requirements of the future.”

Safety advocates also note the advantages of using both HUMS and FDM. “FDM and HUMS both use a flight-data recorder,” says Bob Sheffield, a member of the International Helicopter Safety Foundation’s executive committee, “so it’s efficient for manufacturers to provide them as an integrated package.”

HUMS Return on Investment

With HUMS now becoming available for the small and typically price-sensitive operator, the obvious question is whether this is a tool that can be financially justified, even considering its many advantages.

GPMS’s Swayze reports that in 2019, the company sought to answer that question by working with Arlington, Texas–based aviation consulting firm Conklin & de Decker. Using customer experience as a reference point and basing the model around the single-­engine Bell 407, three distinct areas of value were identified (see Figure 1), providing a $45,920 annual return on the operator’s HUMS investment.

“This financial return exists outside of maintenance credits and extended time between overhauls,” Swayze explains. “In other words, we found substantial return on investment in a product that allows operators to perform both scheduled and HUMS-dependent condition-based maintenance in parallel.”

“HUMS pays for itself,” says Sheffield, “not just in identifying equipment that’s about to fail but in routine maintenance work like RTB.

“An operator’s existing workforce is usually sufficient to integrate both FDM and HUMS in its work practices because FDM calls on pilot resources and HUMS calls on maintenance resources.”

Going forward, says Swayze, GPMS anticipates that there will be more demand to incorporate both FDM and HUMS into all helicopter operations. “The [US] National Transportation Safety Board has recently pushed for FDM and recording devices,” he says, “and several OEMs have or are moving to include FDM as standard equipment.” At the same time, he says, “if the value of FDM plus HUMS is greater, we think many operators will choose to have both capabilities on board and working in parallel.

“These technologies enable data-driven decision-making,” Swayze adds. “Once operators have seen for themselves how they can use the data coming off of their aircraft, I don’t know of anyone who’d want to do without it.”

HUMS Case Study: Amplifying Efficiencies

Reuben Mann of SKYTRAC Systems explains how his Real-Time HUMS product helped one operator solve several challenges resulting from his legacy HUMS installation, including:

• A 20- to 25-minute download process to get the data card from the aircraft to the station, including sign-off
• A loss of up to 15% of weekly data per aircraft
• Approximately 100 man-hours each week to collect FDM data fleetwide.

“On this one project,” says Mann, “we were able to eliminate this time-consuming HUMS and FDM data download process and complete it within minutes after landing. At the same time, we enabled the HUMS data to be analyzed while the aircraft was flying, as we now host the Leonardo HUMS library on the aircraft. It monitors all defined HUMS parameters in real time and, if an exceedance occurs, it immediately lets someone on the ground know.”

Mann says the data that’s automatically downloaded after each flight helps operators plan for parts and repairs ahead of time. “SKYTRAC reduces both turnaround and unscheduled maintenance downtimes while also improving the safety of flight.”

HUMS Case Study: Enabling Predictive Maintenance

At Duke Energy in Charlotte, North Carolina, the GPMS Foresight system warned of an emerging problem with an ACC duplex bearing on the company’s Bell 407. According to a report from the Duke maintenance team, the predicted remaining useful life estimate indicated that the component had about 50 hours before risking collateral damage.

Because the aircraft was coming in for a 2,500-hour inspection, the team decided to replace the bearing proactively. Following disassembly of the main gearbox, scuffing was noted on the ACC ball element and metallic fuzz on the chip detector.

Although the degraded bearing may not have caused a safety event, Foresight’s capability to detect a problem and provide prognostics enabled maintenance to optimally schedule the part’s removal and move from a reactive to a predictive footing.