Way back some 35 years ago, with the ignorance of youth and a bank account unhindered by responsibility, I decided to learn to fly a helicopter. It wasn’t like I was planning on a career as a helicopter pilot, I just wanted to experience what it was like to move in all three dimensions with a freedom unachievable in a fixed-wing airplane. The scope of the challenge really came to light during my first helicopter pre-flight. This was an amazingly complex machine. And every component had to be checked—and not with just the cursory glance you might be tempted to give an airplane—no, every step is critical. “There’s no such thing as a gliding helicopter,” my instructor said. I remember those early pre-flights taking longer than the flight lesson itself.
Those incredible hours not only gave me an unwavering love of helicopters and what they can do, but also a continually growing respect and admiration for the people who maintain them—a responsibility that has only grown in scale and importance as the size and sophistication of helicopters has continued to advance.
|A maintenance test pilot performs engine vibration checks using the Goodrich helicopter health management system cockpit display unit on a 101st Airborne Black Hawk in Balad, Iraq. Courtesy of Goodrich|
A ‘Healthy’ Helicopter
Whether it’s flying or fixing, every element of a helicopter’s operation is elegantly complex. And thankfully, as with many aspects of our industry, technology has been developed to help optimize both the performance and maintenance of key components and systems.
One of the most notable has been the development of health and usage monitoring systems (HUMS) for helicopters. “HUMS was originally born out of a requirement to improve safety,” explained Steve Boakes, business development director for GE Aviation Systems. “In 1986, a commercial Chinook crashed in the North Sea, killing all but two passengers. It had no monitoring equipment on board except for a cockpit voice recorder.”
Many feel that the circumstances surrounding this accident was the straw that broke the proverbial camel’s back and got the industry to look for a technological solution to reducing mechanical failures in helicopters. “It was tragic, but the positive that came out of it was it actually accelerated the implementation of HUMS in helicopters,” Boakes added.
In its simplest form, a HUMS installation is comprised of a series of strategically placed sensors and other ancillary systems that can provide on-demand information pertaining to the condition of the helicopter’s key components and structure. “It’s an acquisition system that really detects two types of data—usage and vibration,” said Marc Brodeur, business development director, vehicle health monitoring systems for Goodrich Sensors and Integration Systems. “It provides analysis and some mechanical diagnostics of component operation, rotor track and balance, and other systems.”
Boakes said that while the typical HUMS system is capable of collecting a lot of data, by far the most important relates to monitoring vibration characteristics among the aircraft’s key components.
“Vibration has been proven to give the earliest indication of incipient mechanical defects—defects that are growing slowly over time,” he said. “If you measure temperature, for example, you will see a minute change in a bearing’s temperature about two minutes before you have a catastrophic failure. That’s no good to you.”
With vibration monitoring, operators can receive “many tens-of-hours, or even more than a 100 hours’ warning,” Boakes continued. “That means you have the luxury of being able to monitor and analyze the data between flights.”
|Health management systems use sophisticated algorithms to pinpoint the source of drive train degradation such as this spline adapter. Early corrective action increases operational readiness. Courtesy of Goodrich|
For example, GE can “diagnose faults right down to individual gear teeth,” he added. “We can tell you which one is failing and when it’s probably going to go ‘bang’. We can do that on any critical rotating component on the aircraft.”
All the suppliers’ representatives Rotor & Wing contacted for this article stressed that the real benefit of any HUMS package is not the data it collects, but what the maintenance technicians can do with that information. After all, as Boakes explained, “HUMS is basically a pipeline from the sensor all the way to a (maintenance) action.”
While any good engineer can develop a data acquisition and collection system—basically the brains of a HUMS installation—the true art is what you ultimately do with all that data you collect.
“Some of the initial systems were pretty rudimentary in terms of what they were able to collect for data—it might have been a simple piece of vibration data right at the gear box,” explained TK Kallenbach, vice president of product management for Honeywell Aerospace. “As we have matured the technology we have gotten more and more sensors throughout the engine and drive train and we are now better able to fuse that data and see what the system is telling us.”
“One of the requirements of a HUMS installation today is to manage the information very carefully. And make sure that information is communicated to the guy with a spanner [crescent wrench] in his hand who’s trying to make sure this one aircraft is airborne today,” Boakes said. “That’s really one of the fundamental challenges of HUMS—making the information we collect easy to understand for the ultimate user.” Kallenbach said he thinks Honeywell has “been able to get to the point with our systems where we can actually give a chief maintenance officer a pretty specific maintenance action that he can take in terms of an adjustment to the rotor system or gear system and actually adjust a problem out way before a failure occurs.” He continues: “We can get down to the point of, ‘Here’s your helicopter’s serial number. Go adjust this particular point five degrees and change the balance a little bit.’ It’s pretty amazing.”
No matter how you look at it, the ability to better understand and use the information that the HUMS collects is a major leap forward in today’s systems’ capabilities. The HUMS isn’t just smart about what it collects, but how it distributes the data to the end user.
“The GE system actually does a lot of processing right on the aircraft, whereas a lot of earlier systems left the processing for collection and analysis off the aircraft,” Boakes said. “We learned that we really don’t want to download much information at all when everything is fine. You just want detail when you think there’s a problem.”
“The moment the aircraft lands it has a suite of data that can be converted to usable information that will tell the maintenance crews what the trends on the aircraft are for every major component,” he added. “Our HUMS automatically ‘floats’ those problems to the surface of the information at the ground station. The screen will automatically indicate where those problems are. The maintenance technician doesn’t have to go looking for them—the problems will come looking for him.”
The ability to clearly understand what all the data can tell a helicopter’s maintenance team doesn’t only help with making the right repair at the right time, it can also help reduce an aircraft’s overall maintenance requirements.
Brodeur explained that the Goodrich HUMS installed on Army UH-60 Black Hawks has enabled field mechanics to dramatically reduce some key inspections.
“They used to do 50- and 120-hour vibe checks,” he said. “Now because of our HUMS we’ve been able to automate those checks and don’t have to do a separate inspection. That reduces their maintenance burden. They want to work toward not having to take something off at an hour level—they want to leave it on and just remove it when it needs changing.”
While improving safety is the undisputed goal of every HUMS installation, the ability to pro-actively monitor a component’s health and possibly extend its useful life is a close second.
“Because we can discern the health or amount of usage a particular component has left and in a highly disciplined way, we are a lot better off in terms of reducing unscheduled maintenance occurrences,” Kallenbach said. “That was brought home to me when I learned how much money an unscheduled failure can cost a company if they strand a helicopter on an oil platform in the North Sea.”
“They have to figure out how to get that helicopter back to where the service can actually be done,” he added. “The cost is absolutely astronomical to the operator and the platform.”
Kallenbach said that by and large, most HUMS operators have figured out how to use the system to greatly reduce unscheduled maintenance, while at the same time, safely extend the lifespan of many of the aircraft’s systems.
“We can detect a potential failure earlier in the cycle and replace a minor component instead of waiting for a catastrophic event and then having to replace an entire power train,” he said. “You always want to extend the hours safely—you just need to ensure you can extend that interval in a way that doesn’t cause another problem.”
While reducing unscheduled maintenance and extending the life of “healthy” components are two major benefits to a HUMS installation, there’s another one that will pay off with significant savings in maintenance—especially by reducing the search time for the elusive “no fault found.”
Some helicopter operators have expressed that in extreme cases, up to 30 percent of every hour spent on maintenance is actually spent searching for faults that don’t exist. With HUMS, the operator can actually reduce the number to just a few percent—that’s a huge savings.
To put that in perspective, in 2007 the aviation consulting firm AeroStrategy pegged the helicopter airframe maintenance business to be upwards of $1.5 billion a year. No matter how one looks at it, 30 percent of $1.5 billion presents many hundreds of millions of good reasons to install HUMS. One of the key enablers to minimizing instances of no-fault-found has been to improve the reliability of the unit’s sensors and components. Today’s HUMS sensors are extremely robust and reliable. The levels of missed alarm rates (MARs) and false alarm rates (FARs) are typically just a few percent, according to Boakes. “We’re trying to crush them down to zero ideally, but zero is a goal—not a fact.”
“These systems have a tendency to be set for action as opposed to being set for inaction,” Kallenbach said. “Most of these systems are designed to be kind of fail-safe—you don’t want to get stuck or have a failure because it didn’t tell you to check something because it thought it was too early.”
Brodeur said that another benefit of HUMS usage comes from enabling the helicopter’s manufacturer to spot maintenance trends and report them to other operators in the fleet. “Sikorsky’s customer service center uses the information that comes off of our system found on the S-92 and S-76D helicopters to track the maintenance needs of their customers in the field. They’re effectively using HUMS to make sure the fleets are being maintained and are operational,” he said.
“When they see something on one particular aircraft they are able to go out and look at the rest of the HUMS-equipped fleet for potential problems based on the data set and their analysis of that information,” Brodeur said. “Sikorsky has made some great strides in terms of making sure they keep their operators happy.” Another tool found in today’s HUMS installations is the ability to collect data, including cyclic stick position, the attitude and speed of the aircraft, and other flight parameters to help identify exactly what was happening when a specific event occurred.
It not only aids in identifying aircraft operational procedures that may need improvement, it also helps the HUMS selectively alter its alarm parameters to eliminate false alerts.
“The HUMS is able to understand what condition the aircraft is in and apply the right thresholds for each component under the right conditions,” Boakes said.
“Components can be undergoing very different loads under each condition and rates of stress, so their vibration characteristics, temperatures and pressures will vary in accordance with a high-stress position or condition versus just taxiing on the ground.”
“So,” he continued, “depending on the how the aircraft is flown, the threshold you apply to the data in order to [find out] if you have an alert are different all the time.”
|The fundamental challenge of HUMS is making the information collected easy to understand for the ultimate end user. Courtesy of GE Aviation Systems|
While today’s health and usage monitoring systems have come a long way from the first installations way back in 1991, we’re still just learning what these amazing systems can do. The future is pretty exciting.
“Condition-based maintenance is clearly the direction that HUMS is going,” Brodeur said. “The ‘Holy Grail’ comes when you get to prognostics as opposed to mechanical diagnostics. You want to be able to say with real authority that a particular component has X amount of hours left.”
Goodrich is “doing some very exciting things with prognostics and patented algorithms that will take us to the next level of condition-based maintenance,” he added. “The ability to increase time between overhauls (TBOs) on the aircraft’s components—that’s the goal. But that won’t happen unless you are working closely with the aircraft’s OEM on that.”
Boakes noted that the ability to monitor the aircraft’s flight regime is also going to help achieve the goal of condition-based maintenance.
“If you are flying an aircraft gently—if you’re not stressing it, then the components are going to last a lot longer and can stay on the aircraft safely. If you’re stressing the aircraft beyond the standard, then be aware that you will have to replace these components more often,” he said.
“You have this elegant scenario where you don’t have to replace components too early to compromise cost, but you don’t replace them too late as to compromise safety,” Boakes said. “That’s where HUMS is going.”
Whether a helicopter’s operator is looking to add greater levels of safety or reduce its overall maintenance costs, HUMS can help. After all, as Brodeur put it, “If you don’t provide benefits to the end-user, it’s just a hunk of weight that increases fuel burn.”