Right Part, Right Place, Right Time   

By By Mike Hangge (mjHangge) | July 1, 2014
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The tip speed of an H-60 tail rotor is nearly 700 fps. Without proper balancing, excessive tail rotor vibrations can insidiously cause equipment damage and airframe fatigue. Photo courtesy PEO Aviation, U.S. Army

If you own or operate a helicopter, you’d probably like to boost your company’s profit margin. Right? Wouldn’t hurt to reduce your maintenance expenditures either. It’d be great to simplify your logistics and improve your aircraft performance, too. Is that all? What if I could do all that and improve your company’s future flight safety record just as a bonus?



Of course you are. Who wouldn’t be? I’ve just offered you everything short of a spa day at the Fountain of Youth. And I’d throw that in too – if I wasn’t convinced that health and usage management systems (HUMS) and helicopter flight data monitoring (HFDM) systems could fulfill every promise I’ve just made.

According to George Grove, vice president of HeliTune, there are only four choices with HUMS – install something, inspect something, replace something, or do nothing. If a company chooses not to install a HUMS and/or HFDM system (do nothing), then they will continue to blindly inspect and replace parts simply for the sake of meeting artificial time requirements created to protect the aircraft, but not necessarily the pocketbook of its owner. Don’t mistake me to say that there isn’t good cause to continue inspecting and replacing parts for the sake of safety. What I’m saying is there’s a better way to operate that will keep your aircraft and your pocketbook a little safer.



By now, most owners and operators in the aviation field have either used or, at the very least, heard of HUMS and HFDM systems. The first systems were installed in the early 1990s and – even before we were partying like it was 1999 – the UK CAA (Civil Aviation Authority) noted that “the first generation HUMS… has already demonstrated the ability to identify potentially hazardous and catastrophic failure modes, and has already reduced fatal accident statistics.”

In the broadest strokes, HUMS is exactly what the name implies – a System that Monitors the Health and Usage of the aircraft. The concept of HUMS is actually pretty simple – it monitors the vibrations, systems, and usage of a helicopter to better predict the condition of its critical components. HUMS can detect the slightest defects or wear simply by listening to two types of information – vibrations and system data. By doing this, HUMS can actually help predict the future maintenance needs of the aircraft.

Even on the smoothest helicopter, there are thousands of rotating and vibrating parts. Mechanical vibrations can be caused by several different sources such as distortion, looseness, wear and friction. Sustained or excessive vibrations can cause issues such as fatigue cracks, component wear, increased maintenance, corrosion, and crew/passenger discomfort. The most obvious and apparent source of vibrations in a helicopter is the big fan that blows the big air. The low frequency, one-per-vibrations of the main rotor permeates the entire airframe and can literally shake a helicopter apart. But gone are the days of balancing blades with chalk and broomsticks as the black arts of rotor smoothing are just one function of HUMS.

Closer to the true advantages of the systems are the abilities to detect mid- and high-frequency vibrations. The mid-frequency, mid-range vibrations of the driveshafts and gearboxes can be monitored through usage spectrum analyses to determine the load cycle calculations and proactively estimate the remaining component safe life, while also reactively detecting propagating component failure before catastrophic damages or seizure occurs. The high frequency short-range vibrations of the many bearings in a helicopter are vital for an aircraft’s safe operations, but their conditions are seldom monitored in a non-HUMS helicopter. It is exactly that ability that makes HUMS such a powerfully predictive maintenance tool.

More extensive HUMS add the ability to acquire directly from an aircraft’s DCUs (data collection units) to combine information such as the health of the engines, temperatures, component cycles, and pressures.

Eventually, this may also be combined with information from strain gauges to complete the total picture of an aircraft’s health.

The science of main rotor track and balance is still little more than a frustrating black art without the help of HUMS. Photo courtesy PEO Aviation, U.S. Army

In flight, a network of accelerometers continuously monitors the various vibrations of the helicopter. An onboard acquisition unit then collects the data from its sensor network – either automatically in pre-determined flight regimes or manually as the crew commands. Though most of the diagnostics and reporting will be performed between flights, some alerts may be signaled for potential exceedances in flight. After landing, the data is transferred to ground stations – currently data cards or flash drives, but the future will certainly appear suspiciously more like wi-fi. The information is collected and evaluated by the ground crew, which will help facilitate the “right part, right place, right time” method of logistics and maintenance.

The time for these systems has finally come. They’ve benefitted from the unbelievable speed of technological advances and survived the trials of youth. Numerous companies, including several OEMs, have decided that time-based maintenance is an outdated concept and that now is the time to begin actively moving toward a condition-based maintenance mindset. Many of them have joined the search for HUMS perfection.

Honeywell’s Sky Connect Tracker III will provide real-time vehicle health status and maintenance alerts for operators and pilots. It will also report information such as fuel status, aircraft system status and limit alarms to enable ground maintenance and operations personnel to instantaneously see when a HUMS maintenance alert is flagged. In late 2006, Sikorsky opened its FMOC (Fleet Management Operations Center) to monitor the entire global fleet of S-92 operators.

By doing this, Sikorsky maximized the maintenance power of S-92 owners/operators by providing the individual companies with the power and knowledge of an entire fleet.

FAA has granted life extensions for expensive and maintenance-intensive items such as the S-92’s main rotor hub because of the efficiencies gained by the FMOC. This will be the future of aircraft production as more aircraft such as the S-76D and more companies such as Bell and AgustaWestland offer similar services.

The future evolutions of helicopters will not be billed to HUMS alone, though. The two faces of HUMS – Health and Usage – are just one side of the coin that will pay for an over-arching maintenance/safety program.

An effective HFDM program is the other side of that coin and will provide our future counterparts with the gifts of both foresight and hindsight.



Like HUMS, an HFDM system is also just as simple as its name implies – it is a system that Monitors the Flight Data of a Helicopter. Simplicity in a helicopter acronym – crazy, I know!

But what data does it monitor and what is that data used for? Is HFDM just aviation’s reenactment of George Orwell’s 1984? Well… no… though the sales brochures may not sway those of us who wear tin foil hats. Yes, some will see it as another step toward the nanny nation and yes, their arguments may not be completely without merit.

Illustration explaining condition-based maintenance from a Honeywell Infographic about HUMS. Image courtesy of Honeywell Aerospace

My question to naysayers is whether professional oversight of our expansive cockpit cubicles is such a bad thing? We need to face the fact that even one aviation accident is too many.

We now live in a time when the most miniscule aviation incidents are streamed live via the world’s news agencies and the cost in lives, money, and reputation are so expensive that it could sink even the most successful aviation companies. Let us now embrace the fact that any amount of prevention may not be considered enough.

With that thought in mind, we must also address the causes of aviation accidents. Though the chances of dying in a helicopter are significantly less than winning the lottery, the odds that said accident will be blamed on me – the pilot – are better than three in four. Sure, maintenance and weather make up the greater amount of the other one in four, but why shouldn’t I just own the responsibility now and focus on how to keep from having the accident?

I can’t honestly think of a reason. Don’t get me wrong – I understand the concerns. After all, I’ve been flying for the military for over a quarter century and, for most of that time, I’d have preferred grounding over having my mother watch every turn I take.

However, I’ve matured and mellowed (is it funny that both are synonyms for aged?) and have learned to respect the old adage that “there are old pilots and there are bold pilots, but there are no old, bold pilots.” I’m now concerned more about how to meet my great-grandchildren than the momentary rush of a negative g-maneuver. An HFDM program may protect the reputations of those of us piloting within the limits and restrictions of our aircraft and protect the passengers of those who aren’t. Even more, and I hate to confess this, the truth of today’s litigious world is that an HFDM might also be the only defense to protect a company’s assets.

Honeywell’s Sky Connect Tracker family. Photo courtesy of Honeywell

But the objective of an effective HFDM program isn’t to watch over shoulders when an accident occurs, as much as it might seem to be so.

An effective program might actually prevent those accidents from ever occurring by detecting adverse trends in flight operations and by providing a systematic method of accessing, analyzing, and acting upon digital flight data before they lead to an accident. Additionally, this information and insight can be used to reduce operational costs and significantly enhance training effectiveness and maintenance procedures.

And that’s exactly why FAA has taken such extraordinary measures to protect the sanctity of digital data acquired by systems such as an HFDM. By affording the data of approved programs a certain level of “anonymity,” FAA has effectively promoted a more open dialogue between pilots and management to focus on improving operations and safety. Aviation, among many other industries, has begun to realize the value of a “Culture of Safety.”

This culture is the very bedrock upon which a safe aviation organization must be built and is a mixed brew of individual and group values, attitudes, leadership, and behaviors.

In a long line of cultures, the concept of a “Just Culture” might be near the top as it promotes an environment of safety and excellence, both in attitude and structure, in which personnel feel comfortable disclosing errors – including their own – in an environment of interactive questioning, resistance to complacency, personal accountability, and corporate self-regulation.

The actual operation of an HFDM system is fairly simple. The hardware records the interaction between the operator to the aircraft and its systems, while the software provides the tools to reactively analyze that interaction after an incident has occurred or to proactively analyze it in hopes of avoiding the incident itself.

North Flight Data Systems’ QAR, or Quick Access Recorder. Photo courtesy of North Flight Data Systems

An acquisition unit such as North Flight Data System’s MFDAU (multi-function data acquisition unit) combines internally generated information from its self-contained AHRS and pitot-static systems with an extensive amount of externally monitored data that can be individually tailored to each user’s needs.

Though this data is stored in crash-protected memory for evaluation after the flight, North FDS has also designed their systems so that vital information can be transmitted in flight to any compatible satcom system in the event of an emergency. Information from HFDM programs is unique since it provides objective data that otherwise is not available.

In the end, an HFDM offers owners a certain level of knowledge of how their helicopters are being operated while offering rule-compliant potential employees an opportunity to evaluate the devotion of a company to safe operations.


The Choice…

In the end, every helicopter owner and operator only has four choices with HUMS and HFDM systems – install something, inspect something, replace something, or do nothing at all. I’d like to say that it’s an easy choice, but each company must balance the benefits added against the costs incurred and smaller companies may have the most difficulty with these decisions.

No matter what the choice is today, the future decisions will certainly include HUMS and HFDM systems. Just as the value of vehicular airbags were once argued, HUMS and HFDM systems will eventually win over their critics and become standard on all new helicopters. As they become more credible and well-rounded maintenance tools, the thought of continuing to inspect or replace simply for the sake of abiding by conservative maintenance intervals will seem just as outdated as keeping a 1984 Chrysler K Car around for its performance numbers.


In the September issue, Rotor & Wing’s HUMS series will go into more specific details about the potential savings of the two systems and how the ambiguity inherent in the systems’ current descriptions has created problems for potential companies. In addition, the series will delve further into what has kept the systems apart for so long and how their marriage will finally merge the kingdoms of safety and maintenance into one.

Related: Maintenance News

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