If US Army helicopter pilots and mechanics tend to brood and worry a bit more than their ground-bound brethren, they have a good excuse - they labour under the perfectly reasonable assumption that if mechanical trouble is not already menacing the machines in their charge, it is about to.

There is hope, however. The army is bolstering its UH-60 Black Hawk, AH-64 Apache and CH-47 Chinook maintenance programmes with a new generation of digital source collection (DSC) technologies, including vibration detectors and other embedded sensors able to detect deterioration in power train components long before a catastropic failures occurs.

 © US Army
The dawn of a new age of sensor technology could prevent the loss of US Army helicopters


"If you can catch a bad part before it fails completely, you can turn an unscheduled maintenance event into a scheduled one, which is a lot less painful," says Chris Smith, condition based maintenance director for the US Army Aviation and Missile Command at the Redstone Arsenal in Alabama. "Not only does it prevent accidents and save lives, it allows us to plan maintenance proactively and helps eliminate ugly, expensive surprises."


More than 360 army helicopters (about 12% of the fleet) have received DSC equipment to date and early reports from the field show promise. Smith compares two 38-aircraft UH-60 Black Hawk battalions recently deployed to Iraq - one with DSC and the other without.

The DSC-equipped unit raised its readiness level by 5% or 1,500 flight hours, a situation he describes as, "the rough equivalent of having two additional aircraft over a 12-month period".

The other two aircraft types show similar, albeit less dramatic, gains. Apache units participating in the DSC programme have seen a readiness rise of around 3%, while the Chinook battalions report a modest 1% improvement. However small, Smith says, these increases translate into additional flight hours per aircraft for the same expenditure of time, manpower, parts and other resources.

"None of this is earth-shattering, but anything you can do to cut your NMC [non mission capable] rate due to maintenance is a big plus," he says.

"The real benefit comes in at the maintenance test flight stage. We just don't have to do as many of these flights, and when you reduce them by 5% - that's five out of every hundred hours - it adds up quickly. Now, we can give those hours back to the army. Traditionally, NMC rates due to maintenance were about 18-to-24% depending on the aircraft and the theatre. We've dropped that to 14-to-18%."

Digital source collection is a relatively new play on a proven theme. It is an evolution of the aircraft health & usage monitoring systems that debuted in the early 1990s aboard civil helicopters supporting the North Sea offshore oil industry. Initial HUMS research dates back to the 1970s when the UK's Ministry of Defence first wondered if vibration monitoring could help to detect faults in the highly stressed propulsion and control systems of military helicopters.

Previously, tracking the internal health of a helicopter was, at best, an inexact science. Monitoring oil pressures and temperatures provided only limited information and "chip detectors" - essentially electrified magnets positioned to intercept metallic debris shed by a disintegrating bearing or turbine blade - warn only of imminent failure. Similarly, monitoring flight performance parameters, such as air speed and RPM, only provides an indication of trouble long after the time for preventative action has passed.

Before health monitoring emerged, "condition-based" maintenance practices required parts to be replaced at prescribed time intervals, regardless of their actual condition. Often these milestones were based on wastefully conservative criteria, in many cases tied to arbitrary time-in-service thresholds. Many engines and transmissions subject to this regime were scrapped well before their time, squandering unit budgets and countless labour man-hours.


HUMS, in contrast, shifted the focus from the theoretical to the actual condition of parts, while providing a reliable backup to the aircraft's cockpit warning systems.

Over time, HUMS has become an everyday maintenance management tool that helps maintainers reduce expense by extracting maximum life from certain key components and systems. Often, it serves as a substitute for more invasive aircraft inspection procedures. Smith, a former Apache test pilot and maintenance officer, describes how HUMS prevents what doctors (and mechanics) call "unnecessary surgery".

"There's an inspection of the UH-60 that requires you to disconnect the drive shaft and test it for wear and vibration. It's a big, dirty job, requiring considerable manpower, know-how, and a lot of support equipment. If you can put a sensor on that drive shaft - we're doing this now - you don't have to disconnect it. Now we're moving into the diagnostic area, where the system tells the maintainers what they have to do, or don't have to do."

Not all HUMS are created equal, but according to General Electric Aviation Systems, which fielded some of the first HUMS as Smiths Aerospace and now has more than 400 units in service on civil and military helicopters, a good one must meet certain baseline criteria. It must be permanently installed on the helicopter, GEAS says, and come with a data acquisition/processing unit, a cockpit control unit, accelerometers and a blade tracker.

GEAS also believes a quality HUMS should be integrated with the helicopter's flight data recorder, and perform vibration-based monitoring of the aircraft's engines, transmission and rotors, including rotor track and balance.

The system should also record aircraft and component time and all parameter exceedances, and have a ground station in the loop to provide alerts and support detailed data analysis.


The US military is acquiring DSC systems of various levels of sophistication, from multiple sources. According to Smith, it recently approved $105 million for an additional 437 DSC systems for the army rotary-wing fleet. The allocation includes 294 Goodrich Integrated Vehicle Health Management systems for the UH-60, and 143 Intelligent Automation Corp (now owned by Honeywell) Modernized Signal Processing Units for the AH-64 fleet.

Kip Freeman, business development manager for the Goodrich VHM systems unit, says his company recently won a large contract to furnish IVHM systems for army Chinooks. IAC, however, whose system was already selected for MH-47Gs of the 160th Special Operations Aviation Regiment (SOAR), is protesting the award. "The army is indicating there may be another competition," Freeman says.

One explanation for the relatively open nature of the army's DSC acquisition plan is the reality that its various helicopter models entered service in different eras, and each represents a different level of technical sophistication. Each requires its own unique DSC system, because data acquisition needs vary so widely from type to type.

The power train of the UH-60, for example, has changed little since 1983, when it represented the pinnacle of mechanical technology. Any DSC for it must, by necessity, be a fairly complex system that reaches deep into primary structure.

1000 hours

While the components of the Goodrich IVHM for a Black Hawk weigh only about 9kg (20lb), installing the equipment takes around 1,000 man-hours. This is typically done during "reset," the army's term for a comprehensive teardown and rebuild of aircraft returning from the combat zone.

The Apache, by comparison, is a more modern design, even though it only trailed the Black Hawk into service by a few years.AH-64s come equipped with a relatively modern electronic data bus that functions similarly to a local area network, collecting and distributing signals from the various vibration detectors. For this reason, AH-64s don't need the full IVHM treatment - they can get by with the MSPU, which is essentially a data recorder connected to the LAN.

In April 2007, the Apache community experienced first-hand the potential of data collection. A DSC-equipped AH-64A - aircraft number 314 - crashed after its tail rotor swash plate mast bearing failed. The crew escaped, which is generally the exception in this type of failure-induced accident, but the aircraft was a total loss. Called in after the crash, Smith's team pulled 314's DSC data and compared its tail rotor swash plate vibration level to other DSC-equipped AH-64As.

"When we did, it was obvious that 314's vibration [level] was higher than the others - except one," Smith says. "Aircraft 118's data indicated that it had a much higher vibration level in the same band as 314s. We called the unit, had them ground the aircraft, remove the bearing and send it to us for teardown and analysis. We found that the bearing was running extremely hot, as it had corroded to the point that failure was impending."

What does the future hold for DSC? The technology is moving forward rapidly new applications beckon and the various vendors are exploring new ways to use the sensor package. One wants to use it to accurately predict the exact life remaining in components another sees it as a means of verifying the integrity of the composite structures now so prevalent in aircraft construction.

There's also a push to downsize the health monitoring hardware to fit smaller rotary-wing platforms. Boeing MH-6 Little Birds of the 160th SOAR were recently equipped with a removable version of the IAC MSPU, and Goodrich has developed a scaled-down version of its IVHM system, called Vigor, for the midsize helicopter market.

For the army's three main helicopters - the Black Hawk, Apache and Chinook - further expansion of the DSC effort is planned, both in aircraft numbers equipped and individual power train components monitored. Of 174 critical parts deemed to be worth watching on the three types, Smith says, only 18 are regularly monitored using DSC. And, due to deployment needs, simply getting the aircraft into the shop for a refit can be a challenge.

"Synchronizing the fielding is probably the biggest headache we face," Smith says. "We have an industrial age mentality out there, that we're trying to move into the information age. It's happening, but there's still plenty to worry about."

Source: Flight International