System health management units

Every pilot is familiar with the preflight inspection. It’s a fundamental tenet of aviation safety that is taught as lesson one on day one of flight training. Set aside thoughts of fly-by-wire systems and transport-category or business jets for a second.

Think back to conducting that first preflight inspection on a general aviation or military trainer in which the flight controls were actuated by cables and pulleys.

You moved the surfaces by hand to check for proper rigging and condition. You looked at hinges, bolts, and pins to verify airworthiness. You listened intently as you checked the flight controls because you could hear a binding or slacking cable by how it sounded.

Fuel was checked by physically removing the fuel cap and looking in the tank. Some of us were taught that if you can touch the fuel with the tip of your fingers, it meant the tanks held at least a certain quantity, despite what the somewhat inaccurate analog fuel gauges read.

Decades later it turns out that using human fingers as a dipstick for 100LL was not the smartest idea from a health perspective, but it’s a good example of how practices eventually evolve over time.

Collins Pro Line Fusion provides an integrated diagnostic readout of problems via a maintenance toggle/switch. It also records flight control inputs, such as rudder deflection duration.

As a pilot progressed upwards in training and professional responsibilities, the size and complexity of the aircraft increased.

Airplanes got bigger, but preflight inspections got smaller. It’s not that pilots were less interested in the preflight.

It’s that airplanes got to the point where pilots couldn’t really identify defects from just walking around the airplane.

Furthermore, some control surfaces and points of inspection are simply inaccessible from a pilot’s perspective, for example on an aircraft with a high T-tail.

It’s a safe assumption that most Gulfstream G650 pilots have never gazed intently into the intake of a pitot tube like a Cessna 172 pilot does. A preflight on a modern jet aircraft today consists largely of a look for leaks, a verification that nothing ran into the plane overnight, and a check to see that all the covers and chocks are removed.

Modern aircraft maintenance

Today, the irony is that an aircraft mechanic no longer looks like the person of yesteryear. The day of greasy overalls and a tool kit in hand has given way to polo shirts and khaki pants. Instead of a tool belt, a flashlight, and a mirror, the mechanic – for the most part now called a technician – comes to the aircraft equipped with an iPad, computer, and cellphone.

Systemic knowledge is no longer the norm. This is true for pilots as well. An intimate knowledge of the physical dynamics (for instance, which valves should open and when) has been replaced with line replaceable units (LRUs).

Flip the toggle or turn the switch labeled maintenance and the avionics suite, such as the Collins Pro Line Fusion, provides a comprehensive, integrated diagnostic readout of what the problem is. Rather than swapping a mechanical part, a technician often swaps out an LRU or hits reset to solve the issue.

Modern maintenance is about electrons, not screwdrivers and safety wire. And those electrons can be monitored in the form of a health management unit (HMU), which adds a whole new dimension to aircraft maintenance.

The foundations of HMUs date back to the space program in the early 1980s. At that time, the goal of the program was to ensure mission objectives were met and the spacecraft was protected from failures that could result in a loss of the craft or its occupants.

HMU and artificial intelligence

HMU technology advancement has been correlated consistently with advances in artificial intelligence (AI). In its early years, AI was dubbed “machine learning,” and the models were very spartan compared to today.

In the 1990s, the aerospace industry embraced the concept of an HMU for transport-category aircraft. The Boeing 747 was the first to use a centralized maintenance computer (CMC) that could diagnose the health of multiple LRUs. Technically, these were AI-based, as they applied rule-based logic. Subsequent improvements incorporated model-based failure propagational algorithms, which were embedded into the Boeing 777.

In the 2000s, Honeywell created the first modular system that used a data-driven approach for diagnostics. The Honeywell Primus Epic Aircraft Diagnostic System (ADMS) contained a database that could be accessed by maintenance technicians and that could be used to streamline maintenance along with historical maintenance records.

The goal of any HMU is to reduce MRO costs and increase aircraft utilization. Said another way, the HMU aims to avoid an aircraft on ground (AOG) scenario. In the long run, it’s far better to replace a part or component at the corporate hangar before it fails. It’s also more cost-effective when done under the supervision of one’s own maintenance team.

Predicting failure

The most salient advantage of the HMU is predictability of failure. If something breaks, a pilot reports it to the maintenance department. This is called a reactive response. But the HMU has the capability to record data prior to the actual failure.

Honeywell is a pioneer in diagnostic systems. It created a modular system in the 2000s
that featured a database for maintenance technicians to streamline repairs using historical records.

Through data analysis or comparison with known models, the system identifies when the item is likely to fail.

Failures can begin so subtly that pilots may not even be aware that a component is close to failure or is working intermittently.

HMUs can detect anomalies with pinpoint accuracy, exceeding a pilot’s capacity to detect them. For instance, transient voltages at specific LED bulbs at specific seats in the cabin can be detected, recorded, and analyzed. It’s that precise.

There’s another benefit to HMUs that relates to how pilots communicate with maintenance personnel when discussing anomalies or conferring on write-ups. On one end of the spectrum are pilots who never write anything up, no matter how bad the condition is.

Then there are pilots who write everything up, no matter how good its condition might be, and regardless of whether the situation could be a fluke, transitory, or intermittent. Most pilots are somewhere in the middle and use good judgment to determine whether something needs corrective action from the maintenance team.

Even so, there’s wide variability in the write-up itself. Saying something is “broken” or “inoperative” really doesn’t help much with determining root cause. And, from a technical standpoint, those terms are diagnostic, which is out of the pilot’s purview.

(L) Textron’s Aircraft Recording System (AReS), installed on Citation jets, offers wiring diagrams, visual troubleshooting, and flight data monitoring, linking to GE’s C-FOQA service for analysis. (R) Garmin PlaneSync enables pilots to share flight logs, engine health, maintenance updates, and flight analysis with third-party maintenance providers for in-depth advice.

But the maintenance department wants the facts. What was indicated (on the EICAS, for instance), when was it noticed, and under what conditions? A write up of “RIGHT PACK not working” when translated into mechanic speak may be better written as, “after gear retraction just after liftoff, PACK FAIL appeared in amber on EICAS. Reset pack per checklist.

PACK FAIL reappeared and remained illuminated.” Not only will an HMU provide all this information in terms the technician can understand, but it will identify the precursors to why the pack failed, like an overpressure or overheat condition.

The HMU is a great pilot/mechanic interface, and one that eliminates subjective comments in favor of objective actionable items. It records a history of things that don’t ever get written up, and gives the maintenance team the tools to analyze things that always get written up and to look for trends that indicate worsening conditions.

(As an example: Is it really that bad? Or is Captain DeMarco – known as “No-Go DeMarco” because he looks for reasons not to fly – not squawking deficiencies appropriately?)

Keep in mind that the HMU does not record flight parameters in the same way that a quick access recorder (QAR) would. The QAR is used to deliver flight operational quality assurance (FOQA) data regarding things like where the aircraft touched down and at what speed. This, in turn, is used to measure compliance with standards and best practices.

However, the HMU can (and does) detect and record flight control inputs and the duration of those inputs. In my case, a post-flight review of the maintenance page of the Collins Pro Line Fusion avionics suite showed that the airplane recorded exactly when and for how long rudder input was applied during a crosswind landing.

Of course, the HMU has no way of knowing that the airplane was landing in a crosswind. It only annotated the deflection without “judgment.”

An HMU is not designed to critique pilot technique, but it’s still capable of determining the fact that the brakes on the left main are getting replaced faster than those on the right side is because a certain captain tends to press harder on one pedal than another. Big Brother is always seemingly watching in a modern airplane.

Popular HMUs

One of the more popular HMUs is produced by General Electric (GE). GE calls it the Integrated Vehicle Health Management (IVHM) system.

IVHM dates to 2011, when GE received a contract to install it in the Gulfstream G650 as part of the PlaneConnect health and trend monitoring system. To date, the GE system has been installed on 400-plus aircraft across 8 aircraft types. The system can be configured to parameters specified by the customer (up to 10,000) and has a storage capacity of 115 Gb, which equates to 200 flight hours.

From a technical perspective, the unit weighs 4.3 lb and incorporates health data recording, a maintenance communications manager, a data load and file server, and a remote parameter display.

Supported interfaces include Ethernet, ARINC 429, 717, and 664, and RSS-422 ports. Technicians can download data physically from the aircraft to a laptop using the Ethernet connection, but the unit also has the capability to transmit through satcom and cellular networks.

The Textron version of an HMU is the Aircraft Recording System (AReS). The system is installed on Citation jets that range from the CJ1+ to the Longitude and Citation X+. Local access is possible with a laptop.

Textron recommends using a dedicated laptop running Windows for local access and download, and points out that anti-virus, pop-up blockers, and VPNs should be turned off for accessibility. A recent user video produced by Cessna also points out that the system prefers older laptops.

A nice feature of the system is that it can display wiring diagrams. When combined with the squawk, this provides visual troubleshooting. The most recent production version of AReS is the 3, but version 2 exists in a multitude of aircraft.

A June 2025 press release from Textron announced additional flight data monitoring capabilities for aircraft equipped with an AReS system.  Users can now use Textron’s proprietary data-reporting ecosystem to link to GE’s C-FOQA service. Again, all aircraft data can be highly correlated and used for multiple purposes.

Bombardier uses the GE HMS as the backbone of the monitoring system but uses a proprietary product called the Smart Link Plus as the user interface. The Global 7500 and Challenger 3500 are delivered with the interface installed, and “classic” Bombardier aircraft are eligible for retrofit.

Access occurs by downloading the myMaintenance app, which is connected to the Internet via a personal device. Technicians can convert aircraft data into actionable procedures in real time to enable what Bombardier describes as “faster data-driven maintenance decisions.”

Nearly all Garmin-equipped aircraft have some sort of embedded monitoring system that can be used to transfer flight data for maintenance purposes. The Garmin PlaneSync service enables pilots to share flight logs and data (including engine health, maintenance updates, and flight analysis) with 3rd parties who can provide in-depth maintenance advice.

Conclusion

One of the challenges that exist between pilots and maintenance technicians concerns the implicit trust between them. Given everything that can go wrong with, say, a toaster, it’s a wonder anyone gets on a modern aircraft with its thousands of unseen moving parts.

Historically, we’ve had to wait for those parts to break before we do something about them.

As pilots, we’ve been on the reactive end of those consequences, but HMUs and data collection offer operational efficiencies and provide another layer of defense against catastrophic failure and undesirable aircraft states.