Predictive Maintenance 101

Danny Green with Tim Hobgood  —  Contributing Authors

I can’t figure out why people look at me so funny when they ask me what I do for a living and I tell them I’m a vibration analyst. When I explain (in laymen’s terms) exactly what it is that I do and why I do it, it tends to make sense. Either that or they’re just being polite or bored and trying to get away as quickly as possible.

What is an aviation vibration analyst? What does one do and why does one do it? I’ll do my best to justify my existence.

Simply put, aircraft are equipped with vibration health monitoring systems (VHMS). Vibration data is monitored by sensors installed at specific locations throughout the airframe, engines and gearboxes. During the aircraft’s flight, the data is collected, stored in a “black box” for later download by the maintenance specialist, then sent to the analyst for review.  The analyst’s goal is to find something wearing out, identify it and have it repaired or replaced before it actually does.


While this sounds great, it also sounds expensive and time consuming, not to mention more personnel and equipment to purchase and maintain! Where is the justification? 

First, depending on whom you are flying for or where you’re flying, it may be a contract or regulatory requirement, but we’ll discuss that later. Secondly and most importantly is a concept that is not used enough in aviation: “predictive maintenance” or a “predictive maintenance program.” Predictive maintenance programs are common in industry and manufacturing and have been for a long time ­— but in aviation, it is still a fledgling concept. After talking to some manufacturing people at a vibration training class I attended (only two aviation people in the entire class including myself), it made perfect sense. Having an unplanned shut down of an assembly line or plant due to a worn out pump, a bad bearing, a misaligned shaft or a failed compressor could be catastrophic, have safety implications, cost millions in lost production, labor and sales. This is obvious justification for a maintenance program that monitors the health of these components on a regularly-scheduled basis, identifies changes in their condition prior to failure and allows scheduled component changes in advance of failure on scheduled shutdown or during regularly scheduled maintenance. Think of the production benefits, safety benefits, labor savings, ease of parts scheduling and reduction in operating expenses. With a program like this, it makes perfect sense, right?

Why not aviation? We have a multi-million dollar helicopter worth several thousand dollars per hour to the customer and the lives of nine passengers and two crew members flying 150 miles offshore. While it’s not doggie kibbles, diapers or widgets on an assembly line, I think it’s still pretty important and might warrant, at minimum, the same attention. 

While I jest in my example, I try to emphasize the importance of this program.  I understand as an A&P/IA that in aviation, we have preventative maintenance programs, scheduled inspections, ADs, SBs, CEBs, CCRs, RIIs, time life parts, the FAA and the NTSB — what more could we possibly need? While these are excellent and certainly time-proven tools that continue to be the cornerstones of our industry, it is imperative that we take advantage of all of the tools and technologies available to us. 

What is a helicopter but a mass of rotating parts? Rotating parts obviously rotate but in doing so they also vibrate. Rotating components vibrate one way when they’re healthy, another when they’re wearing, still another when they’re misaligned and yet another when they’re loose. Knowing the difference and understanding what they are saying in advance is the basis of the program. Some vibrations are low frequency, such as main rotor head components. Others are medium frequency vibrations (such as main rotor 4/rev) and yet others are high frequency vibrations (such as tail rotor components, drive shafts or some engine components). By knowing a component’s normal frequency/RPM (usually available in Chapter 18 of the maintenance manual) and its amplitude or amount of vibration, monitoring the component is relatively simple given the proper equipment and knowledge.

Equipment Options

While a scheduled program using a portable kit in critical areas i.e. hanger bearings, drive shafts, gearboxes and hydraulic pumps would be considered a predictive maintenance program in industry, for the purpose of this article we are only going to discuss on-board systems such as vibration health monitoring systems (VHMS) and health usage monitoring systems (HUMS).

The first and most critical step to setting up your program would be to determine what equipment is available for your particular aircraft. The equipment may be offered as an add-on kit available from the airframe manufacturer or aftermarket as an STCd kit. Of these offerings, if available, do you want VHMS, HUMS, FDM, FOQA or HOMP?

HUMS, FDM, FOQA, HOMP, CBM, VHMS and VHM indicators are just a few acronyms that help keep you confused. Some terms have evolved in time. You may have been in a conversation that went a little like this: “Hey, did you hear that the new helicopter we are getting will be equipped with a HUMS? Will it be able to provide FOQA or HOMP data for FDM?“ “No, it’s just going to be a VHM system giving the maintenance specialist VHM indicators when he downloads the system. Yeah, the future is moving toward CBM.” What the heck did they say? Let’s take a look at each one of these to understand this conversation.


Health Usage Monitor System (HUMS)

In the early 1990s, HUMS was a new aviation tool for measuring vibrations of helicopter rotating components. With sensors mounted on transmissions, gear boxes, engines, bearings and high-speed shafts, we are able to measure, analyze and trend the data for increased or decreased vibration.  A big plus is that the systems were able to perform rotor track and balance (RTB) without a maintenance specialist installing or removing sensors and wires, thus saving time. When you used the term HUMS, the maintenance specialist in the field thought vibration monitoring, period. 

Since their introduction, HUMS in helicopter aviation systems have evolved rapidly. HUMS systems are providing much more than just vibration today. Systems are now being tied into aircraft and engine computers, global position indicators, collecting engine parameters, airspeed, altitude and acceleration, all in an effort to improve safety of flight. From my readings we will very soon be able provide oil particle detection!


Flight Data Monitoring (FDM), Flight Operations Quality Assurance (FOQA) and Helicopter Operations Monitoring Program (HOMP)

What do these three have in common? The Civil Aviation Authority (CAA), in Civil Aviation Publication 739 (CAP739), defines FDM as a systematic, pro-active and non-punitive use of flight data from routine operations to improve aviation safety.  The Federal Aviation Administration (FAA) defines FOQA as a voluntary safety program designed to improve aviation safety through the proactive use of flight-recorded data. Operators will use the data to identify and correct deficiencies in all areas of flight operations. Properly used, data can reduce or eliminate safety risks, as well as minimize deviations from regulations.  Defining HOMP, it is a term that is used in conjunction with both FDM and FOQA. In laymen’s terms, all three are basically the same. Recapping, FDM, FOQA and HOMP are programs that can use HUMS equipment to collect flight operations data from engine start to engine shut down.  The data is then downloaded, reviewed and used to improve aviation safety.

Condition Based Maintenance (CBM)

CBM is a term used with condition monitoring, meaning to perform maintenance when it is required based on one or more monitored indicators. Excessive or increasing vibration in a gear box with metal detected in the oil would warrant maintenance actions. Increasing or high vibrations alone in a monitored high-speed shaft would be cause for maintenance actions. CBM is also used to extend maintenance inspection times and reduce downtime once again by monitoring the health of the aircraft. The old saying “If it ain’t broke, don’t fix it” can be applied to CBM. When you tear down, remove and replace components you are actually contributing to wear and tear. As monitoring equipment becomes better, the possibility of replacing time change components may be a thing of the past.


Vibration Health Monitoring Systems (VHMS)

VHMS are strictly installed on helicopters to measure vibration. By using sensors installed in strategic locations and directions, the system measures the amount of vibration rotating components produce. The vibration data is analyzed and statistical alerts or alarms are created.


Cap 753 Vibration Health Monitoring (VHM)

VHM is the CAA’s guidance for vibration monitoring systems installed on helicopters. It covers VHM design and operational requirements. It also covers VHM indicators which are alerts, either preliminary in nature to allow maintenance to investigate a potential problem, or a warning that the alert is more serious and further flights should be avoided until maintenance actions are taken to correct the problem. Today, CAP 753 is probably the guiding document in the industry with HUMS manufacturers meeting or exceeding the requirements. Hopefully this will help you join in that next conversation either to embrace HUMS or complain about it.

Trending vs. Alerts/Alarms

Whether you are going to install and use HUMS due to contractual requirements or because of regulations, you have to decide how you are going to use it. As stated earlier, VHM programs have indicators or alerts which are statistical levels used to indicate problems, potential or otherwise. These alerts may come from the airframe manufacturer if the VHM unit was purchased as their kit, or generated statistically using algorithms and other mathematical formulas designed by some really smart people tucked away in some dark room if using an STCd kit. Either way, you will probably see something like a go-no-go red, yellow or green light or colored lines on a graph as indicators. The indications may range from good, above goal, caution to exceed. You may also have your fleet average, aircraft average, fleet and aircraft averages, plus three sigma VHM alert, etc. Regardless, you have to decide what you are going to do with the information generated.


While the requirements (contractual or otherwise) may just require personnel to review or download the collected data at specific intervals, check for alerts and or advisories and address as necessary, my experience has shown while this is a “good” practice, it may not be the best. It assumes that all of the levels are correct, both high and low, that your particular operation is comfortable with those levels and that nothing will fail prior to reaching those levels.

Our experience has shown that by increasing the frequency of data downloads (ours are done daily) and reviewing and trending the data, the success of your program will increase exponentially. By comparing the data first to itself looking for suspicious increases, then comparing it against the aircraft’s component average and finally against your fleet’s average (assuming multiple aircraft), potential failures show themselves very early on. Using this technique allows you the benefit of investigating potential failures early and making intelligent decisions in advance, allowing time to line up parts and maintenance on your terms and not the other way around. It is much easier to change a component in the hangar during a scheduled inspection than on an offshore platform while tying up their only heli-deck on break night — or worse, after it’s fished from the Gulf of Mexico and  hopefully in one piece! 

I have numerous examples of rotor head elastomerics that I have seen a week to 10 days before they could be found by visual inspection or by pilot squawk. I have found worn hanger bearings, a brand new tail rotor drive shaft fresh from the manufacturer well out of balance, hydraulic pumps on single hydraulic pump aircraft wearing, and the list goes on and on. They were all caught well below any alert levels but were all well on their way. Is trending an exact science? Not even close. It takes some time to understand what you are seeing and why. It takes a few false alarms and some misses, but even during that growth period it is still a superior way to conduct your program while maximizing your potential.

 HVHM is here to stay. While there currently isn’t an FAA requirement, CAP 753 is driving the requirements in the oil and gas industry worldwide. Hopefully it spills over to other helicopter applications as well. When used properly and to its fullest extent, VHM is an excellent predictive maintenance tool that reduces wear and tear on airframes, avionics and passenger/crew comfort, not to mention the savings in man hours, parts, downtime, leasing costs and most importantly, safety.  


Danny Green
Is an A&P with an IA and the aviation vibration data analyst for Chevron’s Gulf of Mexico aircraft operations located in Picayune, MS. He monitors 16 aircraft daily, including AW139s, Bell 430s and Bell 427s, and soon to be two Bell 429s.  

Timothy N. Hobgood
Is a retired USAF aircraft maintenance C-130 vibration program manager at Kirkland AFB. His current position is as a Honeywell field service engineer supporting ZING HUMS