Roots of Reliability-Centered Maintenance

February 11th, 2014 by Mike Busch

Last month, I discussed the pioneering WWII-era work of the eminent British scientist C.H. Waddington, who discovered that the scheduled preventive maintenance (PM) being performed on RAF B-24 bombers was actually doing more harm than good, and that drastically cutting back on such PM resulted in spectacular improvement in dispatch reliability of those aircraft. Two decades later, a pair of brilliant American engineers at United Airlines—Stan Nowlan and Howard Heap—independently rediscovered the utter wrongheadedness of traditional scheduled PM, and took things to the next level by formulating a rigorous engineering methodology for creating an optimal maintenance program to maximize safety and dispatch reliability while minimizing cost and downtime. Their approach became known as “Reliability-Centered Maintenance” (RCM), and revolutionized the way maintenance is done in the airline industry, military aviation, high-end bizjets, space flight, and numerous non-aviation applications from nuclear power plants to auto factories.

RCM wear-out curve

The traditional approach to PM assumes that most components start out reliable, and then at some point start becoming unreliable as they age

The “useful life” fallacy

Nowlan and Heap showed the fallacy of two fundamental principles underlying traditional scheduled PM:

  • Components start off being reliable, but their reliability deteriorates with age.
  • The useful life of components can be established statistically, so components can be retired or overhauled before they fail.

It turns out that both of these principles are wrong. To quote Nowlan and Heap:

“One of the underlying assumptions of maintenance theory has always been that there is a fundamental cause-and-effect relationship between scheduled maintenance and operating reliability. This assumption was based on the intuitive belief that because mechanical parts wear out, the reliability of any equipment is directly related to operating age. It therefore followed that the more frequently equipment was overhauled, the better protected it was against the likelihood of failure. The only problem was in determining what age limit was necessary to assure reliable operation. “In the case of aircraft it was also commonly assumed that all reliability problems were directly related to operating safety. Over the years, however, it was found that many types of failures could not be prevented no matter how intensive the maintenance activities. [Aircraft] designers were able to cope with this problem, not by preventing failures, but by preventing such failures from affecting safety. In most aircraft essential functions are protected by redundancy features which ensure that, in the event of a failure, the necessary function will still be available from some other source.

RCM six curves

RCM researchers found that only 2% of aircraft components have failures that are predominantly age-related (curve B), and that 68% have failures that are primarily infant mortality (curve F).

“Despite the time-honored belief that reliability was directly related to the intervals between scheduled overhauls, searching studies based on actuarial analysis of failure data suggested that the traditional hard-time policies were, apart from their expense, ineffective in controlling failure rates. This was not because the intervals were not short enough, and surely not because the tear down inspections were not sufficiently thorough. Rather, it was because, contrary to expectations, for many items the likelihood of failure did not in fact increase with increasing age. Consequently a maintenance policy based exclusively on some maximum operating age would, no matter what the age limit, have little or no effect on the failure rate.”

[F. Stanley Nowlan and Howard F. Heap, “Reliability-Centered Maintenance” 1978, DoD Report Number AD-A066579.]

Winning the war by picking our battles

FMEAAnother traditional maintenance fallacy was the intuitive notion that aircraft component failures are dangerous and need to be prevented through PM. A major focus of RCM was to identify the ways that various components fail, and then evaluate the frequency and consequences of those failures. This is known as “Failure Modes and Effects Analysis” (FMEA). Researchers found that while certain failure modes have serious consequences that can compromise safety (e.g., a cracked wing spar), the overwhelming majority of component failures have no safety impact and have consequences that are quite acceptable (e.g., a failed #2 comm radio or #3 hydraulic pump). Under the RCM philosophy, it makes no sense whatsoever to perform PM on components whose failure has acceptable consequences; the optimal maintenance approach for such components is simply to leave them alone, wait until they fail, and then replace or repair them when they do. This strategy is known as “run to failure” and is a major tenet of RCM.

A maintenance revolution…

Jet airliner

The 747, DC-10 and L-1011 were the first airliners that had RCM-based maintenance programs.

As a direct result of this research, airline maintenance practices changed radically. RCM-inspired maintenance programs were developed for the Boeing 747, Douglas DC-10 and Lockheed L-1011, and for all subsequent airliners. The contrast with the traditional (pre-RCM) maintenance programs for the Boeing 707 and 727 and Douglas DC-8 was astonishing. The vast majority of component TBOs and life-limits were abandoned in favor of an on-condition approach based on monitoring the actual condition of engines and other components and keeping them in service until their condition demonstrably deteriorated to an unacceptable degree. For example, DC-8 had 339 components with TBOs or life limits, whereas the DC-10 had only seven—and none of them were engines. (Research showed clearly that overhauling engines at a specific TBO didn’t make them safer, and actually did the opposite.) In addition, the amount of scheduled maintenance was drastically reduced. For example, the DC-8 maintenance program required 4,000,000 labor hours of major structural inspections during the aircraft’s first 20,000 hours in service, while the 747 maintenance program called for only 66,000 labor hours, a reduction of nearly two orders of magnitude.

Greybeard AMTs.

Owner-flown GA, particularly piston GA, is the only remaining segment of aviation that does things the bad old-fashioned way.

Of course, these changes saved the airlines a king’s ransom in reduced maintenance costs and scheduled downtime. At the same time, the airplanes had far fewer maintenance squawks and much better dispatch reliability. (This was the same phenomenon that the RAF experienced during WWII when they followed Waddington’s advice to slash scheduled PM.)

…that hasnt yet reached piston GA

Today, there’s only one segment of aviation that has NOT adopted the enlightened RCM approach to maintenance, and still does scheduled PM the bad old-fashioned way. Sadly, that segment is owner-flown GA—particularly piston GA—at the bottom of the aviation food chain where a lot of us hang out. I’ll offer some thoughts about that next month.

Mike Busch

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 7,500-plus hour pilot and CFI, an aircraft owner for 45 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviator. Mike's book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from Amazon.com in paperback and Kindle versions.

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The opinions expressed by the bloggers do not reflect AOPA’s position on any topic.

  • Robert Mudd

    I agree with Mr. Bush. I have attended his talks at AirVenture and just listened to web seminar of his last week. He is a great source of information.

    The most popular engine in GA today is the Rotax 912/914 series of engines. Rotax mandates time based maintenance, 5 year Rubber parts change, no matter the flight hours, etc. As long as manufactures do this the RCM approach is dead. Who is going to convince manufacturers that RCM is the way to go?

    Robert Mudd
    A&P I.A.

    • http://www.savvyaviator.com/ Mike Busch

      Lycoming and Continental both state clearly that there are no life-limited parts in any of their engines. I do not deal with any Rotax-powered aircraft in my maintenance practice, but my understanding is that there are no life limits on Rotax engines that are prescribed in an Airworthiness Limitations section of the Rotax AMM or ICA, and only such life limits (or ones mandated by AD) need be complied with by regulation. I could be wrong, but that’s what my Rotax-informed colleagues tell me.

      I am familiar with one horrific situation relating to Rotax engines, and that’s the Rotax installation on the Stemme motorglider. It turns out that Stemme does specify a calendar-time TBO for the Rotax engine on the Stemme motorglider, and specifies it in an Airworthiness Limitations section of the aircraft AMM, which makes it compulsory by regulation. This is extraordinary, because almost all other aircraft manufacturers are silent on the question of engine TBO, deferring such matters to the engine manufacturer (as seems logical). Stemme didn’t do that, and of course this is tragic because the Stemme engines hardly ever run (it’s a motorGLIDER after all), so owners are facing mandatory overhauls at 12 years with only 100 or so hours time-in-service on the engine. Now that’s genuinely idiotic.

  • Larry Stencel

    As a long time A&P, pilot and GA aircraft owner retired from both the USAF and a major airplane manufacturer … who takes care of his own airplanes, I totally agree with BOTH Mike Busch’s article AND the comment — above — by Robert Mudd. I have owned a ’75 C172 for almost 30 years but — for varying reasons — don’t fly it nearly enough. That said … every year … year after year … I have to take the thing apart and find very little to worry about. Still, to satisfy both the requirements AND the IA’s I work with … gotta do it.

    The C172 is one of the safest airplanes around and most people never consider that about 1 in 10 GA airplanes ever built is a Skyhawk or it’s C170 cousin. Aside from ensuring that the engine gets a good look/see annually, MOST — but not all — C172s don’t need to be taken apart every year and they DO suffer from this MO of maintenance. I would MUCH rather see an annual engine inspection — by an A&P — and a five year major look/see approach by an IA … much like the RCM model above. Skyhawks are NOT falling out of the sky for maintenance related issues. In MY case, my airplanes are hangared which is not considered in the current maintenance model either … but ought to be.

    The bad news is that the FAA is living a myopic existence stuck in a time warp. GA is dying from a thousand razor blade cuts and they’re trying to manage GA as though it’s a part 121 or part 135 operation … which it isn’t. We could commiserate all day long about the FAA but unless and until THEY get their collective acts together and decide that proactivity is the only thing that’s going to save GA, aviation in the US will continue its decline and inevitable demise. Little Chinese pilots will be flying our airliners if we ain’t careful because the FAA will have driven all (aging) US pilots into extinction.

    Why couldn’t we compromise? Why couldn’t the FAA run a test on — say — 500 GA airplanes … 10 in each State. Why couldn’t we try the above idea and see where it takes us in about five years. Well … that makes too much sense SO … it won’t happen. Besides, the GA airplane manufacturers have no vested interest in such an idea. That said, I’d be willing to bet big bucks that such a system could work AND the test would substantiate same.

    The idea DOES have merit … it DOES make sense … so, it ain’t gonna “fly.” Pardon the pun.

    LARRY STENCEL
    Palm Coast, FL

    • joe

      Excellent analysis by Mike and Larry! Changes from the FAA won’t happen until someone, or some group (AOPA, EAA, Etc) beat them up to the point that they finally remember to SERVE the aviation community, and not DICTATE to the aviation community. Fix the culture at the FAA, and our industry could flourish! -joe

    • http://www.savvyaviator.com/ Mike Busch

      “I would MUCH rather see an annual engine inspection — by an A&P — and a five year major look/see approach by an IA I would MUCH rather see an annual engine inspection — by an A&P — and a five year major look/see approach by an IA.”

      Well, I don’t think I’d go quite so far, because there are some airframe items on a 172 that need to be inspected more often than once every five years — things like tires, wheels, brakes, flap rollers, seat tracks, etc. — but I’ll agree wholeheartedly that performing a disassembly inspection every 12 months on a low-utilization owner-flown Skyhawk is gross overkill. Our little airplanes really should have phased inspection programs like the big boys do, where some things get inspected more often than other things.

  • http://www.summitaviation.com Jon Woellhaf

    Right on, Mike! As always.

  • Pete Whelan

    As someone who supported himself by working on automobiles for decades I agree with the RCM concept 98%. After all the first rule of mechanics is ‘if it ain’t broke, don’t fix it’. Or, possibly the corollary, ‘if it ain’t broke, fix it till it is’. I have certainly had my share of annual inspection related failures/breakages, etc. to the point that after an annual, I would devote a Saturday to the post annual pre-flight inspection.
    The 2% I would take exception to is if there were no such thing as PM, mechanics would starve. The other issue is, down at the bottom end where the piston engines live, some things like oil changes and spark plug reconditioning are pretty necessary at predictable intervals.
    That said, excellent article.

    • http://www.savvyaviator.com/ Mike Busch

      “The 2% I would take exception to is if there were no such thing as PM, mechanics would starve.”

      No, I disagree. In the RCM-inspired maintenance programs I design for my managed-maintenance clients who fly piston GA airplanes, we deemphasize scheduled PM but we compensate for that by doing a lot more inspection and condition monitoring. And of course we still do some scheduled PM like 50-hour oil changes, 500-hour magneto teardown inspections, and annual wheel bearing lubrication (to name a few).

  • Sam

    Mr. Bush,

    In my opnion the FAA like other government agencies and have become “self serving” orginizations. The public is not in ther perview anymore. Not only is GA aviation lost with them, try to get something done on the Aviation Medical side of the house. Some review boards only meet every sixty days and the poor and I do mean poor pilot has to go without work while waiting. Not a problen the the FAA they get paid no matter what.

    I do agree with you and the others but once a year to do an inspection is a smart thing to do. Some owner operators will not spend a dime on maintenance if they don’t have to and do not peform any maintenance theirselves.

    • http://www.savvyaviator.com/ Mike Busch

      Do we really need to design a maintenance program around that “lowest common denominator” operator of whom you speak, Sam? Doesn’t that inappropriately penalize the majority of operators who are responsible?

      Unfortunately, there a great deal of lowest-common-denominator thinking behind the manufacturer’s maintenance recommendations in this segment of aviation. The manufacturer’s don’t trust the maintainers, and therefore seek to take all the judgment calls away from them by “cookbooking” everything. This is the antithesis of condition-based maintenance.

      The FAA generally just follows the lead of the manufacturers.

      The only folks who have incentive to drag GA maintenance kicking and screaming into the 21st Century are the aircraft owners and operators. But they are a disjoint and disorganized group who have no leverage unless they band together and get on the same page. As a group, owners have enormous power to vote with their feet and their credit cards. Individually, they have very little leverage.

  • http://Ellsaviation.com Steve Ells

    I like Mike’s research, his facile brain, and his clear writing. And I’m sure RCM is the way to go. I’m really looking forward to his suggestions on how our/my industry should/can implement the practice. Please address how to change the FAA, the mindset of technicians who will without a doubt, if there’s a fatality following work they’ve performed will answer the “Did you follow the airframe/engine manufacturer’s service requirements,” in court, and how each shop/ technician will acquire the knowledge base required to focus in on the weak points, for lack of a better term, on each airframe/ engine in the GA fleet.

    • http://www.savvyaviator.com/ Mike Busch

      Steve, fortunately we don’t really need to change the FAA. Unlike the CAAs in most other countries, the FAA gives Part 91 (non-commercial) operators enormous latitude in how to maintain their aircraft. The FAA has consistently taken the position that manufacturer-prescribed inspection, overhaul and replacement intervals are mere suggestions and that compliance is not required by regulation. So I think we already have most of the regulatory wiggle room we need.

      Changing the mindset of technicians is an extremely difficult problem. As you point out, today’s AMTs often seem more concerned with fears of civil liability than with safety and regulatory compliance. And in this nation’s wildly litigious environment (which makes us the laughingstock of the world), who can blame them?

      Of course, I deal with this issue every day because I’m the guy who is constantly advising my managed-maintenance clients to do things the RCM way rather than the manufacturer’s prescribed way (and doing it very vocally and visibly). I believe that there are two was of dealing with the risk of civil liability: (1) avoid liability by “playing it safe” or (2) insuring against liability and then “doing the right thing” without fear. I choose the latter approach in my practice.

  • http://Gordianknotandordertheinsanitytostop.AOPAandtheCongressionalGeneralAviationCaucus-areyoulistening? Duane

    Great post and concepts. Of course, the obvious reason that private aircraft owners don’t benefit from 1940s and 1950s maintenance innovations (???) in RCM is that we are at the bottom end of the aviation food chain, hence we have the least financial power in the aviation business … and of course, the FAA will never reform itself in any serious manner.

    Which is why Congress has to step in on matters such as the now-mandated FAR Part 23 certification revision law (which FAA will likely do its best to subvert, and which will therefore likely require additional Congressional oversight and action to enforce) … as well as the bill now working its way through Congress to mandate the “drivers license medical” for VFR pilots, with certain limitations.

    It is well past time for aviation organizations such as AOPA and others to convince Congress to order a top to bottom review and reform of all existing FAA regulations affecting general aviation and specifically private, non-commercial aviation. Everything in the FAR needs to be “sunsetted” and justified and overhauled and brought into the 21st century, enforced by significant outside oversight of FAA. That includes not only aircraft certification (now underway), but also airworthiness-related maintenance rules, airman certification and training, and flight rules.

    We are still trying to operate a 21st century world of aeronautics with 1930s rules, and there are just way too many shortcomings in the present regime that simply cannot be patched up effectively.

    We all know intuitively that Mike’s “Curve F” applies much more frequently to aircraft parts and systems than does “Curve D” … that an engine just out of the overhaul shop is much more likely to fail than one with 500 hours on it … or even one with 2,000 hours on it. Annual (calendar-based) inspections should be replaced with on-condition analyses for redundant parts and systems, and operating-hour-based inspections and replacements for non-redundant systems. Much of what’s required to be inspected every year is just silly, such as looking for internal airframe corrosion. Ditto for 24-month IFR-certifications and transponder checks. Your radios either work well, or it’s pretty obvious (to you and to ATC) that they don’t. Will such checks even be relevant in the age of ADS-B?

    As for new aircraft designs, it should not be difficult to develop new piston aircraft engines that would run virtually trouble free many more hours than the standard 1,800-2,000 hour TBO we’ve had for 60-70 years. After all, 40 years ago few automobile engines made it to 100,000 miles without a major overhaul, while today’s high tech engines frequently go hundreds of thousands of miles with only minor maintenance.

    But of course FAA certification is a huge and expensive barrier for engine manufacturers that clearly stands in the way of new engine designs (along with low demand … i.e., the relatively small numbers of aircraft in the fleet). New tech engines mimicking much of the technology developed in the marine and automotive worlds of the last 50 years would not only be more reliable (i.e., electronic computerized engine monitoring and controls, or as we in aviation call them, FADEC), but modern electronic instrumentation could provide a great deal more engine operating data … data that the pilot would not necessarily need to monitor in flight, but which would provide trending and alarm indications (such as for low compression, high temps, improper fuel mixture, etc.) and would be easily downloaded for ground analysis by a mechanic (perhaps via a WiFi connection to the internet!), much as modern car mechanics grab a download from the now-ubiquitous automotive computers.

    But again, we’re all stuck in a regulatory (and economic) mindset based upon 1930s technology.

    Somebody has to cut the Gordian knot here and get Congress to order a top to bottom revision of all FAA rules affecting GA.

    • http://www.savvyaviator.com/ Mike Busch

      Duane, see my previous comment to Steve Ells. I don’t think we’re really stuck in a regulatory mindset, because at least for non-commercial Part 91 operators, the current FARs give us enormous flexibility to design our own maintenance programs and ignore the recommendations of the manufacturers if we so choose. The real sticking point is the “defensive maintenance” mindset of aviation maintenance technicians and repair stations who are scared to death of being sued if they depart from manufacturer-prescribed inspection, overhaul and replacement intervals (even though the FAA says it’s okay to do that). This is exactly the same problem we face in “defensive medicine” where medical practitioners overprescribe tests not because they believe they’re necessary but because they’re afraid of a malpractice suit if they don’t cover every possible base.

  • Jon Carlson

    Love the concept, believe in it. But I am bound by regulation to follow the manufacturer’s inspection program (twin turbine, see 91.409f(4)), and on an older airframe they have zero – actually less than zero – incentive to make any real changes.

    This would need to be carried out by changes to the FAR maintenance requirements in order to have any kind of real effect. Fingers crossed that can happen, I just hope it is sometime in my lifetime (and I am not old!!).

    • http://www.savvyaviator.com/ Mike Busch

      Indeed, Jon, that is why I explicitly decline to accept multiengine turbine airplanes and large airplanes > 12,500 lbs. MGTOW into my managed maintenance practice. 91.409 doesn’t provide adequate wiggle room once those dotted lines are crossed. But for anything from an RV8 to a PC-12, we can do pretty much anything we want. So that’s the sandbox I choose to play in.

      I’m really excited about the Cirrus SF-50 Vision Jet precisely because it will have just one engine and so from a regulatory standpoint it’s just like a Cessna 172.

  • Larry Stencel

    ALL of the comments seem to agree with Mike. ALL of the comments seem to commiserate the “ways” of the FAA. ALL of us … pilots, mechanics and aircraft owners need to band together under one of the alphabet soup organizations and start challenging the FAA in court. Just last night, after I put my two cents into this blog, I wrote an email to my Representative thanking him for his support of HR3578 (forcing the FAA to use the Regulatory process on the sleep apnea issue) and asking for his support of HR3578, The GA Pilot Protection Act of 2013. How sad that Congress has to initiate Bills to ‘throttle’ the FAA from its most active constituents.

    On Feb 5, 2014, the Aviation Subcommittee of the Transportation and Infrastructure Committee held a hearing on “The FAA Modernization and Reform Act of 2012: Two Years Later.” That Act is Public Law 112-95. Contained in the Hearing notice is a partial list of review action items. It is astonishing to see that a great number of these items are either incomplete or only partially complete. The FAA was previously operating under continuing resolutions but the Reform Act gave the FAA a predictable funding period. They previously “hid” behind funding as something holding them back. NOW … two years later … they’re STILL not anywhere close to completing the tasks they were assigned. Michael Huerta, Administrator of the FAA; Calvin Scovell, DOT IG; and Gerald Dillingham, Aviation Director of the GAO testified. Seems to me that all they provided was still more hot air to temporarily satiate the Congress.

    Seems to ME that the ONLY way we’re ever going get command of the FAA is to have the Congress cut off it’s funding … period. Perform OR you get no money, Michael.

    That the good Mr Huerta apologized to the AOPA for not acting on the AOPA/EAA request for a waiver to the requirement for the third class medical for SOME categories of pilots and flight. With almost 10 years of performance measuring data on the Sport Pilot area, what more do these people need?

    I realize this is partially off topic of the RCM maintenance model but — in actuality — it has a common denominator … an intransigent FAA who has no vested interest in changing anything. ONLY defunding will get their attention.

    I’d urge all interested individuals to start becoming active contacting their Representatives, et al, and trying to force same.

    With respect to the RCM discussion, I fly a 150HP engine in a 1967 PA28 that is the ORIGINAL engine with 2150 hours TT. When I think about rebuilding it I think … WHY? It’s not pumping oil, it’s using little oil, always has great compression and I know the parts got “married” long ago and are happy. Why break up a happy ‘marriage’ just to say the engine has low TSMO?
    Sounds like the early part of Curve B to me and I’m not doing anything to it except keep its accessories in tip top shape.

    • http://www.savvyaviator.com/ Mike Busch

      Per my previous comments, I don’t really have a big problem with the FAA, particularly for piston and single-engine turbine aircraft. The FARs as they now stand give us enormous flexibility to follow or ignore the manufacturer-specified maintenance programs. With respect to modernizing maintenance practices for owner-flown GA, we should focus on the maintenance providers, not the FAA.

      As for the sleep apnea business and the whole third-class medical mess, that’s a whole other kettle of fish. I’ll let others bloviate about those things and keep my focus on maintenance.

  • Chris Cuneo

    I maintain a Gulfstream G550, with a chapter 5 maintenance program based on a MSG-3 (maintenance steering group) program. They take reliability data and adjust the maintenance intervals accordingly. At least, that’s the theory. In our case, it does not work. If I did not know better, I’d guess the manufacturer is making money on these “too frequent” maintenance requirements. Some of which can only be performed by the manufacturer.

    Our aircraft is now 10 years old, less than 3000 hours total time and has been disgustingly over-maintained. The most obvious example: every one of the $100 Ti screws has been rounded out by hack Gulfstream mechanics. Only to find nothing wrong behind each panel and under each floorboard. The maintenance induced damage is considerable in some areas. Sure, it won’t affect safety of flight to have carbon fiber floorboards pried up with a screwdriver and re-installed filthy with glue residue loaded with dirt all over it. But, that’s not how it was built, and that’s not what I expect. We could put the blame on individual mechanics trying to get the job done quickly. Or, place the blame where it actually belongs. With corporate management and the FAA requiring excessive maintenance. Some years we fly less than 200 hours. Why should we have to disassemble an aircraft after such a short intervall, when they clearly last 40+ years.

    We now have a solid 100 years of maintenance data. Folks, sufficient data exists and we need to use that data to improve reliability and safety.

    • http://www.savvyaviator.com/ Mike Busch

      Why isn’t the Gulfstream owners association (there must be one, right?) beating up on Gulfstream about this?

      That’s how it’s always worked in the air carrier industry; the aircraft owners need to beat up on the manufacturers.

  • http://www.AircraftMaint.com Dan MacDonald

    I agree, to a degree.

    Many GA planes are already on the “fly until it breaks” plan, which can leave an owner stranded at a rural airport on Saturday night with a dead alternator – or worse, a broken landing gear down lock rod end resulting in a collapsed nose gear!

    Although arbitrary overhaul times make little sense, a reliable method is needed to determine when critical parts are excessively worn or near failure. Low-end GA aircraft do not have the redundancy of airliners, and some parts are single points of failure.

    I am a proponent of IRAN, “Inspect and Repair As Necessary”, for many components to prevent AOG situations, especially engine accessories, electric motors, and landing gear bearings/bushings/rod ends.

    For more information, see http://www.AircraftMaint.com

    • http://www.savvyaviator.com/ Mike Busch

      Of course, IRAN is at the very core of RCM. The emphasis shifts from schedule PM to condition monitoring (which is the “I” in IRAN). Then when the I reveals that condition has deteriorated to an unacceptable extent, the RAN kicks in.

      (For those who need a decoder ring, “IRAN” stands for “Inspect and Repair As Necessary” and is a term-of-art in the maintenance field, along with “overhaul” and “rebuilt.”)

  • Keith Wood

    You can wear out parts with tools long before they would wear out from use.

    • http://www.savvyaviator.com/ Mike Busch

      “Fools with tools are still fools, but they can do more damage.”

  • http://www.livingreliability.com/en/ Murray Wiseman
    • http://www.savvyaviator.com/ Mike Busch

      Murray, that’s an excellent essay. What the author refers to as “suspended” data is what Nowlan & Heap referred to as “the Resnikoff Conundrum.”

      The Resnikoff Conundrum states that in order to collect failure data, there must be equipment failures, but failures of critical items such as engines is considered unacceptable because such failures can cause injury and death. This means that the maintenance program for a critical item must be designed without the benefit of failure data which the program is meant to avoid.

      What the essay doesn’t address is “the proprietary data conundrum” which states than the best failure data is possessed by the aircraft and engine manufacturers, but those manufacturers refuse to make that data available for third-party analysis for fear that it will be used in lawsuits against them. This is just one of the many adverse effects of the insanely litigious environment we have in the US aviation industry.

      • Murray

        Mike, the Living RCM (LRCM) process, wherein RCM thinking guides the work order documentation process, gets around the Resnikoff Conundrum. LRCM requires that standards govern the distinction between suspension and potential failure. When a technician reports potential failure it is because a functional failure is imminent (hours or days away). Otherwise, if there is still an indefinite amount of life in the item (part, component, assembly) it is a suspension. We build our prediction models, therefore, upon the experience of potential rather than functional failure, thereby circumventing Resinikov. The advantage of reporting suspensions as such and not mistakenly as failures (which is almost always done in most industries) is that the software algorithm will then account for the uncertainty of when it would have failed.

        • Murray

          On your second issue, Mike, the operators can get as good and even *better* data than the manufacturers’ data, if they introduce some simple work order information practices. First, require the component re-builder to provide specific as-found and replacement information, namely: 1) What parts were replaced, 2)What was the condition of the replaced parts (failed?, or suspended?). If the component comes back to the same client, his software will continue clocking hours on the un-replaced parts and good data will be available for subsequent analysis. If the component goes to some other client, that’s a bit of a problem. It will reduce the performance of the operator’s predictive models since the entire serialized component will have to be considered as-good-as-new, which is not necessarily accurate. Clients must get together and easily solve this problem by exchanging data on the components they send to the rebuild shop.

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