The Waddington Effect

January 14th, 2014 by Mike Busch
Conrad Hal (C.H.) Waddington

C.H. Waddington (1905-1975)

In 1943, a British scientist named Conrad Hal (C.H.) Waddington made a remarkable discovery about aircraft maintenance.  He was a most unlikely person to make this discovery, because he wasn’t an aeronautical engineer or an aircraft mechanic or even a pilot.  Actually, he was a gifted developmental biologist, paleontologist, geneticist, embryologist, philosopher, poet and painter who wasn’t particularly interested in aviation.  But like many other British scientists at that time, his career was interrupted by the outbreak of the Second World War and he found himself pressed into service with the Royal Air Force (RAF).

Waddington wound up reporting to the RAF Coastal Command, heading up a group of fellow scientists in the Coastal Command Operational Research Section.  Its job was to advise the British military on how it could more effectively combat the threat from German submarines.  In that capacity, Waddington and his colleagues developed a series of astonishing recommendations that defied military conventional wisdom of the time.

For example, the bombers used to hunt and kill U-boats were mostly painted black in order to make them difficult to see.  But Waddington’s group ran a series of experiments that proved that bombers painted white were not spotted by the U-boats until they were 20% closer, resulting in a 30% increase in successful sinkings. Waddington’s group also recommended that the depth charges dropped by the bombers be set to explode at a depth of 25 feet instead of 100 feet.  This recommendation—initially resisted strongly by RAF commanders—ultimately resulted in a sevenfold increase in the number of U-boats destroyed.

Consolidated B-24 "Liberator" bomber

Consolidated B-24 “Liberator” bomber

Waddington subsequently turned his attention to the problem of “force readiness” of the bombers.  The Coastal Command’s B-24 “Liberator” bombers were spending an inordinate amount of time in the maintenance shop instead of hunting U-boats.  In July 1943, the two British Liberator squadrons located at Ballykelly, Northern Ireland, consisted of 40 aircraft, but at any given time only about 20 were flight-ready.  The other aircraft were down for any number of reasons, but mostly undergoing or awaiting maintenance—either scheduled or unscheduled—or waiting for replacement parts.

At that time, conventional wisdom held that if more preventive maintenance were performed on each aircraft, fewer problems would arise and more incipient problems would be caught and fixed—and thus fleet readiness would surely improve. It turned out that conventional wisdom was wrong. It would take C.H. Waddington and his Operational Research team to prove just how wrong.

Waddington and his team started gathering data about the scheduled and unscheduled maintenance of these aircraft, and began crunching and analyzing the numbers.  When he plotted the number of unscheduled aircraft repairs as a function of flight time, Waddington discovered something both unexpected and significant: The number of unscheduled repairs spiked sharply right after each aircraft underwent its regular 50-hour scheduled maintenance, and then declined steadily over time until the next scheduled 50-hour maintenance, at which time they spiked up once again.

Waddington Effect graph

When Waddington examined the plot of this repair data, he concluded that the scheduled maintenance (in Waddington’s own words) “tends to INCREASE breakdowns, and this can only be because it is doing positive harm by disturbing a relatively satisfactory state of affairs. There is no sign that the rate of breakdowns is starting to increase again after 40-50 flying hours when the aircraft is coming due for its next scheduled maintenance.” In other words, the observed pattern of unscheduled repairs demonstrated that the scheduled preventive maintenance was actually doing more harm than good, and that the 50-hour preventive maintenance interval was inappropriately short.

The solution proposed by Waddington’s team—and ultimately accepted by the RAF commanders over the howls of the maintenance personnel—was to increase the time interval between scheduled maintenance cycles, and to eliminate all preventive maintenance tasks that couldn’t be demonstrably proven to be beneficial. Once these recommendations were implemented, the number of effective flying hours of the RAF Coastal Command bomber fleet increased by 60 percent!

Fast forward two decades to the 1960s, when a pair of gifted scientists who worked for United Airlines—aeronautical engineer Stanley Nowlan and mathematician Howard Heap—independently rediscovered these principles in their pioneering research on optimizing maintenance that revolutionized the way maintenance is done in air transport, military aviation, high-end bizjets and many non-aviation industrial applications.  They were almost certainly unaware of the work of C.H. Waddington and his colleagues in Britain in the 1940s because that work remained classified until 1973, when Waddington’s meticulously-kept diary of his wartime research activities was declassified and published.

Next time, I’ll discuss the fascinating work of Nowlan and Heap on what came to be known as “Reliability Centered Maintenance.” But for now, I will leave you with the major takeaway from Waddington’s research during World War II: Maintenance isn’t an inherently good thing (like exercise); it’s a necessary evil (like surgery). We have to do it from time to time, but we sure don’t want to do more than absolutely necessary to keep our aircraft safe and reliable. Doing more maintenance than necessary actually degrades safety and reliability.

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.

  • Jong

    This dovetails into tales of woe when flying aircraft right out of “routine” 50/100/annual.

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

      Indeed! I will be doing a future blog post about post-maintenance test flights. By far the most likely time to have an in-flight mechanical failure is on the first flight after maintenance. If I were FAA Administrator for a day (which ain’t gonna happen), I’d change the regs to require mechanics to go up on the first flight after any maintenance they performed.

      • Mike Shiflett

        Hey Mike…back in 1973 I started a avionics technician job at the old Hangar One facility in Atlanta, Ga. My boss, Chuck McCorvey always told us to make sure we did the job right because we’d be the first to ride in it after the repair. As you can tell, to this day those words never have left my mind.

      • Don

        When my Dad was at Biloxi in 1944 no one flew an aircraft straight from the shop. They had a couple of older vets fly everything first. At my glider club the Chief Pilot flies gliders just assembled or maintained. Of course the greatest argument was the old TV show “The A Team”. The pilot would ask about some piece of junk that George Peppard had dug up for a mission. If George went on about all the things that had been fixed, the pilot would resist: “I don’t like airplanes that have been worked on.” Everyone found that line pretty funny except pilots.

  • John R

    Or, in redneck terms, “If it ain’t broke, don’t fix it!

    I had an A&P/IA replace my wing struts with new sealed units to prevent against “possible” corrosion in the struts. He did a good job until I took off and performed my first takeoff, pattern and landing while maintaining a continuous 30-degree cross-controlled yaw to the left! He had forgotten to re-rig the plane; I nearly re-rigged my keester!

    • Larry G

      Or, as some companies seem to go… “If it ain’t broke, we’ll fix it till it is!”

  • Billy C

    Curious. Just wonder if the data collected included causes of increased unscheduled maintenance. Mix match of new and used parts? Mechanic errors? Etc.

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

      Mechanic errors played a big part in it, particularly errors of omission (e.g., failure to tighten or torque, failure to safety).

  • John Worsley

    It also dovetails with the failure rate within 50 or so hours of an overhaul or one of the FAA’s mandatory inspections of parts that have an extremely low failure rate.

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

      Yes, the rate of such “infant-mortality” and “maintenance-induced” failures is much higher than generally believed. I’ll get into that in my blog post next month.

  • Max W

    The Waddington Effect: reminds me of what my A&P has been telling me since I purchase a used LSA two years ago. If it ain’t broke leave it alone (don’t fix it)!

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

      Your A&P definitely has the right idea. Most maintenance should be done on-condition, not on some fixed timetable (although there are a few exceptions).

  • Vernon Lewis

    Generally agree that “less is more” relative to critical war-time periodic inspections but SOME items make sense ANYTIME, like spark plugs, timing, oil and filters, pressures, leaks, fasteners, vibrations, noises, et al, are worthy of attention ASAP after detection. Call it “Ramp Maintenance.”

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

      Most of the items you listed are corrective or on-condition maintenance, not scheduled PM. And with today’s digital engine monitors, even things like spark plugs and mag timing can be done on-condition rather than on a scheduled basis.

  • http://www.michiganlighthouse.com A-Nanny-Moose

    That’s why I get an “annual” done every couple of years. jlw

    • Doug K.

      I’m still a student pilot, pretty close to my private certificate, and shopping around for a 4 place piston single. Just trying to really pin down what expenses are going to be. That has me a little more concerned than the up front cost of the plane.

      How do you avoid the annual every year? Are you flying experimental, LSA, or just rolling the dice that you don’t get ramp checked?

  • http://www.blackhawk.aero M L Moore

    This is exactly what I’ve been trying to tell upper management & my customers for years. They all look at me like I’m nuts. I can think of so many maintenance tasks being performed on a colander schedule that serves no good purpose but to make the operational cost of aircraft ownership more than a lot of people or companies can justify.
    Most all mechanics & repair stations want to make as much money as possible every time a plane is in their shop by making the owner comply with any thing they can dig up. We need to stop and realize that if we also help the operator look out for operating cost bottom line then there might be more customers and surely more return customers to support our businesses. It would surely define our professionalism as maintenance providers.

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

      In my experience, shops and mechanics tend to follow the manufacturer’s (grossly excessive) scheduled preventive maintenance protocols not because of GREED (as you’re suggesting) but rather out of FEAR (of being sued if they fail to follow the manufacturer’s guidance exactly and then someone gets hurt), and to a lesser extent out of HABIT (that’s what they were taught in A&P school). Many owners accuse their shops if being greedy, but I think that’s a bad rap. The problem is defensive maintenance IMHO, not greed.

      • http://www.blackhawk.aero M L Moore

        I didn’t mean to imply greed as the motivation more of get what you can when you can, greed just sounds so deviant. But, I think you are more correct in your assessment.

  • Mike Lawter

    Amen to all this. I always take the plane out for a shakedown flight after mntce before I take passengers, but on some things a test hop isn’t enough. I had a mechanic reuse a castle nut without keying it when a throttle cable was replaced on my T210. The nut vibrated off in flight about 4 hours later which, on the TSIO 520, reduces the power to idle. We were over hostile real estate and it’s truly a miracle that I and my family walked away. The nut was found behind the cowl flap in the bottom of the cowling.

    Just wait if the ECI cylinders have to come off.

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

      I have never understood why some engines in piston singles are configured with a spring that opens the throttle if the control cable becomes disconnected, while others have a spring that closes the throttle and yet others have no spring at all. Seems to me a no-brainer that the throttle should go wide-open if the control fails (in a single). Duh!

  • Chris P.

    Althogh I do agree with the “if it ain’t broke don’t fix it” mentality. There are certain items you just have to do for legality issues. With that said as an A&P/IA who holds the post of Operations/Maintenance Officer in a flying club I have been given the ability to track many issues with our plane and have been able to forcast the schduled maintenance for most of our on condition items which in my opinion has lead to less brake downs. And I personaly fly our aircraft after every major inspection and repair. Which brings up the roll of the owner and mechanic working together. The owner is responsiable for maintenace but the mechainc should educate the owner which in my opinon if that relationship builds you will actually reduce your overall maintenance costs. Bottom Line as an owner/operatior find a mechanic you trust and work with him. Mechanics working with and comunicating with your customers will build a bigger customer base and creat a safer GA fleet. Just thrwing in my two cents worth.

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

      As an A&P/IA who holds the post of Operations/Maintenance Officer in a flying club, I’m guessing you see things more closely through the eyes of the owner/operator and have somewhat different (and better) motivations than the typical GA mechanic working in the typical GA maintenance shop.

  • Mr. GIB

    As an owner-operator of a warbird for 3yrs now, this hits very close to home. However, I live by a slightly different credo than the traditional one:

    The Enemy of Good is Better.

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

      I like that!

  • Clement Flatt

    I too agree with “if it ain’t broke don’t fix it”…. You can look at it, but don’t touch. Most GA aircraft do not fly enough during the year, bad for the aircraft. I like to do an in-depth visual for corrosion and try not to disassemble more than I have to. I personally have someone look behind me as a second set of eyes, whether it is another mechanic, owner, friend, etc… it doesn’t catch everything, but it helps. A lot of mechanics don’t have anyone check, (mentality, I know what I am doing). Some get in a hurry or use old locking hardware…. why??? Don’t know, it will get done when it gets done and hardware is cheaper than an airplane or a funeral.

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

      Clement, your point about “a second set of eyes” is really important. If you visit an airplane factory, you’ll see that virtually all tasks have three sets of eyes associated with them: The technicians work in pairs and then an inspector comes in to review their work. If you visit a Citation or Gulfstream service center, you’ll see the same thing. In contrast, the vast majority of maintenance work performed on piston GA airplanes involves only one set of eyes: the technician doing the work. In small GA shops, you’ll find one person who keeps switching between wearing his IA (inspector) hat and his A&P (technician) hat. For all intents and purposes, that’s like having a plumber or roofer or electrician do work on your house, then come back the next day wearing a building inspector’s uniform and approving his own work. This is something that occurs and is tolerated only at the extreme low end of the aviation food chain (piston GA), and would never be tolerated in the world of air carriers or bizjets or military aviation.

  • Chris Cuneo

    I am a DOM of a high end corporate flight department, operating a G550 and other toys. Gulfstream uses MSG3 to adjust the maintenance schedule based on real world results. The maintenance is still too frequent and too intensive. Having just completed our 9th yearly inspection on the G550, very few of the items inspected needed any action at all. And, none of them could in any way impact safety. Would we have been better off simply not touching it at all? The answer is a resounding YES! The maintenance induced squawks, the missing parts, the damaged components, the expense of a nearly useless inspection all add up to excessive maintenance.

    Put another way, the crew and I know what items need to be addressed. Those items are placed on the “list” prior to arrival. Possibly 30 items, ranging from touch up paint to glitchy avionics. Yet, the maintenance crew found only 2 actual squawks. Not including time change items.

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

      Chris, that’s a remarkable and important observation coming from someone in your position. MSG3 (Air Transport Association Maintenance Steering Group Report, version 3) provides a reliability-centered methodology for creating an optimal maintenance program that reduces the amount of preventive maintenance to the essential minimum required to obtain the necessary level of safety and dispatch reliability. But the quality of the resulting maintenance program depends a lot on the judgment calls made by the program developers and how they choose to follow the MSG3 guidance. I’ll be discussing this stuff in detail in my next few posts to the AOPA Opinion Leaders Blog.

      Three of the most important principles of developing an optimal maintenance program are:

      1. To emphasize inspection and on-condition maintenance, and to eliminate most scheduled preventive maintenance and component TBOs and life limits.

      2. To perform inspection using the least invasive methods possible. (E.g., borescope inspections, engine monitor data analysis, spectrographic oil analysis, scanning electron microscope evaluation of oil filter contents, etc.) To use a medical analogy, we want to be diagnosing the patient using CT scans and MRIs and blood labs, not by performing exploratory surgery.

      3. To assess the consequences of various component failures, to limit preventive maintenance to preventing failures that really matter (e.g., failures that could hurt or kill someone), and to avoid doing preventive maintenance intended to prevent failures that are acceptable (as most component failures are). If the failure of a component doesn’t create a genuine safety-of-flight hazard, then it’s generally best to permit that component to “run to failure” (i.e., leave it alone until it fails, then replace it).

  • http://tenhiflyers.com M. Jeffrey Holley

    to summarize the Waddington effect in a colloquialism: “If it ain’t broke, don’t fix it!”

  • http://www.multiplyleadership.com Mark Jones Jr.

    Best post I’ve read yet in the “Opinion Leaders” blog. Less opinion and more facts–loved it!

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

      Thanks for the kind words, Mark. I’ll try to keep posting more of the same every month.

  • Grant Bright

    Mike, so glad to read this. I follow your column in ABS.

    Been an airplane owner for 40 years. Only had one shop try to “like make it new.” Acceptable wear and not fixing “un broke ” parts is very useful.

    Thanks for your SAVY sage :-) Grant

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

      You must have some old issues of ABS Magazine. I haven’t written for ABS in years.

  • Bob K

    In the software business this is much more pronounced.

    The word “update” actually means “the software will keep changing itself until your system stops working” With software playing an increasing role in aviation / avionics – watch out. My advice is not to accept automatic updates ever, and incorporate updates only once proven – and if they actually produce a benefit that your existing package does not have.

  • Joe Gutierrez

    A good friend of mine picked up his Cessna 182 from an annual, where the repair receipt ( after the large bill was paid) said that the control linkages had been “adjusted”. After take off the airplane flew straight into a mountain without deviating or making any attempt to turn or avoid..Almost like the controls were locked. He use to hanger his airplane in my hanger, “a really fine Gentleman”.

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

      There have been plenty of accidents (mostly fatal) caused by the failure of the pilot to remove the control lock before takeoff. And there have been some caused by mechanics who misrigged primary flight controls. But I’ve never heard of a situation where a control lock or a misrigged flight control resulted in an airplane flying straight and level. My bet would be pilot incapacitation coupled with an obedient autopilot. Just sayin…

  • Mike Hart

    I bought a C180 this year with an O-470S engine with 750 hours since overhaul. One jug had low compression (high 50s) when I bought it. I hoped flying it would resurrect the problem. After 80 hours of flight time, the Jug was below 40 and leaking past the valve. The opposing jug was now in the 50s leaking at the rings. It seemed like a no brainer to fix these two, But I also had two in the low 60s and two in the 70s.

    I chose to pull 4 jugs and leave the ones in the 70s on. But the jugs I pulled could not be rebuilt due to pitting and shelving. Subsequently, I chose to yank the remaining jugs and just go with a full top-end overhaul . All jugs proved to be out of spec for rebuilding.

    Following the idea of minimal maintenance and leaving things alone. Should I have pulled the perfectly good jugs in the 70s? They were making power and the oil analysis showed nothing unusual, but they also had fundamental issues (shelving and pitting) Just curious because my inclination was to keep them on, but when they proved to be out of tolerance, I decided it was better to re-baseline the whole engine.

    Wondering what WWMBD – what would Mike Busch do?

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

    WWMBD? Well, if it were my airplane, I would NOT have pulled any jug that didn’t absolutely have to be pulled.

    For your O-470-S, the go/no-go compression threshold is defined by a master orifice, and generally the no-go-threshold is between 40 and 45 for most compression test sets. If the cylinder is at or above the master orifice threshold, there’s no need to pull the cylinder (at least on the basis of compression).

    I have one cylinder on my own airplane that has measured 45/80 for the past two years (with a master orifice reading of 44/80) and I haven’t pulled it because I don’t have to. Leakage is clearly past the exhaust valve, but the exhaust valve shows absolutely no evidence of a hot spot under the borescope, and my engine monitor shows absolutely no evidence that the exhaust valve is leaking when the engine is actually running (which is what really matters).

    If the compression drops below 44/80 and I can’t bring it back up by flying the airplane and re-checking the cylinder hot, I still won’t pull the cylinder if I don’t absolutely have to. I would first try dropping the exhaust manifold, pulling the valve springs, and lapping the valve to the seat in place with some valve grinding compound and an electric drill. This quite likely would reduce the leakage to the point that the cylinder is back up well above the master orifice threshold.

    The only things that would convince me to pull this particular cylinder would be an exhaust valve that was clearly burned under the borescope, or if there was some other obvious defect (e.g., a head crack) that forced my hand.

    It’s important to understand two things: (1) Nobody ever fell out of the sky because of a low-compression cylinder. (2) There is absolutely zero correlation between compression and performance; a cylinder can measure 0/80 and still produce full rated power. (In fact, I’ve flown with some 0/80 cylinders and there was no way of detecting that in flight.) Low compression is absolutely not a safety-of-flight issue. It’s purely a technical airworthiness issue, and is only that because of a brain-damaged FAA regulation (Part 43 Appendix D) that should have been revised long ago.

    Similarly, the fact that a cylinder measures beyond service limits is completely irrelevant unless you remove the cylinder from the engine. As long as the cylinder is mounted on the engine, we could care less whether it’s within service limits or not. A beyond-service-limits cylinder can be perfectly functional, can have compressions in the high 70s, and can be entirely satisfactory in every meaningful way. The only significance of a cylinder being beyond service limits occurs if the cylinder is removed from the engine. If the cylinder is removed and measures beyond service limits, then it cannot be put back on the engine without doing something to restore service limits (like plating or changing to an oversize piston and rings).

    I absolutely guarantee that every single one of the 12 cylinders on my own airplane is far, far beyond service limits (because 9 out of 12 have 4,700 hours time-in-service, and have no trace of crosshatch visible under the borescope), but that doesn’t bother me one bit. They work great, most have compressions in the 70s, the valves look fine under the borescope, so what’s not to like?

    Reliability-centered maintenance (which is what I practice on my own airplane and on my clients’ airplanes, and what I’ll be discussing in my February blog post) is all about distinguishing stuff that really matters (like burned exhaust valves and head cracks) from the stuff that doesn’t matter (like compression readings and service limits). The compression test is a ridiculous test that is completely unreliable at determining whether or not a cylinder is healthy or sick. The borescope inspection is the gold standard for making that determination. I wouldn’t even bother to perform compression tests at all except for the fact that I’m compelled to do so because the test is written into the FARs (in Part 43 Appendix D).

    Continental Motors wisely issued SB03-3 which says that the old 60/80 compression standard is nonsense and that a cylinder can measure in the high 40s and still be perfectly healthy. SB03-3 requires that a borescope inspection be performed whenever a compression test is performed. SB03-3 further says that if the compression is below the master orifice threshold but the cylinder looks okay under the borescope, the compression readings should be treated as suspect, and the airplane should be flown and the compression re-checked after the flight. I’ve seen cylinders that measured 38/80 on the initial compression test measure 72/80 after a 45-minute flight. (That’s a good indication of just how bogus the compression test is.) The engineers at Continental really understand what’s important and what isn’t, and SB03-3 is without question the best guidance that has ever been written on this subject.

    Incidentally, I have another cylinder on my airplanethat measures in the high 70s but that I know has a serious problem (lead-sludge-fouled rings) causing it to burn a large quantity of oil. That cylinder is one that I do plan to remove because I know it’s sick despite having near-perfect compression. I won’t even bother measuring the cylinder after I remove it, because I know it’s so far beyond service limits that it could never be reworked; it will live out the rest of its life as a lamp base. (The fact that it’s way beyond service limits has nothing at all to do with why I’m going to remove it; the fact that the rings are stuck is the reason I’ve decided that it needs to come off.)

    How do I know that this cylinder is suffering from stuck rings even through its compression is near-perfect? That’s my little secret; I could tell you, but then I’d have to kill you. (Grin.)

  • Mike Hart

    I wish I would have talked to you before making the decision. Ouch.

  • Colleen Keller

    Amen, Mike! 500 hrs past TBO and going strong on my Lycoming. I feel safer this way than behind a new engine, for sure. Thanks for the great article…glad to see you in AOPA Pilot!

  • http://waukeganaviation.com Kirk Swanick

    “If it ain’t broke, don’t fix it”?
    That’s great until it DOES break. But look on the bright side……..the money you save on unneeded maintenance can be better-spent on upping your life insurance payout for your surviving relatives (if they’re not on board) and probably afford the fancy-grade coffin and funeral too!
    Interesting how this contradicts what you have been preaching for years like checking twin Cessna gear rigging every couple of years….if it ain’t broke, don’t fix it, right? Gear goes up, gear goes down?
    I am sensing a double standard here.

    • Roger Ameden

      Kirk – I don’t here Mike suggesting Failure-Based Maintenance. I’m hearing Condition based maintenance. Inspect on age, don’t replace on age. If you inspect and its funky, fix it. Years ago I ran a Reliability and Maintainability shop for a Military helicopter manufacturer. It was a big change, but more than worth it.

      As a simpler example – Every oil change I pull the plug on the back of the vacuum pump and inspect the vanes. Although I did replace it at last engine overhaul, the pump had been in place for over 1,000 hours and none of the vanes were worn to the center circle. Isn’t that better than blindly replacing it every 400 hrs? And I have the added confidence that I know it is not worn out, which just replacing it on schedule doesn’t tell you.

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

      Do I sense just a hint of sarcasm there, Kirk?

      “If it ain’t broke, don’t fix it” is definitely on the right track, but you can’t take it out of context. To implement “if it ain’t broke, don’t fix it” correctly, it must be coupled with a “how do we know whether’s it’s broke or not?” program. We usually call that “inspection.” If you read the stuff I write, you’ll see that I’m very big on inspection (particularly if it can be done non-invasively, like using borescopes and oil analysis and digital engine monitors and that kind of thing that gives lots of valuable information about condition without risking breaking something or doing more harm than good). You’ll also see that I’m really down on doing scheduled preventive maintenance (i.e., fixing stuff whether or not it’s broke) and really up on doing scheduled inspections (i.e., figuring out if it’s broke or not).

      I’m also a big fan of the “run to failure” concept, but strictly in the context of reliability-centered maintenance (which I discuss at length in my next-month blog post here). RCM says that we figure out the various ways things can fail, and then we evaluate the consequences of those various failure modes. If a failure mode has the potential to hurt or kill someone or to do serious collateral damage, then we inspect like mad to make sure that we can detect incipient failures before they become functional failures. On the other hand, failure modes that don’t have serious consequences aren’t worth trying to prevent, so for those kinds of things we just “run to failure” and then “fix it when it breaks.”

      You mentioned the twin Cessna gear rigging. That’s a failure mode that has moderately serious consequences. A gear collapse almost never hurts or kills someone, but it usually results in extensive collateral damage and often turns the aircraft into what the insurance folks call ‘a constructive total loss.” The consequences of a gear collapse are clearly unacceptable, so the proper thing to do is to INSPECT the gear on a regular basis (and as non-invasively as possible) to detect when things are getting out of rig to the point that if things get much worse the aircraft might suffer a gear collapse. I do not preach performing adjustments or repairs to the gear on a regular basis. I preach inspecting it and taking measurements that will identify when its condition is starting to get to the limits of acceptability, and then adjusting or repairing as necessary if the inspection reveals an incipient problem.

      On the other hand, I have two vacuum pumps in my airplane. Either one of them is capable of driving the gyro instruments and the deicing boots. The failure of one pump has totally acceptable consequences (since I have two). Therefore, it’s insane to replace the pumps every 500 hours (the way the pump manufacturer recommends), or even to inspect the pumps every 100 hours. There’s no point in doing ANYTHING to prevent a pump failure, because a pump failure is a totally ho-hum occurrence (since the airplane has two pumps and only one needs to be working). So instead of euthanizing my vacuum pumps ever 500 hours per manufacturer’s guidance, I run them to failure and carry a spare pump and the necessary wrench in my wing locker, so if a pump chooses to fail when I’m in Costa Rica I can slap a new pump on in about 15 minutes.

      The key is to leave stuff alone whenever failure is acceptable, and to inspect like mad (but not replace prophylactically) when failure is not acceptable. We only repair or replace things when doing so is unavoidable (either because a non-critical component failed or because a critical component has been determined to be at the point of incipient failure).

      I’ll be discussing the basic principles of RCM in my February blog post, and then discussing the concrete application of those principles to piston GA airplane in March and April.

  • http://www.airnow.com David Corey

    In my organization (30+ airplanes) we had a saying: Eighty percent of your maintenance is caused by eighty percent of your maintenance.

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

      Hehehe. Good one, David!

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  • Gabriel

    “Eighty percent of your maintenance is caused by eighty percent of your maintenance.” I like that David. Painfully true, unfortunately. Generally the more the plane goes into the shop, the more it needs to keep going into the shop (I think you can say something similar about humans and doctors and probably be just as accurate). Mike liked your little quip, too, but it’s not so funny when it’s Mike’s guys at Savvy that contribute to the problem – or don’t address getting it fixed as promised. Then when they don’t stand behind their service you’ll wish AOPA didn’t endorse them (explicitly or implicitly by letting Mike write these blogs and link to his company to get business out of it). I thought that meant that AOPA was endorsing Savvy because they really make a positive difference, not cause more problems, but then, they are mechanics, too, and isn’t that just what this blog post is about – mechanics causing more problems than they fix?

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