Magazine Articles

Exhaust valves are the most heat-stressed components in your piston aircraft engine, and the most likely to fail prior to TBO. Here’s what you need to know about them. I experienced my first in-flight exhaust valve failure about twenty years ago. The engine started running very rough (as you might expect of a six-cylinder engine that was only running on five cylinders). After I landed, I noticed that the manifold pressure at idle was several inches higher than normal, confirming that something was definitely wrong with the engine. I put the airplane in the hangar, removed the top cowling and the top spark plugs, and performed a differential compression test. Five of the cylinders measured just fine, but one measured 0/80 with a hurricane of air blowing out the exhaust pipe. It was pretty obvious that this jug was going to have to come off. Once I wrestled the cylinder off the engine and looked at the exhaust valve, it was pretty obvious that something was missing (see Fig. 1). A fragment of the exhaust valve face had broken off and departed the premises for parts unknown. Luckily, it opted to depart through the wastegate and to spare the turbocharger […]

Your maintenance shop’s paperwork can make all the difference between a good outcome and a nightmare. When he contacted me, the owner of a pristine turbonormalized A36 Bonanza seemed obviously frustrated: I manage to fly only 50 to 75 hours a year, but my annual inspections have been running between $8,000 and $12,000 every year despite my low flying time. I think my mechanic is very honest and thorough, but I think he spends about 100 hours doing the inspection. Perhaps he is overdoing it? I asked the owner to fax me his invoices from this shop for the past two years so I could review them. He did, and when I reviewed the invoices, I found them profoundly disturbing.  It wasn’t just the totals that bothered me—about $7,000 for the 2008 annual and more than $12,500 for the 2009 annual—but the obscure, perhaps even intentionally cryptic nature of the invoices that made them almost impossible to evaluate. I’ve been looking at maintenance invoices for more than two decades, but these were perhaps the most inscrutable I’ve ever encountered. Let me show you what I mean: This invoice contains an astonishingly detailed description of the work performed that goes on […]

Every pilot understands the notion of “pilot in command.” That’s because we all had some certificated flight instructor (CFI) who mercilessly pounded this essential concept into our heads throughout our pilot training. Hopefully, it stuck. As pilot-in-command (PIC), we are directly responsible for, and the final authority as to, the operation of our aircraft and the safety of our flight. Our command authority so absolute that in the event of an in-flight emergency, the FAA authorizes the PIC to deviate from any rule or regulation to the extent necessary to deal with that emergency. (14 CFR §91.3) In four and a half decades of flying, I’ve overheard quite a few pilots dealing with in-flight emergencies, and have dealt with a few myself. It makes me proud to hear a fellow pilot who takes command of the situation and deals with the emergency decisively. Such decisiveness is “the right stuff” of which PICs are made, and what sets us apart from non-pilots. Conversely, it invariably saddens me to hear a frightened pilot abdicate his PIC authority by throwing himself on the mercy of some faceless air traffic controller or flight service specialist to bail him out of trouble. How pathetic! The […]

How do we assess whether a piston aircraft engine is airworthy? Compression tests and oil consumption are only part of the story—a smaller part than most owners and mechanics think. My friend Bob Moseley is far too humble to call himself a guru, but he knows as much about piston aircraft engines as anyone I’ve ever met. That’s not surprising, because the man has been rebuilding Continental and Lycoming engines for the four decades, so there’s not much about these engines that he hasn’t seen, done, and learned. From 1993 and 1998, “Mose” (as his friends call him) worked for TCM as a field technical representative covering Missouri, Kansas, Iowa, Nebraska, North and South Dakota, Minnesota, and the portion of Canada north of those states. “Then I made someone at the factory mad,” he says, “so they gave me Arkansas.” (Not really, but it always gets a laugh.) These days, Mose and his wife Rita operate a small shop called SkyTEK Inc. located at Fulton, Missouri, about 100 miles west of St. Louis. [http://www.skytekonline.com/] The company offers a wide variety of engine-related services including custom overhauls, prop strike inspections, cylinder work, accessory repairs, fuel injection system setup, and all manner […]

By Mike Busch To apply RCM principles properly to the maintenance of our piston aircraft engines, we need to analyze the failure modes and failure consequences of each major component part of those engines. Last month, we looked at the issue of catastrophic failures of piston aircraft engines, and saw that the predominant risk of such failures is greatest when the engine is young, not when it’s old. This month, we’ll examine the critical components of these engines, how they fail, what the consequences of those failures have on engine operation and safety of flight, and what sort of maintenance actions we can take to deal with those failures effectively and cost-efficiently. Crankshaft It’s hard to think of a more serious piston engine failure mode than a crankshaft failure. If it fails, the engine quits. Yet crankshafts are rarely replaced at overhaul. Lycoming says their crankshafts often remain in service for more than 14,000 hours and 50 years! TCM hasn’t published this sort of data, but TCM crankshafts probably have similar longevity. Crankshafts fail in three ways: (1) infant-mortality failures due to improper material or manufacture; (2) failures following unreported prop strikes; and (3) failures secondary to oil starvation and/or […]

Last month, we examined the principles of RCM used by the airlines and military to achieve cost-effective maintenance. Now, let’s explore how RCM can be applied to our small GA aircraft, and especially to our piston aircraft engines. For three decades, the airlines and military have been using Reliability-Centered Maintenance to slash maintenance cost and improve reliability. Most of these benefits have come from replacing fixed overhaul intervals with on-condition maintenance. Unfortunately, RCM has not trickled down to the low end of the aviation food chain. Maintenance of piston GA aircraft remains largely time-directed rather than condition-directed. Most GA owners dutifully overhaul their engine at TBO, overhaul their prop every 5 to 7 years, and replace their alternators and vacuum pumps every 500 hours, just as Lycoming, TCM, Hartzell, McCauley, Kelly Aerospace and Parker-Hannifin recommend. Bonanza owners have their wing bolts pulled every 5 years. Cirrus owners replace their batteries every 2 years. And the beat goes on… Does any of this make sense? After analyzing reams of operational data from a number of major air carriers, RCM researchers concluded that fixed-interval overhaul or replacement rarely improves safety or reliability, and often makes things worse. When does TBO make sense? […]

A strategy known as “Reliability-Centered Maintenance” has drastically reduced the cost of maintaining transport and military aircraft, while simultaneously improving dispatch reliability. Isn’t it time we applied this approach to piston GA? More than 30 years ago, in 1974, the U.S. Department of Defense commissioned United Airlines to prepare a report on the techniques used by the airline industry to develop cost-efficient maintenance programs for civil airliners. The resulting report, titled Reliability-Centered Maintenance [F. S. Nowlan & H. Heap, National Technical Information Service, 1978] described a radically different approach to aircraft maintenance, based on rigorous analysis of traditional maintenance practices and evaluation of their shortcomings. Traditionally, a major emphasis of aircraft maintenance programs had been defining specific overhaul and retirement intervals (TBOs) in order to achieve a satisfactory level of reliability. However, engineering analysis of reams of operational data from a number of major air carriers produced fascinating insights into the conditions that must exist for scheduled maintenance to be effective. Two discoveries were especially surprising: For example, RCM researchers determined back in the 1970s that scheduled overhauls on turbine engines do not produce any reliability or economic benefit, and that maintaining such powerplants strictly on-condition provides longer life, reduced […]