I know I’ve got a bad reputation, and it isn’t just talk, talk, talk …” So says Freedy Johnston, one of those wonderfully quirky songwriters who’s way too good to ever reach the mainstream. His music oozed into my head the day a friend of mine called to ask about my experiences flying a Mooney 231. He’s been doing the usual rental roulette with high-performance airplanes and was in the early stages of shopping for something of his own.His initial research led him toward the Mooney 201 as the right combination of speed, payload and capability. But as he dug further, he realized that Mooney 231s of a similar vintage could be had for not much more than a 201, and they promised considerably more altitude capability and speed. Plus, he noted, the 231s seemed to be better-equipped year-for-year, and he saw this as getting some nifty electronics for next-to-nothing.But, like a lot of shoppers in this category, he was worried about the 231’s reputation. He’s heard and read that the M20K’s Continental TSIO-360 was ill-tempered, hot-running and prone to all sorts of maladies from quick-wearing cylinders to turbochargers that could grow cracks overnight. “Frankly,” he said, “I’m worried that I’m going to get an airframe I like with an engine I loathe.”
Reputation from Rumor
I spent a lot of time dispelling myths about the 231 and generally easing his fears. In the course of this discussion, I found out that much of what he’d learned about the airplane came second-, third- or even fourth-hand. A snippet here from an A&P who didn’t like working on the airplane, and a chunk there from an FBO that once had a Turbo Arrow on the line … it doesn’t take a lot of misinformation to create a bad, but undeserved, reputation.Piece by piece, I walked him through my 231 experiences — nearly 500 hours of them –that included helping to manage the maintenance and actually work on the airplane. OK, we’ll count those hours only as flight time. I didn’t log the many hours scratching my head thinking about maintenance or cussing while working on anything hung off the back of that engine. But like anything to do with flying, there’s probably a two-to-one ratio of thinking about it to actually doing it. Of course, that could just be me.
My friend had heard that the 231’s fixed-wastegate system was difficult for the pilot to manage. Permit me a trip back to Turbo 101, if only briefly. Turbocharged installations come in three basic types, typically denoted by the kind of turbo-control system, which is necessary to keep the turbocharger’s innards spinning within the desired range and the output from the compressor (fed to the engine’s intake system) within bounds. One of the popular, early iterations used a manually controlled valve — called a wastegate — that let the pilot determine how much of the exhaust stream was routed to the turbo. This system was often referred to as a “second-throttle” type because the manual wastegate control was often a big vernier knob that looked like the throttle. One of the benefits was that the turbo system only worked as hard as the pilot requested. The boost pressure — which is what creates the extra horsepower at altitude, of course — is directly related to the position of the wastegate knob because the main throttle is already wide open. In effect, this setup acts like a normally aspirated engine up to the altitude at which the pilot decides to bring the turbos on line.As good as the manual-wastegate system is — and I’ve come to appreciate its simplicity flying the Turbo Viking — it is a high-workload item. If you’re in turbulent air and the airplane is changing speed often, the boost (manifold pressure) will be in constant flux, as will fuel flow as the injection system tries to keep up. Sometimes air and fuel changes are in sufficient lockstep that you can ignore them, sometimes not. In all, the manual turbo system was a good way to bolt on high-altitude performance without having to rework the whole engine.By the early 1960s, turbocharging became more common, as did the automatic controllers. There are many variations on the theme, but suffice it to say that they all aim to manage the wastegate in such a way that the selected manifold pressure is more or less constant; even, theoretically, if you’re in updrafts and downdrafts or decide to change engine speed or mixture setting. (I say “theoretically” because the T210 I fly still has a slightly annoying wiggle to the MP needle despite just receiving a new wastegate. More research is in order.)So, back to the Mooney 231. Automatic wastegates are expensive and manual systems seem outmoded, so Continental came up with a fixed system in which the turbo is always running but the primary control of manifold pressure is the throttle in the cockpit. For fine-tuning, there is a small bypass tube partly plugged by a threaded bullet that creates a restriction. In the 231, you set the position of this restriction in a flight test, setting up a specified rpm and mixture setting at a given altitude and noting the available manifold pressure. In normal use, the turbo is spinning as fast as it wants, based on the energy in the exhaust stream supplied to it. And there’s nothing on the intake side to mitigate manifold pressure besides the throttle, which is why it’s common to take off with the full 40 inches showing with the throttle out from the panel by an inch or so. Continental and Mooney (and Piper) saved money by making the pilot the wastegate.I’ve flown the 231 quite a bit — and have put in enough time in Turbo Arrows to be comfortable — and confess this extra bit of workload bothered me not at all. But it takes two things: One, practice; and two, a willingness to let the manifold pressure wander up and down a bit from the target. Fussy types who feel the urge to have every needle just so will go absolutely nuts in one of these airplanes.
In the many descriptions of the 231 I’ve read, the issue of the turbo working harder than it needs to comes up a lot. From there, one gets the impression that the turbo runs hot — well, show me one that doesn’t — and is prone to failure. Again, that’s true, but not just for the 231 or the Turbo Arrow. But it’s not as though the automatic systems are letting the turbo loaf. The simplified automatic system on the T210, for example, simply causes the turbo to put out a set amount of boost — a value slightly in excess of the takeoff maximum. It’s a closed-loop system. The wastegate is closed — pushing all the exhaust through the turbo in search of the most boost — until the controller sees its setpoint in what’s called the upper deck. (That’s a fancy term for the intake system between the turbo and the throttle plate you control from the cockpit.) The system doesn’t care if you’re only using 25 inches for cruise; the turbo is humming along, trying to put 37 inches into the upper deck. You have no control over it. And, of course, the harder the turbo works, the hotter the induction air becomes, which in turn saps horsepower and erodes detonation margins.Not so with the fixed system. Want less boost? Reduce engine speed. With less exhaust energy, the turbo spools down a bit, reducing manifold pressure on the other end. I used to fly the 231 around at the lowest engine rpm that would provide the desired boost. Ideally, I’d have the engine ticking along at, say, 2300 or 2350 rpm with the throttle wide-open. The result was lower induction-air temps, lower cylinder-head temps and cooler oil. What’s more, I wouldn’t hesitate to run it at 2324 or 2416 rpm unless smoothness was an issue; there’s nothing magic about engine speed in increments of 50 or 100 rpm.Ultimately, I and the 231’s owner became confident in the engine and its maintainability. By the time the airplane was sold, the engine was slightly over TBO in the first run and doing well — albeit with only two of the six cylinders as new. (We used to joke we had a cylinder-durability test-stand in motion, as there were two original, steel-barreled cylinders, two CermiCrome cylinders and two with channel chrome. This was a decade ago, before Continental resumed making new TSIO-360 cylinders, by the way.) Had I not just bought a house, I would have purchased the airplane and continued to run the engine pending some reason to major it. The airplane was a strong, reliable performer with superior efficiency, even before the advent of GAMIjectors and the (new/old) idea of running lean-of-peak EGT.And this is an engine with a dreadful reputation? A powerplant that hangar lore says won’t ever make TBO and will crack turbo housings often enough to put you on a first-name basis with the overhauler? (In the interests of complete disclosure, we did have to replace a turbo housing just after I started flying it, but the Rajays have a mixed reputation that I think is justified.)
Compared to What?
I think the TSIO-360 was (and is) maligned for the wrong reasons. It is fussier to fly than, say, the IO-360 Lycoming you’d find in a 201 or regular Piper Arrow. I’m sure a lot of the step-up pilots buying early 231s and T-Arrows had a steep learning curve just with the higher speeds and altitude performance, so that smart power management — and, more to the point, perfect temperature management — came much later. What’s more, when these engines were new, we didn’t have engine monitors and data logging like we do today. But it’s not hard to fly; and, at least in the Mooney, it’s sufficiently well-cooled that temperature management wasn’t all that stressful. I will say that I’ve seen one or two 231s with poor engine baffling and misrigged cowl flaps. Turbocharged engines produce a lot of heat that must be wicked away effectively, and the cooling systems are surprisingly sensitive to small things like baffle condition and cowl-flap position.In my view, the 231’s reputation is not deserved. I can’t say for sure about the Turbo Arrow. It’s been my observation that when the manufacturers try to reduce pilot workload by omitting cowl flaps they foolishly leave behind a powerful tool for pilots to keep their engines happy.But I’ve seen it the other way around, too: I have friends with Lycoming O-320s or O-360s — reputed to be the most bulletproof engines in the land — who have had valve-wear issues and camshaft spalling that forced a pre-TBO teardown. In 1998, I purchased a Beech P-35 Bonanza, which is fitted with an engine that has a sterling reputation: the Continental IO-470-N. This 260-hp engine is the precursor to the IO-520, which carries a lesser reputation. (Although I know of one IO-520 that’s flying right now, well over TBO with no symptom beyond high oil consumption to suggest its age.) The word on the street was that the IO-470’s 1500-hour TBO was artificially low, and that many have gone well beyond that figure without trouble.Well, not mine. At about 1550 hours, what appeared to be a small leak in the crankcase adjacent to the number-five cylinder turned out to be a crack. It was in the area of the case that TCM considers “no go.” So we overhauled it, well before I had hoped — but only slightly before I had planned, as I figured an at-TBO engine owes you nothing at all — and discovered that the case was full of cracks. The overhauler — I know him by name, thanks very much — told me that it was good we did the job when we did. “It didn’t have much life left,” he said, referring to the bottom end.Wondering if it was just me — my flying, my bad luck — I went back through the logs. Turns out not one of the engines in my airplane went past TBO; and one of them, not in the airplane at the time, didn’t make it halfway through before being replaced with a typically cryptic note. Part of the reason may be that the airplane had an aftermarket turbo system — the manual-wastegate Rajay system popular in the 1960s — but at least one engine got near TBO with that setup. (I think I know why the previous owner removed the system: Near the end, there were log entries about every 50 hours noting the repair or replacement of some part of the turbo system.)Yes, I hear what you’re saying: These are but two data points among thousands — how could they be valid? Precisely my point. Before you go buying into the bad reputation of any airplane or engine or piece of avionics, you have to gain some perspective. I’m sure there are people who have had terrible problems with a 231 and there are owners who’ve busted TBO with an IO-470.Don’t simply accept the reputation without good, hard data and feedback from a good cross-section of owners.Oh, and one more thing: If you decide to buy, make sure you fly. My 231 experience started badly because the airplane had flown, on average, about 20 hours a year before I arrived. For the first 10 or 15 hours, something broke on every flight. By the end of my time with that wonderful airplane — flying nearly 200 hours a year — it was totally, utterly reliable and completely undeserving of a bad reputation.
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