Mooney M20K 231/252TSE/Encore

Adding a turbo to the M20 brought excellent cross-country ability, along with a dose of engine woes.

Mooneys have always been associated with efficient cross-country transportation. Speed is the priority, and Mooney owners tend to view utility in terms of getting from Point A to Point B ASAP rather than load lifting or roomy accommodations.

Non-turbo cross-country usefulness is hampered, however, by an inability to get up high and avoid some of the weather, and the only way to do that is with turbocharging. Get a slick airframe like the Mooney up in the flight levels, and it can really show its stuff.

It wasnt until fairly late in the game that Mooney offered a viable turbocharged airplane. That aircraft, the M20K, overcame some initial teething troubles to evolve into the 252TSE, which some regard as the best Mooney ever made; as one owner has put it, maximum speed for minimum money.

Cessna tapped the high-altitude market first, with the successful T210 in 1966. Mooney tried to address the demand the following year with the M22 Mustang, a big, 310-HP pressurized single that never caught on with buyers. It was axed in 1970. By the late 1970s, the Mooney line was showing its age. Stiff competition was coming from the other manufacturers in the form of updated designs, and Mooney was scrambling to keep up. Mooney was doing small things like updating instrument panels and making a big deal out of its new throttle quadrant, but Piper was doing big things like putting new wings on the PA-28.

Mooneys big breakthrough came in 1977, when the M20J 201 was introduced. The result of a significant Roy LoPresti-led aerodynamic cleanup of the venerable, but virtuous, M20 airframe, the M20J (aka 201, for its speed in MPH), the airplane marked a major turning point for Mooney. It remains a classic.

The 201 proved very successful, with more than 750 being built in its first two years; more than four and a half times the sales of the previous two years. Clearly, Mooney was back with a winner on its hands.

But it didnt win the war. Mooney still had no turbocharged airplane. Cessna was still turning out T210s, but they were much larger airplanes aimed at a different market. In 1977, though, Piper came out with a turbocharged Arrow and created a real threat. Mooney had to deliver a high-altitude ship or risk loss of market share.

The result appeared in 1979 as the first follow-on to the 201. Dubbed 231 (again, for its top speed), the 231 carried the official designation M20K. It was, essentially, a 201 with a six-cylinder, 210-HP Continental TSIO-360-GB hung out front in place of the 201s 200-HP, four-cylinder Lycoming IO-360. Far less ambitious a project than the Mustang of a decade earlier, it also had far greater potential.

The 201s good reception, combined with the appealing idea of being able to fly high in one, was a powerful draw. Mooney sold 246 the first year; not nearly up to the pace of the Turbo Arrow III, but not bad for the smaller company. Pent-up demand for the 231 was so great that it outsold the 201 nearly two to one in that first year; helped, no doubt, by the fact that the price was only about $5,000 higher.

Differences between the 231 and 201 are few; the cowl is longer and there are some minor aerodynamic refinements. The fuel capacity is eight gallons greater and both gross and empty weights are 160 pounds higher. Design-wise, the 231 was exactly what the buyers were looking for: a turbo 201.

Unfortunately, there were problems with the engine installation that soon hurt the 231s reputation. The problems stemmed from several factors: The new cowling wasnt cooling the engine adequately; the fixed-wastegate turbo required constant attention, was easy to mismanage, put undue stress on the engine and was prone to cracking after less than 1000 hours; the connecting rods were prone to failure; the original magnetos were unpressurized and unreliable; and the cylinders tended to display a variety of problems caused by poor cooling and turbo distress. On top of that, the TBO of the first engines was only 1400 hours (later upped to 1800).

Obsessive attention to operating technique and maintenance could easily avoid these problems, but out in the real world they were proving to be a real headache for owners. Similar problems were affecting the owners of Turbo Arrows, which had a different variant of the Continental TSIO-360. Basically, as good an idea as a turboed single is, the stresses placed on the engine by operating practices and environment make it far more prone to failure than its normally aspirated counterpart.

As word of the engines problems got around, sales started to fall off. Mooney responded in 1982 and 1984 with some fixes and improvements. In 1982, the entire line-M20Ks includes-got the split rear cargo seat while in 1984, a new variant of the engine (the -LB1B, which is approved as a replacement for the -GB) with better cooling and overboost protection was introduced.

Mooney also added some aerodynamic tweaks that added 3 to 5 knots: sealed nose gear doors, a belly pan, a more streamlined tailcone, and removal of one of the vent intakes. There was also an improved alternate air intake system, to address reports of icing-induced power loss.

The fixes helped, but Mooney decided to completely revamp the M20K for the 1986 model year. The changes were significant enough that the airplane got a new name: 252TSE (for Turbo Special Edition). Aside from the higher top speed justifying the new model name, its possible that Mooney was trying to get as far away as possible from the 231s initial bad reputation. The 252, while still an M20K through and through, is significantly different from the 231. Another variant of the engine was fitted, the -MB1. The induction and cooling systems were completely reworked, and a new intercooled, density-controlled, variable wastegate AiResearch turbocharger replaced the original, fixed wastegate Rajay/Rotomaster unit and a tuned induction system.

Other changes included an infinitely adjustable electric cowl flap to replace the original dual, three-position manual flaps. There was a vernier throttle control, a little more elbow room, and new-look radiused windows.

The 231s original 60-amp, 14 volt electrical system was upgraded to a 70-amp, 28 volt system. This was a welcome change, since a fully tricked-out 231 could really push the limits of its electrics. In addition, a dual alternator system was made available and a load meter was added to the panel. An electrically driven backup vacuum pump was made standard equipment.

The 252 also got further aerodynamic tweaking in the form of gear doors that fully enclose the wheels when retracted, and cover the wells when the gear is extended. The 252 also got an increase in gear-extension speed to 140 KIAS, up from 132. Maximum speed with gear extended is 165 knots.

In all, 889 231s were produced between its introduction in 1979 and 1985. The 252, introduced in the middle of the GA slump of the 1980s, is somewhat rarer: production totaled, ironically, 231 by the time it was discontinued. The follow-on airplane, the more powerful M20M TLS, was introduced in 1989 and proved popular enough that the 252 was discontinued after the 1990 model year. Its a real shame, because the 252 was, in a way, the epitome of what makes Mooneys so attractive to so many: maximum performance on minimum power and fuel. The TLS is a fine airplane, but it gets its performance in large part from its far larger (and thirstier… and more expensive to overhaul) engine.

The only other directly competing, commonly available airplanes (four-place turbo retractable) are the Piper Turbo Arrow and Cessna Turbo Skylane RG. Of those two, comparisons to the Arrow are more fair, since it has essentially the same engine. The Cessna has a Lycoming 540 under the cowl, and enjoys a 25-HP advantage over the M20K. In terms of get up and go, the M20K leaves both of them in the dust in every category.

At cruise, the 231 outstrips both of its competitors by roughly 20 knots, despite the fact that the 231-MPH top speed is not really reachable under real-world conditions. Realistic max cruise is about 220 MPH (191 knots). The 252 is about ten knots faster. Both M20Ks win the altitude battle as well, with a maximum operating altitude of 24,000 feet for the 231 and 28,000 for the 252, versus 20,000 for the Cessna and Piper. The Mooneys also outclimb the others by about 150 FPM. These performance figures are impressive, and right up there with the 300-HP Turbo 210. However, in actual use, nobody flies 231s at those speeds and altitudes while the 252 will comfort ably cruise at 22,000 to 24,000 feet. At the lower altitudes, turbocharged airplanes arent that much faster than their normally aspirated siblings: The 231 is only a bit faster than a 201 at these altitudes, and is actually slower below 8,000 feet, due to its draggier cooling system.

At altitude, the 252s cruise performance is competitive with most turbocharged twins (except in fuel consumption, which the book lists as 12.7 GPH at maximum cruise power and which most owners say is 13.5 GPH or lower). It is the hands-down winner in fuel efficiency.

Most owners operate a 252 a bit more conservatively. For instance, one says that 65 percent power results in speed averages of 170 knots at 10,000 feet and 200 at FL210, burning 11.5 GPH. Another says that the book fuel burn yields speeds of 210 to 215 knots at and above FL240; and another, a bit more conservative on altitude, averages 175 knots at 6000 feet or below and 185 at 12,000 feet. The 231s numbers are proportionately lower at high altitudes, though the difference lessens the lower one goes.

With 75.6 gallons of usable fuel, the 252 can climb to FL280 and operate a total of 4.9 hours, or just under 990 nautical miles still air range, with legal 45 minute IFR reserve. The 231 has comparable range and endurance, but cannot fly as high.

231/252 differences
The improved powerplant installation makes for a significant operational difference between the 231 and 252. The engine still produces 210 HP, but it does so at a markedly lower manifold pressure: 36 inches for the 252 versus 40 inches for the 231, thanks to the improvements in the tunded induction, cooling and turbo systems.

The 252s induction and cooling air intakes are separate from one another. Induction air enters through a NACA scoop on the side of the cowling, is turned 90 degrees to minimize induction icing through inertial separation, and passes through a larger, less-restrictive air filter. It is then compressed and run through a 42-square-inch intercooler before entering the engine through a tuned induction manifold. The result of this is dramatically lower temperatures for the induction air: anywhere from 60 degrees at lower altitudes to 120 degrees up high. That, in turn, means more power at higher altitudes. The 231s critical altitude is only 14,000 feet, while the 252s critical altitude is 24,000 feet. In practical terms, all of this means that the 252 can continue to climb at about 1000 FPM all the way up to the mid-20s, can fly higher, and is faster once up there.

Because of the greatly reduced induction air temperatures, the 252s operating manual has just one recommended fuel flow for each power setting. There is no best power/best economy to choose between. Recommended leaning technique is to lean to peak inlet turbine temperature (TIT) or 1650 degrees F, whichever comes first.

By far, the most important difference between the 231 and 252 lies in engine management. The revised powerplant installation in the 252 made an enormous difference to the pilot, and makes the 252 a far more desirable airplane. Because the 231 has a fixed wastegate, the pilot must constantly monitor the manifold pressure gauge and fiddle with the throttle to keep it within limits.

At takeoff, the pilot has to start the roll with the manifold pressure set several inches below takeoff power, since boost increases as the airplane gathers speed during the roll. During climb, continuous attention to the throttle is needed as the manifold pressure drops.

An oddity is that if the pilot has to climb after setting up a stable cruise, its necessary to first decrease the throttle, set RPM, then reset the power. The reason for this is that the increase in RPM for climb causes the manifold pressure to shoot up dramatically. Thanks to its automatic wastegate turbo, the 252 does not suffer from any of these foibles.

Failure to follow the regime is very, very hard on the engine, and is a prime culprit in the 231s poor service history. The situation is exacerbated by the fact that the 231 appeals to pilots who are stepping up from normally aspirated airplanes and who are therefore unused to complicated power management schedules.

The 231 and 252 handle like typical Mooneys: relatively heavy in roll and pitch, with good stability. The feel is quite solid, thanks to the pushrod controls. Pitch authority is a bit greater in the M20K than on other Mooneys, and the larger engine out front makes it somewhat nose-heavy. That can make a proper flare somewhat difficult with a forward CG, but its not nearly as pronounced as some other aircraft.

Stalls are more of an event in Mooneys than in some other aircraft. The break is fairly sharp, and there often is a pronounced wing drop. This is easily counteracted with proper application of the rudder, however. Whatever you do, dont spin one – a one-turn spin can result in the loss of 1000 feet.

Pitch change with gear extension/retraction is slight, and less than that of other Mooneys. However, transition from full-flap landing configuration to properly trimmed go-around takes heavy pressure on the yoke and fast action on the trim. Using the electric trim, anticipation of configuration changes helps smooth out the flight path and pilot effort. Speed control is essential when approaching and landing any Mooney, although the 231/252/TSE is a bit easier to manage than the 201/205/AT/MSE. Proper pitch control also is very important. Approach at too high a speed, and the 252 will float. Try to plant it on the ground, and it will fight back. In the worst cases, porpoising followed by a propeller strike or loss of directional control results.

From a handling standpoint, the best addition ever devised for Mooneys are speed brakes. These are useful for the 231, which does not share the 252s higher gear limits. (Theyre standard on 252s.) While most effective at high speeds and managing descents and keeping engine temperatures where they should be, they also make repetitive squeaker landings possible.

Ground handling isnt as good as with some other airplanes. The airplane is pretty low to the ground, and the Mooneys stretched-out seating position makes visibility on the ground worse than it might otherwise be.

Also, the wing span (36 feet, one inch), combined with the different perspective and the relatively wide turning radius of 41 feet can create problems in ground handling, particularly when taxiing on a crowded ramp. One other caution with respect to ground handling: Many Mooneys suffer from damage to the nose gear trunion when towing turn limits are exceeded via power towing.

On paper, the 231 and 252 have the same loading characteristics. In reality, however, the typical 252 weighs somewhat more, simply because it has more equipment. Neither airplane is a stellar load-hauler. Gross weight is 2900 pounds, and basic empty weight is 1800, usually more.

Real-world, full-fuel payloads are on the order of 400 to 500 pounds, making the M20K a very useful two-place airplane. Thanks to its fuel efficiency and good endurance, though, theres flexibility built into the load-carrying equation. The newest M20K, the Encore, has about 200 pounds of additional load. Staying within the CG is not much of a problem, though the M20Ks nose-heaviness means that more attention must be paid to load distribution than with other Mooneys. And, unlike some Bonanzas, theres not much of a CG shift as fuel is burned.

The baggage compartment is large, with a capacity of 120 pounds. Theres also a hat shelf that can hold 10 pounds. Baggage capacity can be dramatically increased by folding the rear seat backs down together or individually. Actually getting the baggage on board, however, is a bit of a problem. The baggage door is a small hatch located high on the right side of the fuselage so its difficult to get bulky items on board.

Mooneys are fast and efficient because they have low-drag airframes with a small frontal area. That translates directly into cramped quarters: Not many heavy people fly them. The seating position is quite different from that of most airplanes. Its more of a sports-car posture than an upright seating position.

Tall people might have trouble moving their legs, finding it impossible to put their feet flat on the floor without their knees hitting the bottom of the instrument panel. Those of below-average height may find that they cant reach the rudder pedals without a booster cushion behind their backs.

However, if the pilot and passengers fit the airplane, its a fairly nice place to be. Early Mooneys tended to be Spartan in their interior arrangements. However, by the time the 231/252 came along, Mooneys reflected both recognition of the need for eye appeal and true comfort. One of the most important benefits was the original 18-inch fuselage stretch of 1968. This made the rear seats bearable for average-sized humans. Thanks to a bit more elbow room and somewhat plusher finish, the 252 is more comfortable than the 231. The 252 is also quieter, and many feel its the quietest of all Mooneys (credit the induction system).

The panel layout is quite good, with one seemingly obvious feature that has no doubt averted many incidents: The gear selector is located up high in the middle of the panel (easy to find, hard to miss in your scan), while the flap switch is located down low. Seems silly until you realize that a lot of inadvertent gear retractions have occurred in airplanes with gear selectors located where the flap switch usually is (the infamous backwards Beech controls).

The power gauges are all on the far right. While theyre angled towards the pilot, we prefer to see them front-and-center, where the pilot is less likely to miss them in the scan. Engine gauges are well-placed, however, right under the glareshield in front of the pilot. The panel also has a good selection of annunciator lights at the top of the radio stack.

Airframe-wise, Mooneys are relatively trouble-free. Long-standing caveats include the potential for corrosion of the cabin frame tubes (particularly if the windows develop leaks), and fuel tank sealant that can break down after a number of years in service, especially with high toluene-based fuels in some locales.

Systems in general are simple and robust. The steel gear legs gear have no oleo struts, relying instead on rubber donuts for shock absorption. Theres no complex electro-hydraulic system driving the gear as is found on Cessnas – Mooneys are electro-mechanical. The flaps, too, are electric, both are relatively trouble free.

The powerplant, however, is another matter. The difficulties fall into several distinct categories: magnetos, con rods, cylinders and turbos. The original 231s had unpressurized magnetos but all have been replaced and the fix came from the factory with the 1982 models, which appeared to solve the problem. Most if not all earlier airplanes have been retrofitted, but check to make sure.

The connecting rods in early airplanes were understrength, resulting in random failures. All should have been replaced by now but it pays to check: The suspect rods are Continental part number 626119, and have a C logo with a circle around it.

The inadequate cooling, unsophisticated turbo and finicky engine management requirements resulted in a large proportion of cylinder problems in the 231. These include the full litany: worn and burned valves, broken rings and cracked jugs. The turbo is a direct contributor to the problems. Since it has a fixed wastegate, the engine is always boosted whether it needs it or not. The induction air is hot, and the cooling system isnt up to the task of carrying that heat away. Finally, pilot foulups can result in overboosting.

The 231s turbo suffers from this abuse. Without the relief provided by an automatic wastegate, the turbo is working all the time. Life is usually measured in the 400 to 700 hour range and theres an AD mandating an inspection for cracks every 200 hours.The 252 does not suffer from these problems, though it has had difficulties with its tuned induction system in the form of cracks and tube failures at the mounting flanges. Continental finally came up with a fix involving flexible tubing after a number of attempts.

Even though the 252s installation of the TSIO-360 is far less troublesome than the original installation on the 231, its important to remember that turbocharged engines are operated in a far more stressful environment than their normally aspirated counterparts. Temperatures and stresses are far more extreme, and thorough inspections of the induction, turbo and exhaust systems, and anything mounted near them is in order.

Mods, Club
By far the most desirable mods for the 231 are those that make it more like the 252. These include intercoolers from TurboPlus and the Merlyn Black Magic automatic wastegate. Reports are that both work as advertised, and go a long way towards eliminating the 231s engine problems. They also make the 231 perform almost as well as a 252.

For those who want to go even further, Mod Works can essentially transform a 231 into a 252, complete with a new -MB1 engine and 252 cowl with electric cowl flaps, intercooler, automatic wastegate… the whole nine yards. In addition, contributor Coy Jacob operates Mooney Mart, a highly respected Mooney shop/brokerage in Venice, Florida and offers a Mooney buyers kit.

Currently, the only association is the Mooney Aircraft Pilots Association, which publishes the MAPA Log. If you are a first-time shopper for a Mooney, MAPA is a good place to start your search.

Owner Comments
What a great airplane! I have either owned or operated a Turbo Arrow, a Bonanza and a Baron and now for 3-1/2 years, a 1979 Mooney 231. The Mooney is faster than the Turbo Arrow, which has essentially the same engine. The price tag is a heck of a lot better than the Bonanza and the fuel burn better than the Baron. Whats not to like? I flight plan at 157 knots, but regularly catch the westerly winds in the low teens and see 200-knot groundspeeds.

Operating expenses (fuel, maintenance, annual, engine allowance) run around $120/hour (130 hours/year) plus insurance and hanger. Plan on a top overhaul before TBO. Ive had great maintenance support from the folks at Byerly Aviation in Peoria and I made it look new with a top notch paint job by Perfect Finish of Hamilton, Ohio. The Mooney Aircraft and Pilots Association is a great support organization and their annual homecoming at the factory is a real hoot.

-Bill Tucker
via e-mail

I purchased a 252TSE in January 1998. I have put about 230 hours on it since then. My use of the airplane is unusual. I use the airplane to go between Murfreesboro, Tenn., and Sundance Airpark near OKC.

The 252 is perfect for this purpose. I almost always fly by myself, occasionally with my wife. Weather permitting I make the trip almost every week. I usually go westbound fairly low and eastbound fairly high (FL190 to FL230). My best time so far (eastbound) is 2:28 flight time.

I love the airplane, but would not recommend it to anyone who was unwilling to fly at altitude. Down low (below 8000 feet) its actually slower than a normally aspirated Mooney. The built in oxygen is an absolute necessity and it is a pain to get filled because not everyone is equipped.

I decided on the 252 as the best airplane for my purpose based on Coy Jacoobs Mooney buyers kit. I had never flown a Mooney before making the decision to buy one. I had Mike Jones (of Mike Jones Aircraft in Murfreesboro, Tennessee) help me find and buy the airplane. On his recommendation I had Dugosh Flying Service in Kerrville, Texas perform the pre-buy inspection.

What a wonderful group of people. Among other things, they showed me how to perform an emergency gear extension while the airplane was on jacks. After that experience it was not much of a decision on where to have an annual. As long as I own a Mooney, it will go to Dugosh for its annual and any other major maintenance.

I generally cruise at max cruise (why else would you own a Mooney?), slightly rich of peak and burn 13.8 GPH at all altitudes. True airspeeds run about 155 at 3000 to 4000 feet to just under 190 knots at FL230.

-Ernest Betancourt Lascassas, Tenn.

My partner and I bought a 1981 M20K 231 in August 1998. Before we bought the airplane, we considered several others including mainly Bonanzas and Mooneys, including big bore conversions. We are both relatively low time pilots so we were committed to sticking with a high performance single. Most of our missions are flown with one or two people in the airplane so either bird would work.

During the process of looking, I had the good fortune to be able to fly several new and very late model Mooneys. I very quickly became a fan. To my initial surprise, for most of my missions, it was a better airplane. The first thing that I found that I liked was that the stiffer control inputs made for a more stable IFR platform. The Mooneys were all faster and were able to climb better. There were no practical CG loading issues. My passengers liked being able to sit facing forward with their baggage stowed in a dedicated baggage area rather than in their laps the way it is in the A36.

Another big selling point for me with these airplanes was the TKS deicing system, which dramatically improved my dispatch rate. We live in Minnesota and find that icing is an issue for more than half of the year.

For anyone who pays their own bills, the figures are very compelling. The Mooney was much cheaper to acquire and is much cheaper to operate and maintain than anything else that we could find with similar performance. After shopping around, we settled on a run-out 231. It had new paint but needed a new interior, engine and some radios. We also added TKS. Initial purchase price was $75,000; by the time we were done bringing the airplane up to snuff, wed spent about $160,000. Total annual costs run a bit over $16,000.

Money was very important to us. While we realized that we would not recoup our investment dollar for dollar, we feel very good about the equipment that we have now and when we looked at both cost and utility, considering dispatch rate, comfort with our ability to handle a variety of conditions and the distances that we would fly, pride of ownership and residual value, we felt that the investment was justified.

We are both able to use the airplane some for our business and, in so doing, to make it pay for itself. We feel really great about the value when we compare the Mooney to the other airplanes that we considered. Our path required a lot of our time and was frustrating because the airplane has been down so much during our first months of ownership. The benefits of this approach were that we now have it the way that we really want it and that we have greater confidence in the various systems.

There is a Mooney mailing list on the Internet that was a very important resource for us. This is an experience that I can recommend heartily. I think that one needs to take the responsibility to be a good and well-informed buyer much like we do when we operate these things. In my case, the real expense of flying is the time that is required. I have been lucky enough to integrate my flying into my life in such a way that there is not a significant amount of incremental expense. Owning the Mooney has actually reduced the expense considerably.

-Name witheld
via e-mail

Also With This Article
Click here to view charts for Resale Values, Payload Compared and Prices Compared.
Click here to view the Mooney M20K 252TSE features guide.