If age mellows people, the same might be said for airplanes, at least if the airplane in question is Mooney’s M20K series. The airplane arrived in the GA market at a time when turbocharging was relatively new and the demand for high-flying aircraft was thin.
Mooney didn’t get the M20K’s turbocharging system right on the first try and the airplane developed a reputation as a maintenance hog. Thirty years later, that reputation has been mostly burnished and the fact that the M20K bores along between 160 and 200 knots on relatively little fuel has improved the model’s used price.
Still, the cabin is small and with a single door, hard to get into. For that reason and others, Mooneys have a bit of cult status to them. They are in no way everyman’s airplane in the way that a Cessna or a Piper is. But if cruising fast yet miserly is your desire, the M20K models—the 231, the 252 and the Encore—are strong contenders.
Model History
Mooney came into the turbocharging game relatively late compared to other manufacturers. In 1966, Cessna pioneered the market with the T210 and made a strong showing in the single-engine, high-altitude market. Beech brought out the V35TC in 1966, but it was never as strong a seller as the A36.
Mooney wasn’t completely flat footed during the 1960s, introducing the 310-HP M22 Mustang in 1967, a big brute of an airplane that was, to some, as ugly as it was unsuccessful.
Through the 1970s, Mooney did we’ll with small, efficient airplanes powered by Lycoming four-bangers. Mooney’s big breakthrough came in 1977, when the M20J 201 was introduced as the fruit of a clever Roy LoPresti-led aerodynamic cleanup of the venerable F-model. The 201—named for its maximum speed in miles per hour—marked a turning point for Mooney, even if the claimed speed was somewhat optimistic. In 1977, Piper had the Turbo Arrow and Mooney realized it needed to compete in this market.
The result appeared in 1979 as the 231—again, named for its top speed—or M20K. It was essentially a 201 with a six-cylinder, 210-HP Continental TSIO-360-GB in place of the 201’s 200-HP Lycoming IO-360.
The airframe had a lot going for it. It was strongly built of welded 4130 steel, the gear system was all but indestructible and the handling was mannerly, easily flown by a pilot with minimal retrac experience. By modern standards, Mooney had a smash hit on its hands. It sold 246 airplanes the first year, outdistancing the 201 by nearly two to one. The fact that the two airplanes were so similar simplified the build process and likely made the project profitable from the first year.
The differences are in minor aerodynamic refinements. The K-model’s fuel capacity is 10 gallons more than the J-model, and both empty and gross weights are 160 pounds higher. Design-wise, the 231 was exactly what the buyers were looking for: a turbocharged 201.
But if buyers were hoping for the 201’s excellent dispatch rate, they got something less. Problems with the 231’s Continental engine were several fold and hurt the model’s initial reputation. The new cowling didn’t cool the engine adequately; the fixed-wastegate turbo required constant attention and was easy to mismanage; overboosting and high heat put undue stress on the engine, and it was prone to cracking cylinders and cases.
The connecting rods were prone to failure and the original magnetos were unpressurized, and would arc at high altitude. On top of all this, the TBO of the first engines was a miserly 1400 hours, later upped to 1800 hours, where it still stands.
Even with all these faults—and they were considerable—some owners achieved impressive maintenance reliability by obsessive attention to operating technique. Specifically, that meant careful leaning and attention to cowl flaps and preventive maintenance of the turbo. But not all owners were so careful—premature engine crumps were common.
REFINEMENTS
With a couple of years of experience under its belt, Mooney undertook some improvements, adding a split rear cargo seat in 1982, while in 1984, a new variant of the engine—the LB1B, which is approved as a replacement for the GB—was introduced with better cooling and overboost protection. Mooney also included 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. The alternate air intake system changed to address reports of icing-induced power loss.
While these fixes certainly helped, the improvements were hardly night and day. By 1986, further retooling produced the 252TSE for Turbo Special Edition. The 252, while still an M20K, is significantly different from the 231. Another variant of the engine was fitted, the -MB1. The induction and cooling systems were reworked and a new intercooled, density-controlled, variable wastegate AiResearch turbocharger replaced the original, fixed wastegate Rajay/Rotomaster unit. Other changes included infinitely adjustable electric cowl flaps to replace the original dual manual flaps. There was a vernier throttle control, more elbow room and new-look radiused windows.
The 231’s original 60-amp, 14-volt electrical system was upgraded to a 70-amp, 28-volt system. This was much needed, since a fully loaded K-model could max out the electrics long before the days of moving maps. 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 knots, up from 132 knots. Maximum speed with gear extended is 165 knots for the 252. 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 less numerous. Production totaled, ironically, 231 airplanes. The K-model made a brief resurgence in 1997 as the Encore, when Mooney was going through yet another of its many reorganizations. But it was not to be and the model was dropped again in 1998. Meanwhile, the so-called long-body models, specifically the M20M TLS and later the M20R Ovation and Acclaim eventually came to dominate the Mooney line.
Performance
The K-model lives in a league of its own when measured against the narrow market segment of four-place, turbo retractables. At cruise, the 231 outstrips its competitors—the turbo Arrow, the 182 RG and Commander TC—by roughly 20 knots, despite the fact that the 231 MPH (196 knots) top speed isn’t reachable under real-world conditions and probably at all.
Realistic max cruise is about 190 knots for the 231, but 170 to 175 knots is more like it. The 252 is about 10 knots faster, thanks to intercooling. Both M20Ks win the altitude battle as well, with a maximum operating altitude of 24,000 feet for the 231 and 28,000 feet for the 252, versus 20,000 feet for the Cessna and Piper. The Mooneys outclimb the others by about 150 FPM.
Due to physiological considerations, however, high teens to low 20s are the airplane’s best envelope. At lower altitudes, turbocharged airplanes aren’t much faster than their normally aspirated siblings.
In fact, the 231 is actually slower than the 201 below 8000 feet, due to cooling drag. The J-model will also outclimb the K-model below 8000 feet.
Many owners operate 252s conservatively. One owner told us that 65 percent power yields 170 knots at 10,000 feet and 200 knots at FL210, burning 11.5 GPH. The 231’s numbers are proportionately lower at high altitudes, although 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 reserves. The 231 has comparable range and endurance, but can’t fly as high.
231/252: how they differ
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 tuned induction, cooling and the turbo system.
The 252’s 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’s then compressed and run through a 42-square-inch intercooler.
The result is dramatically lower temperatures for the induction air, from 60 degrees F at lower altitudes to 120 degrees F up high. That means more power at higher altitudes and a wider detonation margin. The 231’s critical altitude is only 14,000 feet, while the 252’s critical altitude is 24,000 feet. In practical terms, this means that the 252 can continue to climb at about 1000 FPM into the mid-20s, can fly higher and is faster once up there.
The most important difference between the 231 and 252 lies in engine management. The revised powerplant installation in the 252 made an enormous difference and makes the 252 a more desirable airplane. Because the 231 has a fixed wastegate, the pilot must constantly monitor manifold pressure and fiddle with the throttle to keep it within limits. Bootstrapping and overboosting are constant worries. Thanks to its automatic wastegate, the 252 doesn’t suffer these foibles.
Handling
The 231/252 series handle like typical Mooneys: relatively heavy in roll and pitch, with good stability. The K-models have greater pitch authority, thanks to a slightly larger elevator and the longer engine makes it somewhat nose heavy. That can make flaring a challenge with a forward CG, but nothing like, say, a Cessna 182.
Pitch change with gear extension/retraction is slight, but flap extension produces a nose-down moment. Transition from full flaps to trimmed for go-around takes heavy pressure on the yoke and fast action on the trim. Using the electric trim, anticipation of configuration changes helps reduce pilot effort.
Speed control is essential when approaching and landing any Mooney. Approach too fast and the K-model will float. Try to plant it on the ground and it will fight back, porpoising vigorously and striking the prop if uncorrected. This is a common accident for all Mooneys, not just the K-model.
Because of its ability to fly fast, some owners say the best addition ever devised for Mooneys are speed brakes. These are especially useful for the 231, which doesn’t have the 252’s higher gear limits. (Speed brakes are standard on 252s.)
Ground handling isn’t great. The airplane is low slung and the Mooney’s stretched-out seating position hinders visibility on the ground. It also makes gaining purchase on the brakes difficult. The wing span (36 feet, 1 inch), combined with the wide turning radius of 41 feet, makes negotiating a crowded ramp challenging. One other caution: Many Mooneys suffer damage to the nose gear trunion when towing turn limits are exceeded via power towing. Owners learn to watch the ramp rats carefully.
Payload/cabin
On paper, the 231 and 252 have the same loading characteristics. In reality, however, the typical 252 weighs 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 pounds, usually more. Real-world, full-fuel payloads are on the order of 400 to 500 pounds, making the M20K a useful two-place airplane, with generous baggage. Thanks to its fuel efficiency and good endurance, however, there’s flexibility built into the load-carrying equation. The latest M20K, the Encore, has about 200 pounds of additional load, thanks to beefier landing gear. Staying within the CG is easy and there’s no worry of aft-tending CG as fuel is burned off.
The baggage compartment is large, with a capacity of 120 pounds, although the high sill door makes it difficult to wrestle large objects into the airplane. Baggage capacity can be increased by folding the rear seat backs down together or individually.
Mooneys are fast and efficient because they have low-drag airframes with a small frontal area. That translates into cramped quarters. The seating position is quite different from that of most airplanes. It’s more of a sports-car posture than an upright seating regime. There’s plenty of leg room fore and aft, but less lateral room. Those of below-average height may find that they can’t reach the rudder pedals without a booster cushion behind their backs or pedal extensions.
Early Mooneys tended to be Spartan in interior arrangements. But by the time the 231/252 appeared, Mooney recognized the need for more modern if not luxurious appointments. 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 it’s the quietest of all Mooneys, thanks in part to the induction system and the fact that things quiet down the higher you fly.
The panel layout is quite good, with one seemingly obvious feature that has probably averted many incidents: The gear selector is located high in the middle of the panel so it’s hard to miss. The flap switch is located low on the center console, along with the trim/flap indicators and, in the 252, cowl flap controls.
The power gauges are on the far right and angled toward the pilot. Engine gauges are well-placed, 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.
Maintenance
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 the typical fuel tank leaks that plague all Mooneys. Systems in general are simple and robust. The steel gear legs gear have no oleo struts, relying instead on rubber donuts for shock absorption. These need to be replaced periodically. There’s no complex electro-hydraulic system driving the gear as is found on Cessnas—Mooneys are electromechanical. The flaps, too, are electric, and both are relatively trouble-free.
The powerplant, however, is another matter. Difficulties fall into several categories: magnetos, con rods, cylinders and turbos. Most airplanes have been retrofitted with pressurized mags, but check any used model to be sure. The same applies to connecting rods. The suspect rods are Continental part number 626119 and have a C logo with a circle around it. Only a barn dweller would still have the old ones.
Many turbocharged models encounter mid-run cylinder problems of some sort and the K-model is no different. These include the full litany: worn valves and guides, broken rings and cracked jugs. Mid-time turbo and magneto replacements aren’t uncommon, but they aren’t a sure bet, either. The 231’s fixed wastegate means the turbo is working constantly and the engine is susceptible to overboosting.
The 252 doesn’t suffer these problems, although it had trouble with cracked tubes in the induction system before Continental came up with flexible tubing. Even though the 252’s engine installation is less troublesome than the 231, temperatures and stresses on turbocharged engines are greater than on normally aspirated engines. Regular inspections and proactive maintenance is a must for reliable dispatch rates.
mods/clubs
By far the most desirable mods for the 231 are those that make it more like the 252. These include intercoolers from Aircraft Modifications, Inc. (253-851-6440) and the Merlyn Black Magic upper deck controller; contact www.merlynproducts.com or 800-828-7500. Both work as advertised and help eliminate the 231’s engine problems. They also make the 231 perform almost as we’ll as a 252. We consider the Merlyn a must-have for the 231.
There are fewer speed mods for the K-model than for earlier Mooney types, such as the C, F and J models. Single-piece belly skins, minor speed mods, rudder and elevator hinge covers and oversized bushing kits for the nose gear are available from Lake Aero Style and Repair (www.lasar.com, 800-954-5619).
Precise Flight (www.preciseflight.com, 800-547-2558) offers speed brakes for the K-models. LoPresti Speed Merchants (www.speedmods.com, 800-859-4757) has an HID landing light for the K-model and hub caps with filler valve access holes Mod Works, another well-known Mooney house, retains some 39 STCs for Mooneys. However, the facility was wiped out during Hurricane Charley in 2004 and is out of operation indefinitely. Mod Works can still be reached at 941-637-6770.
A big-dollar mod, the Rocket conversion, replaces the TSIO-360 with a 305-HP Continental TSIO-520-NB, yielding 220-knot-plus cruise speeds. Although the conversion is discontinued, these turn up on the used market. Contact www.rocketengineering.com. The thirstier engine in that mod will benefit from larger fuel tanks. Monroy Aerospace (www.monroyaero.com) has an STC to raise capacity to 106 gallons. Hartzell offers three-blade prop conversions (www.hartzellprop.com).
Currently, there are two Mooney Associations, the Mooney Aircraft Pilots Association and Mooney Owners of America. Both have membership benefits, technical support and magazines. Contact MAPA at www.mooneypilots.com and MOA at www.mooneyowners.com.
Owner feedback
I purchased my 1981 Mooney 231 when I lived in Colorado. A turbocharged plane was necessary to fly west over the mountains. This made it possible to go to Steamboat Springs for breakfast or lunch, plus it enabled a more direct route to Scottsdale, Arizona.
My 231 has built-in oxygen for all seats and a large oxygen bottle which must be periodically removed from the battery compartment for recertification. This is not an inexpensive proposition and may keep the airplane on the ground, or at least at lower altitudes, until it’s accomplished.
One of the pleasures is seeing the GPS recording in excess of 200 knots ground speed returning from the mountains to the flatlands under normal power settings. Speed brakes are essential to come down from 17,000 without super cooling the engine. However, I used around 150 to 165 knots for normal flight planning.
Learning to land the Mooney was problematic for some reason. Coming in with half flaps and adding a little power prior to touchdown seemed to help, but I am still working on consistency without ballooning or dropping it in. Sometimes it is difficult to judge how high you are above the runway compared to other airplanes. The rudder pedals are small and one needs to make sure that you are not landing with your feet on the brakes.
Michael Williams
Horseshoe Bay, Texas
After renting Mooneys for almost 20 years, I became involved in a flying club that had Piper Cherokee 180s (108 knots on 8.2 GPH). My wife scared me on a Cherokee flight asking, “What’s wrong with the plane?” After I checked everything, she responded that it was taking so long to arrive at our destination. That’s why in 2001 we purchased a 1980 Mooney 231.
It has several upgrades, including a three-blade McCauley prop, electric standby vacuum pump, intercooler for the turbocharger, the Garmin GNS530/430 combination in lieu of the standard BendixKing radios and an Aspen PFD.
As this is primarily a go-higher, go-faster plane with a reputation for an engine that runs hot, I changed to GAMI fuel injectors and took out the old mechanical engine gauges after installing a JPI EDM930. The EDM930 is a critical change and allows me to operate LOP in cruise, which totally controls the propensity for hot CHTs. During ROP in cruise, it was not uncommon to see cylinders in the 385-425 degree range. Operating LOP, the temps are 345-380. Flying at 12,000 feet, LOP and at 65 percent power I experience 160-165 knots at 9.7-10.1 GPH. At 14,000 feet the fuel burn is almost the same, but the speed is bumped to 166-170 knots.
The fuel tanks on Mooneys seem to leak at 20-year intervals so I had mine resealed, while also adding two 20-gallon aux tanks. The fuel selector remains simple—left and right—so there is no added complexity. If I can accommodate the weight, I could fly 1400 NM nonstop. I make several six-hour nonstop trips a year from Tampa, Florida, to Traverse City, Michigan, on about 72-76 gallons (no-wind flight planning). That leaves plenty of fuel reserve.
Mechanically, the aircraft is very robust with major components like the landing gear requiring few if any repairs. The landing gear in all the Mooneys I’ve flown has been bulletproof; the electric gear is rapid actuating and useful to slow things in the pattern if the aircraft is not fitted with speed brakes. The K-model can be fitted with either long-range tanks or speed brakes, but not both.
Annual inspections are typically less than $1600 unless there is an issue (which thankfully hasn’t happened in several years). Parts availability has always been good.
As it is hangared in Florida, I had the tubular steel structure checked for rust and corrosion and none was found. The aluminum had only two places of corrosion smaller than the size of a dime around the fuel filler cap. The TSIO-360LB engine was a source of issues, including Continental cylinders which developed cracks between the valves and become non-repairable. My current cylinders have over four years in service without a problem. Zephyr Engines here in Central Florida has been excellent both during and after the installation of the rebuilt engine, offering outstanding support.
Alternators (there is only one on the early 231 models) tend to be a sore spot and I have installed three in 10 years. It is the gear-driven design on the rear of the engine which is subject to heat. Finally, there is an STC for a modern model, which is much lighter and hopefully more durable.
As this is a slick, relatively high-performance machine, I take recurrent training offered through MAPA. It offers a two-day course in various locations, and the course and instructors are truly outstanding. If you plan to operate an aircraft like this, recurrent training is a must.
I really like my 231. It is fun to fly, it can fly approaches at 130-plus knots when required to sequence in with the jets, it is durable—with great parts availability—and it can sip gas (while LOP) and still make decent speeds.
Bob Cochell
Tampa, Florida
My Mooney 231, N10162, was originally showcased in a Mooney company product photo shoot. Purchased approximately eight years ago following an extensive renovation in Texas and ferried to Connecticut, I quickly completed type-specific training with local designated examiner Wally Moran. Our 231 had approximately 700 hours total time when we took delivery, plus it had new paint and a new interior.
We fly the airplane approximately 50 hours per year, and as an instrument-rated commercial pilot I typically complete a minimum of two IPCs annually, as we’ll as at least one FAA Wings program phase.
We base the aircraft at Hartford Brainard Airport in Connecticut and have it maintained by Total Aircraft Maintenance on the field. N10162 boasts an intercooler, RayJay turbocharger, King KFC200 autopilot, a Garmin GNS430, plus an HSI system.
We make good use of the oxygen system, since the aircraft can cruise up to 24,000 feet. We typically cruise at 170 knots at 58 percent power while burning roughly 10.5 GPH.
The aircraft is always hangared. Including hangar fees, insurance, annual inspections, plus our careful attention to avionics and maintenance work, the hourly cost of operation is approximately $275.
With a comfortable interior—and with styling similar to a sports car—my wife Cheryl and I typically enjoy flights from Connecticut to Maine and Nantucket, Massachusetts. So far our longest flights have been to Michigan and Minnesota.
Like so many Mooney pilots we adore our 231’s speed and efficiency. We live by the Mooney creed: We love to fly—fast.
Tony Crespi
Hartford, Connecticut
