Mooney M20K

Few airplanes fly as fast or as far on as little gas as the K-model Mooneys. The 252 is the most refined version of the line.

Although they like to complain about the high cost of avgas and maintenance, most aircraft owners pay only passing lip service to true efficiency. They simply pay what it takes to keep the airplane in gas, oil and parts and let the dollars fall where they may.

Mooney owners are different. They gravitate toward the type because its not expensive to buy and goes respectably fast on relatively little gas.Turbocharging can sweeten such efficient performance.

Mooney got into turbocharging relatively late compared to other manufacturers and the first efforts proved somewhat wanting. But that first turbo model, the M20K or 231, overcame initial teething troubles to evolve into the 252TSE, which some regard as the best Mooney ever made.

Model History
Cessna tapped the high-altitude market first with the successful T210 in 1966. Mooney addressed the demand in 1967 with the M22 Mustang, a big, ugly 310-HP pressurized single that never caught on and was axed in 1970. Through the 1970s, Mooney did we’ll with small, efficient airplanes powered by Lycoming four-bangers. Mooneys 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.

Still, Mooney had no turbo model and when Piper put a blower on the Arrow in 1977, the Kerrville crew got busy with plans of their own. 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 201s 200-HP Lycoming IO-360.

The 201s mannerly handling, speed and comfort combined with the appeal of being able to fly high was a powerful draw. Mooney sold 246 the first year, outstripping the 201 by nearly two to one. Differences between the 231 and 201 are few; the cowl is longer and there are some minor aerodynamic refinements. Fuel capacity is 10 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 that hurt the 231s reputation. These were several fold. The new cowling didnt 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 thus 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.

Obsessive attention to operating technique and maintenance could avoid these problems with the so-called GB engine, but in the real world, they were proving to be a pain for owners and gave the airplane a dark reputation it still suffers.

Mooney refined the line in 1982, adding a split rear cargo seat 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. There was also an improved alternate air intake system, to address reports of icing-induced power loss.

The fixes helped but more was needed, hence the emergence of the 252TSE for Turbo Special Edition in 1986. 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, three-position manual flaps. There was a vernier throttle control, 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 much needed, since a fully loaded K-model could max out the electrics long before the days of moving maps. An electrically driven back-up 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 rarer: production totaled, ironically, 231 by the time it was discontinued.

The follow-on airplane, the more powerful M20M TLS, appeared in 1989 and proved popular enough that the 252 was discontinued after 1990. It was revived briefly in 1997 as the Encore, then dropped again in 1998. True Mooney enthusiasts mourn the 252s demise, for it was the epitome of what makes Mooneys attractive: maximum performance on minimum fuel.

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.

Realistic max cruise is about 190 knots for the 231, but 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 also outclimb the others by about 150 FPM.

In actual use, few owners fly 231s at those speeds and altitude, due to physiological considerations. High teens to low 20s are the airplanes safest envelope. At lower altitudes, turbocharged airplanes arent much faster than their normally aspirated siblings. In fact, the 231 is actually slower than the 201 below 8000 feet, due to cooling drag.

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 231s 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 cant 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 tuned 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. Its 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. The 231s critical altitude is only 14,000 feet, while the 252s 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. Thanks to its automatic wastegate, the 252 doesnt suffer these foibles.

The 231/252 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, however, 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.

Because of its ability to fly fast, the best addition ever devised for Mooneys are speed brakes. These are especially useful for the 231, which doesnt have the 252s higher gear limits. (Speed brakes are standard on 252s.) Ground handling isn’t great. The airplane is low slung and the Mooneys stretched-out seating position makes visibility on the ground worse than it might otherwise be. Also, 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 with respect to ground handling: Many Mooneys suffer damage to the nose gear trunion when towing turn limits are exceeded via power towing.

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. 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. 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. Its more of a sports-car posture than an upright seating regime. There’s plenty of leg room fore-and-aft, but not much shoulder room. Those of below-average height may find that they cant 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 its the quietest of all Mooneys, thanks in part to the induction system.

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 its 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, although we would prefer them front and center. 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.

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 electro-mechanical. The flaps, too, are electric, both are relatively trouble-free.

The powerplant, however, is another matter. Difficulties fall into several categories: magnetos, con rods, cylinders and turbos. By now, few airplanes havent 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.

Most turbocharged models encounter mid-run cylinder problems of some sort and the K-model is no different. These include the full litany: worn valve and guides, broken rings and cracked jugs. Mid-time turbo and magneto replacements arent uncommon. The 231s lack of an automatic wastegate means the turbo is working constantly and the engine is susceptible to overboosting.

The 252 doesnt suffer these problems, although it had trouble with cracked tubes in the induction system before Continental came up with flexible tubing. Even though the 252s 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 or 800-828-7500. Both work as advertised and help eliminate the 231s 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 (, 800-954-5619).

Precise Flight (, 800-547-2558) offers speed brakes for the K-models. LoPresti Speed Merchants (, 800-859-4757) has an HID landing light system for K-model and hub caps with filler valve access holes.

Mooney Mart claims to sell a range of speed mods, including gear doors and aileron gap seals. Contact or 941-484-0801. We advise caution in dealing with this shop; some of the claims on its Web site are substantially out of date. Read the sites disclaimer carefully.

Mod Works, another we’ll known Mooney house, retains some 39 STCs for Mooneys. However, the facility was damaged 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. Contact The thirstier engine in that mod will benefit from larger fuel tanks. Monroy Aerospace ( has an STC to raise capacity to 106 gallons. Hartzell offers three-blade prop conversions. Contact

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 and MOA at

Owner Feedback
My first Mooney was a 1982 Model 231 that I purchased in 1984 when it had accumulated about 1400 hours. I flew it about 500 hours over the next three years and when the engine had 1900 hours on it, I converted it to a Rocket 305. I put another 500 hours on that engine and then traded it in on a new 2000 Mooney Bravo.

The 231 is a fabulous airplane. Mooney is one of the few manufacturers whose POH numbers are routinely attainable and this bird didnt disappoint.Normal indicated airspeed was in the range of 140 to 145 knots, which translates into about 140 to 190 knots true, depending on altitude.

These numbers were achieved at 70 percent power burning about 11.5 GPH with the original GB engine, plus a Merlyn automatic wastegate and a TurboPlus intercooler, both of which I had installed immediately after purchasing the airplane.

The GB engine is known for running hot and although the intercooler and wastegate helped, temperature management was still critical. One day when departing Las Vegas, I could only climb at about 200 feet per minute in order to keep the nose low enough to provide cooling. Nevertheless, for the most part, the airplane climbed about 900 FPM in cruise climb configuration of 110 knots. A ham-fisted pilot abusing the throttle and ignoring the temperatures can crump a GB engine in under 400hours. My engine had compressions still in the 70s after 1900 hours when it was exchanged for the Rocket conversion.

Annuals were typically in the $1500 to $2000 range when performed by local mechanics and about $1000 more when done by the big Mooney shops. Every Mooney should see a real Mooney expert at least every couple years. My favorite is Top Gun Aviation in Stockton, California. In addition to being a Mooney factory authorized service center, they annual over 100 Mooneys a year, so they are real experts.

When I first bought the 231, I only had 90 hours total time, none inMooneys. The insurance company required 10 hours of dual and the first year premium was $2100 with a $100,000 hull value and $1 million smooth liability limits. By the time I converted to the Rocket, my hull value was up to $175,000 but the insurance had only risen to $2300 due to my increased experience.

The Rocket conversion makes for an unbelievable performer, but its not without cost. The conversion itself runs about $80,000. Then there’s the increased fuel burn (20 GPH versus 11.5 GPH). Cruise climbs at 135 knots showed 1800 FPM at the new 3200-pound gross weight, up from 2900 pounds pre-conversion. Airspeed jumped up some 30 to 40 knots, depending on altitude.

The only bad news is a substantially increased tendency to yaw in turbulence due to the barbell effect of putting a much bigger engine in a small airframe and trying to balance it by stuffing two huge batteries in the back of the tail cone. I put up with the yaw for a while and then traded for a new Mooney Bravo, because the long body Mooneys are much more stable in the yaw axis than the Rocket.

Steve Brault
Lakewood, Washington

I have owned a 1982 Model 231 for seven years and 800 hours. After considerable effort searching for a high-performance single, the 231 was by far the best all-around performer and most capable aircraft for the money. I chose a 1982 model because I wanted the reclining, removable split-back bucket seats, built-in oxygen and upgraded digital radios, all standard that year.

The airplane has few negative qualities and is stable in instrument conditions. Landings are simple as long as you keep the speed around 70 to 75 knots over the threshold; anything faster and the airplane will float in ground effect.

The Continental TSIO-360 engine has a bad reputation but I think the majority of the issues are unfounded. My aircraft has the LB1 engine, Merlyn controller and a Performance Plus intercooler. My CHTs are typically between 325 to 350 degrees F in cruise and I typically run 65 percent power on 11.5 GPH. This equates to a 1500-degree turbine inlet temperature on the engine monitor. I have had no cylinder or engine issues to speak of while running the engine conservatively.

I typically see 165 knots true at 65 percent power and 8000 feet. The speed increases to 180 knots true at 17,500 feet, still at 65 percent power and 11.5 gallons. The aircraft is capable of almost 190 knots at this altitude at 75 percent power, although I opt for 65 percent.

Another perceived negative is that the Mooney is short on room. My brother is 6-feet 5-inches and weighs 240 pounds and I am 6-feet 1-inch and 190 pounds. We both fit comfortably in the cockpit for long trips. Getting into the aircraft is not the easiest, but once inside, I find the legroom to be fantastic.

A few items when considering a 231: Make sure Service Bulletin 208 A and B is done. This has to do with insulation installed at the factory around the steel tubular structure. If the old insulation is not removed, the structure can rust. Be sure to have a reputable Mooney shop do the pre-purchase, since many items can get overlooked by those not familiar with the Mooney.

The costs to operate and maintain are reasonable for the performance of the aircraft. My insurance for $1 million of smooth is $2300 a year.Maintenance typically runs around $4000 to $4500 a year without any major surprises.

An engine overhaul is expensive and typically cost around $30,000. Many 231s tend to need some expensive care and feeding prior to reaching TBO. A top overhaul for $10,000 and a turbo may need to be replaced along the way if not operated properly.

I highly recommend The Mooney Aircraft Pilots Association. In addition, every year or two, I try to attend the Mooney Safety Foundations three-day training class. It is top-notch, run by a great group of people and the lessons learned are valuable. Insurance will typically give you a break if you attend a class.

Mark Desautelle
Norwalk, Connecticut

Ive owned my 1980 Mooney 231 for 15 years, having previously owned a V-tail Bonanza. The aircraft has been a pleasure to fly and is an excellent performer, particularly in the flight levels. Any problems have been focused on the Continental power plant and whilst an upgrade to the LB1 version greatly improved the overheating concerns, these engines nearly always require a mid-life top overhaul.

Im certain that much of the problem starts with brand new cylinder kits from TCM that don’t meet specifications. To bring them up to specification, my mechanic reworks the cylinders, valves and seats when new and this seems to improve longevity.

I dream of the day when Toyota, Honda or BMW bring out an aviation engine for the 231. TCM quality and lack of support is, in my view, a serious issue. The add-on Merlyn wastegate is a must for any 231, as is an engine monitoring system.

Notwithstanding the engine issues, Im very happy with my 231. Its reputation as the sports car of general aviation is we’ll founded.

Russell Kelly
Mitta Mitta, Australia

Also With This Article
“Mooney M20k/231/252 Specs and Charts”
“Accident Scan: Mixed Causes”