Used Aircraft Guide
July 2015 Issue
Mooney’s M20K demands careful engine management and maintenance, but the payback comes in the form of impressive speed and efficiency.
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.
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 well 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.
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.
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.