The Mooney 231

To say that Mooney batted one right out of the park with the 201's 1976 debut somewhat understates the matter. This much-tweaked M20 tore out of Kerrville, Texas, nearly 20 knots fleeter than the previous 200-horsepower M20F. Greater efficiency and handsome new looks brought the 201 sales success that had always seemed to elude the M20 series. About the only thing pilots asked for was more — more speed, greater flexibility, and additional payload.

Mooney answered quickly, trotting out the 231 as a 1979 model. Gaining its moniker from its top speed in miles per hour (as did the 201), the 231 or M20K carried all the aerodynamic tweaks performed to turn the 201 from utility infielder to star batter. With turbocharging and an additional 10 hp over the 201, the 231 got not just newfound speed, but the ability to fly where the M20J was simply out of its league — up high, where the turbines roam.

Before getting caught up in the performance figures and beginning shopping for a 231, ask yourself if turbocharging is for you. Pilots flying out of mountain strips, or those who need to cross the Rockies with some frequency, will find turbocharging a must-have. On the other hand, those of you living in the East may well discover that a blower adds more expense and potential maintenance problems than the occasionally used flexibility is worth — you are prime 201 candidates instead.

Turbocharging came to the M20 in much the same way as it did to Piper's Arrow in the late 1970s. Tossing the 201's rugged and highly regarded Lycoming IO-360, Mooney fitted a fixed-wastegate version of the Continental TSIO-360; a -GB1 up through 1983 and the -LB1B until the model's demise with the 1986 introduction of the 252. Mooney's engineers rejected a turbocharged IO-360 in favor of the Continental six, concerned about the reliability of a turbocharged four-cylinder engine — this was an understandable worry given the generally unimpressive reliability of the aftermarket add-on turbo systems fitted to the M20s during the 1960s and early 1970s. That Continental made its engine extremely price competitive sealed the deal.

Good economics for Mooney, but perhaps a black hole of cash for owners, goes the hangar lore. Pilot lounge talk calls the 231 a maintenance hog, a title earned by the engine and turbo system's spotty reliability record. In truth, the 231 is no worse than other high- performance turbocharged aircraft if it is properly maintained and carefully flown.

In return for additional power-management work load, the 231 offers excellent performance. At 8,000 feet, the turbo M20, at 166 knots true, manages to outrun the 201 but not by much. The extra 10 hp barely makes up for the extra cooling drag and greater weight of the 231. Take the M20K up to 12,000 feet, and the performance advantage decisively swings its way. Count on 173 knots true at 75 percent and 163 knots at 65 percent. True airspeeds keep climbing with altitude right to the 231's certified ceiling of 24,000 feet, where 75 percent will get you 188 knots and 65 percent earns 178 knots. (Its namesake 231-mph/200-knot top speed comes at full power, 14,000 feet, and a petroleum-guzzling 24-gph fuel flow — not a very realistic day-to-day cruise setting, in other words.)

Weight plays a role in the 231's performance; expect to add about a knot or two for each 100 pounds under maximum gross weight. Fuel flows are consistent at 12.7 gph at 75 percent and 10.9 gph at 65 percent.

Flight planning takes a bit more time with the 231 because you should carefully consider cruising altitudes with respect to wind. Generally, the airplane needs a substantial tailwind to make cruising at or near its maximum altitude worthwhile. Remember, the 231 averages only about 800 feet per minute in cruise-climb through 12,000 feet, tapering to about 500 fpm at Flight Level 240, and it's burning about 18 gph in the climb. To come out on the right end of the efficiency scale, the tailwinds must be sufficiently strong to offset the time and fuel used to get to altitude. Most 231 owners elect to fly between 12,000 feet and FL200.

Pre-1983 M20Ks had 73 gallons of usable fuel aboard, and later models, with revised fuel pickup tubes, carry 75.6 gallons usable. It takes some training of line personnel and a bit of patience to completely fill the 231's tanks. In a situation similar to the Cessna 210's, it's easy to leave 5 gallons or so out of each tank when you think each is full. With the tanks full, the 231 has good endurance; it will stay aloft for nearly six hours with an hour's reserve at moderate cruise power settings.

Unfortunately, filling those tanks to the brim illuminates one of the 231's greatest shortcomings — and one common to most Mooneys — limited payload. Typical empty weights of well-equipped 231s (and most are well equipped) run around 1,950 pounds. Against a maximum gross weight of 2,900 pounds, that leaves just under 500 pounds for the cabin with full fuel.

Handling is garden-variety Mooney, which is to say heavy in roll, light in yaw (but with exceptionally good yaw stability), and moderately hefty in pitch. Pilots moving from a 201 will immediately notice the extra weight of the turbo powerplant hung off the firewall; full-flap landings with one or two aboard often require nearly full-nose-up elevator trim. Though it hardly inspires fighter-jock fantasies, the 231 is a stable and predictable instrument-flight platform.

A couple of 231 quirks ought to be noted. There is a substantial pitch change with the last third of the flaps, but adding nose-up trim along with full flaps helps. Be ready in the case of a go-around for the airplane will pitch up dramatically if you add power and retract the flaps immediately. Another quirk of the Mooney comes on landing. Much has been written on how to land a Mooney, but the simple fact remains that speed control is the greatest aid to squeakers on the 231. Don't get carried away adding margins for gusty winds or heavy loads; 70 to 75 knots over the threshold works well for most conditions. Although the airplane will feel somewhat mushy at 70 knots, there is usually plenty of energy left once you get it into ground effect.

When shopping for a 231, peruse the logs carefully for mention of a propeller strike. It will enter a crowd-gathering porpoise if you try to force it on — a maneuver only exacerbated by the bouncy rubber doughnuts on the gear. If the history of any 231 you're interested in buying discloses life on a rental line, in a large flying club, or at the hands of many pilots, shop with care. It's easy to abuse the turbo Continental — easier even if you don't happen to own it — and engine life can be affected by pilot technique. Field experience suggests that the real time between overhauls on the TSIO-360-GB is lower than the published 1,800-hour figure; many aircraft for sale have had overhauls in the 1,200- to 1,400- hour range.

Otherwise, shopping for a 231 is pretty straightforward. Most of the usual Mooney problem areas are common to the 231, including leaking fuel cells (sloshing the wet-wing tanks takes expertise and patience, so finding the right shop is essential) and corrosion in the steel-tube cage (addressed in Mooney Service Bulletin M20-208A). Airframe-wise, the M20 series is generally plenty rugged.

Mooney made relatively few changes to the 231 during its seven- year, 880-unit production run. The engine remained the same up through the end of the 1983 model year, when the LB1B version came aboard. It has stronger connecting rods and a larger throttle body than the -GB1 (and is approved for installation in any 231). The later engine tends to run cooler and is somewhat less sensitive to throttle movement.

Modifications for the 231's -GB1 or -LB1B engine include an intercooler and an automatic wastegate. Turboplus, Incorporated, of Gig Harbor, Washington (800/742-4202), offers an intercooler for the 231 — it's $4,500 plus installation — which is said to reduce induction-air temperature and allow the same cruise power to be made at reduced manifold pressures. Also, Merlyn Products of Spokane, Washington (509/838-7500), sells an automatic wastegate that plugs in where the fixed plug is normally found in the 231. The main advantage is reduced upper/lower deck pressure differential, which keeps the turbo from having to work any harder than it has to; critical altitude also gets a real boost, to 19,000 feet, with the system. It costs $2,300 plus installation. Precise Flight in Sunriver, Oregon (800/547-2558), makes speed brakes for the 231 series, at a cost of $2,900 plus installation. With the speed brakes, slowing down and coming down at the same time is much easier, and they help some of the Mooney's tendency to float in the landing flare.

According to the Aircraft Bluebook-Price Digest, current retail selling prices for the 231 range from $62,500 for a 1979 model to $99,000 for a 1985 airplane. These prices reflect mid-time engines and typical equipment, which usually includes a full IFR radio stack, horizontal situation indicator, and some type of area navigation system. Autopilots are common in the 231; most frequently found are the Bendix/King KAP 200, KFC 200, and KFC 150 models.

Mooney in 1986 introduced the follow-on to the 231, called the 252; it was largely considered to be a 231 with all the problems fixed. It received a new variant of the TSIO-360 (the MB1) with an automatic wastegate, intercooling, revised induction-air arrangement (to eliminate the frequent ram-air icing of the 231's system), and a tuned-induction system. The airplane also got a revised electrical system — a 24-volt setup with optional dual alternators, as opposed to the 231's single 70-amp, 14- volt electrics. The 231 is on the verge of being underpowered in the electrical department. Unfortunately, the 252 was hugely more expensive at its introduction and maintains a lofty margin over the 231 in the used market. A first-year 252 sells for $130,000, according to the Bluebook. Such a great price for the 252 marks it as a star player compared to the 231. But the price gap back to the 231 makes the earlier turbo M20 a good value, sort of a utility infielder against the home-run-hitting 252.

Flying Behind the TSIO-360

Tips on handling the 231's powerplant

To hear some pilots talk, managing the Mooney 231's TSIO-360 powerplant is tantamount to patting your head and rubbing your belly simultaneously: It can be done but not without great concentration. Put that one away in the exaggeration file. With some understanding of the 231's turbo system and a bit of practice, the TSIO-360 becomes easy to understand and manage.

Central to the 231's unusual behavior is the fixed-wastegate turbo system. All turbo systems have a wastegate of some kind, whose job it is to route some of the exhaust gas past the turbine section, limiting production of boost. It employs a ground-adjustable plug threaded into a small bypass pipe in the exhaust system. When screwed all the way in, the bypass is closed, and all the exhaust gas goes into the turbo. Normally, a critical altitude test determines where your mechanic is supposed to set the plug; the book calls for the 231 engine to make full power up to 14,000 feet density altitude.

Like the other fixed-wastegate systems, the 231's demands a deft hand on the throttle to keep the manifold pressure needle from swinging like a rookie batter at outside pitches. Minute changes in throttle position, propeller speed, airspeed, temperature, and altitude will move the pointer from the desired mark.

Takeoffs are normally accomplished with the throttle at about half-travel at sea level. As the airplane accelerates and ram-air pressure builds, the boost will increase, in some cases calling for a slight throttle reduction around rotation time.

Though you could climb at full power, few 231 owners do because the engine is making copious amounts of heat and consuming great quantities of fuel. Instead, cruise-climb ought to be initiated at a safe altitude, using the book-recommended settings of 34 inches, 2,600 rpm, and about 18 gph. (As an alternative, you can use 35 inches and 2,500 rpm; this yields about the same power and fuel flow, with significantly less noise.)

Cowl-flap management is essential to keeping the TSIO-360 happy. Consider the needs of the engine; don't just open and close the flaps by rote. On cool days, you can ascend at cruise-climb power with the cowl flaps in trail, and the cylinder head temperatures will remain reasonable. On very cold days, you might even have to close the flaps completely in the climb to maintain mid-green-scale CHT. Either way, they will usually have to come open again during high-altitude climb.

Choosing the best cruise power setting means considering a number of variables. Generally, the TSIO-360 likes to run quite oversquare; for example, 31 inches manifold pressure and 2,300 rpm in cruise. Pick the rpm that will offer the least vibration and vary the throttle opening to set the desired percent of power.

All-cylinder EGT and CHT instrumentation is highly desirable with the TSIO-360 because it will help you lean accurately and keep a close eye on cylinder temperatures. Lean smoothly until the first cylinder reaches peak EGT; normally this will be cylinder number five or six. If you continued leaning to peak turbine inlet temperature, the instruments would disclose the numbers five and six running well lean of peak. Typically, engine roughness will accompany this lean-of-peak mixture. For best turbo life, consider running well below the maximum TIT of 1,650 degrees Fahrenheit — more like 1,550 degrees F. Continental says operation at peak TIT is recommended only at 65-percent power or less.

On descent, make the first power reduction slight, and keep adjusting the throttle as you gain airspeed; the manifold pressure will creep up all on its own. Once the cylinders have begun cooling, you can further reduce power. Talks with powerplant engineers suggest that the first power reduction is the most critical in keeping the top end happy.

Finally, don't forget the three-minute cool-down period after landing to make sure the turbo has wicked as much heat to the engine oil as possible before shutting down the lubricant supply. Time spent taxiing counts toward the three-minute rule. — MEC