There’s grim truth to the old clich that when an engine quits in a twin, the second engine only gets you to the scene of the crash. But to fly even that far, you have to avoid rolling the thing due to asymmetric thrust. And thats exactly what Cessna had in mind when it devised the 337 Skymaster.
The notion was to attack the fundamental engine-out handling problems inherent in the design of conventional twins. Lose one, and youre immediately fighting the dreaded Vmc rollover.
So Cessna put both engines on the centerline in a push-pull configuration and the result was that pilots got the safety of the second engine without the penalty of adverse handling. If one quits, identify and feather it and don’t worry about dead foot, dead engine. The FAA even granted the 337 its own class rating that limits pilots to centerline thrust twins only.
Did the idea work? Yes and no. There’s no doubt the Skymaster is as easier to fly than a conventional twin. But its overall accident rate is not much better than conventional twins because, in reality, the Vmc rollover the 337 was designed to avoid doesnt happen that often.
The Skymaster started out as the fixed-gear Model 336. It had the front-rear engine layout, high wing and fixed gear. The 336 was slated to appear in 1962, but a long gestation pushed it back to 1964. Cessna sold only 195 336s in its one year of production. Undaunted, Cessna forged ahead with the retractable-gear 337 Skymaster in 1965 and sold 239 units. In 1967, there was an abortive attempt to build a lower-powered version, the 327, but it proved too slow and the project was dropped.
The original 337s borrowed the complex and troublesome hydraulic landing gear system from the 210. In 1973, it was replaced with a somewhat less convoluted electro-hydraulic system that, while simpler, more reliable and better from a maintenance standpoint, is still not as robust as the electro-mechanical system on a Mooney or Baron.
Early 337s also had a problematic fuel system with multiple tanks that was replaced in 1973 by a superior, less complicated system. A turbocharged version, the T-337B, appeared in 1967, but was dropped in 1972 with the addition to the Skymaster line of the almost revolutionary pressurized P-337 version, with up-rated 225-HP engines. The turbo was revived in 1978, but Skymaster sales had begun slipping by then. Cessna pulled the plug following the 1980 model year, after a total Skymaster production run of 2058, plus 332 pressurized versions. In addition, about 500 Skymasters were built for the U.S. Air Force and saw action in Vietnam as the O-2A. It boasts structural beefups, hard points and extra windows. These airplanes appear on the used market and may we’ll be the least expensive warbirds available.
Major model changes were few but as the model history chart shows, there were many designation changes. Some airframe changes starting about 1970 made maintenance a little easier with the addition of access panels and the airframe was lightened a bit. That had a small impact on useful load. The interior arrangement also changed through the years, with various combinations of seat mounting. During its years in production, the 337s gross weight crept up, starting at 4200 pounds and eventually making it to 4630 pounds, but max landing weight was limited to 4400 pounds. (The P-337, with its 30 extra horsepower, had a takeoff weight of 4700 pounds and max landing weight of 4465 pounds.)
Market Scan
With gas prices hovering in the $4 to $5 range, twin prices are as soft as ever and the 337 is no exception. On the up side, most of the depreciation has been squeezed out of these airframes but the tarnished reverse side of that coin is that 337s cant be counted on to increase much in value. But that said, a 337 is a lot of airplane for the money. One reason prices are low, beside gasoline prices, are that the 337 has a reputation for being a maintenance hog, largely undeserved, and as twins go, they arent all that fast. The Skymaster doesnt perform much better than a Cessna 210, but it has two of everything to maintain and replace, which drives up ownership costs.
Scanning the Fall 2005 Aircraft Bluebook Price Digest, we find that the earlier Skymasters sell for $50,000 in reasonable shape, a heck of a good bargain for a twin. Pressurized and turbocharged models sell for essentially the same prices, for equivalent years. The pressurized Skymasters remain the best buys in the relatively small pressurized market segment.
Buyers should beware, however, that buying a cheap twin is not the same as operating a twin cheaply. A hangar queen will eat through a bunch of money if it needs remedial work and, in any case, find a shop that knows the breed to do the pre-buy and maintain the airplane going forward.
Performance, Handling
Although they hold their own, Skymasters arent Barons in the speed department, although the turbocharged models do respectably for pilots willing to take them into the teens. Owners tell us they plan between 150 and 155 knots, although some insist the airplanes are capable of 160 knots.The turbo and pressurized models will push 190 to 200 knots at 20,000 feet, their maximum certified altitude. At the non-oxygen middle altitudes, 170 to 180 knots is a typical speed for the turbo models, which isn’t bad.
Since Skymasters have small displacement six-cylinder engines, fuel burns tend to be reasonable, ranging from 15 GPH to 22 GPH total, with 19 to 20 GPH typical for a 150- to 155-knot cruise. For comparison, a Twin Comanche will do about the same speed on 100 less horsepower and a lot less gas. Efficiency is not a Skymaster hallmark.
Rate of climb ranges from a modest 1300 FPM in the old 336 to a lethargic 940 FPM in the last 337H models. Were unaware of any other twin-engine airplane with a book rate of climb below 1000 FPM; even the old 150-HP Apache had a book climb of 1250 FPM with both engines running. Some of our respondents own airplanes with the Robertson STOL kit installed and speak highly of it for its ability to boost the airplanes climb rate.
Runway performance, on the other hand, is good. Stall speeds with flaps range from 55 to 62 knots, depending on the gross weight of the particular model-about 10 knots below conventional twins like the 310. As a result, a Skymaster will get off the ground in less than 1000 feet at gross weight-a feat very few other twins can manage. Barrier performance is not quite as good, however; the leisurely climb rate brings the Skymasters 50-foot takeoff figures down to the middle of the light-twin pack.
The single-engine climb rates of all the light twins tend to be very similar-200 to 300 FPM-because engine-out climb rate is a certification point around which the airplane is designed. The FAA requires a certain minimum climb, figured by a formula relating to stall speed, and the manufacturers typically bump up the gross weight to the point at which the airplane just barely meets the FAA minimum. Any excess engine-out climb capability is, in effect, wasted payload. And payload numbers sell airplanes.
Whats surprising is the difference between the front and rear engines.Climb on the front engine only is about 50 FPM less than on the rear.Apparently, when the rear propeller is turning, the airflow over the rear fuselage remains attached. But it separates when the rear prop stops, increasing drag.
Also unusual is the fact that while leaving the gear down produces a climb penalty of a bit over 100 FPM, the act of raising it carries a 240 FPM hit.This is because of Cessnas complicated gear door arrangement, which adds a lot of drag while the gears in transit. In an after-takeoff engine out situation, it may be better to leave the gear down. In normal flight, the Skymaster has typical Cessna handling: heavy in pitch, reasonably responsive ailerons. (The P-model has especially light ailerons.) Pilots praise its IFR stability.
The noteworthy aspect of the Skymasters handling-indeed, the whole reason for the airplanes existence-shows up when an engine fails. Instead of the normal yaw-roll-stall-spin scenario that too often follows engine failure in a normal twin, the Skymaster continues to fly straight ahead. An unprepared or rusty pilot can take his time and concentrate on the task of identifying and feathering the prop on the failed engine, without worrying about losing control.
Payload, Range
A Cessna press release from the 1970s describes the Skymaster as a full six-place airplane with nearly a ton of useful load. This is utter bunk. One owner accurately describes the two rear seats as a joke. And the press release conveniently forgets to mention that with the fifth and sixth seats installed, there’s no baggage space, nor is there a baggage door. (If you must carry six people and bags, a belly cargo pod was an option on later models.) Consider the Skymaster a nice four-placer.
Real-world useful loads run around 1500 pounds-not bad at all-and several hundred pounds more than a Twin Comanche, for example. Standard fuel is 88 gallons and that still leaves around 900 pounds available for payload, more than enough for those four passengers and lots of baggage. Standard fuel is just adequate, however, unless you throttle back-a bit more than three hours with IFR reserves at fast cruise.
Pre-1973 airplanes with long-range tanks had a four-tank fuel system, while later airplanes had a two-tank system with a single fill point on each wing; simpler and safer. The long-range tanks-148 gallons in 1975 to 1980 models, 123 gallons in earlier models-solve the range limitations nicely, at the expense of payload, of course. One owner reports that with long-range tanks full, he has seven-plus hours at 150 knots with 650 pounds of payload (three people and bags). Not a bad compromise.
Oddly, the P-337 is allowed only five people; it was certified under different rules that require an emergency exit in a six-seat airplane. Rather than put in the exit, Cessna simply limited the seating to five. Regarding the seats, early P-Skymasters had a middle seat that hinged up and to the side to get at the back row, but these seats didnt slide fore and aft.Access to the rear seats in other Skymasters requires an awkward scramble over the center row.
The Skymaster is a roomy airplane and visibility is excellent, about as good as it gets in any light airplane, single or twin. The view down is unlimited, of course, and the wing is set far enough back that it doesnt block upward vision, as it does in most Cessna singles. The good visibility is not only a safety feature, but it adds to the feeling of roominess in the cockpit as well.
Although its relatively roomy, the Skymaster is also quite noisy, according to owners weve heard from. Both engines are attached directly to the passenger compartment and sympathetic vibration can be a problem, particularly without the prop synchronizers. Conventional twins are quieter by far.
Maintenance
Heres the potential Skymaster nightmare: Runaway maintenance costs, particularly in the turbo and pressurized models. The Skymaster was the most complex aircraft ever engineered and manufactured by Cessnas Pawnee Division, which otherwise built only Cessna singles. The evidence suggests the Pawnee Division simply wasnt up to the task, particularly in the 1975 to 1980 period when production was growing rapidly and Cessna was plagued with an epidemic of design, engineering and production problems.
For example, the pressurized Skymaster was initially such a disaster that the first years production was recalled to the factory for complete remanufacture and modification. As one owner noted, Cessna had to pack everything into the fuselage, not having the luxury of sticking systems out in the wings or even into a tailcone. As a result, access is difficult.
Most of the Skymasters problems seem to be in the systems. The basic airframe is stout, with a rugged strut-braced wing. There are remarkably few ADs on the airplane. And remember that the military version of the Skymaster, the O-2 observation aircraft, did plenty of rough duty in Vietnam, often flying home with bullet holes and worse.
Closely examine the logbooks and service records of any Skymaster considered for purchase. And be sure to keep a sharp lookout for these chronic Skymaster problems: Oil leaks in the rear engine. Loose rocker box covers, a chronically leaky quick-drain and a sloppy breather tube seem to be the main culprits.
Defective landing gear switches. Cheap plastic switches plagued the Skymaster during the mid-1970s. In fact, the entire landing gear system is a challenge to maintain and proactive maintenance and inspection will help.Water leaks around the windshield were a chronic problem in older models.Check for possible rain damage and check the condition of the windshield sealant.
Hot-running rear engine. This a subject of some debate. One Skymaster buff we talked to swears that the hot rear engine problem is a red herring due to defective gauge installations. He told us precisely calibrated digital six-probe CHT gauges show that the engine usually doesnt run hot. But careful attention to the rear engines baffling and cowling will keep it cool.
Defective paint jobs. When Cessna switched to DuPont Imron on the Skymaster (as we’ll as the other Pawnee models) in 1977, it ignored DuPont requirements for metal preparation and priming, using a cheap, quick wash primer instead of the required alodyne and epoxy primer. The result was an epidemic of filiform corrosion, particularly in warm, humid coastal areas. By now, most of the surviving airplanes will have been painted at least once.But check the logs and inspect the paint carefully.
Mods, Groups
The Riley Rocket was a popular Skymaster mod and included upgrades to 310-HP TSIO-520 engines, intercoolers, three-blade props and air conditioning.Rockets come on the market now and again, a premium price over stock models.For more, check out www.skymaster.com.
Other mods include vortex generators from Micro Aerodynamics (www.microaero.com/ and 800-677-2370), intercoolers from American Aviation (www.americanaviationinc.com and 800-423-0476. Horton and Sierra Industries have STOL kits and other mods. Contact Horton at www.airsport.com and Sierra Industries at www.sijet.com, or 888-835-9377.
Cessnas seem generally blessed with good owner organizations, perhaps because the company abandoned the piston market in 1986 and stayed out of it until 1997. The clubs and groups have proven to be as good as it gets when it comes to support.
Every Cessna owner should join the Cessna Pilots Association (www.cessna.org, 805-922-2580). The organization offers the usual benefits, including an insurance program, monthly newsletter and fly-ins, and has a wealth of Skymaster-specific information. For a useful if unofficial Skymaster site, see www.skymaster.org.uk.
Owner Feedback
I bought a 1973 pressurized, Riley-modified Cessna (Skyrocket) in May of 2000. I have flown it 630 hours in over five years. The airframe has 2700 hours and was zero-timed by Riley International in 1991. The airplane came from Cessna with a front hot prop, boots and windshield hot plate. I subsequently replaced the Loran with a Bendix/King KLN94 GPS because it was a minimum change to the wiring and my panel real estate sells at a premium. I was the airplanes third post-Riley owner when I bought it with 640 hours on both engines and the refurbished airframe. I put factory remans (TSIO-360-CBs) on it at 1060 engine hours in 2003. The engine and turbocharger replacements cost about $105,000 for parts and labor. Prop and governor overhaul was also due and cost another $15,000.
The P337 is great for my missions. I commute up to six times a month, once or twice on the average, to Southern California over the Sierras from Reno, plus other trips in the mountain west. The airplane cruises at 185 knots true at 18,700 feet (its single-engine service ceiling) and does this on 24 GPH at 2400 RPM and 30 inches MP. You can go faster, but you’ll burn a disproportionate amount of fuel in level flight.
I usually give the rear engine 2 to 4 PPH more fuel flow to keep both TIT readings at 1500 degrees, equal to that of the front engine when leaned using the GEM. This allows me to close the rear cowl flaps completely and lose up to 2 or 3 knots of drag. Normally, with 123 gallons usable, I land before four hours have elapsed.
If you throttle back, both the POH and my burn rates suggest you can get eight to 10 hours endurance, but neither my bladder nor the decay of pressurization at altitude under low power make this attractive. Under normal loading and full fuel at sea level, handling is similar to a Cessna 210, except for the Skyrockets intercooled climb rate, which can be north of 1800 FPM. The airplane is stable, responsive and maneuverable anywhere in its cabin-limited regime up to 20,000 feet. With the Horton STOL kit, Vso is a hard-to-believe-but-real 48 knots.
In the two years prior to replacing the engines, I averaged $40,000 per year in combined annual and maintenance expenses. I bought some gyros, fixed some radios and did a pair of progressive overhauls on the original engines–cylinders, pushrod tube seals, main seals, turbochargers, variable controllers, fuel lines and many other goodies.
Maintenance bills have dropped substantially with the new engines and accessories on line. In 2004, I spent $12,500 on maintenance and inspections, but some was for new engine bugs, so to be fair, figure $12,000 for annual, oil changes and stuff that happens. I should have re-engined earlier, but the engines 1400-hour TBO was my elusive goal.
My insurance coverage costs about $5500 and even with a commercial/MEL/instrument certificate and 2000-plus hours, I take annual refresher training to get that rate. The training is excellent, but it mandates another $1300 annual expense.
Here are some points to consider when you think P337:
Its a twin. It drinks like one, spends like one and requires that you have the MEL certificate to fly one.
It offers excellent visibility up and down. The pilot sits we’ll forward of the leading edge and due to its mass distribution, dihedral and twin rudders, it is very stable. Its a fine instrument platform and has great redundancy.
Its engines experience different operating environments. For instance, they drive different diameter props. One sees undisturbed airflow and gets conventional cooling. The other is mounted in tandem but with a vertically offset centerline. It experiences unique vibration, relies on ducted cooling and moves already disturbed air. Other things being equal for the two, the rear engine runs hotter. You manage this with cowl flaps, mixture or power adjustments. Continual attention to engine management in different flight conditions is a key to aircraft performance and longevity.
Cramming two turbocharged, intercooled engines, air conditioning, a pressurized cabin and retractable landing gear into one fuselage necessitated compromises. One of those was made at the expense of maintenance accessibility. You will pay extra for that in labor over time. Also, you get a relatively small cabin for a twin.
The Skyrockets pressurization is great, but adds weight, requires maintenance and mandates annual insurance refresher training. My cabin heater doesnt worl very well. The Riley air conditioner does.
Certificated payload is what it is. At 4700 pounds gross, my useful load is 1170 pounds. Subtract full fuel of 750 pounds and you have 420 pounds to fill four seats and baggage area. Plan accordingly.
Despite equipment and performance, no Skymaster is certified for known ice.
Not every shop is acquainted with Skymaster idiosyncrasies. You will pay to educate someone somewhere sometime. Such investment opportunities arise from landing gear, flap cables, engine accessories, electrical system balancing, Cessna fuel system computers, avionics and turbocharger tuning.Further, the aircraft is nosewheel light, posing ground handling problems for the uninitiated, unwary or common-sense challenged.
Support organizations include Cessna Pilots Association, Superskyrocket LLC: www.superskyrocket.com/ (successor to Riley International Corporation in Carlsbad, CA); and a UK organization: www.skymaster.org.uk.
Owning a Skymaster is all about the safety margin that comes with centerline thrust. Even so, its just as capable of biting a pilot as any airplane. That shows up in the airplanes landing accident stats. But especially on takeoff and to a lesser extent in cruise, it does give you a little breathing room that conventional twins just cant offer.
If you lose an engine in the takeoff decision zone and your gear is retracted, add full power, identify, verify and feather while continuing to climb at about 300 FPM on either engine-no asymmetric thrust, no Vmc. If you have an engine failure in cruise, life is even better. Look forward to an inconvenience on the ground, not a potentially full-blown emergency in the air.
On either engine, a feathered and fuel-light P337 lands like a 182. Those properties of the design accumulate dividends as extra safety margin if we pilots don’t become complacent and squander them with reckless decisions like single-engine takeoffs-which are prohibited-or by flying non-airworthy or badly maintained equipment. Bottom line: Those of us who own and love the airplane believe the extra margin of safety we get from it is worth the bucks it costs us in maintenance as our engines and airframes age.
Jeff Nash
Incline Village, Nevada
I purchased a 1968 T-337C in 2002 after strong consideration was given for all others in the class. As a former Twin Comanche owner, I wanted a non-pressurized replacement twin with predictable landing characteristics, a great single-engine ceiling, some deicing capability and a great view. Not only does the Cessna exceed my expectations, it is a dream to hand fly and can be as solid as you need for precision instrument flying.
If you feel a need for speed, go to a Cessna 310. The T337 is all about seeing the world in true comfort at a leisurely 145 knots true below 6000 feet and 165 knots in the mid-teens.
Thats cruising at 22 to 23 inches and 2350 rpm at 20 GPH total. I do drag around a 34-pound cargo pod and the typical slurry of antennas slowing the speed down a bit from book. Removing the cargo pod speeds it up 3 MPH … wow.
The beauty of the turbo system is the automatic wastegate that is basically just that…just set the throttles and play with the prop synchronization and mixture controls. Speaking of engines, the Continental TSIO-360A engines will eventually leak oil from numerous places.
The most common areas are the oil quick drain, rocker covers, oil filter adapter and magneto/alternator seals. The overboard oil and fuel drains on the rear engine underneath the belly leave a mess around the aft cowl flaps, and extends to the aft propeller and then blowing onto the stabilator.
As far as engine cooling goes, the aft engine is no problem as the cowl flaps are substantially larger than the non-turbo model and they generally arent required on standard days. The front engine cowl flaps are more of an issue in the summertime, particularly with the cargo pod installed. They normally need to be dropped a notch enroute and require the full open setting for takeoff and climb.
Cost wise, insurance at $95,000 hull value runs about $2900 for six seats. Turbo overhauls are about $1900. New props from McCauley are near $14,000 each. Engine overhaul costs vary, but hang around $25,000 complete.It is easier to R&R the rear engine, since its on a mount vice the front engine which attaches to the aircraft framework.
A must-have STC is a rear-engine access panel from the aft cabin that is marketed by Owen Bell at Aviation Enterprises in Nashville. After installation, it immediately calms down the most nervous A&P and will pay for itself in reduced labor costs.
One gotcha is the electric cowl flap motor replacements at $2000 each.There are manual replacements available for retrofit. The fuel gauges and sensors are remarkably crappy and virtually unrepairable. Otherwise, replacement parts for most other systems have been readily available. Annual costs range from $1500 to $3000 for inspection alone, depending on your location. Naturally that doesnt include parts or labor time to fix the broken stuff they find.
When I bought the airplane, avgas was about $2 per gallon and now its about $4. I fly it about 50 to 60 hours per year and you can do the math.The beauty of the airplane is that you can put it away for a long period and not feel behind the speed/power curve when you take to the blue again as you may otherwise find in a Baron/310/P210.
If youre looking for a pilot and passenger-pleasing aerial SUV that will maintain 17,000 feet eastbound over Kalispell, Montana on one engine at night, this is the E-ticket ride. Lastly no, we don’t shut down and feather the front engine to save gas as there is no prop accumulator for restart.Check out the 1978 movie video BAT 21 with Gene Hackman and Danny Glover for super aerial footage of the O-2 version.
Steve Bulwicz
Eden Prairie, Minnesota
Also With This Article
“Cessna 337 Skymaster Charts and Specs”
“Vietnam O-2 Tour: A Year of Flying Dangerously”
“Accident Scan: Oddball Causes”
