198211-1964-73 Cessna 337 Skymasters.pdf - Aero Resources Inc

slightly higher than, those of com- parable light twins. Even the much- maligned 336, according to the pilot's operating handbook, can manage a gross-weight single-engine climb of 355 fpm. And this is what the book refers to as the front-engine-only climb rate. Since the rear (pusher) en- gine is aerodynamically more effec- tive, the rear-engine only rate of climb is listed as 420 fpm. These fig- ures rose to 360 and 450 fpm for the 337, then sank slightly to 295 and 375 fpm (due to higher gross weight) in SKYMASTERS the turbocharged 337s. Single-engine climb in the pressurized 337s is claimed as 375 fpm. Service ceilings also are listed as slightly higher than the competition. As with single-engine climb rates, single-engine service ceilings are higher in rear-engine-only operations. Handling, climb and performance problems? While the Skymasters have shortcomings in these areas, they are minor and not enough to be the rea- sons for the airplane's fate. The 337 series cruises only a tad slower than similar light twins,carries comparable loads (there is even an optional 13 cu- bic foot, 300-pound capacity cargo pod, made available in 1966) and does this while burning a total of no more than 23 gph, at 75-percent power. Not bad, considering the airplane's rela- tively high empty weight and the dis- placement of the engines. Ill-suited for high-altitude opera- tions? This is not the reason either. Oxygen systems are available for all models, and a buyer can choose from turbocharged or pressurized versions: The SkYlI/astcr plwtographed for this article, oWlled by Dr. Robert Poole of Washillgtoll, D.C., is a 1965 Model 337. The power quadrallt call muse nmfusioll at first. The right levers cOlltrol the rear el1gille (just rell/ell/ber I~ight-Rear); the left levers, the frollt clIgil1e. 38 • NOVEM8EH 1982 •.•. . -I ••••••••••••••• A>. Cesslla 1l'CIltol'a/Joard to d.'mollstrate the Skymaster's ellgille-out directiollal stability. This 1963 promotiollal photograph shows a 336 takillg off 1l'ith the frollt propella feathered. Such StlllltS helped ellcourage some pilots to do the same, with IIIlsuccessful resuIts. Lack of all-weather capability? Per- haps. Leading-edge deice boots and propeller anti-ice systems are options that are available on all Skymasters. . But an approved windshield heating system is not. (One Skymaster manual recommends stretching your arm out the vent window and using the con- trol lock to scrape ice off the wind- shield.) For this reason, the Skymaster never received approval for flight into known icing conditions. In 1977, weather radar was first made an op- tion. A Bendix RDR-160 could be in- stalled, its antenna housed in a fiber- glass pod slung under the right wing. Unless an owner receives a supple- mental type certificate, however, the radar option is not available to own- ers of earlier Skymasters. For these airplanes, Stormscopes have become a popular panel addition. So Skymasters can be equipped to negotiate adverse weather conditions. Perhaps the answer lies in the Sky- master's systems, safety record, main- tenance peculiarities or airworthiness directives. Until 1973, Sky masters were equipped with engine-driven hydrau- lic systems. Then a switch was made to a simpler, lighter, electrically driven hydraulic power pack, a heavy-duty derivative of the gear sys- tem used in the Cessna 210. In the pre-1973 Skymasters, a single hydraulic pump is mounted on the front engine. If the front engine fails, the landing gear has to be extended manually, using the emergency pump handle stowed beneath the floor to the right of the pilot's seat. In 1967 Cessna offered an optional, second hydraulic pump for the rear engine. Besides providing system redundancy, this second pump reduces gear retrac- tion time by 40 percent. A schematic diagram of the hydrau- lic system looks like a plumber's nightmare. The system uses a complex network of pressure sequencing switches and priority valves, all de- signed to ensure that the landing gear and their doors operate in the correct order. For all its complexity, though, the hydraulic system never brought about any airworthiness directives and has not generated an unusually high number of service bulletins. The electro-hydraulic system also has had a relatively uneventful service his- tory, having been more or less per- fected through lessons learned in its 210 application. Beginning in 1974, Cessna tacked a nitrogen bottle onto the electro-hydraulic system. This pre- vents the gear doors from falling open when the airplane is unused for long periods of time. As with the landing gear systems in much larger airplanes, Skymasters with engine-driven hydraulic systems have a gear selector handle that snaps back into a neutral position once the cycle sequence is completed. This in- dicates that the hydraulic pressure has stopped building and that, together with an amber (Up) or green (Down) indicator light, the gear and their doors are locked in the proper posi- tion. A handle that remains in the Up or Down position serves as a warning: The pump(s) is still pressurizing the system and overheating may occur. The pilot should return the selector handle to neutral so that pressure may be relieved. A pilot can use this feature of the engine-driven hydraulic system to make a safe landing when there is not a positive gear-down-and-Iocked indi- cation. The handbook advises landing with the front engine at no less than 1,000 rpm and with one hand holding the selector handle in the Down posi- tion. By keeping high pressure in the AOPA PILOT • 39 system, this technique can prevent an unlocked gear (particularly the nose gear) from collapsing under the weight of the airplane. The most frequent complaint about the Skymaster's electrical system has to do with the avionics access panel, located just forward of the wind- shield. It seems the panel leaked wa- ter, shorting out the avionics-usually at the worst possible moment. Cessna service letter ME80-22 gave recom- mended methods for better sealing the access panel. There are also com- plaints about the early alternator sys- tems' ability to handle a large electri- cal load. Prior to the 1967 models, Skymasters have 28-volt, 30-ampere alternators. After that time higher ca- pacity, 38-ampere alternators were made standard. Electrical load balancing between the two alternators has been a prob- lem for some owners. Skymasters have no automatic paralleling func- tion, which means that load imbal- ances can occur. With all electrical equipment turned on, one of the al- ternator warning lights may come on due to an unequal demand on the sys- tem. The only way to optimize load sharing between alternators is to keep 40 • NOVEMBER 1982 SKYMASTERS So comfortable and spacious, you can stretch out and relax. So what if the engine just quit? both voltage regulators adjusted to within .60 volts of each other. The pre- 1975 fuel system also re- quires special knowledge. The fuel boost pumps work off the main tanks only; fuel from the auxiliary tanks feeds to the engines by gravity. There have been several accidents involving Skymasters whose engines (yes, both of them) simultaneously quit after their pilots switched tanks. Analysis often revealed that the pilots ran the main tanks dry. Switching to the aux- iliary tanks mayor may not permit a restart once the engines have taken the last of the fuel from the main tanks. Air drawn through the engine- driven fuel pumps can cause cavita- tion, preventing the immediate trans- fer of fuel from the auxiliary tanks' lines. To make matters worse, one study revealed that it can take as much as 29 seconds for fuel to travel \ .1- from the auxiliary tanks to the en- gines. With no boost pumps to hasten the fuel flow from the auxiliary tanks, the swirling vacuum of air caused by the suction in the engine-driven pumps momentarily can starve the engines. This is why takeoffs and landings must be made with the fuel selectors on the main tanks only. Design changes relating to fuel problems include an enlargement of the fuel lines' diameters in 1968, the installation of a capacitance-type fuel measurement system in 1970 (float gaugf's werf' previously uSf'd) and a switch to an interconnected-tank de- sign in 1975. This newer fuel system permits boost pump operation through the entire system, since all tanks share the same plumbing. There is another interesting footnote-both engines cannot share the same tank. Fuel flow is inadequate for high- power conditions. For these reasons fuel mismanage- ment is the single greatest cause of Skymaster accidents. A look at the 1978 through 1980 Skymaster accident files reveals no other remarkable trends. During this period there were a total of 74 accidents, 19 involving fatalities; IS were blamed on misman- Base price new Current market value AOPA Pilot Operations/Equipment Category' 1964 Cessna 336 Sky master $39,950 $10,600 to $14,750 IFR 1965 Cessna 337 Super Skymaster $39,950 $13,400 to $18,500 IFR 1970 T337E Cessna Turbo Super Skym..ster $56,995 $23,400 to $31,500 IFR 1973 T337G Cessna Pressurized Skymaster $78,500 $34,600 to $46,000 IFR 8,200 ft 8,200 ft 9,500 ft 10,200 ft 1,395 ft 1,465 ft 655 ft 575 ft Limiting and Recommended Airspeeds 63 KIAS 68 KIAS 90 KIAS 96 KIAS 74 KIAS 77 KIAS 83 KIAS 86 KIAS 130 KIAS 130 KIAS 139 KIAS 139 KIAS 165 KIAS 165 KIAS N/A 139 KIAS 190 KIAS 190 KIAS 62 KIAS 64 KIAS 52 KIAS 55 KIAS 1,022lb 794lb N/O N/O 558 Ib (552 usable) 93 gal (92 usable) 786 Ib (768 usable) 131 gal (128 usable) 10 qt 365 Ib, 17 cu ft Vx (Best angle of climb, ~ flaps) Vy (Best rate of climb) Vxse (Best single-engine angle of climb) Vyse (Best single-engine rate of climb) Va (Design maneuvering) Vfe (Max flap extended) Vno (Max structural cruising) Vie (Max gear extended) Vne (Never exceed) VSI (Stall clean) Vso (Stall in landing configuration) 2 ContinentaIIO-360-A, 210 hp @ 2,800 rpm 1,500 hr 2 McCauley 2-blade, constant speed, full-feathering, 76-in dia 38 ft 29 ft 7 in 9 ft 4 in 201 sq ft 19.4 Ib/sq ft 9.3 Ib/hp 4 (5 or 6 opt) 7 ft 9 in 3 ft 2 in 4 ft 4 in 2,320 lb 3,900 Ib 1,5801b 2 Continental TSIO-360-C, 225 hp @ 2,800 rpm 1,400 hr 2 McCauley 2-blade, constant speed, full-feathering, 76-in dia 38 ft 2 in 29 ft 10 in 9 £I 2 in 202.5 sq ft 23.2 lb/sq £I 10.4 Ib/hp 4 (5 opt) 7 £I 9 in 3 £I 2 in 4 ft 4 in 2,900 Ib 4,700 Ib 1,8001b 1,2221b 1,0501b 9941b N/A 4,630 Ib 4,700 Ib 4,400 Ib 4,465 lb 558 Ib (552 usable) 750 Ib (738 usable) 93 gal (92 usable) 125 gal (123 usable) 786 Ib (768 usable) N/A 131 gal (128 usable) 10 qt 10 qt 365 Ib, 17 cu £I 365 Ib, 17 cu £I 1,000 £I 945 ft 1,675 ft 1,500 ft N/O 2,925 £I 1,105 fpm 1,250 fpm 295 fpm 375 fpm 375 fpm 375 fpm 168 kt (10,000 £I) 190 kt (12,000 £I) 72 pph/12 gph 79 pphll3 gph 652 nm (10,000 £I) 891 nm (12,000 ft) 904 nm (10,000 ft) N/A 150 kt (24,000 £I) 178 kt (18,000 ft) 42 pph/7 gph 60 pph/l0 gph 996 nm (24,000 ft) 1,064 nm (18,000 £I) 1,347 nm (24,000 ft) N/A 29,300 £I N/O N/O 20,000 £I 14,400 £I 18,700 £I 17,200 ft 18,700 £I 1,650 £I 1,675 £I 700 £I 795 £I 75 KIAS 63 KIAS 93 KIAS 96 KIAS 81 KIAS 78 KIAS 87 KIAS 89 KIAS 133 KIAS 135 KIAS 139 KIAS 139 KIAS 165 KIAS 165 KIAS 196 KIAS 200 KIAS 196 KIAS 200 KIAS 70 KIAS 70 KIAS 61 KIAS 62 KIAS 2 Continental TSIO-360-A, 210 hp @ 2,800 rpm 1,400 hr 2 McCauley 2-blade, constant speed, full-feathering, 76-in dia 38 ft 2 in 29 ft 10 in 9 £I 4 in 202.5 sq ft 22.9 Ib/sq £I Illb/hp 4 (5 or 6 opt) 7 ft 9 in 3 £I 2 in 4 £I 4 in 2,850 Ib 4,630 Ib 1,7801b 805 ft 1,435 ft 2,300 ft 1,300 fpm 360 fpm 450 fpm 167 kt (5,500 ft) 69 pphll1.5 gph 665 nm (5,500 ft) 925 nm (5,500 ft) 150 kt (10,000 ft) 51 pph/8.5 gph 812 nm (10,000 ft) 1,130 nm (10,000 ft) 20,500 ft N/O Specifications 2 ContinentaIIO-360-C/D, 210 hp @ 2,800 rpm 1,500 hr 2 McCauley 2-blade, constant speed, full-feathering, 76-in dia 38 ft 29 ft 9 in 9 ft 4 in 201 sq ft 20.9 Ib/sq ft 10 Ib/hp 4 (5 or 6 opt) 7 ft 9 in 3 ft 2 in 4 ft 4 in 2,625 Ib 4,200 lb 1,5751b 1,017 Ib 7891b N/O N/O 558 Ib (552 usable) 93 gal (92 usable) 786 Ib (768 usable) 131 gal (128 usable) 10 qt 365 Ib, 17 cu ft Performance 625 ft 1,145 ft N/O 1,340 fpm 355 fpm 420 fpm 150 kt (7,000 ft) 66 pphlll gph 650 nm (7,000 ft) 900 nm (7,000 ft) 107 kt (10,000 ft) 42 pph/7 gph 820 nm (10,000 ft) 1,140 nm (10,000 ft) 19,000 ft N/O Powerplants Recommended TBO Propellers Wingspan Length Height Wing area Wing loading Power loading Seats Cabin length Cabin width Cabin height Empty weight Gross weight Useful load Payload w/full fuel std (93 gal) opt (131 gal) Max takeoff weight Max landing weight Fuel capacity, std Fuel capacity, w/opt tanks Oil capacity, ea engine Baggage capacity Takeoff distance, ground roll Takeoff distance, over 50-ft obst Accelerate/stop distance Rate of climb, sea level Single-engine ROC, sea level front engine only rear engine only Cruise speed, 75% power Fuel consumption, ea engine Range @ 75% cruise, no rsv std tank opt tank Cruise speed, 55% power Fuel consumption, ea engine Range @ 55% cruise, no rsv std tank opt tank Service ceiling Max operating altitude Single-engine service ceiling front engine only rear engine only Landing distance over 50-ft obst Landing distance, ground roll All specifications are based on manufacturer's calculations. All performance figures are based on standard day, standard atmosphere, at sea level and gross weight, unless noted. 'Operations/Equipment Category reflects these aircraft models' maximum potential. See June 1982 Pilot, p. 92. N/A-not applicable, N/O-not obtained. • agement of fuel (i.e., both engines quit). Of this number, though, only two accidents caused fatalities. The rest of the accidents ran the usual gamut: continuing VFR into adverse weather, fuel contamination, im- proper recovery from bounced land- ings and various mechanical difficul- ties. Two accidents were caused by pilots attempting single-engine take- offs (neither caused fatalities), one in- volved a pilot with no multi-engine or centerline thrust rating, and three were the result of overgrossed Sky- masters hauling marijuana (two fatali- ties). Absent were stall-related acci- dents. For a fleet of 2,386 airplanes, this is a very good accident record. Bad news, such as the double-en- gine failures, travels fast. But in the aviation world, rumors of mainte- nance quirks and mysterious glitches travel even faster. Ask any pilot and he will tell you that Skymasters leak engine oil all over the place and there is nothing you can do about it. This leakage usually has three main causes. When you fill a Skymaster's engines to their lO-quart maximum, oil is blown out the breather. Service Letter ME74-2 advised owners to fill their engines to eight quarts instead of ten and to remove the 10-quart mark from the dip sticks. Another cause is the two-piece ac- cessory case gasket. These gaskets seal the alternator and tachometer drive shafts, but leave a plugged hole be- tween the shafts without a gasket. The solution, according to mechanics, is to install a one-piece gasket that covers the entire area. But, Continen- tal does not make a one-piece gasket for this application, so a mechanic must fabricate a custom-made one. The last theory that we know of concerns too much use of the high- boost pump. Engine starts should be made with the boost pumps on Low. Too many pilots become impatient if the engine does not fire immediately, then go to high-boost operation. This generates an overflow of fuel, w.hich is vented at the rear of the fuselage, beneath the propeller. Since this fuel has been mixing with residual oil in the intake manifold-the product of piston blow-by-the result is an oily fuselage. There are other squawks that have become Sky master legend: • The engine cases crack, particularly on those engines with front-mounted alternators. With the 337H models, the Skymaster's Continental engines come with heavier crankcases. • The rear engine still overheats, in spite of the 337's larger intake scoop. Intake air is not the problem in flight. Pilots must learn to fly with the rear cowl flaps open at all times; other- wise, airflow over the cylinders will be inadequate. In 1969, the cowl flap controls went to a three-position tog- gle switch and a pair of blue lights. When the cowl flaps are either fully open or closed, the blue lights will illuminate. While taxiing, the trick is to use the front engine. Its blast of air keeps the rear engine cool. Many pi- lots are tempted to taxi with the rear engine only-not just to save fuel, but because rear propeller ground clear- ance is better. • In 1968 voltmeters and ammeters were replaced by idiot lights. They come on and go off when they are not supposed to. Their diodes and transistors have a reputation for burn- ing up. Check the sealing on the avi- onics access panel. • The book says to start the rear en- gine first, to ensure that people will not walk into the propeller after the pilot shouts "Clear" with the front engine running. But in cold tempera- tures, the battery cannot power the rear engine through more than a few turns. Long-time Sky master owners know to start the front engine first under these conditions. The battery is on the front firewall; too much juice is lost through the long cable to the rear engine. • There is no proper way to load and secure baggage. It was not until 1971 that Cessna came up with optional tie-down latch plates and seat-rail at- tach points for cargo restraint. Those who have not installed these fixtures must leave baggage unsecured. As for airworthiness directives, the Skymasters have few of substance. AD 71-17-8 calls for increasing the rear engine's idle rpm. At idle, it is diffi- cult to hear the rear engine while wearing earplugs or a headset and there is always the danger that a pilot unknowingly may attempt a takeoff using the front engine only. This is why the handbook advises beginning the takeoff roll by leading with rear engine power. This is a way to check that the rear engine is running . Along the same line, AD 77-8-5 re- quires a placard stating "Do Not Initi- The optional cargo pods expand the SkYlllaster's utility, but cost you speed and looks. ate Single Engine Takeoff." This is ironic, since early promotional photo- graphs showed a Skymaster being pushed into the air on one engine. Accident records have shown that Cessna may have set a bad example. AD 78-9-5 and AD 81-13-10 R1 are potentially the most expensive direc- tives. The former requires an inspec- tion of the wing front and rear spar lower caps for cracking. The latter mandates an inspection or replace- ment of certain tachometer and oil pump drive gear assemblies. An objective analysis shows that no one shortcoming can explain the pop- ular stigma attached to the Skymaster. Rather, it is an unfortunate combina- tion of marketing strategy and atti- tudes. Priced to undercut the twin-en- gine competition as a new airplane, it was apparent that customers wanted to have more speed and a conven- tional design rather than safety at a bargain price. Many purchasers were undoubtedly drawn to the speedier Mooneys, Comanches and Bonanzas. Anything but be caught dead in an airplane with the looks of a Sky- master, even if it is a twin. The same holds true today. As used airplanes, Skymasters can be bought for a song. The 336, probably on its way to an Edsel-style revaluation as a rare cult object, can be purchased for about $10,000 to $15,000. Blue book values climb slowly through the years and top out at between $72,000 and $90,000 for a 1980 pressurized 337. The Skymaster's biggest single ob- stacle to public ac'ceptance appears to be rooted in the very design goals that brought it about in the first place: safety. Macho types look down on them. What good is a big, powerful- yet unthreatening-twin-engine air- plane? If it denies you the opportu- nity to pose as a pilot with the Right Stuff, ready and able to prevent a deadly spin should an engine fail on takeoff, what psychological value can such an airplane have? Skymaster freaks, of course, are im- mune to this logic. They lavish great care on their airplanes, take pride in owning such a unique machine and even may fit their airplanes out with the very popular Robertson STOL (short takeoff and landing) kit. Own- ers also may share news, run ad- vertisements and give maintenance advice to their brethren through the 337 Newsletter, published three or four times a year by O.R. Whitaker. To be included on the newsletter's mailing list, write Whitaker Enterprises, Incor- porated, Post Office Box 1274, Liberal, Kansas 67901. There is no charge, but contributions are appreciated. Thirteen years after the Skymaster was introduced, the National Trans- portation Safety Board issued a special report entitled "Light Twin-Engine Aircraft Accidents Following Engine Fail ures, 1972-1976." The report showed that, within the space of four years, there were 21 fatal accidents following engine failures in Piper Apaches and Aztecs, two such fatal ac- cidents in Twin Comanches, 16 in Beech Travel Airs and Barons and 10 in Cessna 31Os. Skymasters had only six such accidents. Ahead of its time in many ways, the Skymaster, like the Cessna Cardinal, was billed as an airplane of the fu- ture. But no one was convinced except Cessna's higher-ups and the type of customers who buy old two-stroke Saabs or Dodge Valiants. In a contest between glamour/speed and ugly/ practical, there is only one choice for the buyer who wants to invest in such an emotional object as an airplane. 0