THE FLYING CROWN
After World War II the private aviation sector began to grow in the United States, as technology from military aircraft were applied to civilian designs. During the course of this blog, we'll follow the development of the first Cessna twin-engine private aircraft built after World War II, which became an icon of civil aviation-the versatile Cessna 310.
The 310 first flew in January, 1953, with deliveries starting the following year. In contrast to earlier private aircraft designs, the 310 was built around sleek, modern lines coupled with innovative features, such as an engine exhaust augmenter and wingtip fuel tanks. Cessna entered the light twin market a year after the Piper Apache, though the Cessna aircraft offered 110 more engine horsepower. The 310 was also more aerodynamically structured than the Apache. Speaking of horsepower, the Cessna 310 went through a number of engine upgrades during its twenty-eight year production life. Starting with pressure-carbureted 240-hp Continentals, the 310 gained fuel injection and an additional 20 hp per side with the C model in 1959. Several variants of the IO-470 were used through the 1975 model year in the 310Q. The overall opinion of the IO-470 has always been positive, with the model functioning well in the 310. Both owners and mechanics are pleased with the 470s, as the number of service reports for 470-powered aircraft is proportionally lower than that of other engines, such as IO-520 and TSIO-520 versions. Of interest is the fact that through the life of the 310, engine power was only increased by 45 hp per engine, a more efficient comparison with that of the Beechcraft light twins, which began with 180-hp Lycomings in the Travel Air and culminated with a pair of turbocharged 380-hp monsters in the 56 TC.
Among the details of the early 310 models are multiple rear side windows, a straight tail and wingtip fuel tanks, called "tuna tanks" named for their shape. When the 310B model was introduced in 1958, it was built with a 100 lb. increase in gross weight. The following year, the 310C model entered service with an engine change from the 240 hp Continental engines on the earlier models to a fuel-injected 260-hp Continental IO-470D. The time between overhauls (TBO) was 1,500 hours, as with previous models. The 310C was also slightly heavier than the 310B. The primary feature of the 310D model, introduced in 1960 was a swept tail design, as Cessna did across its entire fleet of aircraft. In 1962 the 310 received a major change in the wingtip tank design, with the canted "Stabilia-Tip" tanks, which were stated to be more efficient than the old design. Part of the rationale for the canted design was due to the old bladder equipped tanks, which had a fuel pickup, causing a buildup of unusable fuel. The 310G with its swept fin, short nose and bladderless all-metal canted tanks was perhaps one of the most aerodynamic light twins ever built. With the 310H model, introduced in 1963, the cabin size in the 310 was increased from five passengers to six. The 310I, introduced the following year, offered three-bladed props, as well as optional wing tanks and auxiliary lockers. The most important difference in the 310I over previous models was the switch from the overwing exhaust design (which was prone to corrosion) to an underwing system eliminating the problem. A new engine, the IO-470-U came with the I model. The 470-U engine could produce 260 hp, as with the H model along with 1,500 hrs. time between overhauls (TBO). The 310K model entered the market in 1966 with an IO-470-V engine. The 310K was also equipped with extended, one-piece aft windows on each fuselage side. Cessna took a major step in 1969 when it first offered a turbocharged version of the 310 in addition to the normally aspirated 310P version. The T310P sported a 285 hp Continental TSIO-520-B engine(1400 hour TBO), three-bladed props and a 5,400-pound gross weight, compared to the normally aspirated 310P's 260 hp IO-470V Continentals, optional three-bladed props and 5,200-pound gross weight.
Flying the Cessna 310 is relatively easy. Cessna has painstakingly attempted to ensure a smooth transition in models over the years, making it simple for pilots to adapt to new aircraft. The 310 flies similar to other Cessna planes, although with more weight and greater speed. Due to the aircraft design, the roll aspect is the lightest while pitch response is the heaviest with good stick force per G and longitudinal stability. Pilots graduating from other light twin planes will recognize the 310 as a more substantial aircraft than the Travel Air or Piper Apache, in both size and speed. As for those progressing through the Cessna line, a 210 pilot would appreciate the 310's heavier controls, higher wing loading, as well as greater landing speeds. One performance aspect of the 310 which may cause a few anxious moments for a new pilot is the single-engine method near a runway. The all-electric landing gear is slow to stow. This, in addition to the plane's propensity to roll, caused by the tip-mounted fuel tanks, can make for an intense few minutes following the loss of engine power. Experienced 310 pilots suggest the only way to keep the aircraft from rocking up and down is to minimize aileron inputs. The 310 is a fast plane, compared to other light twins. The 240 hp engines will give the 310 a respectable performance of 210 mph cruising speed and a climb rate of 1,600 ft. per minute. The heavier weight in later models almost offsets the increased power. For example, the 310Q equipped with 260 hp engines, turns in a performance of 214 mph at the same altitude (7,500 ft.), with the 285 hp versions rated at 219 mph at 7,500 ft. The turbocharged 310s are the fastest, with a performance of 228 mph at 10,000 ft. and 253 mph at 20,000 ft.
For its many virtues, the 310 has a few flaws. The fuel system can be confusing at times. The first model only offered the 50 gal. wingtip tanks, designed to shear off in the event of a crash. It wasn't until the 310B model came out in 1958 that Cessna installed two 20 gal. auxiliary tanks in each wing. Wing locker tanks first became optional in 1967, again with a 20 gal. capacity in each tank. In 1973, the tanks were enlarged to 63 gal. each, meaning 310s had varying capacities of 100, 140, 163, 183 or 203 gallons each. A long-range aircraft may have as many as six tanks and ten fuel pumps. Operating the tanks is another problem as the 310s pull more fuel from the tanks than the engines will use. Because of this, both the pressure carburetor and fuel injection models have a return line from the engine compartments. The problem is, this connection only returns fuel to the main tank on the same side as the engine. This means that using fuel from either the auxiliary tank on the same side, or the main tank on the opposite side, about half of the fuel flow will be moving to the same-side main tank. When the main tank reaches capacity from the return fuel, any excess is sent overboard. Due to the locker to main tank arrangement, the pilot can't cross-feed auxiliary fuel. Planes equipped with locker tanks must draw down fuel from the main tank on the side to the engine in order for the locker tank to supply the main tank on that side. They may crossfeed the opposite engine to minimize any lateral imbalance. The other system prone to flaws on the 310 is its landing gear. Since the aircraft is higher than many private aircraft, as well as using a tricycle system, the landing gear is of prime importance. People familiar with the 310 relate three weak points in the gear system. The nosegear idler bellcrank, situated under the pilot's feet is probably the worst, since it always fails at retraction and always means two prop and two engine teardowns. The main-gear torque tubes and the landing-gear door actuator bellcrank are two other issues with the 310. If the torque tube fails, it does so during the retraction sequence, leaving the associated main gear down and locked. To remedy this, extend the rest of the gear and land. If the inner landing gear door actuator bellcrank fails, the inner door hangs in the air.
The Cessna 310 was in production for twenty-eight years. During that time over a dozen versions were built of the plane. The aircraft entered service in the 1950's, when light twin private planes were first beginning to make their mark in the aviation community. During most of its production run, continual updates were added to the plane. By the 1960's military versions of the 310 were in use, with the 310 also serving as an executive aircraft. Even today, one doesn't have to look far to find one at a local airport. Though we have a number of business jets in today's aviation market, the 310 was a worthy forerunner.