When Beech developed its big T-tailed King Air 200 in 1973 it named it the Super King Air, and using the word super has proved to be right on target 35 years later. More than 2,000 King Air 200s have been delivered, making it the most popular turbine-powered business airplane of all time. And over the years Beech has made more than 2,000 improvements to the design, both big and small. One of the most important of those refinements is an engine switch on the new B200GT, giving the turboprop more speed and higher climb rates.
Since 1981, when the B200 series replaced the original King Air 200 in production, the Pratt & Whitney PT6 turboprop engines have been flat rated at 850 shaft horsepower (SHP). The new engines on the B200GT raise that flat rating to a much higher level to deliver the increased speed and climb rates.
As you probably know, turbine engines have a number of operating limitations including rpm, pressure and internal temperature. Each limitation is critical for safe engine operation and long engine life, but in a free turbine engine like a PT6 it is temperature that caps power output. Because the internal temperature of the hot gases flowing over the turbine sets the maximum power output of the engine, it is said to have a certain "thermodynamic rating," meaning that it is capable of delivering that much power at its maximum inter-turbine temperature (ITT).
However, other parts of the engine -- particularly the gearbox that reduces turbine rpm to a value usable by a propeller -- often have a maximum rating lower than the thermodynamic potential of the engine. Limiting power to the propeller to match the capability of the gearbox or to the ability of the airplane to use the power is what is meant by flat-rating an engine. In other words, the full potential of the engine at low altitudes is restrained to suit the gearbox and airframe, but that reserve of power becomes available at higher altitudes for faster climb and cruise.
The venerable PT6 turboprop engine is divided into two main sections, one to generate the hot high-pressure gas stream and another that captures the energy in that gas to turn the propeller. The power producing part of the engine is not coupled directly to the propeller, and that is why it is called a "free turbine" design.
The way a PT6 operates is that air is drawn into the rear of the engine and compressed through multiple stages as the air flows forward. Beyond the compressor is the combustion section where fuel is sprayed into the compressed air and ignited. The rapidly expanding gas caused by the fire surges forward over turbines that are connected to the compressor sections so the whole process of compression, combustion and hot gas generation can be maintained.
The hot exhaust coming out of the gas generator section of a PT6 is analogous to the exhaust blast of a jet engine. But instead of using the exhaust stream to propel the airplane, the gases are forced over a separate turbine in the front of the PT6 engine. That power turbine is connected to the gearbox that reduces the turbine speed down to propeller rpm, and the gases then exit the exhaust stacks at the front of the engine. Because there is no mechanical link between the two sections of the engine a PT6 can idle with its propeller turning very slowly in the feathered position, or the prop can even be braked to a stop while the gas producing section of the engine continues to operate.
The King Air 200 really doesn't need more than the 850 shp that has been delivered to its propellers since the very first model, but it can use as much of that power as possible all the way up to its ceiling of 35,000 feet. The PT6A-42 engine in the previous model of the 200 can deliver maximum cruise power up to altitudes in the high teens, depending on air temperature, but above that, climb rate and cruise speed peak and then begin to slow. The engine in the new B200GT produces maximum power into the high 20s where cruise speed can be 20 or more knots faster and climb rates several hundred feet per minute higher.
To get this improvement at high altitude Pratt took the gas generator part of the PT6A-60A engine that powers the bigger King Air 350 with 1,050 shp rating and mated it to the gearbox section of the Dash 42 engine in the previous model of the 200. The new engine, dubbed a PT6A-52, is still limited, or flat rated, to 850 shp at the propeller, but the engine has the thermodynamic ability to produce well over 1,000 shp.
The reason you need to start with more power potential at takeoff than you can use is that as the air thins at altitude, power drops off. The compressor section of a turbine engine has a finite ability to compress air to the necessary value. With the air becoming less dense as you climb -- the atmosphere loses half of its density by 18,000 feet -- the compressor simply can't deliver as much air by weight to the combustion section. And the air coming out of the compressor is hotter and thus it burns hotter, so the engine temperature limits are reached with less punch from the hot gases out of the combustor. The result is power drops off with altitude.
The bottom line is that the new Dash 52 engine in the B200GT leaves the runway with loads of power potential in reserve, and that potential is converted to airspeed and climb up high. Because both sections of the new engine have long track records in the King Air fleet they can enter service with a full 3,600-hour overhaul interval while an all-new engine would need to demonstrate its reliability over time. The Dash 52 engine was initially reserved for new production, but in the past few weeks Blackhawk has received an STC to install the engine on existing King Air 200s with no requirement for other airframe modifications. A flat-rated Dash 61 engine is also available for retrofit as part of a Raisbeck modification package.
Because the big changes in the B200GT are under the cowling it's very difficult to identify the airplane compared to previous B200 models. The only external clue -- other than the 200GT graphics -- for real King Air experts is the shape of the exhaust stacks, which are now virtually identical to those on the bigger model 350. But inside the B200GT are several important improvements borrowed from the 350, including newly styled seats with more comfort and adjustments; cabin sidewalls with integrated armrests that make the cabin functionally wider; stronger and more durable tables between the seats; and tuned dynamic vibration absorbers and better sound insulation system.
