There are two important measurements of turbine engine power output. The one published in basic specifications, and tossed around when pilots get together to talk, is the takeoff power rating. And that's important. But a more difficult to define, and equally important, measure of turbine enginepower is how much is available at useful cruise altitude. The new TBM 850 has the same takeoff power rating as the previous model, but delivers considerably more thrust at high cruise altitude to increase its effective cruise speed by 25 knots or more.
The actual cruise power increase in the new TBM 850 is variable when you consider altitude and air temperature for the comparison, but it is nominally 150 shaft horsepower (SHP), thus the name change from TBM 700 to 850. Because the extra fuel needed to generate the extra power up high is offset by the cruise speed increase, range remains about the same with no fuel capacity increase. And when boring into a strong headwind, the higher true airspeeds actually improve range because a smaller percentage of the true airspeed is lost to the wind.
The TBM 700 could bump up against 300 knots true airspeed at standard air temperature in the mid-20 altitudes, but when cruising at the more fuel-efficient altitudes near the airplane's 31,000-foot ceiling, the speed dropped down to about 250 to 260 knots. The new 850 can top 300 knots at any weight, hit 320 knots at optimum altitude and maximum weight, and can come close to 320 knots true at its ceiling at typical cruise weight. The key to the speed increase is a new version of the Pratt & Whitney turboprop engine, and a different way of tapping bleed air for pressurization. The new 850 uses the PT6A-66D engine, which is capable of producing 1,825 shp at sea level. The engine is in the same family of PT6 engines used on the big Beech 1900 regional airliners and the new speedy Piaggio Avanti twin turboprop pusher. The PT6A-64 that powers the TBM 700 can generate 1,580 shp at sea level, so you can see the size of the potential power increase.
A turboprop engine, like any engine, is nothing more than an air pump. The greater mass of air that an engine can compress and burn, the more power it makes. As air density thins at altitude, the engine has less mass of air to work withand power output drops. That's why so many turbine engines are flat rated, meaning that they can make much more power at sea level than the airplane can use. But you need to start with a reserve of power potential to have the power you want left up high for cruise. And the new TBM 850 has a bigger flat rating margin than any other airplane I can think of. The 850 at sea level has more than double the power potential available than it will use for takeoff and initial climb.
But the engineers at Pratt and TBM worked even more magic to obtain additional power and efficiency at cruise by modifying the engine-bleed air system that pressurizes the cabin. As in many large jets, the new 850 has a dual-bleed air tap that provides a reliable and steady source of pressurization at all power settings at any altitude in the operating envelope.
To pressurize the cabin in a turbine-powered airplane, engineers tap high-pressure air from the engine compressor section before the air enters the combustion chamber. This is called "bleeding" air, and it's an accurate description because the high-pressure air that goes to the cabin doesn't go to the engine burner section, so less power is produced. The engine is literally bled of power.
The new Dash 66D engine in the 850 has four stages of axial compressors plus a final stage of centrifugal compression. Obviously, the pressure is increased by each stage of the process. When the engine is operating at cruise power, only a small portion of the compressed air must be bled to maintain the 6.2 psi cabin pressure and keep the TBM cabin pumped up. But, the rules require-and every pilot would want-that the cabin pressure must be maintained when the engine power is low. In other words, you must be able to chop the power at the certified ceiling without the cabin altitude climbing. To meet that requirement, the TBM 700 tapped bleed air from a higher pressure section of the compressor than was needed for normal cruise and descent and that used more engine power. The new 850 has two engine-bleed taps to satisfy normal cruise power pressurization and the low power high-altitude condition.
