Thursday, March 1, 2007
TBM 850 Scorching The Airways With Style
Meet Socata’s answer to the very light jet
Whoa, the simulator at SimCom never accelerated like this! I’ve just advanced the throttle of N850LA, a brand-new EADS Socata TBM 850 with barely 100 hours, and I feel like I’ve floored the gas pedal in a candy-apple red 1969 Chevy Camaro with a big-block V8. Sure, the sound is different, but I’m pinned to my seat all the same." />
So what makes a TBM 850 an 850 and not a TBM 700, which until last year, had been in production since 1991? The TBM 850, as I mentioned during our takeoff, is identical to the TBM 700 around the airport in that the engine is torque-limited to 100% torque with a 10% buffer, maxing out at 700 shaft horsepower, a measure of the power delivered to the propeller shaft. On climbout, when the 850 detent is selected, max allowable torque is bumped up to 121.4%, and 850 shaft horsepower becomes available. One flies the TBM like a jet—by the book. As such, TBM pilots consult a table in the POH to set climb power according to a schedule to avoid engine and/or gearbox damage from excessive torque or ITT temps.
Another distinction is that the TBM 700’s PT6 bleeds air from only one station, P3; in the 850, however, Socata engineers tap bleed air from two stations, P3 and P2.5. P3 air is higher-pressure air and is bled from the combustion chamber inlet after the centrifugal impeller. P2.5 air is lower pressure, tapped from an earlier compressor stage, and only kicks in above 80% torque. Naturally, the farther air travels into a turbine engine, the more compressed it becomes. This compressed bleed air is tapped for cabin pressurization, to heat and cool the cabin, and to cycle the deice boots. It also controls the Woodward fuel control unit. During typical cruise settings, by tapping only P2.5 above 80% torque, less compressed air is bled, and the engine can produce more power, contributing to the 850’s speed advantage over the 700.
What this all means to you and me, in real-world numbers, I found out on my flights from DeKalb Peachtree Airport (PDK) near Atlanta to Savannah, Ga., and back. Climbing out of PDK, the outside air temperature was 12 degrees Celsius and we were burning fuel to the tune of 79 gallons per hour.
At FL270 and about 74 miles from Savannah, I consulted the Max Cruise (ISA+5) chart and set torque to 107%. The prop was already at 2,000 rpm, the recommended setting for all operations. It was minus-21 degrees C and we were indicating 200 knots, truing 313 or Mach .506 and going through gas at 59.5 gph—about three gallons better than book. Had the outside temp been standard ISA, at our lighter weight, the book says we’d be truing 319 and burning 64.6 gph of Jet A. In the TBM 700C2 under the same ISA +5 conditions, the book says we’d be scooting along at 294 knots true and burning Jet A at 52 gph.
With the pressurization set to 28,000 feet—altitude +1,000—the TBM’s 6.2 max-pressurization differential was resulting in an 8,200-foot cabin at FL270. As we neared SAV, we requested and were granted an emergency descent from ATC to roughly 8,000 feet. To mountain climbers on peaks like Everest, the death zone starts at 26,000 feet. It’s called the death zone because above 26,000 feet, or FL260 to us pilots, there’s not enough oxygen to sustain human life. Had we experienced some kind of pressurization failure, such a descent would get us down to breathable levels right quick. Pushing the nose over and banking a bit to spill lift, we nudged the 266-knot red line and clocked 8,000+ fpm down on our way to thicker air.
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