Page 12 - March 2016

It is a gray, windy winter after- noon and I find myself weaving back and forth across the runway’s white centerline like a drunken sailor,rapidly accelerating through 90 knots on the takeoff roll in a Cessna Conquest I. It has been awhile since I have flown turboprops and Tim, the check pilot sitting next to me, says “It really pulls to the left, doesn’t it?...I think it’s torque from the Blackhawk mod.” No mention that it could be the pilot, which was very kind of him. After mostly flying either business jets or my own Cessna 340 for the past couple of years, I am back in a turboprop and the asymmetry in power being delivered to the widely-spaced propellers by the modified airplane’s PT6s has caught me by surprise.
Over the next couple of flights, I gradually tame the weaving. Then I spend some time thinking about the differences between piston twins, jets and turboprops.
Under normal circumstances in most piston twins, even if the engines do not power up evenly, there is not enough thrust available to significantly alter the direction the airplane is going on takeoff. And jets, with their closely-spaced engines, pretty much track where you have them pointed. Turboprops have enough power, however, that even slight differences in the rate the engines power up can (unless well anticipated) cause all kinds of directional problems. This can make keeping the nosewheel exactly on the white line during the takeoff roll, a basic expectation for professional pilots, difficult to manage sometimes.
But the differences go well beyond that, and they are evident throughout the flight. With most piston twins, you just push the throttles slowly to the stops and hope there is enough power output to top the green lines on manifold pressure and RPM gauges. With the FADEC systems on newer jets, both engines come on line at a nearly-identical rate and
10 • TWIN & TURBINE
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nicely limit their output to whatever the computer mandates. But with turboprops you really must watch the small two-inch torque gauges very closely to prevent them from going over redline. And the need to glance back and forth between the runway and gauges, while making small adjustments to the power levers, just makes tracking the white line all the more difficult.
Even once you’re airborne, trouble around the vertical axis is not over. Unless you have the torque gauges matched exactly, the thrust asymmetry gives the passengers in the aft seats all kinds of odd sensations when you fiddle around with the throttles, rudder trim and yaw damper, trying to balance things out. And, of course, just about the time you get it perfect, the controller gives you an altitude change, and the dance starts all over. Piston twins just don’t make that much power and the engines on business jets are usually aft and closely-spaced enough to minimize yaw problems.
Climb Management
When all these initial issues are resolved and you are finally climbing to cruise altitude, you can be sitting there relaxed and sipping coffee, but getting puzzled as to why the turboprop’s airspeed is tapering off with the autopilot set for a given
rate of climb. Then the light dawns, and the memory returns. These are not turbocharged pistons that automatically maintain manifold pressure all the way up, nor are they jets that have computers to worry about that sort of thing for you. The PT6s require continued throttle advancement in order to maintain power during a climb, and you must personally take care of it by gradually pushing the levers up to keep torque constant during the climb, or the whole thing just slows down. So, you switch the coffee cup to your left hand, and make the needed power adjustments with your right.
Then, passing through about 18,000 feet, you may find that, with the torque set for climb, the inter-turbine temperatures (ITT) can start to hit the redline. Ah, yes, you remember. There is the business of the PT6s being torque- limited when below about 18,000 feet, but temperature-limited when above that altitude. So, you now move your focus from the torque meters to the ITT gauges, located in the middle of the vertical engine instrument cluster. However, with the Blackhawk mod in the Conquest the temperatures stay well under redline all the way up to the high 20s, even while pulling maximum torque.
Now in cruise, you need to decide what RPM the props should be at, and, unlike jets where you don’t worry about it at all, or in pistons where your choice is limited by power output, you have quite a range to select from. Your options are between 1,600 and 1,900 RPM, and the setting you choose can make a big difference in the level of vibration and noise for those sitting in the back. Further, due
by Kevin Ware
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MARCH 2016