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 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.
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 to the varied harmonics in any given airframe, a lower RPM is not always better. Sometimes, quite oddly, 1,700 can seem less noisy than 1,600 for those sitting aft of the cockpit. So, it is wise to ask the passengers what setting they like best, before you start dinking around too much with the propeller RPM controls. Besides, too much unannounced variation will just confirm the negative opinion of your piloting skills that was formed during the takeoff roll.
As High As You Can Go
You are now at FL280 and going along nicely with the passengers reading magazines, but then the cloud tops gradually start coming up to your flight level, causing a bouncy ride. In the jet, you would be higher to start with, and can nearly always climb above the tops, even if it requires going to FL 450. But, unlike jets, many of the turboprops are not RVSM equipped, and few have the power output to perform well at those levels. So there you stay, bouncing along in and out of the sunlight. A piston passenger would expect this sort of thing, but in a “jet prop” they might wonder if you are doing this just to get them upset.
Finally, it is time to start down, and this is one area where turboprops really shine. Unlike pistons, you need not worry at all about shock cooling, so just bring the power back and point the nose down. And, unlike jets, those big four-blade propellers out there make wonderful speed brakes, so spoilers, with their buffeting noise, are not required. Down you go, with everything going along just fine, until the over-speed horn sounds off, reminding you that the power increases all by itself as you descend unless you continuously pull the throttles back…the reverse of what was required during the climb.
Now, when it comes to entering the pattern…do you leave the propellers set at 1,700 RPM, or push them up to 1,900 as the checklist suggests?Push them up and the passengers will wonder what all the noise is about; in addition, the extra drag will cause a sudden and firm touchdown when you pull the power back during the flare. But, if you leave them set at a quiet 1,700 RPM you will need to remember to push them back up before applying reverse thrust or attempting a go-around. All things to consider that are not an issue in jets.
Having successfully managed a reasonable landing, you taxi in and find the airplane is going way too fast at idle power. Of course, you could ride the brakes, but that is an expensive idea, and you just might wear them out and they won’t be there when really needed. Your other options are pulling the propeller on one engine (usually the right) back into feather. This slows things down to a comfortable pace, but creates a disharmony some people find objectionable. It also looks odd, one propeller turning like crazy and the other just barely moving. This can cause the passengers to wonder if you landed just before theengine quit. The other option is to pull both power levers back into Beta range, or even slightly reversed. You can completely stop the airplane by doing this, and even back up a bit if not too worried about FOD. It does, however, cause that loud propeller buzz you often hear from big commuter turboprops at large airports. Big airplane buzz must be good, so that is what you choose.
Once parked, shutting down the PT6’s is reasonably simple. There is no long wait for the cylinder head temperatures to cool down, and with the propellers barely turning there is no concern about prop blast or jet exhaust blowing something over behind you while you work through the shutdown checklist. So, you just wait for the ITTs to drop below 610 degrees (which they almost always are), then pull the red fuel-control levers aft, which promptly shuts the engines down with a satisfying whine.
When you exit the airplane there is that smell of burnt kerosene wafting aft toward you, something pilots find particularly intoxicating. And so, nostrils flaring, you wander away thinking…great machines, those turboprops.•T&T