Transitioning from Light Twins to Turboprops (How Tough Could It Be?)

The BE-76 Duchess is a 3,900-pound airplane with seating for four (including crew). Power is by Lycoming at 180 horsepower per side. At recommended cruise power settings, it's good for about 750 nm (with reserves) at around 150 KTAS at 10,000 feet.

The BE-300 Super King Air is a 14,000-pound airplane which is certified to carry up to 15 people (including crew) but is usually configured with seating for eight. Power by Pratt & Whitney Canada is 1,050 shaft horsepower per side assisted by 158 lbs per side of jet thrust from the exhaust for a total of 1,113 equivalent shaft horsepower per side. At normal cruise power settings, it's good for about 1,600 nm (with reserves) at around 300 KTAS at FL300.

The Duchess uses the venerable Lycoming O-360 four-cylinder reciprocating engine. It's a bulletproof powerplant, familiar to anyone who's flown a mix of GA singles. A pair of them will sip 100LL at the leisurely rate (compared to the King Air) of 17 gph (102 pph) in normal cruise. With four on board at 150 KTAS, the consumption rate is about 0.17 lbs./passenger mile.

The Super King Air 300/350 uses the reverse-flow, modular, free-turbine Pratt & Whitney PT6A-60A turboprop powerplant.

"Reverse-flow" means that the air enters at the rear of the engine, flows forward through the gas generator (compressor and combustion) section, through the power section, and exits at the front. "Free-turbine" means that the propeller is not connected via a drive shaft to the back half of the engine. Energy is transferred from the gas generator section to the power section by the forward flow of air through the power turbines (just like the old "Fluid Drive" transmission in your grandfather's Buick except that air, not transmission fluid, is doing the work).

The free-turbine design is less efficient than a direct shaft-drive design, but the PT6A has diehard supporters because of its offsetting benefits. The engine is easy to light off and suffers fewer hot starts because the starter/generator doesn't have to rotate the huge four-blade propeller in order to get the fire going out back. The design also allowed Pratt to build the engine in two bolt-together modules (hence "modular"). The power section is easily disconnected from the gas generator section which helps keep maintenance costs under control. Finally, it's less noisy than the old Garrett "grenades," now marketed by Honeywell.

A pair of Pratts consume about 590 lbs. per hour of Jet A in cruise at FL300. With eight on board and cruising at 300 KTAS, the burn rate is about 0.25 pounds per passenger mile.

Starting the Duchess is just like starting any other Lycoming O-360-powered GA airplane. You just do it twice.

Ever wanted to burn up $25,000 in six seconds? Here's your chance. If ITT (interstage turbine temperature) exceeds 1,000 degrees Centigrade for more than five seconds during the start sequence, someone gets to pay for a hot section tear-down and inspection.

The number one concern at light off is the chance of a "hot start." Turbine engines depend on a cushion of air to cool the combustion chamber. Only about 40 percent of the energy generated through combustion is used to enhance forward motion. The remaining 60 percent is used to compress air and drive it through the engine. Further, only 25 percent of the compressed airflow is combusted. The remaining 75 percent is used to center the flame in the combustion chamber, for internal cooling or is dumped overboard. It doesn't sound efficient but that's how it works. If there is insufficient airflow to keep ITT under control (which usually occurs because a tired battery isn't keeping the compressor RPMs up) you've got a hot start.

The secondary concern is the chance of a "hung start." The PT6A compressor section consists of four sequential compressors (stages). Three axial stages (which look like pinwheels or hybrids of normal kitchen fans) feed the final centrifugal stage. At lower RPMs, the axial stages are more efficient at providing air than the centrifugal stage is at receiving it. To prevent a high-pressure bottleneck and consequent compressor stall of the axial stages, there is a compressor bleed valve that dumps the excess air overboard. If that valve sticks in the closed position, the bottleneck will occur, compressor RPM (N1) won't make it past 32%, the axial stages will stall, and the poor old PT6A will be shaking and baking.

The starting process therefore commands the pilot's complete attention. It's about a 20-second drill. The first half consists of a well-rehearsed scan of a several indicators to confirm adequate and appropriate battery power, engine lubrication, ignition, N1 and fuel flow. Then comes light off and all eyes are focused on the ITT and N1 gages (with one hand poised to move the condition lever to "Fuel Cut Off") until rollback of ITT temps and an advance of N1 through 50% confirm a normal start.

Then, just like in the Duchess, you do it all over again.

On the Duchess, propeller RPM is controlled by the standard governor with rotating flyweights that raise or lower a pilot valve depending on prop RPM. In an underspeed condition, the pilot valve rises, opening the valve that causes governor oil to flow to the prop hub, the prop blades to flatten, and the RPMs to increase. An overspeed condition works in converse. Finally, since it's a trainer, there's also a nitrogen accumulator to assist in unfeathering a prop in flight for those occasions when you were "just kidding" when you feathered it in the first place.

Contrary to the constant-speed systems on light singles, governor oil pressure moves the propeller blades toward fine pitch. That way the blades move to coarse (and feather) when you lose an engine (and oil pressure). A centrifugal latch pin assembly prevents the blades from twisting to feather when the engine is shut down. That makes the next start an easier proposition.

The propeller system on the Super King Air is similar but sports a couple extra layers of bells and whistles. Propeller RPM is controlled by the primary governor, which is exactly like the one you're used to and has a governing range of 1,450 RPM to 1,700 RPM.

The first layer of complexities arises from the need to provide systems redundancy. There are two additional governors that back-up the primary governor to protect against propeller overspeeds. The hydraulic overspeed governor looks and acts just like the primary governor except that it has only one setting — 1,768 RPM. If the primary governor fails, the overspeed governor will dump oil pressure from the propeller hub as necessary to maintain 1,768 RPM.

The second backup is the fuel-topping governor. In normal forward flight, the fuel-topping governor kicks in at 106 percent of the RPM value selected by the pilot (1,802 RPM with the prop levers forward). In the event of a 106-percent overspeed, a valve reduces air pressure in the line that sends P-3 bleed air to the fuel control unit (FCU) sensor. The FCU is misled to believe that less air is flowing through the engine and that there is a consequent need to reduce the volume of fuel it is delivering to the burner can. The fuel-topping governor accomplishes automatically what GA pilots are supposed to do manually — it reduces engine power until prop RPMs are within an acceptable range.

The second layer of complexities arises because the minimum blade angle acceptable for flight is not acceptable for ground operations and visa versa. For flight operations, the low pitch stop is 13 degrees. Anything less could lead to excessive drag and pitch-down attitudes. For normal ground operations, the low pitch stop is reduced to 1 degree. Anything more and you'd smoke the brakes to keep taxi speeds under 50 mph at the minimum RPM required to avoid the resonance range.

There is a third range of pitch settings for ground fine (down to -2 degrees) and a fourth for reverse pitch for engine braking action (down to -14 degrees). This is all accomplished with the ground low pitch solenoid, the beta valve and a system of rods, levers, springs, a carbon block and a feedback ring which is impossible to explain without a functional model of the system. Suffice it to say that, compared to the Duchess where the low pitch stop is simply the unit's mechanical limit, it is an intricate device.

The one complication that doesn't exist is a centrifugal latch pin assembly to prevent the props from feathering on shutdown. It's a free-turbine design, so the PT6A doesn't care whether or not the props are feathered when you throw the start switch.

The Duchess carries 51.5 gallons of fuel in each wing for a total capacity of 103 gallons (618 lbs.). It's a pretty basic system except for the eight fuel drain locations and the crossfeed system.

Each tank feeds the engine on that wing unless crossfeed is selected (single-engine operations only). To crossfeed from left to right the left fuel selector must be "Off." Then it's simply right auxiliary pump "On," right selector "Crossfeed," right auxiliary pump "Off." The engine-driven fuel pump on the right engine will draw fuel from the left tank.

The Super King Air carries 3,611 lbs. of fuel, which is the equivalent of the Duchess loaded with full fuel, two adults up front and junior in the rear. Fuel is divided between two main tanks (which actually consist of ten separate wing cells and two nacelle tanks) and two auxiliary tanks. Although that all sounds complicated, the wing cells dutifully gravity feed into their respective nacelle tanks without pilot input. The major complications are auxiliary fuel usage and crossfeed operations.

When you start talking about the location of 3,611 lbs. of weight in an airframe, the design engineers start to care about where you put it. In the case of the King Air, the engineers want the weight out on the wings to reduce bending loads so auxiliary fuel usage is a LIFO (last-in-first-out) operation. No fuel in the auxiliary tanks unless the mains are full.

Because of wing dihedral, the auxiliary tanks are downhill from the nacelle tanks, so a system is needed to motate that fuel uphill to the engine. That is accomplished by the motive flow system. When the aux tanks have fuel, the motive flow valve is open so that fuel from the main tanks is routed through a venturi siphon pump in the aux tanks. Courtesy of Dr. Bernoulli, the fuel stream sucks up Jet A from the aux tank until a sensor detects that the tank is empty and closes the motive flow valve.

Crossfeed operations serve the same function in the Super King Air as they do in the Duchess and in much the same way. The differences are a variety of valves that automatically open or close when you throw the switch and the corresponding annunciators that confirm the status of the valves. You don't need to know all that now, but you will before you can fly the plane.

There are a couple other issues relating to the differences between types of fuel and operating environments. For example, Jet A is heavier than avgas and suspends water much more readily. Suspended water supports the growth of micro-organisms. If your fuel sample looks like it was taken from a fraternity swimming pool, it's time to add some biocide. You may also need to add some Prist if you're operating in temperatures below -40 degrees F.

Depending on the year of manufacture, the Duchess has one or two batteries and two alternators feeding 12 or 28 volts into a three-bus electrical system. There are current limiters, circuit breakers and a fuse to protect each bus from electrical failures elsewhere in the system. The starters run off the battery bus. All other equipment is powered by the number one (left) or number two (right) bus. They receive current from their respective alternators but are tied together to facilitate load-sharing during normal operations.

There are two connections between the left and right busses. Both busses tie into the battery bus through lines that are protected by current limiters and the ISO circuit breakers. There is also a direct bus-to-bus connection that is protected by the bus-tie fuse. A short circuit in either bus is supposed to pop the same-side ISO breaker and blow the bus-tie fuse before the excess current fries the opposite bus or its associated equipment.

The status of the ISO circuit breakers is apparent on a visual inspection. A thorough pre-flight will include a check to assure that the bus-tie fuse has not blown.

In the Super King Air, a 24-volt nickel-cadmium battery and a pair of 28-volt starter/generators feed DC power into five busses and two inverters. Most of the electrical systems draw 28 volt DC current from one or the other of the busses. The "glass cockpit" avionics equipment draws 115 volts AC from whichever inverter the pilot has selected.

Most systems, including the number one avionics bus, draw from the "triple-fed" bus — so named because it is powered by the battery and both starter-generators. All the lines feeding the triple-fed bus are protected by current limiters and diodes. The diodes make the triple-fed bus an electronic roach motel — electrons can come in but they can't get get out. That's a good thing when you're trying to isolate electrical systems.

The other major DC systems are powered by the left generator bus, the right generator bus or the center bus. Under normal conditions, the busses are tied together. Each line is guarded by current limiters and each bus tie relay is controlled by a Hall Effect Device (HED). The HEDs function something like the GFI circuit in your bathroom. They open the circuit IMMEDIATELY in the event of a power surge. They accomplish that by sensing the magnetic field upstream in the line. A surge in current causes a surge in the magnetic field which opens the bus tie before the excess current can wreak its havoc. A thorough preflight check of a Super King Air will include a check of all the current limiters and bus ties.

In the Duchess, cooling is by ambient air. Heating is accomplished with a Janitrol gas-fired heater that converts Avgas into BTUs. All you need to know is that it burns two-thirds of a gallon per hour off the right tank. Management of cabin pressure is simple enough. At 8,000 feet MSL you have an 8,000-foot cabin; at 12,000 feet MSL you have a 12,000-foot cabin. You get the idea...

The King Air is known-icing certified for operations up to FL350 and, therefore, has systems to pressurize and heat the cabin, melt the ice off the brakes, inflate the de-ice boots, retract the de-ice boots and pressurize the landing gear hydraulic fluid. They all run off P-3 bleed air.

"P-3" is a reference to the location in the PT6A where the bleed air valve is located — just downstream from the third compressor stage. P-3 bleed air is compressed to 90-120 PSI and (as a consequence of having been compressed) is heated to up to 700 degrees Fahrenheit. The BTUs are is used for brake deicing, defogging windows, and warming the cabin. The PSIs are used to pressurize the cabin, inflate the de-ice boots, and pressurize the hydraulics. At the back end of the system, the P-3 air is exhausted through a venturi that supplies the vacuum pressure for retracting the de-ice boots and powering the vacuum gyros.

All these systems require close study, the cabin pressurization system in particular. Ever felt like killing one of the ground crew? Just have him open the airstair door with a couple PSI residual cabin pressure. There are a lot of square inches to that door and it could flatten him like a pancake. Pilots who aren't feeling homicidal will open the pilot storm window to equalize cabin pressure before anyone gets close to the door.

Standard GA.

With all its equipment and associated caution and warning lights, the annunciators in a Super King Air are treated as a distinct system. There are so many of them that Beech installed annunciators to warn you of an annunciator. There are master warning and master caution annunciators, 13 warning annunciators, 20 caution annunciators, and 17 advisory annunciators. They tell you exactly what is going on, but only if you have a comprehensive understanding of the systems.

For example, a "L No Fuel XFR" light might mean that you should select "Aux Transfer Override" on the left side to correct for a failed circuit card in the auxiliary fuel transfer system. However, a "L Fuel XFR" light is also a normal auxiliary fuel system indication if: 1) there is low fuel pressure on that side, 2) the ignition system is energized on that side (manually or through auto-ignition), or 3) fuel is being crossfed to that side. Got it? One down — 49 to go.

It takes a bit longer to get a light twin into position-and-hold than a GA single because you have two engines and two props to check (plus feather checks). Still, you can measure the time with a stop watch.

You can use an hourglass to time a thorough first-of-the-day pre-flight check of a Super King Air. At FlightSafety, a first run through the pre-flight checklist in 45 minutes is considered to be a good time.

"Everything Forward; Everything Up; Identify; Verify; Feather; Secure"

Passers-by probably thought you were nuts as you walked around chanting that mantra during your initial multi training. Lose an engine on a light twin and you need to get the unproductive propeller feathered post haste in order to maintain any semblance of performance. If it happens right after rotation, there may not even be time for the checklist. If the nose yaws to the left, feather the left prop — now. "Yaw left — feather left." "Yaw right — feather right."

Here's one place where all those extra systems pay off: The Super King Air's autofeather and rudder boost systems make dealing with engine failures a snap. (Both systems are required to be operational for flight.)

When armed, the autofeather system senses the difference between the torque outputs of the two engines. If the prop RPMs are above 88 percent, the operating engine is producing at least 17 percent torque, and the inoperative engine's torque falls below 10 percent, autofeather feathers the prop on the inoperative engine.

The rudder boost system in the BE-300 senses the volume of air flowing through each engine. When the difference in P-3 bleed air exceeds 40 PSI, the system applies up to 107 lbs. of rudder pressure toward the operating engine. Without this system, the rudder pressure required to overcome asymmetrical thrust at Vmc could exceed 150 LBS.

These systems are the reason that mere mortals can handle V1 cuts in an aircraft producing 1,113 ESHP/side. When an engine fails at rotation, the rudder boost system will automatically lead with about a 50 percent rudder input toward the operating engine. The autofeather system then feathers the propeller on the inoperative engine. The pilot "follows" the rudder boost system by applying full rudder on the forward rudder peddle and follows the flight director command bars which have been preset to the correct climb attitude. That yields a blue-line climb. Confirm that the inoperative propeller has feathered, climb to 1,000 AGL, and take another look at the world. It's like magic.

There you have it in a nutshell — a very tiny nutshell. We've skipped over some major systems, including the flap, landing gear, instrumentation, navigation, emergency oxygen, and fire suppression systems, and have given scant attention to others. Still, you should have an idea of what you're in for if you have a hankering to fly turboprops. It's all about mastering the systems and there are a whole lot of systems to a Super King Air 300/350.

Underneath all that, it's still an airplane — a really great airplane.