Top Turboprop Series: King Air 90 and Turbine Duke

Top Turboprop Series: King Air 90 and Turbine Duke

Top Turboprop Series: King Air 90 and Turbine Duke

Photos Courtesy of Clint Goff

If you’ve read my writings, you know that I love the PA46 series of airplanes. They are designed for the owner who craves efficiency and speed – both characteristics I find appealing in an airplane. So, it is no wonder that I really appreciate the JetPROP conversion provided by Rocket Engineering headquartered in Spokane, Washington.

I fly the JetPROP almost daily as an instructor in this tight-knit community, so unsurprisingly, I also entered the Beechcraft Royal Turbine Duke world. The same Rocket Engineering that owns the JetPROP Supplemental Type Certificate (STC) also owns the STC for the Royal Turbine Duke. Coupled with my background in other Beechcraft models, it was a natural migration for me to become an instructor in the Royal Turbine Duke. 

Royal Turbine Duke 

Rocket Engineering has a long history of taking a piston airplane with “good bones” and adding horsepower. The JetPROP and Royal Turbine Duke are no exception. The Beechcraft Duke is a distinctive, good looking six-seat airplane that is built with Beechcraft’s usual penchant for strength. Beechcraft airplanes have a well-deserved and well-respected reputation for being “overbuilt.” The Duke is no different. 

However, the Duke has a problem – er – two problems. The piston engines on the Duke are expensive maintenance hogs. 

Beechcraft’s goal was to make the Duke perform, so they incorporated the biggest piston engines they could find, selecting the 380 horsepower Lycoming TIO-541. But, too much is asked of the big piston engine on the Duke. Just about any turbocharged piston airplane engine is really an engine that was originally designed for normal aspiration, and the turbocharging makes the engine work harder as aircraft designers seek more horsepower. In the Duke, the big turbocharged engine developed a reputation for poor dispatch reliability and expensive maintenance invoices. So, the Duke became known as a really good airplane with really expensive engines.

What’s the best way to fix the piston problem on an otherwise rugged, functional and well-built airframe? Bolt on PT6s. That’s what Rocket Engineering figured how to do and created the Royal Turbine Duke. 

The PT6-35 engine is installed in the place of the big Lycoming and everything changes. So, how’s the Turbine Duke perform? In short, the change in performance is spectacular.

Advancing the power levers of the Royal Turbine Duke is downright fun. The acceleration is brisk. No, that’s not strong enough language…the acceleration is amusement-park-ride spectacular. The Turbine Duke literally leaps forward and jumps in the air. At my home airport (KJSO) with a 5,000-foot runway, I can start at one end of the runway, take off, climb and reach nearly 2,000 AGL when I get to the other end of the runway. The angle of climb is greater than my Pitts S2B, an airplane known for ridiculous performance. The rate of climb when using full takeoff power easily tops 3,000 feet per minute and is oftentimes much more in a lesser-loaded Royal Turbine Duke. 

Of course, such tremendous climb comes with a steep deck angle. A “normal” climb in a Royal Turbine Duke is completed with higher airspeeds and lower deck angles. But, even so, the Royal Turbine Duke is a remarkable performer that is vastly overpowered. 

Reaching the cruise altitudes in the mid-20s is easy and happens very quickly. A Royal Turbine Duke can reach FL250 in less than 10 minutes. Then, that tremendous rotate of climb is converted into forward speed, lots of forward speed. I regularly see cruise speeds of 290 KTAS, and when the temperatures are right, I can see 300-plus KTAS.

So, with all of this good news, why isn’t the Royal Turbine Duke a hot commodity on the market? Why have there been relatively few (about 20) conversions of the Royal Turbine Duke when there have been over 330 JetPROP conversions? The answer lies in the competition.

If you love efficiency, you probably will not love the Royal Turbine Duke. Basically, the Royal Turbine Duke is the exact same conversion as the popular JetPROP times two. Whereas the JetPROP burns 32 gph to obtain 260 KTAS, the Royal Turbine Duke will burn 64 gallons to cruise at 290 KTAS. The JetPROP is simply more efficient.

If you are willing to feed a twin, though, efficiency is not your thing. Power and redundancy are your thing. And the Royal Turbine Duke has gobs of excess horsepower. But, there are lots of other airplanes to consider if you like power and want six seats, and there’s even another Beechcraft product to consider – the King Air C90.

King Air C90

Admittedly, the C90 is not nearly as sexy, fast or sleek as the Turbine Duke. It won’t climb as quickly, won’t cruise as fast and won’t turn heads on the ramp. It is a ho-hum performer comparatively, but it is a performer that has no gaps in performance. It is not the fastest, but it is  reasonably fast. It doesn’t carry the most, but it carries a lot. It doesn’t have the best short field capabilities, but it’ll easily handle most paved runways in the world. That is why the Royal Turbine Duke doesn’t sell as well. The Royal Duke wins in climb rate, cruise speed and short-field performance. But, the C90 beats the Duke in every other category – and in those categories where the C90 is in second place (climb, cruise and short-field performance), the C90 is no slouch.

If you have four people and bags, want to go 800 nm routinely and you prefer a multi-engine steed, you’ll probably look closely at the Royal Turbine Duke. But, you’ll also look at the King Air C90. And, the C90 will carry more, have a bigger cabin, have more seats, have a lot more availability of already-trained pilots in the local area, have a toilet and be a “middle of the road investment.” It’ll show up to FL250 later than the Duke and arrive at the destination later than the Duke but it is safe, reliable, big and comfortable.

Let’s use an automobile analogy. What would you use as your daily driver if you could only have one vehicle? I drive a Ford F150 and I love it for its four big doors give access to a big dual cab, all the load carrying capability that I ever need, a smooth ride and decent gas mileage. It is good for just about everything I do from a long highway drive to my parents’ house, going to the grocery store or carrying clients to lunch. 

But, I also used to enjoy driving my Ford Mustang with the 5.0 liter 480HP V8 engine. I could leave everyone behind on a green light, zip around traffic with ease and stop on a dime. It was a bunch of fun. But, if I had bunch of stuff to carry, it might not fit. If I had more than two people riding along, someone was cramped in the back seat. And, the road noise from the big engine could be tiresome on a long trip.

You’ve probably already figured it out – the C90 is like the F150 and the Turbine Duke is like the Mustang. Which is better? It depends upon your penchant.

I have several customers who simply love their Royal Turbine Duke and would never trade the performance. Interestingly, all of these owners drive a four-door truck in their everyday life, but they want a sports car when it comes to aviation. They love the climb performance that gets them above the bumps in mere single-digit minutes. And, they love going fast. The 290 KTAS in a Turbine Duke is a lot faster than the 220 KTAS in a C90. There’s simply no way they’d give up those 70 knots. 

The Decision

Which should you buy? If you think “mission creep” (meaning you anticipate carrying more later in ownership) may occur, or if you want a “herd purchase,” or if you carry lots of bags, strongly consider the C90.

If you have a smaller hangar or live in the desert areas of the western United States (and want a super-fast climb to get above the bumps), or if you love raw power, strongly consider the Royal Turbine Duke.

Both are Beechcraft-strong, both are powered by a PT6 and both are great airplanes. Me? I fly both routinely and I just love the multi-engine Beechcraft products, all of them. The real question is, “Do you want an F150 or
a Mustang?” 

About the Author


  • Avatar
    James Roberts August 2, 2020 at 12:11 pm

    The TIO-541 engine on the Duke was designed from day 1 to be a turbocharged engine producing 380hp. Please correct your statement to the contrary or provide a citation to explain your bizarre claim to the contrary.

  • Avatar
    Odessa Choppo September 3, 2020 at 6:57 pm

    Excellent article. I’m buying a B60 within the next 60 days and have already begun talks with Rocket Engineering for conversion. I will convert the panel to glass first.

Leave a Reply