There’re lots of reason to love the JetPROP conversion, but one of the best is the fact that it retains the baggage compartment. Absolutely functional, it is a great place to carry additional baggage, throw a bag or two from the aft baggage when countering an aft CG situation, and a handy place to put all the “preflight items” (rags, oil, etc). An additional functionality, it provides protection from freezing fuel.
Jet A fuel has a freezing point of -41C, and Jet A1 has a freezing point of -47C. In the PA46 world, we always add a Fuel System Icing Inhibitor (FSII), and the most readily found FSII is “Prist,” which can help to ensure that the small amount of water in the jet fuel doesn’t freeze.
Water in fuel? Freezing? Yup…and it’s a very real threat. All jet fuel can have small amounts of water in the fuel. Normally the microscopic water in the fuel means absolutely nothing to a turbine engine because there’s not a large enough concentration to impact the operation of the turbine. But, when it freezes, a problem develops.
The liquid water droplets in the fuel flow easily through the system and burn in the turbine with no challenges. But, when the droplets freeze, they turn into a bunch of “marbles” that can then concentrate and stop up a fuel filter. A stopped up fuel filter can easily lead to fuel exhaustion and a flame-out.
How does an airplane defeat the frozen water marbles? The best answer is to always use Prist when purchasing fuel. The second answer is that most airplanes have fuel heaters at some point in the fuel system prior to the fuel filters. In the Meridian, warm fuel is returned to wing tanks and then monitored by a “fuel temperature gauge” in the cockpit.
In the JetPROP there are no fuel heaters prior to the fuel filter. So, how does the JetPROP ensure that the water marbles don’t clog the filter? The answer is the header tank and its location. The JetPROP header tank is in the forward baggage compartment, and the forward baggage compartment receives warmth from the engine which is just in front of the baggage compartment. How warm does the baggage compartment get? You might be surprised. It is a lot warmer that I thought it would be.
I bought a household thermometer that bluetooths data from a remote sensor and stuck the remote sensor in the baggage compartment during flight at various altitudes and temperatures.
Here’s a spreadsheet that shows the temperature of the baggage compartment:
As can be seen, the baggage compartment temperatures never go below freezing, even after being airborne for a long time. And, the the “frozen marbles” need MUCH lower temperatures than freezing (32F) to stop up a fuel filter. So…the JetPROP has one of the coolest (er…warmest) ways to ensure that the engine fuel filter won’t clog with frozen marbles. It’s another reason that I think the JetPROP is a singularly fantastic turbine conversion.