I have received numerous questions from farm and ranch pilots about using #1 diesel fuel in aircraft with diesel cycle piston engines. The reasoning behind these questions concerns the availability and price of Jet A in rural agricultural areas. #1 diesel is cheaper and more readily available.
On the surface — and in theory — the answer would be yes, but the devil is in the details.
Last winter we had a significant snowfall and then very cold temperatures. I normally move snow with my diesel skid loader, which I fuel with a 50/50 mix of #1 and #2 diesel fuels. Because of the low temperatures, I added some diesel fuel additive and a couple of additional gallons of straight #1 fuel.
I then went out and started to play in the snow. After about a half an hour, the engine chugged once and died, and would not restart. Since it was sitting in the middle of the driveway, I dragged it back to the shop and turned the heat on. The next morning, it still would not start. After a lot of work, I replaced the fuel filter and it started.
Most gelling problems concern wax in the fuel. When an additive is used, it will lower the pour point of the fuel, but not the cloud point. But once the fuel is cooled below the cloud point, it will plug the fuel filter if the filter is not heated.
For example, on my diesel V-8 pickup, the fuel filter is mounted in the deep V of the engine. Once the engine is warmed up, it will keep the fuel above the cloud point. In my skid loader, the fuel filter is mounted away from the engine with no heat.
The second problem is that with normal wax in the fuel, once you heat the filter above the cloud point, the wax goes back into a solution, so why did my skid loader not start after a night in the heated shop?
After I replaced the fuel filter, I cut the old one apart. The filter media was coated in a thick substance that did not “melt” when I heated it. I assume that the fuel I was using had been contaminated with some bio-diesel and the substance on my filter came from improperly processed bio-fuel.
So let’s get back to the original question. There are several problems with using #1 diesel fuel in an aircraft.
The first is a common problem with aircraft. The engine and the airframe are manufactured by two different companies. We see this in many planes where the oil drain is right above the front suspension or other equipment. With diesel powered aircraft, the big question is where did they mount the fuel filter? Is it heated or out in the cold wind?
The second major problem is how can you be sure that the #1 diesel meets the proper cold temperature properties and is not contaminated with some bio-diesel that has not been properly processed?
We have seen some #1 diesel from southern states that are very marginal on cloud point, so that can cause filter plugging, especially at altitude.
But the biggest problem is from the bio-diesel. The major concern is that a fuel supplier has some residues left from improperly processed bio-diesel and it could get into your fuel supply. Some fuel supply companies use a tank for bio-diesel in the summer and #1 diesel in the winter, leaving a small amount of residual in the tank after the switch over.
So what is the bottom line? Technically, #1 diesel can be used in a diesel cycle piston engine aircraft. But there are several concerns that can really bite you, especially in colder weather and/or at altitude.
Switching back to Jet A in the winter may not help because you would have to completely drain the tanks and flush them out to get rid of all of the residual.
So I guess the pilot needs to be the final judge.
I just keep thinking about my skid loader sitting in my driveway and my wife having to drive around it to go shopping. My job rating that month was not good. Think of having that problem at 6,000 feet.