Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985.
In my last post, How long can fuel be safely stored?, I talked about the storage life of aviation and automotive fuels. Reader Steve Phillips asked, “I would love to hear the same facts and cautions about Jet A. I have been unable to find reliable advice through online searches. Many say Prist (a fuel treatment additive) needs to be added at the nozzle, while Phillips 66 Aviation claims it can be safely added to a transport load of fuel and does not need to be added at the nozzle. The claim is Prist will settle out over time.”
We have stored Jet A samples for many years under dry, ideal conditions and observed only a small deterioration in thermal stability with age.
The main concern with Jet A is the water phase. In all large storage tanks, water is present or will form due to condensation. In large flat-bottom storage tanks, the water drain is designed to be the lowest point in the tank. However, after a short time the bottom settles and the water drain no longer drains all the water from the tank.
Two things happen in the water phase. First, any surface active ingredients in the fuel will migrate to the water. These surfactants are things like fuel injector detergents, pipeline additives, etc. As they acuminate, they can get to a high enough concentration that they will disarm the filter separator, which can allow a slug of water through the system and into an aircraft.
The other big problem with the water in Jet A is that when it sits, small microbes or little bugs start to grow and multiply very quickly. These bugs can then be pumped with the fuel into the tanks of an aircraft. If they are not removed quickly, they multiply very rapidly and can literally start to eat the tank for lunch. If they go unchecked, in a very short time the tank will be ruined and need replacement. This is a very expensive procedure for most aircraft.
Prist is an approved fuel treatment that is supposed to absorb all of the water so that surfactants and bugs do not cause you any problems. The concern with Prist is that if it is not mixed completely, it can cause problems. There can also be problems if it is in a tank with a large slug of free water in the bottom of the tank. For this reason, many fuel suppliers add the Prist as fuel is dispensed. Phillips feels that with proper sumping and filter maintenance, it can safely add the Prist to the transport.
The key here is that either system works if the proper quality assurance (QA) practices are followed at the fuel farm, the line truck, and at the plane. If they are not followed, then neither system works well.
The final answer to the shelf life of Jet A is a big question mark. If Jet A is stored following good QA practices, the sumps are properly located, or in a cone bottom tank, the fuel should last almost indefinitely. If these conditions are not met, then the shelf life can be very short because it depends on what is in the bottom of the tank. Climate also plays a part.
The same is true for your aircraft. The big concern is low usage aircraft, especially in humid climates when the aircraft is not in a heated hanger. Here it is critical that the tanks be sumped every few days even if the plane never flies, because water can condense and settle to the bottom. Then the bugs set in and it can be downhill from there.
It is always the little details that can — and will — get you.
You can contact Ben at Visser@GeneralAviationNews.com.