I recently had a note from a gentleman from California who flies out of a local airport. In the last few months his aircraft has developed several fuel leaks. When he checked around, he discovered that other pilots at the same airport are also experiencing fuel leaks.
Additionally, one of the pilots noticed a higher than normal EGT for his normal fuel flow at cruise. When his engine was inspected, it appeared that several cylinders had run hot and experienced knock or pre-ignition.
On top of all that, the fuel just smelled wrong, according to the pilot. Rather than the regular smell of 100LL, this fuel smelled like “wet socks,” he reported.
What is wrong with the fuel?
It all starts with the aviation fuel system at the airport.
Aviation fuel systems have several differences from autogas fuel systems.
With avgas, the tank is supposed to be installed slightly off level (less than a degree) so that any free water in the tank will run to the low end and can be sumped off. Many of the tanks also have a pocket at that end so that the water is more easily located.
Avgas tanks also are supposed to be installed with floating suction, so that the fuel pulled out of the tank when you are fueling your airplane will be drawn from the top and not pick up any of the contaminants that settle to the bottom of the tank.
Another major difference between aviation and auto fuel tanks is filtration. In an aviation fuel system, there should be a filter separator that removes any free water and any particulates from the fuel.
The problems begin gradually. Over the years, the aviation fuel tanks will settle, which means the water is not drawn to the lowest point of the tank.
Instead, it often collects in a dead space at the bottom of the tank.
Over time, the water at the bottom of the tank attracts — and retains — all kinds of things that are bad for your airplane, such as surfactants, ethanol, microbial growth, rust, dirt, and many other harmful things.
Over the years, the junk and contaminates continue to accumulate because they are more attracted to the water than the fuel. As it becomes more contaminated, the fuel begins to smell bad.
This becomes a big problem when new fuel is added to the tank, stirring up the whole mess. If adequate time is not allowed to let the bad stuff settle back to the bottom of the tank, it can be drawn into the fuel inlet and then into your airplane.
Another potential problem is when the floating suction on the inlet doesn’t float due to a compromise of the float system over time. This allows the float to sink and the crud on the bottom to be drawn into the fuel inlet.
Once this bad fuel is drawn into the inlet, it enters the fuel filter separator and can disarm the water separator part of the system. Then the free water can weaken the particulate filter and allow the contaminates to enter the aircraft tanks.
Here contaminates like ethanol will attack fuel system rubber components, causing leaks, while the microbial growth can attack metal in the fuel system. Meanwhile, particulates can plug passages in your carburetor or injection system. This can lead to bad fuel distribution and an overheated or detonating cylinder.
Where do these contaminates come from?
The water comes from leaks in seals and fuel condensation. Other contaminates come from poor housekeeping and the distribution system.
While many fuel distributors have dedicated trucks to deliver 100LL, not all of them do. Additionally, the fuel distribution system at most distributors have common piping and control valve systems somewhere in their plants.
The bottom line is that water and other contaminants are going to get into the fuel distribution system, so they must be managed properly.
Quality Assurance
Every avgas dispensing system needs to have a comprehensive quality assurance program.
For example, FBO or airport personnel need to check the floating suction EVERY DAY to ensure it is still floating. They also must sump the water draws until clean dry fuel is noted.
In addition, the filter separator needs to be sumped and the differential pressure across it measured and recorded while fuel is being pumped.
I have found many systems where the differential pressure was recorded as zero for a long time, only to find that the filters had been ruptured and were not doing anything.
What can you do to protect your aircraft?
So, what should a pilot do to protect their aircraft?
First, get to know your fuel supplier really well. Review its quality practices. I have seen some EAA chapters do this for their area.
Once you know where you get your fuel is spot on, frequent these suppliers as often as possible.
I know this can be a problem on cross country flights, but use common sense and ask questions. Most FBOs are proud of their systems and will gladly show you or discuss their operations.
Lastly, never — and I do mean never — refuel right after a fuel delivery.
If you do pull up to an airport’s fuel farm and discover it just had a fuel delivery, you’ll have to play a waiting game. How long is up to a variety of factors, including the size of the tank, as well as the amount and type of contaminants.
If it is a relatively clean tank with a working floating suction, you’re looking at about a two-hour wait.
But all bets are off as to how long everything will take to settle if the tank is in any other condition.
So Ben…how does one know when fuel was last delivered? Ask, perhaps, but good luck getting someone on the phone, or if you do, someone who knows the answer.
And folks, sump your fuel top to bottom, not wing, belly, wing or water will migrate past the drain point. Some POH’s have the sequence wrong. FWIW…
Good article, Ben, and good comment, Randy. Operational and safety requirements, beyond federal regs and A/Cs, should also be in the airport’s rules and regs and minimum standards. Even so, too many pilots and FBOs (with understanding that one is too many) see these as bureaucracy and government regulations designed only to interfere with their business or fun. In my days working with GA as one of The Airport People, the great majority of tenants and users understood the need for compliance. It’s those inevitable free spirits, however, who do the “stuff.”
Steve and Ann: good to see the grammar discussion. But about those upper- and lower-case letters…..
I am sick of listening to these alleged experts knowing everything that is wrong with our fuel system’s & dispensaries, yet I haven’t seen any of them write up these problems that could cause a lose of life to the FAA and force the people responsible to correct these things. What is the purpose of writing about all these problem’s if nothing is done to correct them??? If you are an expert and see a potential serious problem and do nothing to correct it, well please don’t say anything ! It doesn’t do any good, does it??? Always comparing apples to oranges, that’s not how it’s handled, saying the auto industry doe’s this and the auto industry does that, not good, only a bad example. What is the sense of being an expert? I think I would rather be a concerned maintenance personnel and correct these problems as they happen. Passing the buck is pretty petty and gets nothing done, aircraft are very different than auto’s. my opinion.
COWS – if you don’t do it, you may get to meet them.
I’m going to put a label on each of my fillers.
Great mnemonic!
I liked that you explained that it would be smart to consider doing some quality assurance tests. That does seem like it would help you know if the fuel you are putting in the plane engine is good. Personally, I would want to get a professional to help me deal with caring for a plane.
Excellent summary, thanks
What about LSA’s using automotive gas?
Always filter your fuel, very very important. MOGAS isn’t maintained to the best standards. Don’t let it set in you tanks more than a few weeks. Use fuel stable if it’s going to set for a while.
Great article Ben. We need more information like this out to the pilots. I inspect airports for a living. You absolutely can’t believe the “stuff” I see on fuel systems that are incorrect. Poor or no inspections, no daily sumping of tanks, no testing fuel when it is delivered, unlocked filler pipes, corroded pipes (if they are rusting on the outside, they are the incorrect pipes and you can bet they are rusting in the inside. Also a bit of advice: 1. Never, never allow someone else to fuel your plane unattended. If you aren’t fueling yourself, at least observe the line service personnel. 2. If you are doing self-fueling, where is the emergency shut-off? It should be outside of the fueling area and clearly marked. I’ve seen them hundreds of feet away and not marked. There’s much more……
Hey Randy…good to see you are still out there protecting us from ourselves! I’d suggest too that anyone using Mogadishu to check for. Alcohol. It’s easy. I’ve found alcohol twice in the Mogas I use in my ‘46 Aeronca.
Mogadishu??
Thanks Ben,an excellent article on avgas and English
Just as a clarification, contaminate and contaminates are verbs, not nouns; it is an action or a process. The correct word for anything contaminating a solution or solid is contaminant(s), a noun, as correctly used towards the end of the article.
hi, steve glad you pay attention to words and usage
please look up the word towards that you used
grammatically, it can be used in the the USA
i had never looked it up previously
i found some interesting information on the two words- toward and towards
oh, what we learn every day
i always use toward
ann
I have found that spell checkers do not like the word “contaminants” and often try to change it to “contaminates”. I teach several environmental courses and it is always a problem with Word and PowerPoint. Not a big deal.
Water in fuel below 32F may be frozen and will go undectected in cold weather. Also, checking fuel at night prevents a proper color determination if the background light source is too faint. Of course it’s not possible to check fuel for either dirt or water just after fueling at a stop enroute. Who will wait half a day in the middle of Wyoming for that check?
After fueling Iet the fuel settle for about twenty minutes, then sump the tanks and fuel strainer, plus smell the fuel .
COWS
Color
Odor
Water
Sediment