How Does Water Get Into Aircraft Fuel Tanks?

If any of you figure out a way to make water burn, please call us. We'd be happy to help you make use of this knowledge. Until then, I think we can all agree that it is not a good idea to put it into aircraft fuel tanks.

Many people just do not understand how water can get into an aircraft and therefore do not understand what they can do to prevent such contamination. Some do not realize how often it happens, and that it can happen to them. We are not talking about a drop or two, we are talking about gallons or even hundreds of gallons.

In our industry we have procedures for checking certain things, but many people do not know why they are performing these checks or what can happen if they do not. Unfortunately, a lack of understanding often leads to complacency, and this can lead to disaster.

In case you are not getting the point, EVERY YEAR LIVES ARE PUT AT RISK BECAUSE REFUELING COMPANIES LIKE YOURS DON'T THINK IT CAN HAPPEN TO THEM! In every case, after the event the operator is shocked and disbelieving. The common comment is, "I thought it couldn't happen here; we have a good QC program, do all the tests and use a good fuel supplier." The same thing CAN happen to you, no matter how good you think your quality control is.

Water, water everywhere

The following are examples of how water has gotten to aircraft. Surely there are other additional ways for this to happen; this is not a complete list of every possible cause.

1. On top of virtually all truck tanks, there is what we call "roll-over protection." This amounts to a dike or dam around the vents and manways. The idea is to prevent damage to these items if the truck rolls over upside down. To drain rain water (or melting snow) from this area, hoses are run down from the corners of the enclosure. Unfortunately, inspection of these drain hoses is often ignored as "no big deal," and they plug with debris (or even ice).

Well it is a very big deal. This has caused many incidents where gross amounts of water (up to 250 gallons) were put into aircraft. I personally spoke to a man who drained 150 gallons of water from an airliner. How did it get into the tank? If the water can't drain off, it "pools" up to 8 inch (or even more) deep. If the manway or vent seals fail, it goes directly into the storage tank. Even if the seals do not leak, when you engage the PTO to pump, the vent (which is submerged in water) opens allowing the water to enter the tank.

Check the drain hoses and manway gaskets and the filter separator water controls as well as checking tank and vessel sumps.

2. The snow had accumulated at an airport, and due to warm days and cool nights, the snow melted during the days and refroze at night. This allowed several inches of water to cover the area where the underground tank was buried. The gauging hatch or the test cable port gasket for the floating suction leaked. Water went directly into the storage tank. The filter separator water controls at the fuel farm and refueler truck both failed. An aircraft crashed.

This has also happened in warm climates during strong rains or floods. In one case, all of the fuel floated out of the tank and the tank was completely filled with water.

Check your tank top connections for a tight fit and proper gaskets; sump your tanks and vessels. Make sure the sample is fuel and not pure water. Check water controls periodically, tank sumps and vessel sumps daily.

3. The ground-water level was high and a structural problem caused a leak in the underground storage tank. Fuel did not leak out, water leaked in. The pilot or engineer of an aircraft being fueled later happened to notice that a fuel tank level indicator suddenly went from almost empty to full in the blink of an eye. If he had not noticed this, caused by an electronic gauge not calibrated for the high conductivity and mass of water, a catastrophe would certainly have occurred Still, more than 300 gallons of water was drained from the aircraft.

Test your water controls and take extra samples from the truck and aircraft sump if the slightest hint of unusual water risk is present.

Tank tightness testing is not just for environmental reasons. If you notice your storage tank levels increasing or not decreasing as expected, double check your tank sump. In this case the water float on the truck had a severed wire and the fuel farm had a failed water float.

4. A new man was assigned to fill the aircraft lavatory water tank. He was not familiar with the particular aircraft and put the water into the overwing fueling connection. It was only caught because his supervisor knew a Beechcraft King Air will not hold 150 gallons of potable water.

The fueling operation is not the only way to get water into the aircraft.

5. The aircraft was being refueled when the refueler cargo tank went empty. The lineman left both of the fuel tank caps off and drove back to the fuel farm for more fuel. He could not refuel because they were unloading a transport trailer (bridger). His shift ended and he went home, forgetting to finish. It rained the next two days and only due to the pilot's preflight check was the error discovered.

When you open a cap, do so only to insert a nozzle. Close it before you depart for any reason, even to do a small task such as picking up a dropped item. ALSO -- If you notice that the cap does not fit tightly, indicating a possibly failed gasket, notify the pilot. Rain water frequently enters aircraft fuel tanks through leaky tank caps.

6. Upon completing a new arm of the hydrant system, a "hydrotest" was performed. To do so, the system was filled with water and pressurized. When no leaks were detected, the system was drained and purged at high flow rates with fuel prior to being commissioned for fuel use. Due to an imperfect purge, and a broken wire on a water probe, more than 50 gallons of water was placed onto a jetliner. Even though the wing tanks were sumped prior to flight as a special precaution, the water was not discovered until the engines performed unevenly in flight and a precautionary landing was made. It turned out the sump sample was pure, clear water, but looked like clean, dry fuel and the jar smelled like fuel from previous sampling.

Carry coffee, tea, "food coloring" or other water based or soluble material with you and drop some into fuel samples. If it sinks to the bottom as a drop, you have fuel. If it dissolves, you have a problem. Do not accept a clear and bright test alone as proof that the sample is good, clean fuel. It can be bad, clean water.

Test your water controls and take extra truck and aircraft sump samples if the slightest hint of unusual water risk is present.

7. The operator decided to steam clean the tank on a refueler truck. He used great care to remove all of the water from the tank after the cleaning operation. However, he neglected to drain the refueler pump and piping. When he put the truck back in service, the operator had a problem getting flow until he ran the engine speed up higher and increased the pump pressure. When he suddenly got flow, he congratulated himself, assuming a valve had been stuck. A later operator noticed low flow and checked the strainer. He found the remains of the burst water absorbing elements from his filter. After careful inspection, it was determined that the pump bypass valve was set at over 100 psi, but miraculously no water entered the aircraft because the filter element largely remained intact and the nozzle strainer caught all debris. This was a 22 gallons per minute avgas truck.

There are many lessons in this one, and all are pretty obvious. Most importantly, recirculate a truck and check sumps, pressures and filter condition carefully after steam cleaning.

8. On an ocean going research vessel, the helicopter was fueled from drums. The aircraft lost power but was fortunately not lost. It was found that rain water, pooling on top of the drums, had been the cause. Due to the need for air to enter the drums as the fuel was pumped out, a vent plug on top of the drum had been loosened and not retightened after the last refueling. As luck would have it, it rained that night. Due to temperature changes, the drum "breathed" vapor out and water in.

Either lay drums over horizontally, or cover them. Use water absorbing filters in such situations for safety.

NOTE: THIS ALSO APPLIES TO "PRIST" OR ANTI-ICING ADDITIVE DRUMS! We know of a case where the bladders in an aircraft had to be replaced for this reason. The "Prist," having just 1 percent water in it, would not dissolve into the fuel and dropped out, destroying the bladders.

In the words of a skilled helicopter pilot who had this "water/drum" contamination happen to him in a similar case set in the mountains of a North American forest, "Helicopters glide like a streamlined brick. A safe landing is only possible if the engine failure occurs at a reasonable altitude, a suitable landing spot is available ahead and below and the pilot can perform a (very tricky) "auto-rotation" landing. The proper procedure is to first kiss the ground and then grab the refueler by the throat." (Your author finds himself in a rare situation of a lack of additional comment).

9. This one is really rare, but worth mentioning A major airport had a "Salt Dryer" in its incoming fuel system. This device is a huge bed of salt which the fuel flows through at a very slow rate. (See Gamgram 27 re: Salt Dryers). The idea is that the salt bed removes not only free water, as does a filter separator, but also significant amounts of dissolved water (Similar to humidity in air). This is a primitive sounding but highly effective device. Unfortunately in this case the flow became too high and water (salt water now) was carried through to the airport fuel system.

All of the water controls on this major international airport failed and all quality control checks missed it until after an aircraft had been fueled and had departed. The aircraft lost one engine on approach to its destination, and two more while taxing on the ground. The salt water damage to the fuel system was devastating.

10. This example is not for fuel system education, but for human nature education. A lineman was about to refuel a King Air. He noticed a "shadow" in the tank and called over his supervisor. Together they drained about 15 gallons of dirty water from the sumps of the aircraft. Upon the pilots return, the proud station manager presented this hero lineman to the pilot along with the buckets of water. Not only did the pilot not appreciate the service, he got mad at the FBO for touching his aircraft. He stated that the drain valves are expensive to replace and if any developed a leak as a result of the draining, the FBO was going to get the bill. He also threatened to call management and report this incident. The station manager calmly told him "Please do that."

We really do not know how the water got into this aircraft, but the two lessons here are to never underestimate the stupidity of anyone regardless of credentials, and don't touch an airplane unless the pilot says to.

Comments and details

Many of you will be thinking "The water controls should have stopped the water," or "The pilots were also at fault." You are right, but it does not matter. It is your job to deliver clean, dry fuel to the aircraft.

Tanks and filter vessels must be sumped. The water controls must be checked properly. Opinions vary on how to properly do this. Some say that purposely injecting water into the sump of a filter separator is the only truly reliable way, as manual testers and "squeeze bulbs" indirectly test the system, and have been known to falsely indicate that the system is in working order when it is not. The only sure way to test a water control is to have enough water in the sump to cause it to work.

The best way to do this is to fill the sump with water (easiest when done when changing elements), and then try to start fuel flow. (This usually means that you squeeze the deadman handle.) If you get fuel flow, stop immediately, your water control has failed. If you get no fuel flow, the water control worked properly.

Either way, put in a measured amount of water, and make sure you get it all out.

NOTE: Do not use city water/drinking water piping to do this, as the refueling pressure may be higher than the water pressure, causing fuel to flow into the water system, not water into the fuel system. We know of two such occurrences. In one case fuel backed up into a building where a fireman was taking a shower. The fireman was not happy with his jet fuel shower.

Others feel pouring water into the vessel sump when changing elements is the best, or removing the float and making sure it floats in water but not fuel. Others actually claim old style manual testers do the job all right (WRONG). Some probes have a push-button tester on them, much better.

No easy answers

We know of flaws in all of these methods, but the big risk is relying on the manual testers on old style float controls or squirt type testers on water probes. The old testers raise the float and test the shutdown system, but do not detect excess friction in the mechanism due to old age or contamination, and certainly cannot detect a failed float. Probes can also be contaminated on the outside with a not-conductive layer of gum, varnish or other contaminant. The squirt tester tests the INSIDE of the probe, where such contamination does not reach. New probes are under development to solve this problem.

Use modern counterweight floats and clean the outside of your water probes when changing elements. It is still a good idea, in our opinion, to test with real water, but be sure to remove it all. NEVER test with water when refueling an aircraft. DEFINITELY get approval from your oil company or airline before running such a test. Measure the water quantity before and after.

If you do find that you have water in an aircraft fuel tank, experience has proven that simply draining the tank sumps will not remove all water. Whether your aircraft is large or small, there is a strong possibility you will have trouble with remaining water. In one case, on a single engine propeller driven avgas aircraft, even rocking the aircraft and draining the sumps repeatedly did not prevent an emergency landing on the next flight. On a large corporate jet, two days of repeated tank sumpings at two separate airports did not prevent a crash landing later.

Jim Gammon is the president of Gammon Technical Products, Inc, a fuel quality control and handling equipment company. The Gamgrams have been written and published on an infrequent basis since 1975 and are periodically updated to keep them current.

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