Aircraft Fuel System Maintenance: Key Issues to Consider

  •   22 May, 2018
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Aircraft Fuel System Maintenance: Key Issues to Consider

Fuel—having enough of it and assuring its steady flow to the engines—is so central to an aircraft’s operation that by many measures, the machine is designed around its fuel’s inflight storage and delivery. Wing and center body tanks are most commonplace and fuel is delivered through a combination of gravity and electrical power. A complex system of valves, pumps and measurement devices must all work together to assure a constant feed and to provide the flight deck crew with the information necessary to safely complete the mission. 

One infamous example of that system breakdown is the ‘Gimli Glider’ incident. On July 23, 1983, an Air Canada 767 crew was planning a flight from Montreal (YUL) to Edmonton, Alberta (YEG). The Fuel Quantity Indicating System (FQIS) had been disabled due to a maintenance issue. Accordingly, a dip tank test was performed, but at the time, there was a lack of standardization of fuel delivery and recording due to the country’s then evolving adoption of the metric system. As a result, what was thought to be a measure of pounds/gallons was actually of liters/kilograms. 


Credit: istockphoto


The flight to Edmonton was uneventful until about two-thirds of the way when the crew got a low-fuel-pressure notification on the left side. Believing the problem was a faulty fuel pump, the pilots shut it down. A few moments later, they got the same indication on the right side, followed by a shut down of the left engine, and then the right. What ensued then can only be described as an astonishing act of airmanship, whereby the pilots glided the powerless aircraft to a safe landing at an old military airstrip in Gimli, Manitoba, which had been converted to a race track.

Modern aircraft fuel systems provide many challenges for maintainers. From delivery and measurement systems, to mechanical structures like tanks, filters, valves and plumbing, all of which require knowledge and experience to keep them functioning properly. In addition to operational wear, the fuel system is under constant attack from contamination. Water and debris intrusion from outside the aircraft is always a concern since it can lead to an insidious attack from a fungus that feeds on the fuel hydrocarbons. There is much trouble for which technicians and those who manage them need to be alert and have a plan of corrective action. 

Fuel Systems

Knowing how much fuel you have on board is key, obviously. Unless you are flying very long distances over water, the days of topping off the tanks for every flight are long gone. For most missions, cost and weight/performance considerations outweigh endurance, making takeoff fuel quantity and fuel burn in flight important measurements. 



Today’s aircraft use electrical capacitance systems to measure the dielectric value of the gap between the plates of the sensor. As fuel quantity changes, so too does the resistance and the result appears in the cockpit indicator. Advanced systems use a compensator to adjust the value depending on fuel temperature since that varies its density. When changing probes or sensors, technicians must calibrate them to ensure they read correctly. The procedure often involves adding a measured quantity of fuel, with a known temperature and density, which is then calibrated through the indicator of a signal conditioning unit. 

For aircraft with multiple tanks, there are multiple probes and measuring units. Balancing the fuel load is aided by gravity and/or pumps. Multiengine aircraft have redundant lines, hoses and valves to ensure supply to each engine. Fuel tanks need to be vented to allow air in and prevent a vacuum, but they also need a check valve to prevent siphoning. Each component needs to be both functioning properly and free from defect, as the smallest leak can allow air into the system, which can result in a flameout. In addition, many aircraft have pressure refueling valves and both check- and shut-off valves to prevent over pressurization of the fuel tanks. For many large aircraft, there is a mechanical backup system where a dipstick can be dropped in the tank to measure fuel.  

Bugs and Fuel

Water and contaminants have always been the enemy within aircraft fuel systems. Terrestrial fuel delivery systems are the first line of defense against the invaders, and generally do a good job. 

As a result, the most common method for water ingress into an aircraft is through condensation. Water vapor settles from the air space above the liquid fuel and sinks to the low spot in a tank, whereupon it can be removed.  


The brown-black debris on the bottom of this tank is made up of fungus and bacteria. This growth can clog fuel filters causing the airplane fuel quantity indication system to read incorrect values, and eventually cause structural corrosion of the aluminum stringers and wing skin. Credit: Boeing


To detect water in a fuel, a small amount of liquid is drained and inspected. The frequency for conducting this procedure varies based upon the operating environment. In general, any aircraft that sits motionless for several days should be checked, but always follow OEM guidelines. A problem for most turbine aircraft greater than water ingress is the existence of a particularly nasty bug — Hormoconis Resinae — a fungus that feeds on hydrocarbon molecules in Jet A. It grows very fast, especially in warm temperatures; it can go dormant when it gets cold, but reactivates when the temperature rise again. The bacterial waste from the bug is extremely corrosive to tanks and it can also eat away sealant resulting in leaks and other serious issues.

A problem with sump checks is that often the sump valve sticks and the checker gets drenched with fuel. Moreover, the check is difficult to conduct on aircraft with a particularly low fuselage. Such realities, over time, can cause some checkers to become lax in their checking. Even though many OEMs allow pilots to perform sump checks as part of their preflight inspection, many defer this task to their maintenance technicians. Regardless, it is especially important that those aircraft traveling frequently overseas or sitting for long periods of time undergo sump checks before launching.

“The entire aircraft is sumped monthly,” says Mark Jones, director of Maintenance and a pilot for a small, IS-BAO registered, mid-western U.S. flight department. “When the aircraft leaves the country it is sumped immediately upon return. We also treat every three months with BioBor JF using the shock method and let it set for at least 24 hr. before adding any additional fuel. When we find anything, we immediately investigate and remedy the situation.” 

Operators should treat fuel with an anti-microbial biostat in accordance with your OEM recommendations. Many manufacturers suggest regular treatment even if there is no indication of bacterial growth. If samples come back positive, you can kill the fungi with a biocide. A typical biocide uses about one quart of chemical per 1,000 gallons of fuel and takes 24 to 36 hr. to kill the microbes. Be sure to follow the manufacturer’s recommendations for post-biocide application. You may need to increase the frequency of drain and filter checks until you are certain your tank is clean. 

Periodic fuel tests include a dye check and microbial growth media. You place the sample in the tube and wait a few days for the result. You can send your sample to the lab as well, but new technology had enabled a much quicker turn-around time (see sidebar). Put a few drops in the analyzer and within ten minutes of so, you have the results. While not cheap, this option can quickly assess the status of your system, a particularly helpful piece of information if you’re planning a long trip or are heading overseas after an extended period of inactivity. Many technicians inspect tanks at the C check, and if finding them clean, assume that condition will continue until the next check. However, the fact is microbial growth can consume a tank in as little as six weeks.  

The sump check also requires a safe way to dispose of the fuel. There was a time when we’d just return a non-contaminated sample back to the tank, but that actually that actually increases the chances of contamination. The best procedure is to hand the samples to the fuel service provider and let it to the disposal. 

“We try to do most of the sumping on our aircraft in our hangar as we do have an environmental company that regularly picks up the used fuel and oil. When traveling internationally we use only airports that have air carrier service and that gives us a good and quality fuel to be put in the aircraft,” Jones says. “When fueling, most FBO’s are more than accommodating to letting us see the strainers and fuel test results.” 

Tank Diving

Besides sump checks, most aircraft have a regular tank inspection requirement as well since problems in the FQIS are also sometimes caused by contamination. The sensitivities of gauging probes are such that even small amounts or crud can cause a discrepancy. 

This additional requirement can involve a detailed visual inspection either directly by opening up the tanks, or by inserting a video scope. You also may need to gain access to your tank if there’s a malfunction with the indicating or supply system. These “tank dives” demand the diver take extra precautions. 

“Anytime you get into a fuel tank, whether it’s either a simple task [such as] replacing a probe or getting inside, make sure there’s a second set of eyes so that no items are left behind,” cautions Mark Goertzen, a Dassault Falcon Jet Tech Rep at Duncan Aviation in Lincoln, Nebraska. Depending on the fuel tank size and configuration, the technicians involved may need breathing apparatus and protective clothing for the task. If the tank is large enough to enter, the team will need Confined Spaces training that includes emergency extraction and safety protocols. 

The key to any tank related maintenance operation is to plan ahead. This is especially true for tanks with limited access and restricted openings. 

“What we do ahead of time is gain access to the tank by pulling probes or panels and use a borescope to look inside the tanks just to get a heads up on what we’re facing. Normally the microbial growth is in the feeder side, which is a small tank where everything accumulates,” Goertzen explains. “In the feeder side there’s usually a lot of plumbing, boost and transfer pumps. To get in there and to be able to clean the bottom of the tank is a big job.”  

Most aircraft tankage systems feature very limited access and small panels, which present technical maintenance challenges, while some have large planks with numerous fasteners, which are time consuming to remove and reinstall. Even with good access, technicians need to be careful when working inside the tank to ensure that they do not create more problems for the systems or themselves. Besides removing microbial growth, the maintenance challenge is mitigating the corrosive effects of its waste. 

“The corrosion is not just on the structure of the airplane,” notes Goertzen, “but in the pipes and components. Then we see a lot of corroded electrical terminal ends that attach onto the fuel probes, because their clad plating gets warn off. 

“This type of capacitance system is very sensitive and does not tolerate electrical resistive connections,” he continued. “So, any kind of corrosion on those terminals ends will signal issues inside the tank.” 

Fuel system deterioration caused by biological growth is a serious matter and difficult to repair. And it seems that over the past decade or so, the problem is getting worse. Perhaps the bugs are getting resistant and adaptive — an organic evolution taking place, unseen, within your wing tanks. Perhaps that’s why many OEMs are becoming more proactive in mandating sump checks and microbial tests. And perhaps science can come up with a new silver bullet to keep tanks clean. 

Until then, however, maintenance technicians need to be trained and gain experience in keeping fuel systems airworthy. Too, pilots should also request samples from fuel trucks and get down and dirty performing sump checks when and where appropriate. Don’t let the bugs trash your tank and clip your wings. 


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