FUEL SYSTEMS – LIFE BLOOD FOR ENGINES
Our bodies need fuel to keep us going. That fuel is the food we eat. Likewise, the engine(s) on our helicopter need “food” to generate the power for us to fly, and that food is fuel. Our helicopter has a dedicated fuel system for each engine, but what exactly is the job of the fuel system?
The helicopter’s fuel system allows the flight crew to pump, manage and deliver fuel to the engine(s). Fuel systems can differ greatly due to different performance characteristics of the helicopter in which they are installed. A single-engine piston helicopter will have a very simple fuel system, while a multi-engine helicopter will have a far more complex system. A helicopter’s fuel system will vary as to what kind of engine(s) are being used, and what are the performance characteristics of the helicopter as to speed, altitude, etc.
In a typical fuel system, fuel is piped through fuel lines to a fuel control valve (usually known as the fuel selector). This valve serves several functions. The first function is to act as a fuel shut-off valve. This is required to provide the flight crew with a means to prevent fuel reaching the engine in case of an engine fire. The second role is to allow the pilot to choose which tank feeds the engine. Many aircraft have the left tank and right tank selection available to the pilot, and many have the “both tanks” position in addition to the left and right. The reason to have the left and right tank option is to allow pilots to balance fuel load and reduce the banking moment. Occasionally the shut-off function is in a different valve located after the fuel selector valve.
After the selector valve, there is usually a fuel filter that can be drained. Drainage points are typically found in each tank (often more than one per tank), at the fuel filter and at the injection pump.
Each fuel tank needs to be vented to allow air in the tank to take the place of burned fuel, otherwise the tank would be in negative pressure, which would result in engine fuel starvation in the long run. The vent also serves to allow for changes in atmospheric pressure with altitude.
With fuel being so important to the operation of the engines, anything that can hurt the fuel system can rob the engine of performance and that can prevent the helicopter from accomplishing its mission. Let’s talk about some of the things that can hurt the helicopter’s fuel system.
With operating costs and budgets a constant concern for helicopter operators, having a fuel leak can not only cost more money to replace the lost fuel, but could be serious enough to ground the helicopter. Needless to say, having a fuel leak is not something that is good for the aircraft, flight crew or maintenance specialists. The danger of fire, explosion and safety of flight is not something to be ignored. Detecting and repairing a fuel leak is best left to experienced, trained mechanics. Here are some things to consider if you believe you have a fuel leak:
• Note any observations about the leak, such as does it occur only when the tank is full? Does it stop when the tank is less than half full, or does it continue to leak?
• After reading the manual in order to determine what type of fuel tank system you have and how to gain access to it, inspect your helicopter’s fuel tank carefully. You should look for any wet areas and note if you smell a strong fuel odor.
• If you’re certain the fuel tank has a leak, take the proper steps to get the leak plugged.
• Always make sure you comply with the manufacturer’s instructions on how to fix your specific type of fuel tank. This will ensure that you don’t run into hazardous situations at the time of the repair.
• Once the repair has been completed, reinspect the fuel tank and check to be certain the leak has been fixed.
Regularly-scheduled maintenance is the best way to prevent fuel leaks, as this ensures that all of the equipment is in good condition and functioning properly. Regular inspections prevent larger, costlier problems from occurring in the future, and it’s best to invest in regular inspections and maintenance to keep everything in working condition rather than having to spend a lot of money to fix a serious problem on short notice.
If you need to install a fuel bladder, let a team of professionals who are well versed in the dynamics of the job handle it in order to ensure it’s done properly. The bladder can be a flexible bag or a rigid unit designed to hold the fuel and which is contoured to the shape of the particular helicopter fuselage. Damage to the bladder can result in a fuel leak and can result in serious problems to the helicopter. Routine inspections and maintenance helps you uncover any such possible leaks in the bladder.
Fuel Tank Corrosion
Corrosion is arguably one of the most common causes of leaks in fuel tanks. The root of this problem is usually bacteria, which contaminates the helicopter’s fuel and, in turn, leads to corrosion in the fuel tank. When fuel gets contaminated, organisms start to feed off the hydrocarbons in the fuel. The waste products that are created during this process lead to acids that can corrode the helicopter’s fuel tank.
Regardless of what caused the tank to leak in the first place, the source must first be determined in order to fix the problem and ensure the safety of the crew and passengers. Fuel tank maintenance personnel should ask the following questions in order to detect and repair the leaks:
• How noticeable is the leak?
• When was the leak picked up?
• Has the helicopter been grounded since the leak was noticed?
• Does the fuel tank only leak when it is full?
• Can you see any damp spots or leaks inside the aircraft or on the aircraft’s exterior?
The problem with detecting and repairing a helicopter’s fuel tank leak is the fact that there are quite a few different fuel systems that all work differently than one another.
Helicopter Fuel Leaks from Corrosion
Fuel system corrosion is often the cause of aircraft fuel leaks and should be treated separately from structural corrosion for several reasons. First is that while the majority of the helicopter’s structure is metal of various types, the fuel system often contains many polymer components causing the types of corrosion to be different. Second, significant sections of the fuel system are internal, requiring different inspection techniques. Lastly, structural corrosion is often caused by helicopter components coming into contact with fluids while the job of the fuel system is to contain many of those same fluids.
While a full fuel system inspection need not be performed before every flight, it should be carried out regularly. One check that can be done before every flight is the normal fuel tank sump check looking for water in the fuel tank. During this check, look not only for the presence of water, but also any discoloration of the fluid which might indicate the presence of microbes. Microbes are one of the areas where fuel system corrosion deviates from that of structural corrosion.
Hydrocarbon-based fuels, especially when mixed with water from condensation or leakage, provide an excellent breeding medium for several strains of bacteria. These bacteria often form a film or a sludge that can actually eat into the materials of the aircraft’s fuel system. Such corrosion by these metal and plastic-eating bacteria can be a significant cause of helicopter fuel leaks.
These leaks, in return, can allow fuel to seep back along lines and into previously-dry internal compartments. As the fuel collects in these areas, it can cause additional corrosion to both the helicopter’s fuel system and its structural components. As a result, corrosion detection and repair should not be limited to the structural areas of the helicopter as it can occur anywhere.
To ensure continued airworthiness, a proper helicopter corrosion detection and repair regimen must be maintained. A visual corrosion inspection should be performed on all external surfaces during the walk around before each flight, in particular to look for fuel leaks. Furthermore, a detailed inspection of internal compartments and the fuel system should be made on a regular basis. Early detection will keep the helicopter flying while minimizing corrosion repair costs and ensuring less down time.
Repair or Else
Among the more common causes for helicopter fuel leaks are stress and/or structural damage, corrosion and sealant degradation. Economic factors often require helicopters to be kept in service longer. This, combined with the fuel system inspection practices of some operators, often aggravates the problem. It is important to have a robust maintenance process ensuring flight safety.
Metal fatigue and structural damage, caused either by hard landings or high cycle times, can also be a common source of helicopter fuel leaks. Hard landings can have a negative effect on fuel system sealants, causing them to lose their bond with the helicopter. Fuel system sealants are also affected by weather, particularly in the case of helicopters parked on the flight line. It is important to recognize that even helicopters kept in hangars can be affected.
Having said all this, the helicopter’s fuel system is the life blood of the engines. If you suspect a problem, don’t wait to report it. If you do not have the talent in house to deal with it, then go out and get the help you need. Don’t let a small problem turn into a big one.