Modern Trends in Aircraft Fuel Systems

Aircraft fuel systems may not be the most glamorous feature of an aircraft. However, their functionality makes them an essential part of all aircrafts. The functional characteristics of these fuel systems play an important role in the design, certification, and operational aspects of both commercial and military aircrafts. The fuel system designs actually have the biggest impact on the operational capability of any aircraft. They encompass a wide range of technologies that are very important, especially when they are looked at, considering the complexities of large transport and high speed military aircraft applications. The last 50 years have seen great changes in the technologies, used in avionics, and major advances in the design of the avionic products, which have had a great impact on the fuel management systems and fuel quantity gauging. This paper seeks to provide a detailed insight on the modern trends in the aircraft fuel systems.

Fuel measurement technologies have been changing over the years and have culminated in the development of the improved in-tank gauging sensor technology. Many alternative sensing techniques have been investigated over the years, and this process has included the use of optics, radar, pressure, and ultrasonic techniques. Hybrid solutions, which combine various aspects of these technologies, have also been investigated. However, ultrasonic gauging is the only technology, apart from the traditional capacitance technology, which can see service in a production program. The Lockheed Martin F-22 military fighter and the Boeing 777 are the only aircrafts that have embraced the ultrasonic approach for in-tank fuel quantity sensing. At the inception of this technology, the obvious benefits, introduced by it, were the minimization of the connector integrity-related and harness shielding problems that are exaggerated in capacitance systems, since the system has to deal with very poor capacitance signals in a hostile environment. However, the overall benefits of the ultrasonic systems have not been appreciated adequately, even with such improvements. All new military and commercial aircrafts have returned to the traditional capacitance technology.

The modern aircraft fuel systems now use high accuracy capacitance gauging systems to measure the properties of the fuel. The system specifically measures the temperature, density, and the permittivity of the fuel on board. In recent years, this requirement has been fulfilled through the use of an integrated sensor package, which is also referred to as the Fuel Properties Measurement Unit (FPMU). It has all the three sensors, through which a sample of the uplifted fuel can be measured. Thus, the important characteristics of the current fuel load and the residual fuel (fuel from the last refuel) can be determined. The location and the quantities of these devices within the fuel tank have to be well considered so as to minimize functional redundancy. This is also essential in helping to avoid issues of contamination mainly from the water that accumulates over time and adheres to the in-tank equipment. The latest approach for the installation of the FPMU is within the aircraft’s refuel gallery. It, however, calls for the need to maintain the accuracies of the measured parameters, even when the flow in both directions is high. This new in-line FPMU technique was used for the first time in Airbus A350 aircraft.

Modern aircrafts also have improved fuel transfer systems. Civil aircrafts must always transfer fuel from the fuselage centre wing tanks to points, where they are consolidated before engine feed. However, some of the JAR/FAR regulations require separate engine feed systems. More recent civil aircrafts such as the Airbus A340 have horizontal stabilizers that may carry up to 7 tonnes of fuel. The fuel must be transferred so as to maintain the aircraft’s centre of gravity within acceptable limits when the plane is cruising. This schedule is often activated automatically when the aircraft exceeds altitudes of FL250. The two main in-flight refuelling techniques that are widely used today are the probe and drogue method, which is mostly preferred by the US Navy and the Royal Air force, and the boom and receptacle technique, which is used almost exclusively by the US Air Force.

In the probe and drogue method, the tanker aircraft trails a refuelling horse that has a large drogue attached to it behind the aircraft. The recipient is fitted with a retractable or fixed probe when it is not in use. The pilot of the receiving aircraft is then tasked with the responsibility of inserting the refuelling probe into the tanker drogue. The fuel is passed to the receiving aircraft only when a positive pressure is exerted on the drogue by the refuelling probe. The gauging system of the recipient and the tanker monitor the transfer of fuel. Contact between the two is broken when the positive pressure between the drogue and the probe is lost, once the receiving aircraft withdraws, when the refuelling operation is complete. The tankers in the Royal Air Force usually operate with one drogue from the centre line of the aircraft and one from the refuelling pods under the wing. It means that a total of three stations is available. This method makes it possible for more than one aircraft to be refuelled at a time.

 
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The modern aircraft fuel systems have the capability of measuring the temperature of the fuel. The fuel temperature is appropriately measured and displayed on the ECAM or EICAS. The display changes to a different colour, usually amber, each time the level of the fuel goes below the cold fuel threshold and annunciates the low fuel temperature to the crew. The appropriate level for this cold threshold is often set at a standard rate, which can either be JET A-1 (-440C) or JET A (-370C). This setting may be customized from time to time if the actual freezing point of the fuel board is known. Both Airbus and Boeing have been equipped with software that can help the flight crew to address the cold fuel issue at the flight planning stage.

Modern military aircrafts have incorporated in-flight or aerial refuelling systems, which have a critical function in their applications. For instance, the strike aircrafts can depart with a consignment of weapons and reach high latitudes, consuming large quantities of the fuel on board. The fuel tanks of these airplanes can be renewed when these aircrafts reach operational altitudes. This extension is important for the mission capability of these aircrafts, and it is acknowledged as an significant force multiplier. The fuel design system is further complicated by the aerial refuelling function through the provision of an on-board hook-up structure with the connections that are fluid-tight and have suitable disconnection capabilities that are safe, especially in cases when unforeseen emergencies occur.

Most of the modern aircrafts are also equipped with two or more fuel cells or tanks. The high wing aircrafts have these cells housed within each wing. The position of the tanks in the wings for the high wing aircrafts facilitates the gravitational flow of the fuel to the engine. However, the low wing aircrafts require fuel pumps to facilitate the flow of the fuel. In both instances, the fuel must be availed to the engine for it to start. Thus, electrical pumps have to be used to propel this fuel to the engine, especially in the low wing aircrafts. After the start of the engine, fuel is then driven to it by the engine feeds, aided by a mechanical fuel pump. All fuel tanks have drain valves, positioned on the lowest points of the tank. They allow pilots to check for the water that may have collected in the fuel tanks and to remove it if needed. This is always done during the check before flights. Another drain is also situated at the bottommost point of the fuel piping structure. These valves must be drained before flights in order to remove any water that may have collected in the fuel pipes.

One of the important advancements in the fuel systems, used in military aircrafts, is the introduction of the air-to-air refuelling capability. This refuelling technique allows the planes to depart with a significantly higher weapon load. The planes may also extend their range or loiter times over the battlefield. Experiments with this system started in the 1920s through the individual biplane fighters that transferred fuel to one another. Most of the systems that were developed later on in the 1950s applied large tanker aircrafts to fuel other aircrafts. The most recent trend involves the use of a “buddy store” refuelling system, mounted on a standard fighter. This trend has brought the notion of aerial refuelling back to its roots.

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The fuel valves, used in modern aircrafts, include hydro-mechanical and electro-mechanical valves that have a wide variation in surge pressure and flow control. The variations are also exhibited in flow characteristics and in supply pressure for typical product manufacturing tolerances. The valves that are motor-operated have varying time frames in opening and closing with difference bigger than two to one. The valves have variations in operating temperatures and electrical power supply. For instance, the control valves, used for refuelling, can cause wide variations in overshoot and surge pressures. The variations are mainly presented in the aerial or ground refuelling tanker systems. Ideally, when a command for closing the valves is ordered, the amount of fuel passing through the valve should remain the same, regardless of variations in the physical configuration of the valve, operating temperature, electrical power, or supply characteristics.

The most auspicious technology that could be established is one that combines the use of micro-electric machines (MEMS) and light. The MEMS devices can be designed to measure density, temperature, pressure, and acceleration when they are excited by light through optical fibres. This new technological approach, which is still being evaluated by specialist corporations within the aerospace industry, would offer a recurring cost approach that is significantly low. The low cost could possibly provide an intrinsically safe and HIRF immune sensing solution, which is very suitable (mainly because of the small size of MEMS sensors) for embedding in composite structures. Therefore, it could be actually the best candidate technology that would be able to operate reliably in harsh environments and meet the today’s stringent regulatory requirements.

Some of the technologies that could be developed to overcome the problems of overshoot control and surge pressure include velocity control valves, which incorporate hydro mechanical design techniques. However, it is an open approach that could improve surge pressure and overshoot performance. This technique may not be particularly accurate or repeatable due to the fact that it depends on the functional tolerances of valves, refuel pressure variations, and overshoot performance. Another option is the use of a constant speed motor-operated valve that applies a voltage reduction technique. This method provides opening/closing times that are relatively constant by minimizing or eliminating the effect of the aircraft power supply stations. It will also help to reduce the functional variation in motor-actuator performance with minimal defects in terms of reliability, weight, or cost. This approach, however, may solve only part of the inconsistency-related problems of the valve, since it does not address the variation of other system factors such as pressure variations. The third technique that could solve the overshoot problem would involve the use of fuel management systems, which could help to shed light on the functional characteristic of each of the control valves and to adjust the anticipatory valve selection process accordingly.

The avionics industry has been experiencing many changes over the years and it is mainly shown in the technologies that have been adopted in the modern aircrafts. Most of the advancements in this industry have been motivated by the trends, incorporated in the military aircrafts over the years. The fuel systems, used in the military aircrafts, have been evolving with the evolution of the engines and other technologies. The modern trends are noted in the techniques, used for fuel measurement, the high accuracy capacitance measurement systems, the in-tank gauging system technology, the fuel transfer systems, and in the air-to-air refuelling capabilities. The carburettor, which was commonly used in the piston-engine aircrafts, has led to the fuel injection technologies, used in jet engines. However, the concept of the self-sealing tank has been improved consistently, since the time it was invented, and it continues to be used in modern aircrafts. The drop tank systems, the air-to-air refuelling and the self-sealing tanks all have deep historical roots, but their concepts have been improved and significantly refined over the years. The avionics industry is not limited to these technologies. Several other technologies that can introduce more modern trends are still being developed. Therefore, more changes should still be expected.

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