Mechanism of hydraulic systems in aircrafts
Hydraulic system in aircrafts consists of a system in which liquid under pressure is utilized to transmit energy. The power from the engine is converted by the hydraulic systems to hydraulic power. Several hydraulic systems existing in an aircraft include the landing gear, brakes doors and control boosters. According to Roskam (2000), some of the components that are part of the hydraulic system include hydraulic fluid reservoir, accumulators, hydraulic pumps, valves, and lines for distribution and controls located in the cockpit to operate the different functions of the hydraulic system.
The hydraulic system has an actuating cylinder that functions to change the hydraulic power to mechanical power. This cylinder has a piston which is regulated by oil pressure. The oil is supplied on both sides of the piston heads (George, 2006). A selector valves control the supply of the high oil pressure to the pistons. Thus, when the selector valves supply the high oil pressure, the piston heads on which the high pressure oil is directed will move. Through the movement, oil on the low pressure side moves to the reservoir. According to Federal Aviation Administration (2009), the presence of a relief valve will aid in providing an outlet for the hydraulic system in situations where there is excessive fluid pressure in the system. The difference in oil pressure is responsible for the movement of the piston, which generates force required to move loads (George, 2006).
The pumps required to execute a certain function will depend on the criticality of the hydraulic system for instance, more pumps will be used during safe flight operations. In this case accumulators may be used to supplement the required hydraulic pressure (Roskam, 2010). The hydraulic system in an aircraft integrates different components to meet the individual requirements of different aircraft. The recommended operating pressure of the hydraulic systems is 3000 psi (Roskam, 2010).
Benefits of Hydraulic systems in Aircrafts
The use of hydraulic systems in aircrafts is to provide for activation of movement within the aircraft. The devices activated by the use of hydraulic systems include flying controls, landing gear, roll spoiler and brakes. According to Fielding (2007), hydraulic power is necessary in the undercarriage. Here the hydraulic power is utilized to retract jacks and for braking. The large doors of the aircraft will also demand to be moved hydraulically.
Roskam (2000) notes that hydraulic power will be used to move the primary flight controls, which include elevator, ailerons, rudder, spoiler, and the stabilizer. Additionally, hydraulic power is necessary to move secondary flight controls such as the flaps, trim controls and the speed brakes. Consequently, the hydraulic power produced from the hydraulic system also actuates the wheel brakes and is used to operate the thrust reversers.
Causes of hydraulic system failure at Aircraft
System Overheating
The hydraulic fluid in the hydraulic system may be overheated because of inefficiencies, which arise when the engines are in operation. According to Adams and Manion (2011), inefficiencies may develop from the presence of engine debris. When the debris in the engine is not contained and have higher masses they can be able to slice, severer or dislodge critical flight components. For instance, when the uncontained debris is travelling at high velocity when the locomotive is in operation, they can be able to dislodge electrical and hydraulic lines in the wing and this can result in the loss of numerous organizations.
Loss of System Pressure
Loss of system pressure results from failure of the engine driven hydraulic pump. Further, since the movement of the pistons in the hydraulic system is from the differential pressure created in the actuating cylinder, failure of the selector valves will cause pressure loss (Erjavec, 2004). According to the Mechanical Failure Prevention Group (1978), low fluid levels in reservoirs causes hydraulic pumps to lose prime, which result in loss of system pressure and flight control.
Auxiliary power unit (APU) failure
The auxiliary power unit supplies electricity, hydraulic power, and pneumatic air. Lack of fuel may cause engines and the auxiliary power unit to fail and since there is no power, the hydraulic systems cannot be controlled (Ostrom and Wihelmsen, 2012).
Hydraulic fluid contamination
The presence of contaminants in the hydraulic fluid can result in component malfunctions and failures of components such as the electromechanical servo-valves. The hydraulic fluid contaminates can include solid particles, wear debris and liquid contaminates lie water, synthetic oil and air. The particles cause wearing of system component surfaces, which may introduce leakage and increase loss of performance leading to the eventual breakdown of the system. According to Abzug and Larrabee (2005), precision high hydraulic pumps, valves, and actuators are sensitive to hydraulic fluid contamination.
Lack of Maintenance
Maintenance performed poorly may interfere with the linkages between components and pulleys in the hydraulic system. Additionally, the lack of replacement of check valves and failure to install filter elements result to decreased system performance and breakdown. According to Ferguson (2013), poor inspection of aircraft may result in failure of recognizing fatigue cracks in engines, which may then cause disintegration of components in the engine resulting to loss of the aircraft hydraulic system.
Defective Design
Defective design arises when certain components are designed in such a way that they are close to each other and in the case of a small mishap such as engine debris finding its way easily to the hydraulic system causing breakdowns.
Manufacturing Designs
Manufacturing designs may originate from metallurgical defects in the materials used to make the system components. Further, such defects may introduce fatigue in the system components, which will result in compromising other components such as the hydraulic lines. This may result from certain components in the hydraulic system having a common manufacturing flaw when there is a failure results in simultaneous failure of other components.
Other causes
Hydraulic system failure may also occur because of wearing out of the hydraulic pumps resulting in failure in a number of ways (Mechanical Prevention group, 1978). When hydraulic fluid leaks into the emergency air bottles during resetting of the system resulting in a decrease in charge of the bottles (Skybrary).
Brief Description
This flight was scheduled from Stapleton International Airport in Denver to O’Hare International Airport in Chicago (Ostrom and Wihelmsen, 2012). While at 37,000 feet and making a shallow right turn, the fan disk of its tail-mounted General electric CF6-6 engine failed. This resulted in the disk crumbling (NASA, 2008). Debris in the engine was not contained by the engine’s housing structure, which resulted in the debris probing the aircraft tail section at several places (Ostrom and Wihelmsen, 2012). Additionally, the crumbled disk also contributed to the piercing of the aircraft tail section. This resulted in the loss of three hydraulic systems, which were positioned close to each other by the piercing effect of the shrapnel from the failed engine (NASA, 2008). The shrapnel pierced the lines of the three hydraulic systems causing the hydraulic fluid to drain. Following this loss of the hydraulic systems, the flight crew used the remaining two engines to maintain control. Steering of the aircraft was achieved by applying power to an engine over the other and altitude was attained by applying power to both engines. Since the flying controls were lost when the hydraulic system failed, the flight crew was forced to land at a high speed at the Sioux Gateway Airport. 111 lives were lost.
Prevention to avoid same accidents
A manufacturing defect in the fan disk was the cause of the accident. However, failure to identify the defect during inspection contributed further to the situation, the accident could have been prevented if the maintenance personnel were attentive to their work. Additionally in the design of aircraft, hydraulic systems need to be close to one another to ensure that in case of one failing the remaining ones can be used for flying control.
Brief Description
Flight 143 was scheduled for Montreal, Quebec to Edmonton, Alberta. Halfway through the flight the aircraft ran out of fuel at 41,000 feet fuel pressure warning was assumed to be a fuel pump failure by the pilots. However, the flight management computer indicated that fuel was still available to complete the flight. Within a few minutes, the cockpit warning system indicated that all engines had broken down. The lack of fuel caused the engine and the auxiliary power unit to fail. Thus, there was no power to supply to the hydraulic systems, which controlled the operation of the flying controls. Landing became a challenge as airflow prevented the lowering of the main landing gear via gravity. The speed of the aircraft was too high to land and this made it difficult to land the aircraft safely as the lack of adequate hydraulic pressure prevented the flap extension, which are used during normal landing conditions to reduce the stall speed of the aircraft for a safe landing (Ostrom and Wihelmsen, 2012). The Captain of the aircraft employed forward slip technique to reduce the altitude and increase the drag to prevent increasing the forward speed. None of the 61 passengers were seriously hurt.
Prevention to avoid same accidents
New engineering technology to avoid hydraulics systems failure
According to Gamauf (2006), modern hydraulic systems need to be designed to include redundancy systems in case of a single component malfunction. The redundancy systems can include independent supply systems or multi-stage components. According to the National Research Council, U.S. (2006) hydraulic systems are being replaced with electrical systems. High-power density electric generators are being integrated into the architecture of aircrafts to provide power in cases where the system may malfunction and to be able to recover from failures. Additionally, to address issues resulting from hydraulic failures, advanced system components consisting of wireless control systems and high efficiency electric motors and generators are being incorporated in the design of aircrafts.
References
Adams, C., & Manion, J. (2011). Improving Survivability of Aircraft from Uncontained Gas
Turbine Engine Failures. Mechanical Engineering, 133(8), 56-57.
Abzug, M. J., & Larrabee, E. E. (2005). Airplane stability and control: A history of the technologies that made aviation possible. Cambridge: Cambridge university press.
Erjavec, J. (2004). TechOne. Clifton Park, NY: Thomson/Delmar Learning.
Federal Aviation Administration. (2009). Pilot's handbook of aeronautical knowledge. New York, NY: Skyhorse Pub.
Ferguson, M. D., & Nelson, S. M. (2013). Aviation safety: A balanced industry approach.
Fielding, J. P. (2007). Introduction to aircraft design. Cambridge: Cambridge University Press.
Gamauf, M. (2006). Aircraft Hydraulic Systems: The Genie in the Bottle. Business & Commercial Aviation, 98 (1), 46-49.
George, F. (2006). How Does It Work? Hydraulic Systems. Business & Commercial Aviation, 99 (3), 76.
Mechanical Failures Prevention Group, M. F. (1978). Detection, diagnosis, and prognosis: proceedings of the 28th meeting of the Mechanical Failures Prevention Group, held at San Antonio, Texas, November 28-30, 1978. Washington: U.S. Dept. of Commerce, National Bureau of Standards.
NASA. (2008). System Failure Case Studies. Retrieved from http://nsc.nasa.gov/SFCS/SystemFailureCaseStudyFile/Download/35
National Research Council (U.S.), & National Research Council (U.S.). (2006). Decadal survey of civil aeronautics: Foundation for the future. Washington, D.C: The National Academies Press
Ostrom, L. T., & Wilhelmsen, C. A. (2012). Risk assessment: Tools, techniques, and their applications. Hoboken, New Jersey: Wiley.
Roskam, J. (2000). Airplane design. Lawrence, Kan: DARcorporation
Skybrary. Hydraulic systems. Retrieved from http://www.skybrary.aero/index.php/Hydraulic_Systems
