Hydraulic system works through the pressurized fluid to create movement and or work and is able to transmit high force with a relatively small volume of hydraulic oil [1]. In aircraft, the hydraulic system is primarily used for actuating landing gear, flaps, and brakes while for a larger aircraft it used for controlling flight, spoilers, thrust reversers. The hydraulic system applies the principle of Pascal’s law which states that pressure exerted anywhere in a closed area is transmitted equally in all directions [2]. Further investigation of the hydraulic system and other important features of the aircraft will be discussed based on the Boeing 777.
The aircraft is constituted with three hydraulic systems; left, center, and right which operates at the hydraulic fluid rate of 3,000 psi [3]. The power used for the left and the right hydraulic system is obtained from two EDPs and assisted by another two ACMPs while for the center hydraulic system is provided by the two AC-electric motor pumps (ACMP) and also assisted by two air-turbine driven pumps (ADP). The center hydraulic system plays major roles of the engine thrust reversers, primary flight controls, landing gears, and slats. However, under dire situations the hydraulic power is obtained from the automated ram-air turbine (RAT) which runs the variable displacement inline pump, thereby supplying the flow to the center flight control system [4].
[3].
The left and Right hydraulic system.
The functionality of the left and right hydraulic system is the same. The left system delivers the hydraulic fluid to the left thrust reverser and also the flight control systems while the right system operates the right thrust reverser, flight control, and normal brake systems. As the shown on the figure below.
[3]
Hydraulic fluid reservoir
The hydraulic pumps have the hydraulic fluid under high pressure supplied from the left and right reservoirs which is pressurized by the bleed air in the pressurization module. The fluid in the standpipe is drawn by the EDP, and in the event of low-level fluid, ACMP is used thereby becoming the only source of hydraulic power. The reservoir in the aircraft has its maintenance through the center servicing point located in the fuselage, moreover, a sample valve used in collecting samples for the purpose of testing, reading of the temperature, pressure transduction, and the drain valve used for draining the reservoir [3].
Pumps
The main pumps used for the left system and right hydraulic systems are the EDPs which functions supportively with the supply shutoff valves. These valves are normally operated when the engine is running. Each EDP contains special solenoid valves whose functions are to raise and to lower the hydraulic fluid pressure of the pump. The pumps in use in Boeing 777 employ a variable displacement inline piston pumps which are made of the first stage impeller pump and second stage piston pump [3]. Each part of the pump operates distinctively as the impeller is the one transmitting the hydraulic fluid at very high pressure to the piston pump. The ACMPs pumps operate only at peak conditions of the system engaging the left and right system
Filter Module
The pressure and the case drain flows of the hydraulic system are serviced by the filter modules of each flow. The return flow of the fluid is cleaned by the return filter module. The system is designed such that when the filter is clogged owing to the contamination, the module is bypassed. The dynamics of heat exchange is made such that the exchanger, located closed to the wing fuel tanks, cools fluid flowing in the ACMP and EDP case drain as the fluid is being moved to the reservoir.
Measurement of the hydraulic Fluid properties
The readings of the pressure, temperature, and the quantity whose signals are received and transmitted to the flight deck by the sensors and transducer in the respective locations of the hydraulic system are indicated and monitored. Reservoirs have sensors and transducers installed on them with the switch connected to the pneumatic line found between the pressurization module and the reservoir. The output pressure of the pumps is recorded by the pressure transducers on the EDP and the ACMP. Overpressurization is regulated by the pressure filter valve located on the EDP [3].
Center hydration system of the aircraft
The center hydration system of the aircraft plays critical roles of supplying the fluid under pressure to run the following systems; nose landing gear actuation and steering, alternate brakes, main landing gear actuation, and steering, trailing edge flaps and slats and also flight controls [3]. Like the left and right hydraulic system, the center hydraulic system contains a reservoir and filters.
Reservoir
The reservoir reserves the hydraulic fluid for the hydraulic pumps. The bleed air pressurizes the fluid through the pressurization module. The fluid in the reservoir is supplied to the ADPs, the RAT, and the ACMP through the standpipe and the landing gear alternate extensions. Other ACMPs are supplied with the fluid from the lowest levels. The ACMPs are the basic center hydraulic system pumps and are manually switched on while the ADPs are the pumps used during peak operations of the system [3]. The RAT provides the emergency source of power to the system controls. The quantity, temperature, and pressure transducers are installed on the reservoir to transmit the sensor signal to the flight deck where it is monitored. The reservoir pressure switch is also connected to the pneumatic line in between the reservoir and the pressurization module [3].
The system filters
The filter cleans the pressure and case drain output, return flow from the system and can also be bypassed if the system clogs. The heat exchanger lowers the temperature of the ACMP case drains to the reservoir. The pump output pressure is measured on both the ACMP and the ADP as it contains the transducers. Temperature transducers take the temperature readings of the case drain pump, and the pressure transducer monitors the pressure of the hydraulic system.
The pressure relief valve prevents the overpressurization of the system on the ADP filter, while that in the ACMP C1 supplies protection of the center hydraulic isolation system(CHIS) from extreme pressure.
The Center Hydraulic Isolation System (CHIS)
The system is automated to provide the engine burst protection and protect the brakes and steering. The electric motors operating the reserve and the nose gear isolation valves of the system is controlled by the relays. During the operation of the CHIS, the leading edge slats are impeded from running.
The bottom of the center of the reservoir supplies the ACMP C1 with approximately 1.2 gallons of the hydraulic fluid while other pipes in the hydraulic system get through the standpipe. The valves are usually open and are closed when the reservoir is low. The operation of the CHIS divides the center hydraulic system into NLG actuation and steering. During the same operation of the CHIS, the leading edge slat is isolated, and the ACMP C1 feeds the brake system [3]. The other pumps from the machine feed the trailing edge flaps, MLG actuation, and the steering, and also the controls of the flight. In the event that there are leakages in the NLG actuation and steering loss of the fluid is minimized. The functioning of the alternate brakes, trailing edge flaps, MLF actuation, and steering, and the PFCS are free from any hindrances [3].
Leakages in the pumps can reduce to the minimum levels causes the loss of the systems from operations, but the alternate brakes remain in service as it gets the power from the ACMP C1. On occasions that the ACMP C1 and the alternate brakes are leaking the entire system is lost [3].
The Nose Gear Isolation Valve
The nose gear Isolation valve is a conditional system that opens when the airspeed is below 60 knots, pump pressures for the RAT, ADP C, and ACMP C below 1,200 psi for half a minute and when the left and right engine revolution is beyond the idle level [3].
When the flight airspeed is below the minimum level, the NLG steering is started, and when the pump levels decreased below 1,200, the NLG actuation and steering is activated. Moreover, the draining hydraulic fluid in the center hydraulic system is isolated by the nose gear isolation system valve [3]. If the latter condition is affected, then the NLG actuation and steering is also activated. The extension of the NLG is caused by the landing gear lever moving downwards.
[3]
The resetting of the Central Hydraulic System
The opening of the valves is automatic and is caused by the quantity of the center system being greater than 0.70 and airspeed is being less than 60 knots for five seconds [3]. It also opens when the engines and their pumps operate for correctly for half a second.
Airworthiness of Boeing 777 aircraft
According to the Skybrary publication, airworthiness refers to a number of factors relating legal and physical suitability of an aircraft on flight safety. Compliance of the safety standards is certified by national bodies overseeing the flights in every region and country. The design of the Boeing aircraft is meant to meet the certification code, manufacturers and operator’s benefits to promote the economy and ensure beneficial venture. The airworthiness certificate is conferred after carrying out the required maintenance by the licensed aircraft engineers [5]. Airworthiness directive of the Boeing company is registered under the Federal Aviation Administration, the actions of issuing and revoking certificates are the mandate of the administrator whose actions are determined after due diligence investigation involving the aircraft and the public through participation [6]
The airworthiness of the aircraft gives responsibility to the flight crew to determine whether the flight is in a safe condition for flight, and can discontinue flight when there is a compromise of mechanical, electrical and or structural states [7]. Also, the aircraft must conform to the standards set out in its certificate of conformity. The documentation of the aircraft must follow the federal regulations remembered by use of the acronym ARROW standing for; A-Airworthiness certificate, R-registration certificate, R-radio certificate, O-Operators manual a placards, and W-weight and balancing of the plane [7].
Complete airworthiness requires the inspection documentation which can be recalled by use of the acronym; AVIATE. A-Airworthiness Directive, V-VOR check, I-inspection at a specified 100-hour flight, A-Altimeter-static, T-Transponder inspection, and E-emergency locator transmitter. The pilot is in charge of the aircraft and the company register and maintains the aircraft [7].
The airworthiness certificate is valid for as long as the maintenance, preventive and alterations are abided by according to the federal aviation regulations. The dates of revocation, termination and or surrendering are determined only by the administrator [7].
Safety standards of the Boeing 777
Safety is the primary consideration of any aircraft all over the world as it is involved in flying people from all works of life from one place to another in the sky. It is the reason the Boeing engineers design with strict consideration to safety and follows the regulatory requirement before certification. The engines are two for safe take-off and landing in the event one fails. The aircraft is structured to withstand one-fifty percent of the possible load it can carry so that in case of an emergency the aircraft can still be safe [8].
The aircraft is passed through rigorous testing to ensure its safety, standard requirement and certification standards. Structural testing is conducted through static and fatigue tests by applying maximum weights beyond the possible loads available to check the capability of the plane. The durability of the aircraft is tested by exposing the plane for up to triple times the expected wear and tear. The new aircraft was tested for many months and years in laboratories, exposure to wind, ice on both ground and flight [9].
The Boeing company conducts regular monitoring of the airplanes to improve safety. The executive heads of the company are among the council overseeing the safety plan during the improvement of design, assembling, and maintenance of the aircraft. Moreover, the Federal Aviation Administration rigorously review the safety compliance and act accordingly [8]. The customers are also a valued resource for as their comments are collected and responded as it is necessary.
According to the world aviation analyst, the Boeing 777 is the safest planes despite the crash of the Malaysian Airlines [10]. The past crashes of the Boeing according to the investigations carried out, there has not been any significant trace to the safety flaw which includes the cases of Malaysian Airline Flight 370 which got disconnected from the air-traffic control, and there was no trace found, the British Airways flight which had the Rolls Royce engine was not the Boeing responsibility. Another British Airways flight that crashed in 2008 was associated with the ice crystals in the fuel. The Asian Flight that killed three people in 2013 had struck a seawall and broke apart [10]. The number of accidents recorded as compared to thousands of Boeing planes in flight each moment in the sky is of a very low percentage which satisfies a high safety standard of Boeing 777 aircraft.
Boeing has invested in new technology, reserve, and innovation to promote the safety standard of the aircraft. The flight deck has the predictive wind shear, vertical situation display showing a potential terrain conflicts and controlled-into-terrain (CFIT) which is a look-ahead terrain avoidance improves safety problems [8].
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[11]
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