A significant component of a spacecraft or aircraft is the landing gear, also referred to as the undercarriage. The gear is a formation that holds the plane on the surface and enables it to take-off, land, and taxi (Federal Aviation Administration, 2012). The design of the landing gear is likely to pose several interferences to the structural design of the aircraft. Boeing 777 encompasses several design parameters that strongly influence the plane’s aerodynamics and configuration design. The aircraft's landing gear forms the leading ever built-in into a commercial aircraft to date. The gear has various important aspects, such as retraction mechanism, brakes, and shock absorber (Komorowski, 2011). Accordingly, the following entry endeavors at discussing Boeing 777 landing gear system while highlighting how it operates.
Constituent Parts of Boeing 777 Landing Gear System
The landing gear for Boeing 777 embraces a typical two-post configuration containing one nose gear and two main landing gears. As compared to the traditional four-wheel units, each of the two gears of the plane consists of six wheels in parallel pairs. The arrangement provides the main landing gear with twelve wheels whose role is better weight distribution on taxi areas and runways as well as avoidance of the necessity for an extra two-wheel gear beneath the hub of the fuselage. Likewise, the six-wheel pattern allows for a more cost-effective brake design (Moaveni, 2011).
Boeing 777’s aft hinges of every main gear are steerable to enhance the turning radius. The hydraulic system at the center supplies hydraulic power for extension, retraction, and steering. While the hydraulic system on the right side powers the regular pedal hydraulic system, the one at the center powers the reverse or alternate pedal hydraulic system. However, both systems provide antiskid protection, but only the regular system provides the auto brake system. Moreover, the tire pressure indicator and the brake temperature examiner display the tire pressure and the brake temperature on the synoptic display of the gear. While the nose wheels do not have brakes, each of the main gear wheels comprises of multiple disc carbon brakes. The brake structure encompasses of normal brake, parking brake, antiskid protection, reserve or alternate brake, auto brake system, and brake accumulator (Mallick, 2007).
Main Landing Gear
The main landing gear bar contains an air-oil buffer. A side brace and a drag brace are responsible for transmitting weight from the strut to the plane structure. When the gear is completely extended, over-center techniques shut both braces. During gear extension and retraction, the gear doors on the main gear wheels open and close. The aircraft’s truck encompasses of three axles. At their ends, the axles harbor a wheel-tire and a brake assembly. The rear axle spins around to steer the main gear (Federal Aviation Administration, 2012).
The principal landing gear utilizes the hydraulic pressure, originating from the central system, to extend and retract. Sequence valves assume the task of controlling the gear and door movement. Side brace and drag brace down lock activators then lock the gear within the broadened position. Sequentially, uplock hooks bolt the gear within the retracted location. The trucks of the principal landing gear tilt at about 130 forward tires up extending the gear. The trucks of the gear tilt at approximately 50 forward tires down, with the gear in transit or up and locked (Moir & Seabridge, 2011).
Alternate extension mechanism employs a dedicated direct current electric hydraulic pump as well as interior hydraulic system liquid for extension of the landing gear. The structure allows for landing gear extension when the core hydraulic system does not have pressure. The alternate extend power pack provides hydraulic pressure that unlocks the landing gear and its doors. Consequently, the gear extends and the doors open using their own weight. After the alternate extension, the gear doors remain open.
When the alternate gear button reads DOWN, the gear uplocks and all doors are released. The landing gear falls freely to the secured position. Nonetheless, the landing gear switch does not influence the alternate extension. Upon using the alternate extension, the position indication on EICAS landing gear portrays the position indication of the expanded gear. Throughout alternate extension, EICAS displays the text GEAR DOOR each of the hydraulically powered door is open. Subsequent to an alternate extension, it can be possible to retract the landing gear via the normal technique if it is working. This can be done by selecting DN before selecting UP (Federal Aviation Administration, 2012).
Ground Door Operation
The alternate extension technique permits one to unbolt the doors when the plane is on the surface. The hydraulic pressure at the core of the system locks the doors.
Nose Landing Gear
This gear strut embraces an air-oil buffer. A folding drag strut transmits weight from the brace to the aircraft structure. When the nose gear is completely retracted or extended, the over-center system of the lock fastens the drag strut. During gear extension and retraction, the nose gear’s forward doors operate hydraulically while the rear doors operate through mechanical links that are attached to the nose gear (Erikson & Steenhuis, 2015).
To facilitate in extension and retraction, the nose landing gear employs hydraulic pressure from the center system. The sequence valves assume the role of controlling landing gear and forward movement (Erikson & Steenhuis, 2015).
Alternate Door Extension
Alternate extension for the nose gear employs hydraulic pressure emerging from the alternate-extend power pack. The land gear and the forward doors use their own weight to extend and open respectively. After alternate extensions, the forward doors do not shut (Erikson & Steenhuis, 2015).
Ground Door Operation
The alternate extension mechanism allows for opening the anterior doors when the plane is on the surface. The frontal doors unfasten using their own weight. However, they shut using the hydraulic pressure emerging from the core system (Erikson & Steenhuis, 2015).
Landing Gear Operation
The landing gear switch is responsible for controlling the landing gear. On the surface, an automatic lock holds the switch in the DN location. The lock can be physically overridden via pressing and clasping the landing gear override switch. During flight, the switch lock is mechanically let loose through ground or air sensing (Erikson & Steenhuis, 2015).
Landing Gear Retraction
Retraction of the landing gear takes place when the lever moves up. The gear doors unlock and the wheels of the main gear tilt, assuming a retract position. As the gear retracts into the tire wells, the position of the EICAS (Engine Indicating and Crew Alerting System) landing gear assumes a white crosshatch signal from a green down signal (Federal Aviation Administration, 2012). After retraction, the uplocks hold the landing gear. Accordingly, the EICAS gear location display adjusts to UP for ten seconds before blanking. With all doors locked and the landing gear pulled in, the hydraulic system depressurizes automatically. In case the standard transit time elapses and a gear is not locked up, the EICAS displays a caution message. Further, the gear position indicator on the EICAS changes to an expanded informal format while the affected gear is displayed as down or in-transit.
Landing Gear Extension
Once the landing gear switch moves to DN, the doors affiliated to the landing gears open, the gear unlocks, and the in-transit signal displays on the EICAS indication. The gear falls freely to the down, locked position without application of hydraulic power. Then, down locks are powered towards the locked point, the key gear trucks incline hydraulically toward the flight position, and the hydraulically activated doors lock. After all gears are secure, the EICAS gear signal displays DOWN. However, if one of the gears is not safely locked, the EICAS displays a caution message saying ‘GEAR DISAGREE’ after the standard transit time. Subsequently, the EICAS signal assumes an expanded non-formal layout, displaying the affected gear as in-transit. If only a single strut on a key gear is bolted after the standard transit time, a caution message for the concerned gear displays on the EICAS. If a hydraulically activated door does not close after the average transit period, the EICAS displays an advisory message reading ‘GEAR DOOR’ (Moaveni, 2011).
Main Gear and Nose Wheel Aft Axle Steering
Boeing 777 is fitted with both main gear and nose wheel rear axle steering (Federal Aviation Administration, 2012). While the reserve hydraulic system has been used to power nose wheel steering, the center hydraulic system has been used to power main gear rear axle steering. A nose wheel navigation rudder for every pilot proves vital in providing prime steering control. On the other hand, rudder pedals offer minimal steering control. The rudders are capable of tilting the nose wheels to a maximum of 700 in any direction. They contain pointers on their assembly that show their position in relation to their neutral location. The tiller pedals may be used in turning the nose wheels to a limit of 70 in any direction. Main gear rear axle steering assumes operation automatically when the angle of the nose wheel navigator exceeds 130 with the aim of reducing tire scrubbing. In case the main gear rear axles fail to lock during takeoff, a warning message displays on the EICAS, along with a takeoff configuration acoustic alert. Similarly, in case the main gear navigation activators are not locked into the central position when a command authorizes so, the EICAS displays an advisory message (Moaveni, 2011).
During landing, the pilot can choose five levels of reducing speed. However, on dry landing strips, the highest autobrake deceleration speed is less than the one produced by complete pedal braking. Subsequent to landing, autobrake use begins when the wheels spun up and thrust switches have retarded to inoperative. Autobrake use takes place shortly after main gear comes to rest. If the pilot selects MAX AUTO, he limits deceleration to the level of autobrake 4 until pitch position rotates to less than 10, then deceleration increases to the level of MAX AUTO. The extent of deceleration can be altered without deactivating the system via revolving the selector. Autobrake pressure can be minimized to sustain the chosen plane deceleration rate. Ultimately, the system offers complete braking until the plane comes to a halt or the system is deactivated (Erikson & Steenhuis, 2015).
The Boeing 777 landing gear system is renowned for performing flawlessly. The unique gear system allows aircrafts to rotate early through changing the rotation focal point from the main axis to the rear axle. As the plane revolves, the nose manages to rise higher. The features of the landing gear allow the plane to take off on short runways.
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