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...Read More
~Posted on
Mar 2018
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).
Normal Operation
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
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).
Normal Operation
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).
Landing
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.
References
Erikson,
S. & Steenhuis, H. (2015). The global
commercial aviation industry. Oxon, OX: Routledge.
Federal
Aviation Administration (2012). Aviation
maintenance technician handbook- airframe volume 2. New York, NY: AMT
Exams.
Federal
Aviation Administration (2012). Federal
aviation regulations/ aeronautical information manual 2013. New York, NY:
Skyhorse Publishing Inc.
Komorowski,
J. (2011). ICAF 2011 structural
integrity: influence of efficiency and green imperatives: Proceedings of the 26th
symposium of the international committee on aeronautical fatigue, Montreal,
Canada, 1-3 June 2011. Canada, CA: Springer Science & Business Media.
Mallick,
P.K. (2007). Fiber-reinforced composites:
materials, manufacturing, and design, 3rd edition. Boca Raton,
FL: CRC Press.
Moaveni,
S. (2011). Engineering fundamentals: an
introduction to engineering SI edition. Stamford, CT: Cengage Learning.
Moir,
I. & Seabridge, A. (2011). Aircraft
systems: mechanical, electrical and avionics subsystems integration. New
York, NY: John Wiley & Sons.