Aoa 737ngx Groundwork Gear Brakes Handout
Short Description
brakes B787...
Description
13
n o i t c e s
Landing Gear and Brakes
The material covered in this document is based off information obtained from the original manufacturers’ Pilot and Maintenance manuals. It is to be used for simulation purposes only.
Copyright © 2012 by Angle of Attack Productions, LLC All rights reserved
Table of Contents
Table of Illustrations 3 4
Figure 13-1. Landing Gear Leaver Figure 13-2. 13-2. Main Landing Landing Gear Retracting Retracting
Extension
4
Figure 13-3. Autobrake Deceleration Rate Table
Retraction
5
System Overview Extension and Retraction
Manual Extension Nose Wheel Steering
8 9
Wheel Braking System
10
Antiskid Protection
11
Autobrake System
12 14
Parking Brakes
6 7 13
System Overview The landing gear system on the 737NGX is made up of two main landing gear in the center fuselage and one nose landing gear in the forward fuselage. fuselage. Each landing gear has a set of two wheels and all landing gear are retractable into the aircraft ’s fuselage. The landing gear is designed to sustain the weight of the aircraft when it is on ground, as well as absorb the load forces when the aircraft touches down during landing. Each landing gear has a series of shock absorbers that compresses when there is weight over them. QUICK TIP: If TIP: If landing gear shock absorbers were rigid, they would immediately break when the aircraft touched down! They have to be able to compress sufciently without damaging themselves or any other components around them. The 737-800’s maximum landing weight is over 65 tons, all that weight must be absorbed by these shock absorbers. The landing gear is extended and retracted with the landing gear lever located in the center forward panel. The lever has three p ositions: ●
UP: UP: Landing gear is fully retracted and secured,
●
OFF:: Landing gear system is free from hydraulic pressure, OFF
●
DOWN: DOWN: Landing gear is commanded to extend.
A lever lock prevents the landing gear lever from accidentally moving to the UP p osition on ground.
Extension and Retraction Extension When the landing gear are in the UP position, position, they are held in place by uplocks. In order to extend the landing gear, normally the pilot not ying pulls the landing gear lever and brings it to the DOWN position. position. Firstly, the landing gear doors open to allow the mechanism to come out of the fuselage.
During this process three red lights, one for each gear, located above the landing gear lever illuminate to indicate either: ● ●
That the related landing is neither down nor locked, That the related landing gear disagrees wit h the selected position in the lever.
Then, Hydraulic pressure from system A is used to extend the landing gear. Aerodynamic loads and gravity also reduce the demand for hydraulic pressure by aiding the extension process. Once the gear are in their DOWN position, mechanical and hydraulic linkages hold it in a locked position in order to prevent them from collapsing upward upon touchdown. QUICK TIP: The TIP: The landing gear locks must also protect
the landing gear from collapsing when the aircraft lands sideways, sideways, such as during a crosswind landing. The maximum allowable demonstrated crosswind component for landing is partly a function of how much side load can the landing gear absorb during a crosswind touchdown. When the gear has come down and is fully locked in position, the red landing gear lights extinguish and three green lights illuminate. These green lights are immediately below the red gear lights and indicate that: The related gear is down and locked. With this, the landing gear warning horn is also deactivated. This warning is discussed in the Caution and Warning Systems lesson. lesson. Additionally, for added redundancy, there is also an independent extra series of green lights in the AFT overhead panel that provide a gear down and locked indication. This is a quick place to look just incase one of the gear lights is out. This will cross verify the gear is in fact down. Just because beca use a light li ght is out, doesn’t mean the gear isn’t down. Although these lights could be checked with the annunciates and light test, test, it ’s much easier for the ight crew
Extension and Retraction to simply look up on that panel during that critical phase of ight, ight, and know if they need to discontinue the approach or if they can continue.
Retraction When the landing gear is down and locked, and the landing gear lever is moved to the UP position, the landing gear begins to retract upwards. After liftoff, liftoff, as soon as the pilot not ying notices a positive rate of climb and change in altitude, he informs the pilot ying by saying “positive rate”, who then commands the pilot not ying to raise the gear by saying “gear up”. At this point when whe n the landing gear gea r lever is selected UP U P, the wheels may still be rotating after being in contact with the runway.
If rotating wheels were retracted into the landing gear wheel well they may cause damage to components. For this reason, when w hen the landing landi ng gear lever lev er is selected selec ted UP, the wheel brakes automatically automatically stop rotation of the main gear wheels. The nose gear wheels are also suppressed with snubbers. If any of the 6 wheels have suffered damage during the
takeoff roll, braking will be difcult. To prevent a damaged spinning tire from damaging any component in the wheel well, the gear stops retracting and free falls back to the down position. This gear can no longer be retracted during that ight.
This is a protection in place for the critical systems that are largely housed in the gear wheel wells, like what you’ve seen in the hydraulics section. Now, once the landing gear have been retracted, they are held in place with uplocks, doors and seals. That completes the retract cycle. Hydraulic pressure from system A is used to retract the main and nose landing gear, however during retraction, the hydraulic system B may also provide pressure. A landing gear transfer valve changes the source of hydraulic pressure when: ●
The aircraft is in the air
●
Landing gear lever is UP
●
One of the main landing gear is not UP
●
Left engine high-pressure compressor (N2) is < 50%
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Hydraulic system B pressure is available
Extension and Retraction Obviously this condition is rare, and would be accompanied by other warnings. But it does ensure that in the chaos of a speedily evolving takeoff sequence that the gear will come up unless there are other limiting factors. Finally, as one last step after takeoff checklist is run, the landing gear lever is placed in the OFF position. position . This isolates all hydraulic power from the landing gear system.
Figure 13-1. Landing Gear Leaver, Leaver, Down Position. Position.
Figure 13-2. Main Landing Landing Gear Retracting.
Manual Extension In order to provide a safe backup for hydraulic system A power loss or system failure, the landing gear system can also be extended manually. There are manual gear release handles guarded by an access door in the ight deck oor that allow for manual gear extension with the landing gear lever in any position. There are three manual gear extension handles, one for each landing gear. To actuate the system, the manual gear handles must be pulled aft. When this occurs, the related landing gear uplock is released and the gear free-falls to a down and locked position. Once again, the gear weight, gravity and aerodynamic forces also aid this extension process. QUICK TIP: Manual TIP: Manual extension of the nose and main landing gear are completely independent from the normal extension system. system. In case the normal extension system is jammed or inoperative, inoperati ve, manual extension ext ension may ma y be used. After the landing gear has been extended manually, it cannot be retracted with this same mechanism. The only way to retract it is if hydraulic system A pressure is available and:
●
The manual extension access door is closed
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Landing gear lever is moved to its DOWN position
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Landing gear is then moved to its UP retracted position
Nose Wheel Steering The nose landing gear wheels may be used to steer the aircraft when it is on ground during taxi, takeoff and landing and the landing gear lever is in the DOWN position. position . There are two ways to steer the nose wheels: ●
With the rudder pedals
●
With the nose wheel steering wheel or tiller bar
The rudder pedals allow for a maximum nose wheel deection of 7º left or right. The nose wheel steering wheel allows for a maximum nose wheel deection of 78º, which is a HUGE difference. During taxi, the nose wheel steering wheel is used, however, during takeoff the rudder pedals are used so that nose wheel deection is lesser and therefore the loads imposed over the nose gear during takeoff are minimized. QUICK TIP: Keep TIP: Keep in mind that the tiller bar for the nose steering is NOT available in ight simulator, simulator, and is rather connected as a function with the rudder. Therefore, tight turns available in the real aircraft will not be available in the simulator version. Nose wheel steering is normally powered by hydraulic
system A. Alternate nose wheel steering is available through the hydraulic system B when: ● ●
●
The aircraft is on ground There is a normal hydraulic uid quantity in the B system reservoir
The Nose Wheel Steering switch is placed to the ALT position
The nose wheel steering switch is a two-position switch located in the left forward panel. It can be selected either: ●
●
NORM: Normal NORM: Normal position where hydraulic system A powers the steering system. This position is guarded ALT: The ALT: The steering system switches to obtain power from hydraulic system B
QUICK TIP: The TIP: The nose wheel steering wheel is directly linked to the steering system, unlike the rudder pedals. This means that if both pedals and the steering wheel are used at the same time, the steering wheel will have priority to control the nose wheel and will override the rudder pedal input.
Wheel Braking System The 737NGX has multiple disc brakes on each of the wheel of the main and landing gear. The nose landing gear does not have any braking capabilities. The upper portion of each rudder pedal controls wheel brakes. brakes. When the top part of the pedal is pressed, the corresponding wheel brakes are actuated. Example: When either the captain or rst ofcer’s left rudder pedal top portion is pressed, the left main landing gear brakes are actuated, similarly for the right pedal. During normal operation, braking power is supplied by hydraulic system B pressure. pressure. Hydraulic system A automatically powers the braking system when hydraulic system B output pressure becomes low or is unavailable. When this happens, the brakes operate under alternate braking. In order to provide added system redundancy, there is a way to achieve wheel braking when both hydraulic systems pressure becomes low or unavailable. There is a system that accumulates hydraulic system B pressure in a Brake Accumulator so that in the event of a hydraulic failure, there is still sufcient pressure for several braking
applications. QUICK TIP: The TIP: The accumulator is capable of supplying six full braking applications, or eight full hours of parking brake application. There is an instrument in the right for ward panel that provides brake accumulator pressure readings. ●
Normal brake accumulator pressure is 3000psi
●
Maximum brake accumulator pressure is 3500psi
Antiskid Protection Both hydraulic systems A & B through normal and alternate braking provide antiskid protection against: ●
Wheel skid
●
Locked wheels
●
Touchdown
●
Hydroplane protection
Under normal braking, when a skid is detected, the antiskid system reduces the related wheel braking pressure until it is no longer skidding. Under alternate braking, when a skid is detected, the antiskid system reduces the related pair of wheels braking pressure until it is no longer skidding. Example: When Example: When alternate braking is in place through hydraulic system A pressure and a skid is detected in one of the wheels of the left main landing gear, the antiskid system reduces braking pressure to both wheels of the left main landing gear. When the speed difference between two wheels is 8 knots or greater, the system treats it as a skid. When the speed difference between two wheels is 25 knots or greater,
the system treats it as a locked wheel. In both cases the antiskid protection activates. activates. For added system reliability, antiskid protection is also available when both hydraulic systems have failed. To notify the pilots of an antiskid failure, as it is usually transparent and automatic, an antiskid inoperative light in the center forward panel illuminates when a fault is detected in the antiskid system.
Autobrake System Not only can wheel brakes be operated with the rudder pedals but also automatically through the autobrake system. This system uses hydraulic system B pressure to provide deceleration during a rejected takeoff (RTO) or after touchdown during landing. The autobrake system can essentially place the wheels in a near skid condition, where braking is maximum. For a pilot to try this manually, he or she would make a disaster of it. This system ensures that when maximum braking is needed, it can be achieved efciently and safely. During rejected takeoffs, maximum deceleration is provided by this system when thrust levers are retarded to idle above 90 knots. If a rejected takeoff occurs below 90 knots, autobraking will remain armed but will not activate and manual braking will be necessary. During landings, deceleration rates are selectable by the pilots with the AUTOBRAKE Selector in the center forward panel. Autobrakes only activate when both thrust levers are at the IDLE position and position and the main wheels start to spin due to ground contact. If necessary, autobrakes may also be selected when on ground and at above 30 knots of ground speed, however, this practice is not recommended.
QUICK TIP: The TIP: The maximum achievable autobrake deceleration rate on a dry runway is lower than the maximum possible manual braking. The autobrake selector has six positions: ●
RTO
●
OFF
●
1
●
2
●
3
●
MAX
These positions will be discussed later during FlightWork FlightWork and GroundWork when we’re planning our landing distances based on the turnoffs we’ll need to take. The level of autobraking needed in these cases will be essential to calculate. There is also an AUTOBRAKE DISARM LIGHT above LIGHT above the autobrake selector. This light comes on when the autobrake system is switched disarmed from one or several of the following conditions: ●
The Speed Brake lever is moved to a DOWN detent
Autobrake System ●
Manual braking is applied during an RTO
●
Thrust levers are advanced during an RTO
●
There is a fault in the autobrake system
This light also comes on briey when the RTO mode is selected on ground because the system per forms checks to test the autobrakes. A nice little reassurance for the crew that RTO will be available.
The autobrake system is designed to bring the airplane to a complete stop, unless the pilot disarms it by either switching the autobrake selector switch to the OFF position, position , or: ●
Moving the speedbrake lever to the DOWN detent
●
Advancing the thrust levers
●
Applying manual braking
A slight tap of the brakes, for example, will disable the system. From there on out, braking is controlled by the pilot. Each autobrake position has a specic target deceleration rate, outlined in the following table. We won’t go over these in detail now, as we mentioned before that we’ll talk about the practical application of these numbers
later when we’re actually ying the aircraft:
Autobrake Position
Deceleration Rate (ft/s2)
1 2 3 MAX/RTO
4 5 7.2 14>80kts, 12
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