aoa_777_groundwork_landing-gear_transcript.pdf

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Landing Gear and Brakes System Script Writer: Roshan P. Bhojwani

Lesson Introduction Welcome to the PMDG 777-200LR Groundwork Landing Gear & Brakes system lesson, from Angle of Attack. In this lesson we intend to teach you all of the aspects related to the safe and correct operation of the landing gear and the wheel braking systems. Most aircraft systems aboard the 777 are of prime importance for the safe completion of a flight, and the Landing Gear and Brakes system is no exception. Failure to properly understand the gear and braking system has led to many runway excursion incidents in the Aviation history, as well as a few unfortunate belly landings. For this reason, today you will learn a great amount of detail in the following topics:

- Overview of the 777 landing gear system, - Landing gear operation, retraction and extension processes. - Alternate gear extension, also known as gravity extension, - Nose-wheel steering, - Main-wheel steering, - Normal braking, - Alternate braking, - Brake hydraulic accumulator, - Autobrakes, - Anti-skid protection system, - and last but not least, the Parking Brakes.

- Throughout the lesson well be talking about controls and indicators related to this system.

Without any further delay, lets start by discussing the 777s landing gear system.

Landing Gear System Overview The landing gear is designed to support to hold the weight of the aircraft while it is on the ground. It has to be rigid enough to support a fully loaded aircraft at maximum takeoff weight, and more. Just so you get a feel of perspective, the 777-200LRs MTOW is in the order of 345 tonnes, thats about the weight of almost 5000 adults put together. Now on ground, all that weight is concentrated upon 3 main gears shaped altogether like a tricycle. The nose landing gear is the one located near the nose, and the main landing gear is made up of two sets of wheels near the center fuselage, one being on each side. The landing gear as a whole also has to be able to absorb the loads imposed when the aircraft lands, and safely transfer those loads without collapsing and breaking the aircraft structure. It is a fairly difficult task, but a series of shock absorbers make the job a bit easier.

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Nose landing gear is a two-wheel gear that has a steerable axis for movement on ground. Well talk about nose-wheel steering a bit later. There are no wheel brakes on the nose gear. In contrast, main landing gears have a six-wheel truck each. Three axles of two wheels on each landing gear are placed in tandem, where the aft axle is also steerable in order to support the nose-wheel steering and therefore reduce the required turning radius for the aircraft. All wheels on the main gear have brakes, as well as a few protection mechanisms in case one wheel was to burst or skid in difficult runway conditions. The entire main gear is normally tilted upwards a couple of degrees. To reduce the enormous form drag that landing gears create, these are made retractable. Once the aircraft lifts off, it may be raised and stowed inside the fuselage, in compartments known as wheel wells. During the approach to landing, the landing gears are extended and locked. Movable doors aerodynamically seal the wheel wells when the gears retract. Each main gear has four doors, and so does the nose landing gear. This retraction and extension operation is fairly straightforward, however there are a few considerations related to each process, which is why were going to talk about them in detail now.

Landing Gear System Operation And so, the landing gear is electrically commanded with the landing gear lever, located in the center forward panel. The system is then hydraulically energized by the center hydraulic system, which provides the required muscle to operate the gear. The lever has two simple positions: UP: Landing gear is commanded to retract DN: Landing gear is commanded to extend. A lever lock doesnt allow the lever to move form the DN position when the aircraft is on ground, however this may be overridden by pushing the LOCK OVERRIDE switch next to the landing gear lever. In flight, this lock is automatically released and the gear may be commanded UP. How does the aircraft tell when it is on ground or in flight? Simple, when the air/ground system senses weight on the wheels in the main landing gear beams, it communicates with most aircraft systems and informs that it is on ground. When that weight disappears, the system deduces it is in flight. Moving on, the flight crew can monitor the landing gear position with indications shown on the EICAS display. Indications for the three gears under normal conditions are shown in only one box. When any non-normal condition develops, the three gears segregate and have their own indicating boxes. During normal conditions, the indications have the following structure: DOWN: illuminates in green confirming all three landing gears are down and locked UP: illuminates in white showing all landing gears are up and locked. This indication disappears after 10s. White crosshatch: indicated one or more landing gears are in transit to their commanded position. Blank: indicates all three landing gear position indicators are inoperative.

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In a very similar manner, the non-normal indications for each individual gear also have the following structure: DN: illuminates in green showing the related gear is down and locked. UP: illuminates in white showing the related gear is up and locked. White crosshatch: shows the related landing gear is in transit to its commanded position. Blank: indicates the related landing gear position indicator is inoperative. Lets now move on to the landing gear retraction sequence. When the landing gear is moved UP after takeoff or a go-around, the retraction sequence begins. Firstly all doors in the fuselage open, then the main gear trucks tilt downwards so that they gear axis becomes parallel to the fuselage. The EICAS gear position indicator changes from DOWN to the crosshatch indicating that it is in-transit. Both main gears and the nose gear retract almost simultaneously into their wheel wells. The doors then close, and once the process is complete, uplocks hold the gear in place and the EICAS display changes to UP for ten seconds, disappearing after that. Finally, when all gears and doors are fully retracted and locked in place, the landing gear hydraulic system is automatically depressurized. Moving on to the other side of the coin, landing gear extension. When the gear lever is moved to DOWN during approach, the extension sequence begins by opening all gear doors. The gear uplocks are released, the EICAS gear indicator changes to the white crosshatch to illustrate the gear are in transit. During this time, the landing gears are allowed to free-fall by the force of gravity, minimizing the need for hydraulic power from the center system. Once the gear reaches the DOWN position, three important things happen:

- All gear doors close, - Gear downlocks are energized to lock the gear and prevent it from collapsing upon touchdown, - The main gear trucks tilt backwards a few degrees.

Once all three things have occurred, the EICAS gear indicators changes from transit to the DN position. When the gear lever is moved UP or DOWN, and after a normal transit time, if any gear is not either up and locked, or down and locked, the EICAS displays the caution message GEAR DISAGREE. When this occurs, the gear position indicators we talked about earlier change to the expanded non-normal format, so the flight crew can identify which gear is the one presenting problems. Similarly, if after normal gear transit time any of the hydraulic doors is not closed and locked, the EICAS displays the advisory GEAR DOOR. Any further failures to these, or multiple GEAR DISAGREE or DOOR conditions demand the need for an alternate way of extending the landing gear. Were going to be talking about this now.

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Landing Gear Alternate Extension Landing gear alternate extension, also known sometimes as extension by gravity, is an independent alternative that pilots have for when the normal extension system presents faults or when center hydraulic system pressure is low. A DC electric powered hydraulic pump allows this process to happen. A guarded switch in the center forward panel named ALTN GEAR operates the alternate system, and has the following positions:

- NORM: normal position, the landing gear is under normal operation. - DOWN: the landing gear is extended by alternate means.

Note that alternate extension may be commanded with the normal landing gear lever in any position. So, once the alternate switch has been commanded DOWN, all gear uplocks are released and all gear doors are opened. The landing gear extends to the down and locked position aerodynamically and using its own weight. Whenever the alternate system is used, the EICAS always switches the gear position indications to the expanded non-normal mode and the GEAR DOOR advisory is always displayed, because doors remain open. In alternate operation, the gear may only be extended. In order to retract it, the normal system can be used if it is operative. Selecting DN and then UP would initiate a normal retraction sequence. Weve talked about the landing gear and its characteristics, as well as how to operate it. Another of the very useful capabilities of the landing gear allows manoeuvring on ground by means of steerable wheels.

Nose & Main Wheel Steering The 777 is equipped with nose wheel steering and also main gear aft axle steering, meaning the last two-wheel row on each main gear is steerable. This is to support nose wheel steering and reduce the minimum aircraft turning radius. Both wheel steering systems are powered by the center hydraulic system. The nose wheel steering system is also further powered by the reserve hydraulic system, because it is more essential than main wheel steering. In fact, only when the nose wheel is steered left or right more than 13 from its neutral position does the main wheel steering come into action. This happens automatically, and is done to reduce tire wear-and-tear from excessive scrubbing. So how do we operate the steering system? Either of two ways. Primary steering control is provided by a nose wheel steering tiller control, available for both pilots in the left and right sidewalls. Rotation of the tiller is translated to rotation of the nose wheel in the desired direction, up to a maximum of 70 left or right. Remember that if the nose wheel is steered past 13, main gear steering automatically comes into place and steers the main gear aft axles as a function of nose wheel position. Tiller position is shown in an indicator at the bottom of the tiller. Secondary steering control is provided with the use of rudder pedals. Both pilots have a set of rudder pedals that when fully pressed into either direction, allow a 7 turn in the nose wheel, in the respective direction. Note that when

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rudder pedals are used for steering, the main gear steering doest not automatically engage. It is only slaved to the tillers. If both the tiller and the rudder pedals are simultaneously used, the tiller takes priority and overrides all rudder pedal input. This segregation is there so that during taxi, it is the steering tiller that is used to manoeuvre the aircraft, however during the takeoff or landing roll where aircraft airspeed is much higher, the tiller would turn the wheel far enough that high loads may be imposed over the nose gear structure, increasing the risk of it breaking. Thats why in these phases we maintain directional control of the aircraft with the rudder pedals and leave that tiller alone. Lets now move on to the final segment of this video. As weve seen, the landing gear allows the aircraft to roll on ground as well as maintain directional control. In this portion, well talk about the wheel braking system, which is also housed within the landing gear system. It is extremely important and vital function to limit taxi speed and streamline ground traffic, as well as its main purpose of providing an important deceleration rate for the aircraft to slow down once it has landed. Ineffective braking has led to many runway excursion incidents during aviation history, and more so in Flight Simulator.

Normal Braking System The 777 braking system is divided into the following areas:

- Normal braking, - Alternate braking, - Reserve braking, - Brake accumulator, - Anti-skid system, - Parking brakes.

Normal braking is achieved with a hydraulic brake assembly in every wheel of each main gear truck. Nose gear wheels do not have ground brakes fitted to them except for little wheel-spin brakes for when the nose gear is being retracted. Normal braking uses hydraulic pressure from the right hydraulic system and is operated with either pair of rudder pedals in the cockpit. Pressing the top portion of the left rudder pedal will activate the mechanism that engages the left wheel brakes, similarly for when the right pedal is pressed, or both are pressed together.

Alternate Braking System When right hydraulic system pressure is low or when normal braking valves present a significant failure, the system automatically reverts to alternate mode. In this mode, the center hydraulic system supplies pressure for brake operation. Control and operation of the brakes is exactly the same except that when pilots press the pedals, alternate valves are used to control braking.

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Assuming the case where center hydraulic system pressure also was to fall below normal limits, alternate braking would not be able to operate. In this situation, a center hydraulic isolation valve, or CHIS, isolates a determined amount of center hydraulic power from the other services in order to dedicate it to braking, in what is known as a reserve braking condition. For more information on the CHIS, check out the 777 Hydraulic System GroundWork lesson. Isolation of the C1 primary pump, which is an ACMP, allows this segregation of power in order to dedicate it to reserve braking. If a hydraulic fluid leak were to occur downstream of the CHIS lines, the available and isolated center system power would potentially fall to nil. This is obviously a highly improbable case, yet redundancy must be built into the system. When this happens, meaning when Center and Right hydraulic system pressures are unavailable, the EICAS displays the advisory BRAKE SOURCE and the BRAKE SOURCE light in the left forward panel illuminates in amber to alert the flight crew. Also, a brake accumulator comes into action. The brake accumulator is located in the lines of the normal braking system and essentially performs the following functions:

- Stores hydraulic fluid under pressure from the right system, - Dampens pressure fluctuations, - Provides an emergency supply of fluid to the braking system in the event of a failure.

Summing up, if normal, alternate and reserve braking becomes unavailable, an emergency source of braking is the accumulator, which guarantees several braking applications at maximum rate of deceleration. Accumulator pressure is indicated on a dial in the left forward panel. Speaking about deceleration rates, well now discuss the autobrake system.

Autobrakes As the name clearly implies it, autobrakes are a means by which the system provides automatic braking at a deceleration rate that is selectable by the flight crew. This mechanism is useful during landings to ease pilot workload and allow attention to other critical areas of the touchdown and rollout, but is even more important for the case of rejected takeoffs (RTOs). RTOs are intrinsically complicated manoeuvres due to the rapid response and quick decisions that have to be taken at relatively high airspeeds. Autobrakes, which only operate when the normal braking system is functioning, are selectable with the AUTOBRAKE selector in the center forward panel, having the following positions:

- OFF: deactivates and resets the autobrakes, - DISARM: disengages the system and releases brake pressure,

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- 1,2,3,4, MAX AUTO: selects the desired deceleration rate. Keep in mind that the maximum automatically achievable deceleration rate is lower than that produced by full pedal braking.

- RTO: arms the autobrakes so that they automatically apply maximum brake pressure in a RTO condition, when the thrust levers are retarded to IDLE and the aircraft groundspeed is above 85 knots.

The EICAS displayes the selected autobrake setting as AUTOBRAKE then followed by 1 through 4, MAX or RTO respectively. Whenever the autobrake system is disarmed or is inoperative, such as during alternate braking, the EICAS displays the AUTOBRAKE advisory. The system works under the following logic: Once the aircraft has landed, the autobrakes only come into place if both thrust levers are IDLE and the wheels have spun up to a determined speed. If MAX AUTO is selected, it will only activate if the main gear has touched down and the aircraft pitch angle is lesser than 1, until which the system only allows AUTOBRAKE 4. During the rollout, rotating the autobrake selector, without risk of disarming the system, can easily change the deceleration rate. Note that to maintain the selected deceleration rate, autobrake pressure is reduced as items like the ground spoilers and thrust reversers come into place and contribute to deceleration. The system is designed to provide braking until a full aircraft stop, or until the system is disarmed. To disarm autobrakes, you must perform either or a combination of the following:

- Apply manual braking with the pedals, - Advance either thrust lever after touchdown, - Move speedbrake lever to the DOWN detent.

When the system disarms, the selector automatically moves to the DISARM position and releases brake pressure. Lets now move on to the last few segments of this lesson.

Anti-skid Protection System Optimum and efficient braking is important to the operation of all aircraft, but more so when you are trying to land an aircraft as heavy as a few hundred tons at about 150mph on a wet & slippery runway. In bad weather, flight crew may select and expect automatic or manual braking but is unable to sense if all brakes are effectively doing their job. Wheels sometimes lock and slip on the ground, therefore it is important to have a protection against such a condition. The 777 is protected by an anti-skid system, which includes the following functions:

- Wheel skid control, - Locked wheel protection, - Hydroplaning and touchdown protection, - Taxi brake release.

The antiskid system works by monitoring each wheels deceleration rate and comparing it to the rest. When it notices that one of the wheels is considerably slower than the others, it assumes the wheel is locked or is skidding and reduces its brake pressure until skidding stops.

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Antiskid is available during normal, alternate and reserve braking. The system logic differs slightly in all three cases. During normal braking, each main gear wheel is provided with individual antiskid protection. During alternate braking, the system provides antiskid protection to tandem wheel pairs for the forward and middle axle wheels. The aft axle wheels remain individually controlled. Another function of the antiskid system is taxi brake release. When taxiing at or below 45 knots, and during each taxi brake application, the antiskid system releases the brakes of one axle pair of each main landing gear. The system sequences through the axle pairs in order to minimize the number of brake applications by each brake, thus extending brake service life and minimizing tire wear. Very smart indeed. Now assuming the aircraft has landed, safely decelerated and taxied off the runway into its respective gate, another system is activated when the aircraft is stationary and parked. Namely, parking brakes.

Parking Brakes It is easy to set parking brakes. Both brake pedals must be fully pressed, while pulling the parking brake lever UP and then releasing the pedals. This mechanically latches the pedals in the pressed position and closes the parking brake valve. The parking brake lever is located on the control stand. Parking brakes are available during normal, alternate and accumulator braking.

Lesson Summary In this lesson, we walked through all the characteristics and modes of operation of the 777-200LRs landing gear and brakes system. We started with a discussion about the landing gear and how we may operate it normally, or during a non-normal condition. Following that, the we talked about the main gear wheel brakes and all the considerations related to normal, alternate and reserve braking, as well as the emergency source of braking power through the brake accumulator. An impressive amount of redundancy is built into the system, but only because it is an absolutely essential and critical set of components in order to safely slow the aircraft down to taxi speed, after it has landed. You are encouraged to jump into the sim and have a look ad how the aircraft behaves with different autobrake settings, or while braking on a wet runway. Take a look at the landing gear synoptic display in the MFD too. What do you see? We may find indications of brake temperature, gear door status, tire pressure indications, and indications for faulty conditions. This lesson wraps up the controls section. There is surely a lot to learn from Hydraulics, the PFCS, HLCS and landing gear & brakes. Stay tuned for the following lesson, where we will talk about one of the primordial sources of power and energy for the 777s systems: bleed air. Until then, thanks for watching and Throttle On!

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aoa_777_groundwork_landing-gear_transcript.pdf

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