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The Extension And Retraction Engineering Essay


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For the analysis of a B777-200ERs landing gear, the components are split in two main sections: the main landing gear and the nose landing gear (2.1.2). In these sections a detailed and zoomed overview will be given about the technical specifications of the landing gear itself. The focus of these mechanical analysis lies on the way the landing gear can retract or extend. After this mechanical analysis is given, there will be a detailed overview of the cockpit panel itself that is used to operate the landing gear (2.1.3). At the end, the situation will be observed when the landing gear (because of the explained reasons) refuses to extend or retract and there will be explained how pilots of a b777-200ER are able to solve this kind of problems (2.1.4).

Main Gear

The extension/retraction system of the Main Landing Gear (MLG) retract and extends not only the landing gear, but it also opens and closes the main gear doors. The hydraulic system of the Boeing 777-200ER is providing pressure to several components which makes the landing gear extend or retract. Hydraulic pressure is provided to the retract actuator (I.1.1.a). When the retract actuator is extended, the MLG side and drag brace down lock actuator are been driven (I.1.1.b). For preventing that the MLG doors fall down, a lock actuator and an uplock assembly is used (I.1.1.c). For releasing the doors; a door actuator (I.1.1.d) and a door-operated sequence valve (I.1.1.e) are being pressurized. To accommodate the landing gear properly and to make the landing as smooth as possible a truck positioned actuator (I.1.1.f) is used. This can turn the truck several degrees. When all the components work together, the MLG can run its retraction (I.1.1.g) or extension (I.1.1.h) sequence.

Retract actuator


Retractor actuator

Shock strut

Retract fitting


Fig 1.1 MLG restraction actuator

The MLG is operated by the central hydraulic system of the 777-200. The hydraulic system provides pressure through an auto slat priority valve and a bypass valve (also called a MLG selector). These valves operate the MLG. The force which is required to retract the MLG is supplied by the retract actuator (fig 1.1)(1). This actuator retracts to raise the gear and extends when the gear is extending. The retract actuator is a hydraulic driven piston type actuator, but it is not pressurized during the gear extension. A flow restrictor in the retract port union controls the gear extension rate. The amounts of pressure from the valves go to the actuator which operates the retraction of the MLG. The retraction actuator (which has a dry weight of approximately 109 kilograms) is located in the inboard of the main landing gear shock strut (2). The rod end is attached to the inboard side of the shock strut, where the head end of the actuator is attached to the retract fitting (3), which is mounted on the trunnion (4) which is a cylindrical protrusion used as a pivoting point.

MLG side brace and drag brace downlock actuators

The purpose of these actuators is to lock and unlock the over-center toggles with are between the side brace and the drag brace lock during extension and retraction. Both actuators are identical and interchangeable. Both actuators are hydraulic cylinder; with extend to lock the toggles during gear extension. They retract to unlock the toggles during gear retraction. The toggles are connected to the actuator by the rod end of these actuators. The head end of the actuators is attached to the upper lock links.

Lock actuator and uplock assembly

When the MLG is completely retracted, the hydraulic system is depressurized, this is to prevent hydraulic failures. But to prevent the MLG doors of falling down an uplock hook is combined with the door lock actuator. The uplock hook uses over-centre locking to hold the doors closed. The hook is held in position by two springs. The door lock actuator retracts to unlock the hook and extends when the hook has to be locked. The MLG is hold in position by a MLG uplock assembly (appendix ##)(1). This uses over-center locking to keep the MLG up and locked. The uplock assembly is hold in position by two springs. During retraction of the MLG, the roller on the MLG shock strut engages the hook in the uplock assembly. By this motion of the MLG, the uplock mechanism moves toward the over-center locked position. A MLG uplock-operation sequence (2) valve is supplying the pressure to the extend side of the uplock actuator (3), which makes sure that het uplock mechanism is moving towards the over-center position.

MLG door actuator

Wing/fuselage junction is used by Boeing to store the landing gear of the Boeing 777-200ER. This means the hollow part between the wing and the body of the plane is being used as a storage room for the MLG. For opening and closing of the landing gear doors, door actuator areinstalled (fig x)(1). The MLG door actuator is a hydraulic piston-type actuator, which extends and retracts to open and close the MLG door. In the door, flow restrictors control the rate of door operation. These flow restrictors are located in the extend en retract port unions. The actuator speed at the ends of the actuator travel is controlled by snubbers in the rod and the head end of the actuator.

MLG door-operated sequence valve

The hydraulic fluid which comes from the central hydraulic system is controlled by the MLG door-operated sequence valve (fig x)(2). This valve is a two-position valve, namely; Door open and Door not open. The valve is being operated by a mechanical linkage from the MLG and is located on the keel beam aft on the MLG door actuator.



Door actuator

Door-operated sequence valve

Fig x door operating

Truck positioned actuator


5 degrees


Fig. 1.4 STOW position truck


13 Degrees


Fig 1.3 Truck Tilt

To ensure the landing is as smooth as possible, the MLG of the Boeing 777-200ER is movable. A MLG truck positioned actuator is able to move the MLG truck thirteen degrees forward wheels up (also called TILT) (fig 1.3). To turn the truck into a thirteen degrees angle (1), hydraulic pressure goes to both sides of a floating piston and the rod side of the piston (2). This retracts the actuator to move the truck thirteen degrees forward, wheel up. During a gear retraction the actuator also moves the truck approximately five degrees forward wheels down (STOW) (fig 1.4). These five degrees (1) are therefore to improve the storage of the gear. The MLG will stay in this position until the gear is down and locked due to a gear extension. For turning the truck into STOW position, hydraulic pressure goes between the floating piston and the rod side of the main piston (2). This makes the pistons to move apart until the floating piston is touching the stops on the cylinder rod end. This moves the truck to STOW five degrees downward wheels down position.

Retraction sequence

Retraction of the MLG starts when the pilot moves the landing gear in the UP position. This means that the MLG selector/bypass valve is also moving to the UP position. In that case, the retract lines are filled with hydraulic pressure. The pressure goes to the MLG drag brace actuator and the side brace actuator, which are starting to retract to make sure the landing gear is unlocking. After the landing gear is starting to unlock, hydraulic pressure is moving towards the MLG truck actuator, which turns the landing gear in the STOW position. Sequence valves are being supplied with hydraulic pressure. The valves retracting the MLG door actuator to unlock the main door. The priority/relief valve is than opening the door, by providing also pressure to the door actuator. When the doors are almost all the way open, the MLG door-operated sequence valves moves to the OPEN position. In this position, pressure goes to the MLG retract actuator for retracting the landing gear. When this happens, the MLG drag brace-operated sequence valves moves to the NOT DOWN position. When the MLG goes into the wheel well, a roller on the landing gear strut, moves the MLG uplock mechanism and the MLG uplock-operated sequence valve to the LOCKED position. The pressure will then be removed from the MLG truck positioned actuator. The landing will be kept in the STOW position, by some trapped hydraulic pressure in the positioned actuator. The MLG door-operated sequence valve is also providing pressure to the brake system components for the gear retract braking function. When the MLG is finally retracted, pressure goes to the close side of the MLG door actuator and the lock side of the MLG door lock actuator, which starts the doors to close. A roller on the door starts to move the MLG door uplock mechanism to the locked position, when the doors are almost closed. The uplock mechanism over-centre moves to the locked position by pressure of the MLG door lock actuator.

Extension sequence

All the components come together for opening the MLG doors and let the MLG out. The pilot is selecting the DOWN position with the landing gear lever. Hydraulic pressure is permitted to go to the extend lines, when the MLG selector/bypass valve is moving to the down position.

The doors open, by pressurizing the MLG drag brace-operated sequence valve and the uplock sequence valve. The pressure goes to the MLG truck positioned actuator to hold the truck in the STOW position. The same pressure also goes to the door lock actuator. The actuator gets extend pressure, which opens the MLG door. When the door is opening, the door-operating sequence valve is moving to the open position. Pressure goes to the MLG uplock actuator to unlock the MLG uplock. The MLG extends by its own weight and airloads. This means no hydraulic pressure is being used for bring down the MLG. The MLG drag brace-operated sequence valve moves to down position, when the MLG is at twenty degrees from full extension. Hydraulic pressure is than been used to retract the MLG truck positioned actuator, which moves the MLG truck to the TILT position. The MLG drag brace and side brace downlock actuator are locking the gear down, with the use of hydraulic pressure. MLG uplock-operated sequence valve and the MLG door release/safety valve module are getting pressure from the MLG brace-operated sequence valve. The pressure is providing the door actuator and the MLG door lock actuator with closes the MLG doors. When the doors are at the end of closing, the MLG door uplock mechanism moves with the use of a roller on the door to the locked position. With the help pressure of the door lock actuator, the uplock mechanism is over-centred in the locked position.

Nose Landing Gear

The nose landing gear (NLG) is part of the landing gear of the Boeing 777-200ER with the main landing gear and is for instance also responsible for handling all the pressure while the plane is braking, Both the main landing gear and the nose landing gear will need to be able to retract and extend. To do this, the NLG needs some specific components and uses the centre hydraulic system supply pressure to operate. All of the following components are needed by the Nose Landing Gear before it can retract or extend and they will be explained: the NLG retract actuator (2.1.1.a), the NLG operated sequence valve (2.1.1.b), the NLG lock actuator (2.1.1.c), the NLG door actuator (2.1.1.d) and the NLG door-operated sequence valve (2.1.1.e). At last, a final view of the process will be given (2.1.1.f).

NLG Retract Actuator

Before the NLG (Fig x) can retract, it will need force to do so. The retract actuator (1) will supply the NLG enough force to retract as shown in figure x. It is a two-position piston-type actuator. The actuator retracts to raise the NLG and extends to lower the NLG; the rate of extension is controlled by a flow restrictor. While the gear is extended, the retract actuator is unpressurized and has a weight of approximately 43 kilograms. It is located in the top right corner of the aft bulkhead of the wheel well.

NLG-Operated Sequence Valve

The NLG-operated sequence valve (2) is a three-position valve that means it has three different modes: Gear up, Gear down and Gear in transit. The purpose of the NLG-operated sequence valve is to make sure the doors of the nose landing gear will not move while the landing gear is in transit. So with this sequence valve, the doors will only move while the gear is retracted or extended.

Lock Actuator

The lock actuator (3) in the nose landing gear serves as a guard. Before the NLG can retract, the lock actuator retracts first. Once retracted, the NLG locking mechanism becomes unlocked and the NLG then will be able to retract as well. The lock actuator also has another task. When the NLG is retracted or extended, the actuator will lock the situation so the NLG will stay retracted or extended thanks to the over-centre locking mechanism. The hydraulic pressure is still up at the retract side of the NLG lock actuator when the gear is still in transit.

NLG Door-Operated Sequence Valve

The NLG Door-Operated Sequence Valve (4) is a two-position valve that can open and close. It controls the hydraulic flow to the NLG lock and retract-actuators. This control makes sure that only when the doors are open, the NLG can extend. It is located on the left of the NLG wheel well.

Fig X Nose Landing Gear B777-200ER

1) Retract Actuator

2) NLG-Operated Sequence Valve

3) Lock Actuator

4) NLG Door-Operated Sequence Valve

NLG Door Actuator

The NLG is a two-position hydraulic actuator that extends to open the NLG doors and retracts to close the NLG doors. The door actuator prevents the doors from collapsing. It opens and closes the doors and uses a lock to hold the doors in the wanted position. This makes sure that the NLG retracts or extends only when the NLG doors are open. The NLG Door Actuator avoids problems when the NLG wants to extend, but the doors are not open yet.

Fix X | NLG Wheel Well (Forward Look)

1) NLG Door Actuator

Retraction sequence

After the knowledge is given about each specific component, an overview can be realized about the retraction sequence. The following conditions are active when the command for gear retraction can be given (fig x). The NLG is down and locked (1), the NLG doors are closed: NLG is down and locked, the NLG-operated sequence valve is on gear down position and the NLG door-operated sequence valve is closed. The control command is where the line begins, the pilot selects landing gear UP on the landing gear control lever, then the hydraulic pressure will flow to the retract lines.

Once the hydraulic pressure has arrived, the pressure goes to the NLG lock actuator. The actuator will start to unlock the downlock to start the process of retraction. Also the NLG-operated sequence valve will receive hydraulic pressure and will pass on the pressure towards the door actuator. Then the door actuator will extend and the doors will be opened.

Once the doors are almost fully open, the NLG-door operated sequence valve also goes to condition open. Then the hydraulic pressure will go to the NLG retract actuator and the NLG will start to retract (2). The NLG-operated sequence valve moves to the gear in transit position when the NLG starts to retract.

After the NLG is retracted, the NLG-operated sequence valve moves to the up position. The hydraulic pressure now will go to the retract side of the NLG door actuator and the NLG doors will start to close. The lock inside the NLG door actuator will lock the NLG when the doors are closed (3).

NLG Down and locked

NLG in transit

NLG up and locked

Cockpit Layout

The Boeing 777-200ER alternate gear extension method is displayed in an understandable way. The whole gear section panel of the cockpit is displayed below in (figure x). The pilot can command the gear to retract or extend by moving the Landing Gear Lever that will give an up command or a down command (1). Moving it down will make the landing gear extend, moving it up will make the landing gear retract.

Also on the panel is the autobrake selector, this is a switch that can take multiple positions (2). When the switch is turned to off, the autobrakes will be deactivated and reset. Disarm will disengage the autobrake system and releases the brake pressure. Finally there are 5 positions for the rate of deceleration. Max auto will give the command to reach the highest deceleration rate.

On the down right corner of the panel is the Alternate Gear Switch located (3). A safeguard that will prevent accidental activating of the switch covers this switch. When the switch is turned to NORM (stands for normal), the landing gear lever operates normally. When the switch is turned to DOWN, the landing gear extends by the alternate gear extension system. This option may be selected with the landing gear lever in any position.

At last there is a Landing Gear Lever Lock Override Switch on the Landing Gear Control Panel (4). This button has only one function: when the pilot wants to change the situation of the landing gear. Before the pilot can change the position of the landing gear lever, this button has to be pushed first. The Landing Gear Lever Override Switch is a safeguard for changing the situation of the landing gear by accident. The button has to be pushed first so the landing gear lever lock will be released.

Fig X

1) Landing Gear Lever

2) Autobrake Selector

3) Alternate Gear Switch

4) Landing Gear Lever Lock Override Switch

Alternate gear extension

The centre hydraulic system provides pressure to extract are retract the MLG. When, due to a fail, there is no more hydraulic pressure, the pilot can extract the MLG by using the alternate gear extension. The alternate gear extension is operating by the power it receivesfrom the alternate gear power pack (I.1.4.a). For opening the MLG doors, the system is operated by two valves; the release and safety valve (I.1.4.b). These valves regulated the amount of pressure that is produced by the power pack.

Power pack

The alternate system works independently of the normal extension and retraction system. This system is being powered by the hot battery bus for operating and controlling it. At the alternate system, a power pack is installed, which is controlled by the relays of the Electrical load Management System (ELMS). The power pack has a hydraulic pump which is operated by an electrical motor. This electrical motor is a 28 volt dc hot battery bus which supplies power to the motor. The power pack has internal hydraulic routing to let the hydraulic fluid flow back into a oversize supply tube. This oversize supply tube supplies the hydraulic pump in the power pack with fluid from the centre hydraulic system.

Release/Safety Valve

The hydraulic pump pressurizes fluid, which is being transported by door release/safety valves, which unlocks the MLG doors. The release/safety valves are also connected to the normal MLG extension/retraction system. Hydraulic pressure (powered by the power pack) extends the door release valve. The release valve releases the uplock mechanism in the MLG, which releases the over-centred lock. The NLG will be retracted when the release actuator extends a rod to push the crank. The landing gear will hereby extend, but this happens due to the air loads and its own weight. When the landing gear is in full extend an alternate extend hydraulic pressure switch stops the extends the power pack when the extend cycle is complete. The pressure stop switch is located on the ceiling in the NLG wheel well. It lies in the hydraulic line between the NLG door release/safety valve module and the NLG alternate uplock release actuator. This actuator is the last component which gets hydraulic pressure from the power pack. The stop switch is opening, when there is 2100 psi in the pressure line. This removes the electrical power used by the alternate extend power pack. In that case, the power pack stops operating. When the pressure is below 1700 psi, the pressure stop switch closes, which resets the alternate extension system. The safety valve which is also operated by the power pack has two positions; SAFE and NORMAL. The safety valve is standing in position NORMAL during a normal operation of the landing gear extension and retraction system. In this position, pressure to close the door, can go straight to the door actuator, which can close the door. The safety switch will go to the safe position, when the door release actuator is extending. This prevents that pressure which is required to close the door, will not be transferred to the door actuator.

All die getalletjes moeten verwezen worden naar de plaatjes/stukken/etc.

NLG deel: Plaatjes volgens sjabloon.

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