How Landing Gear Works | Part 7 : Main Gear Retraction And Extension

[00:00:00]In the seventh installment of the Boeing 77 landing gear series, we will explore the main landing gear retraction and extension system. The landing gear control lever is used to operate the gears, but the lever remains locked in the down position when the aircraft is on the ground. To operate the landing gear, the aircraft must be in the air.

[00:00:21]Let's understand how the control lever unlocks when the aircraft transitions.

[00:00:25]The main landing gear is supported by the landing gear beam inside the wing structure. When the aircraft is in the air, the beam is in an unloaded condition and remains relatively straight. As the aircraft hits the ground, the landing gear bears the weight, causing the beam to bend. To determine whether the aircraft is on the ground or in the air, a load sensor is installed to measure the bending stress.

[00:00:49]The sensor is a variable reluctant strain measurement device. It has two parts mounted independently on the beam and connected by flexible bellows. One side of the sensor has the soft iron target which is placed between the two coils attached to the other side. The coils are connected in series and a voltage is applied across them. When the aircraft is in the air and the beam is unloaded, the air gap between the target and the coils is equal. Therefore, the voltage drop in each coil is the same.

[00:01:18]By knowing the applied voltage and measuring the voltage across an individual coil, the target position can be determined. When the aircraft is on the ground, the bending of the beam causes the target to move closer to one coil. The change in the air gap increases the inductance of this coil and reduces the others. The sensor measures the change in voltage to indicate that the aircraft is on the ground. The load sensor is extremely responsive and determines the transition from air to ground and vice versa.

[00:01:48]Instantaneously the sensor signal is received by the weight on wheels logic cards in the avionics compartment. The weight on wheels computer determines the aircraft mode and controls the air ground relay inside the electrical power management panel. The air ground relay is normally open and receives the main power from the electrical power panel.

[00:02:11]In the air mode, the computer operates the relay and allows the lever unlock power to reach the landing gear control module.

[00:02:18]The lever module also receives the selector valve power directly from the power panel.

[00:02:25]The landing gear control lever module is in the cockpit and is locked in the down position by a lock solenoid.

[00:02:32]The unlock power operates the solenoid and releases the lock to allow the lever operation.

[00:02:39]The lever controls two switches, the gear down switch and the gear up switch.

[00:02:44]When the lever is down, it operates the down switch and allows the gear down power. When the lever is moved up, it operates the up switch and the gear up power is applied. The power from the gear up switch reaches the autooff relay in the power management panel. The normally closed relay will send the power to the main landing gear selector valve. When the lever is in the down position, the power from the gear down switch is directly connected to the valve. The selector valve is in the main gear wheel. Well, the aircraft has three independent hydraulic systems and the center system powers the main gear retraction and extension. The valve has three main components. The down solenoid operated valve, the up solenoid operated valve, and the selector valve. Each solenoid operated valve has two input lines and the selector valve has three input lines. When the lever is in the down position, power is applied to the down solenoid which operates the valve and keeps the pressure line closed. As a result, the control pressure to the selector valve is applied through the upsenoid operated valve. This maintains the spool of the selector valve to the left and the pressure is applied to the down lines of the main landing gears.

[00:03:56]When the lever is moved to the up position, the power is removed from the downsoid and applied to the ups solenoid. Both valves will operate to switch the control pressure application.

[00:04:08]Now the control pressure from the down solenoid operated valve will move the selector valve spool to the right. This applies pressure to the up lines and removes the pressure from the down lines. The pressure from the selector valve is used to operate the main landing gears. Since both gear operations are similar, we will only consider the left main gear components.

[00:04:28]During retraction, the main landing gear and its door have to follow a sequence.

[00:04:33]Therefore, the pressure cannot be applied directly to the actuators.

[00:04:36]Instead, they are applied through the three sequence valves.

[00:04:40]The only exception is the landing gear downlock actuators to which the retract pressure is applied directly from the selector valve and the extend side is connected to the drag brace operated sequence valve which will provide the fluid return path. In the initial phase of retraction, the applied pressure will simultaneously operate three systems.

[00:05:00]Let's start with the down lock release function. The aircraft has two downlock actuators installed on the lock links of the drag brace and the side brace assemblies.

[00:05:10]Each brace assembly has an upper brace and a lower brace connected by the hinge. The lower braces are connected to the landing gear shock strut attachment.

[00:05:19]Therefore, the two brace assemblies hold the landing gear in the extended position and the braces are held in the extended position by the lock links. The lock links can be further divided into an upper and a lower lock link connected by a hinge. The head end of the downlock actuator is pivoted to the upper lock link and the rod end is connected to the toggle. The toggle is connected to the lower lock link. Two springs hold the link attachment in the overenter locked position.

[00:05:47]The application of the retract pressure will operate the toggle against the spring force. As the actuator retracts, the lock links start to fold and the toggle to link attachment crosses the over center position. This releases the drag brace assembly down lock.

[00:06:03]Simultaneously, the side brace down lock actuator will operate to release the side brace lock. Since the lower lock link is connected to the brace hinge, the movement of the lock link will allow the braces to fold. The down lock release and folding of the two brace assemblies will allow the landing gear to retract.

[00:06:22]The applied pressure for the landing gear retraction will also perform the truck positioning function.

[00:06:28]The drag brace sequence valve output has the stow and tilt connections.

[00:06:33]The output lines from the sequence valve are connected to the SLG controller. The controller will operate the semi- levered gear hydraulic strut. The strut is designed in a way that when the aircraft is in the air and the landing gear is down, the actuator retracts to move the forward wheels up to the tilt position. But the gear cannot enter the wheel well if the truck is tilted.

[00:06:55]Therefore, when the gear retract command is given, the truck beam is moved to the stow position.

[00:07:01]The head end of the semi-levered actuator is pivoted to the shock strut and the rod end is attached to the front of the truck beam. The actuator has a floating piston and a main piston.

[00:07:13]During gear retraction, the applied stow pressure inside the controller will extend the actuator. A pressure reducing valve drops the applied pressure to 1,000 lb per square in and releases the excess pressure to return.

[00:07:27]The controller has an additional line that applies the stow pressure directly to the actuator.

[00:07:33]The reduced pressure is applied to the head end of the main piston. Inside the main piston, there are slots that allow the reduced pressure to reach the bottom side of the floating piston.

[00:07:43]Since the surface area of the floating piston is greater than that of the main piston to which the reduced pressure is applied, the floating piston retracts and pushes the main piston to its retracted position.

[00:07:55]Therefore, the reduced pressure application tries to maintain the truck in the tilt position. However, during retraction, the direct stow pressure is also applied to the top side of the floating piston. The higher pressure overcomes the reduced pressure and extends the floating piston. Since the floating piston has extended, the main piston is now free to extend.

[00:08:18]As the main piston extends, it will push the front of the truck beam downward, causing it to rotate on its mid-axle pivot. This will cause the truck to move from the 11° forward wheel up tilt position to the 5° forward wheel down Stow position.

[00:08:34]The third concurrent operation will be the opening of the main gear door. Let's look at the door system components.

[00:08:40]Starting with the door actuator. The rod end of the actuator is attached to the door structure and the head end is pivoted to the keelbeam attachment.

[00:08:49]The hinges provide support for opening and closing the door. A mechanical linkage connects the door to the door operated sequence valve. The door also has a roller for lock operation.

[00:09:01]The roller is held by the door-up lock assembly installed in the wheel well.

[00:09:06]To release the door block mechanism, pressure is applied from the gear uplock operated sequence valve to the retract side of the door lock actuator. The main components of the door lock assembly are the lock springs and the uplock hook.

[00:09:20]The uplock hook uses overenter locking to hold the door closed. The lock springs ensure the hook maintains the locked position. When the actuator retracts, it operates the mechanism against the spring force and releases the overenter lock. This disengages the uplock hook from the roller and the door is free to open. Once the mechanism crosses the over center position, the spring force snaps it in the opposite direction, keeping the mechanism ready for the next door closing procedure.

[00:09:50]The door actuator receives the extension pressure from the drag brace sequence valve and the gear uplock sequence valve provides the retract connection.

[00:09:59]The applied pressure will operate the actuator to open the door. The door operation will also activate the mechanical linkage and change the door sequence valve output.

[00:10:09]Currently the valve is in the door not open position and the output lines are not pressurized. As the door opens and is about to reach its final position, it will operate the sequence valve. The change in the door sequence valve position will pressurize the landing gear retract line. The pressure for retracting the landing gear will also apply the landing gear brakes. The right hydraulic system operates the normal brakes and the center system operates the alternate brakes. Therefore, the brakes will be applied using the alternate lines. When the brake pedals are in the neutral position, the brake metering valves remain off. The gear retract pressure will bypass the brake pedals and apply the brakes directly through the alternate lines. Pressure will travel through the alternate anti-skid valve to the anti-skid shuttle valve and then reach the six main wheel brake assemblies. The application of the brakes ensures the wheels stop spinning before they enter the wheel well.

[00:11:07]The main landing gear retract actuator receives the retract pressure from the door sequence valve. But the actuator's extension side is directly connected to the return line of the center hydraulic system which serves as the fluid return path during retraction.

[00:11:22]Before we look at the actuator in action, let's understand the main landing gear support structure. The landing gear uses the four-point support configuration.

[00:11:31]The drag brace assembly is attached to the wing spar and the side brace assembly is attached to the fuselage structure. The other two attachment points are on the trunion.

[00:11:41]The forward end of the trunion is attached to the wing spar and the aft end is attached to the gear beam.

[00:11:47]The trunion is mounted with the help of two spherical bearings. The bearings allow the gear to swing during retraction and extension.

[00:11:56]The side brace assembly pivots and folds with the help of three bearings. Two on the shock strut and one on the fuselage attachment. Spindles connect the upper lock link, the lower side brace and the upper side brace to the bearings. During retraction and extension, the spindles will rotate and allow the folding and unfolding of the side brace.

[00:12:17]The drag brace attachment is similar, but the assembly has an additional linkage that connects to the drag brace sequence valve.

[00:12:25]When the upper drag brace spindle rotates, it will operate the sequence valve.

[00:12:31]The head end of the gear retract actuator is connected to the retract fitting which acts as a pivot for actuator rotation during operation. The rod end of the actuator is connected to the attachment on the shock strut. The applied pressure to the actuator will retract the landing gear.

[00:12:49]Currently, the drag brace sequence valve is pressurizing the door actuator and the SLG strut. In the initial rotation of the landing gear as the drag brace folds, it will operate the connected sequence valve. The sequence valve moves from the gear down to the gear knot down position. The SLG struts stow and tilt connections are closed by the sequence valve spool and the door actuator's line is connected to the return.

[00:13:14]As a result, the extension pressure is released from the door actuator.

[00:13:19]The SLG struts stow pressure gets trapped and the actuator is locked in the extended position.

[00:13:26]The retract actuator will continue to retract the landing gear. As the gear enters the wheel well and the retract actuator is about to reach its full retraction, the gear uplock assembly will be operated. The gear up lock mechanism is similar to the door uplock mechanism and has a gear uplock actuator. The retract line of this actuator is connected to the door sequence valve.

[00:13:49]The uplock mechanism along with the actuator also has lock springs and an uplock hook. In addition, the mechanism has a linkage that is connected to the gear uplock sequence valve.

[00:14:02]The extension line of the uplock actuator is connected to the gear uplock sequence valve. As the retract actuator continues to pull the gear up, the landing gear roller on the shock strut will hit the uplock hook. This will cause the mechanism to operate against the spring force and cross the overcenter position. At the same time, the gear uplock sequence valve will be operated from the gear unlocked to the gear locked position. This will switch the output lines of the sequence valve.

[00:14:30]The uplock actuator will now receive the extension pressure. As the uplock mechanism has crossed the overenter position, the lock spring force and the uplock actuator will operate to engage the hook with the roller. This locks the main landing gear in the retracted position.

[00:14:46]The change in the gear uplock sequence valves output will now apply pressure to the retract side of the door actuator.

[00:14:53]The retract pressure will close the door. As the door starts to close, it will operate the door sequence valve.

[00:15:00]The door not open position of the sequence valve will close the pressure port and release the gear retract pressure.

[00:15:08]This will release the applied pressure from the retract actuator.

[00:15:12]Simultaneously the brake pressure gets released.

[00:15:16]The door actuator will continue to close the door and when it is about to reach its closed position, the door roller will operate the door uplock mechanism.

[00:15:25]When the gear up lock sequence valve output switched, it also reversed the pressure application to the door lock actuator.

[00:15:32]But since the door lock mechanism is in the unlocked position, the actuator cannot extend. Therefore, it has to wait for the door roller to operate the hook and move the mechanism to the overenter position. After this, the actuator and the lock spring can ensure the door gets locked. Once the landing gear and door-up locks engage, the overenter locking held by the lock springs is sufficient to secure the gear in its door. Therefore, the hydraulic pressure applied to the actuators can be released. This will reduce the load on the center hydraulic system and help meet the demands of the other user systems. The hydraulic power is automatically removed by the avionic system of the aircraft and for that the computers need to know the landing gear and its door position. The proximity sensors provide this information. The door closed proximity sensor is installed in the wheel well and a steel target is installed on the door. The sensor is an inductance type proximity sensor that determines whether the target is near or far. Alternating current in the sensor coil generates a magnetic field. The detection circuit inside the sensor monitors the change in inductance to determine the target position. When the target is far, the inductance of the coil is unaffected.

[00:16:45]This provides a door notclosed signal.

[00:16:47]As the target approaches the sensor head, an eddi current is induced on the steel target due to electromagnetic induction. The eddiurren generated opposes the magnetic field of the sensor and reduces its inductance. This change in inductance is picked up by the sensor to determine that the target is near and the door is closed. For determining the landing gear uplock position, a proximity sensor is installed on the gear uplock assembly. The sensor is fixed and the target moves with the uplock mechanism.

[00:17:18]Therefore, the sensor will only provide a target near indication when the uplock mechanism latches onto the landing gear roller.

[00:17:26]Signal from the sensors is monitored by the proximity sensor electronics unit in the avionics compartment. The proximity computer informs the hydraulic interface module that the retraction process is complete.

[00:17:40]Now the hydraulic computer which controls the autooff relay will send a signal to stop the gear up power.

[00:17:48]As the power is removed from the upsenoid valve, the spool returns to open the pressure port. The control pressure in the spring force will overcome the control pressure from the down solenoid valve to move the selector valve spool left. The spool moves until it reaches its center position where the force on each side is equal. This closes the pressure port and releases the pressure from the uplines. The off position of the selector valve will release the pressure from the actuators.

[00:18:16]The door actuators retract pressure is removed.

[00:18:20]The gear up lock and door lock actuators extend side pressure gets released. The drag brace down lock and side brace down lock actuators retract pressure is removed. However, the SLG actuator's stow pressure will remain locked. This will ensure the truck beam holds its stow position and is not free to move inside the wheel well.

[00:18:41]That completes the landing gear retraction process.

[00:18:45]Now let's understand how the landing gear extends.

[00:18:49]To extend the landing gear, the lever is moved to the down position. This applies gear down power to the down solenoid valve and releases the control pressure.

[00:18:58]The control pressure from the upsenoid operated valve moves the selector valve spool to the left. The valve operation will apply pressure to the down lines of the main landing gear.

[00:19:09]The sequence valves now receive pressure on their downline connected ports to start the extension sequence.

[00:19:15]First, the door will be operated. The drag brace operated sequence valve is in the gear not down position and the gear uplock operated sequence valve is in the gear locked position. Due to the pressure in the down lines, the gear up lock sequence valve will apply the retract pressure to the door lock actuator. The drag brace sequence valve will apply the extension pressure to the door actuator and the gear uplock sequence valve will provide the fluid return path. The extension pressure to the door actuator will open the door once the door lock actuator releases the uplock mechanism. The door proximity sensor will provide the door status information.

[00:19:52]As the door opens towards the end of its movement, it will operate the door operated sequence valve.

[00:19:59]Next, the door sequence valve will extend the landing gear. Let's look at the valve operation again in detail. The valve has two output lines. One is connected to the retract side of the gear up lock actuator. The other line is connected to the retract side of the gear retract actuator and the brake lines for gear retract braking. The check valves prevent the output lines from being pressurized when the door is not open. When the door opens and activates the valve, it pressurizes the retract line of the uplock actuator. It also opens the retract line of the retract actuator, allowing the fluid to return during extension.

[00:20:37]The pressure applied to the gear up lock actuator will release the gear uplock mechanism.

[00:20:42]The gear retract actuator, as the name implies, is not pressurized during extension, and the landing gear extends by its own weight. Therefore, the extension line of the retract actuator is directly connected to the center system return.

[00:20:57]In order to control the extension rate of the landing gear, the retract line is installed with a flow restrictor. When the actuator extends, the restrictor limits the return fluid flow rate. This ensures the landing gear extends in a controlled manner and does not freeall.

[00:21:13]When the gear up mechanism gets released, it operates the gear up lock sequence valve. The gear up lock proximity sensor will indicate the gear release condition.

[00:21:24]As the gear up lock sequence valve output is switched, it will reverse the door lock actuator supply line, keeping it ready for the door close operation.

[00:21:32]The sequence valve will also apply pressure to the extension side of the gear uplock actuator, but will not affect the gear uplock mechanism and it will maintain its unlocked position.

[00:21:43]Likewise, the gear up lock sequence valve will apply pressure to the retract side of the door actuator, but the actuator will hold its extended position as the extension pressure is still available from the drag brace sequence valve. Since the extension pressure is applied to the head end, which has a larger surface area compared to the rod end, the extension force is greater.

[00:22:05]The landing gear will continue to extend due to its own weight and its downward rotation will continue to unfold the drag brace and side brace assemblies.

[00:22:13]As the gear drops out of the wheel well and just before it reaches its full extension, the upper drag brace spindle will operate the drag brace sequence valve.

[00:22:23]So let's look at the drag brace sequence valve operation again in detail. When the valve was in the gear knot down position, it was pressurizing the extension line of the door actuator. The stow and tilt connections of the SLG strut were blocked. The tilt line port is also connected to the extension side of the drag brace and side brace down lock actuators.

[00:22:44]As the gear extension operates the sequence valve, it pressurizes the tilt line and the downlock actuator extension lines. Simultaneously, it releases the pressure from the stow line and the door actuator extension line.

[00:23:00]The change in sequence valve output lines will operate the SLG actuator. As the stow pressure is released, it releases the direct pressure applied to the top side of the floating piston.

[00:23:11]The reduced pressure is still applied, but now from the tilt line, a pressure reducing valve drops the tilt line pressure to 1,000 lb per square in.

[00:23:21]Without the stow pressure, the reduced pressure acting on the larger surface area of the floating piston's bottom end will overcome the pressure applied on the head end of the main piston. As a result, the floating piston will retract and simultaneously retract the main piston. The retraction of the SLG actuator will rotate the truck beam from the 5° forward wheels down Stow position to the 11° forward wheels up tilt position.

[00:23:46]At the same time, the downlock actuators will operate to lock the landing gear in the extended position. The actuator's retract line is directly connected to the landing gear up lines and the extension line is pressurized by the drag brace sequence valve. Therefore, only after the drag brace sequence valve is operated to the gear down position will the downlock actuators receive the extension pressure. The proximity computer relies on the downlock sensors to determine that the landing gear is down and locked. There are two downlock sensors, one for each brace assembly.

[00:24:19]The sensor is mounted on the upper lock link and the target is fitted to the toggle. When the lock link is not straight and locked, the sensor will read the target as far during gear extension. The unfolding of the brace assembly causes the link attachment to cross the overenter position. This allows the downlock actuator and the brace spring to operate and lock the drag brace assembly. As the target is now close to the sensor, it will indicate the locked condition.

[00:24:47]Similarly, the side brace lock links will operate and lock the side brace assembly. When both brace assemblies are locked, the landing gear is down and locked. The sensors will send the signal to the proximity computer. Along with the truck positioning function and the down lock operation, the door will start to close. The door lock actuator is ready for the door close operation. As the drag brace sequence valve releases the extension pressure, the door actuator will retract and close the door.

[00:25:17]When the door closes, the door sequence valve currently supplying power to the gear uplock actuator will operate to lock the retract pressure.

[00:25:25]However, the action is inconsequential as it does not change the gear up lock assembly position. The hydraulic power is not removed from the down lines after the extension cycle and the actuators will remain pressurized when the gear is down and locked.

[00:25:41]The proximity computer will use the downlock sensor signal for gear down indication, the door sensor signal for door closed indication, and the uplock sensor signal to inform the hydraulic computer to reset the auto off relay.

[00:25:54]The hydraulic computer removes the control signal and resets the relay for the next retraction cycle.

[00:26:00]That completes the extension sequence.

[00:26:04]As the support structure for the main landing gear is in the wing route section, the retraction and extension of the gear require additional doors to cover this area. Therefore, along with the main door, the landing gear system has three link doors. A drag brace door is hinged to the wing structure and mechanically connected to the drag brace. The door creates a slot for the drag brace assembly movement and will operate when the drag brace folds and unfolds.

[00:26:30]A shock strut door is attached to the shock strut on its outboard side. The door is not hinged and is directly connected to the strut. Therefore, as the shock strut moves during retraction and extension, the door will move along with it. Next to the shock strut door is the trunion door, which is hinged to the wing structure and mechanically connected to the trunion. This door operates due to the rotation of the trunion. Since all three link doors are mechanically connected and operate with the landing gear movement, they do not require hydraulic actuators.

[00:27:03]When a retraction command is given, the hydraulic actuators open the main door and retract the landing gear. Gear movement operates the link doors to close and aerodynamically seal the wing route section.

[00:27:16]When the landing gear extension command is given, the main door opens and the landing gear starts to extend. The link doors will open to create a slot for the gear extension.

[00:27:27]Finally, let's see the entire main landing gear retraction and extension sequence.

[00:27:32]Gear up, gear down.

[00:27:51]In the next chapter of the landing gear series, we will explore the nose gear retraction and extension system. Thanks for watching.