Pratique du Shadowing: How Landing Gear Works | Part 6 : Main Gear Steering - Apprendre l'anglais à l'oral avec la vidéo
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157 phrases
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Executing a sharp turn with a widebody aircraft on the ground is challenging due to its large size
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and the complex nature of its main landing gear.
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Therefore, the nose landing gear steering system on the Boeing 777 is assisted by the main landing gear steering system.
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From the previous chapter, we know the nose gear steering is mechanically controlled using the tiller or the rudder pedals.
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The steering input controls the cable loops.
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When the tiller is rotated counterclockwise, the cable loop tension activates the steering actuators, and the nose wheel turns left.
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A full tiller command results in a 70 -degree left turn.
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Releasing the tiller force returns the nose gear to the neutral position.
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A full clockwise tiller input rotates the nose wheel 70 degrees to the right.
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The tension generated in the steering cable by the left rudder
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pedal input can only turn the nose wheel 7 degrees to the left.
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The full right pedal command turns the nose wheel 7 degrees to the right.
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The main gear steering system, on the other hand, is electronically controlled and requires nose gear position data to operate.
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The tiller command operates its connected cable drum and moves the steering cable.
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The cable loop movement causes the other drum to rotate and operate its connected tiller.
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The pedal input operates the rudder interconnect mechanism and moves the steering cable.
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The steering cable movement will rotate both the cable drums and operate the tillers.
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In either case, the cable drums rotate, and by measuring the degree of rotation, the nose gear position can be determined.
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Therefore, a shaft from the cable drums is connected to the tiller position transducers.
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Each transducer is a rotary variable differential transformer.
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The RVDT has an iron core connected to the shaft, a primary winding, and two secondary windings.
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Alternating current is applied to the primary winding to generate a magnetic field.
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This induces a voltage in the secondary windings due to mutual induction.
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The secondary windings are connected 180 degrees out of phase.
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Therefore, when the core is in the neutral position, equal flux cuts through both windings, and they cancel each other out.
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When the tiller is operated, the core rotates and changes the flux flow.
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This increases the electromotive force in the number 2 secondary coil while the force in the number one coil decreases.
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As a result, a differential voltage is generated.
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For a given shaft position, a corresponding voltage will be generated and by knowing the voltage value, the exact nose wheel steering angle can be determined.
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The signal from the position transducers is sent to the main gear steering control unit.
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The steering computer calculates the nose gear position to control the main gear steering actuators.
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The steering actuators are electrically controlled and hydraulically operated.
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The aircraft has three independent hydraulic systems, and just like the nose gear steering, the center hydraulic system powers the main gear steering.
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Let's see how the actuator functions.
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The external components of the actuator are the arming solenoid valve, unlock solenoid valve, servo valve, and centering valve.
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The main gear steering computer controls the servo and the solenoid valves.
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To control the actuator, the computer requires position feedback, which is provided by the linear variable differential transformer inside the actuator.
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The LVDT has an iron core connected to the actuator rod, two secondary windings, and a primary winding.
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Alternating current is applied to the primary winding to generate a magnetic field.
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This induces a voltage in the secondary windings due to mutual induction.
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The secondary windings are connected 180 degrees out of phase.
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Therefore, when the rod is in the center position, equal flux cuts through both windings and they cancel each other out.
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If the actuator rod retracts, the core moves towards the number two secondary winding and increases the induced voltage.
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This also reduces the voltage on the number one secondary winding.
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The opposite happens when the actuator rod extends.
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By measuring the differential voltage, the computer calculates the exact actuator rod position.
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The actuator rod must remain in the center position when there is no steering input.
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Therefore, the actuator has a locking mechanism that holds the rod in the center position.
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Along with the locking mechanism, the actuator also uses the hydraulic pressure to keep the rod centered.
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The hydraulic pressure from the centering valve is applied to both sides of the actuator rod.
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This acts as a hydraulic lock and prevents inadvertent movement.
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Now, let's see what happens when a steering command is given.
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The main gear steering computer monitors the tiller position.
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When the tiller command crosses a threshold, it activates main gear steering.
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It simultaneously operates the arming and unlock solenoid valves.
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The applied current pulls the solenoid and moves the valve spool against the spring force.
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Pressure from the arming solenoid reaches the control line of the centering valve and, through the unlock solenoid, reaches the actuator lock.
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The applied hydraulic pressure operates the plunger, releases the lock and slides the mechanism against the lock spring.
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As the mechanical lock gets released, the control pressure to the centering valve moves the spool.
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This disconnects the pressure line to the actuator and connects it to the servo valve's return line.
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As a result, the hydraulic pressure gets released and the actuator rod is free to move.
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At the same time, the servo valve gets connected to the hydraulic pressure.
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The steering computer will now control the servo valve to operate the actuator.
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For the left steering command, it will apply hydraulic pressure to the head end of the actuator.
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The pressure will extend the actuator, and once it reaches the desired position, the computer will use the LVDT feedback to operate the servo valve and release the pressure.
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When the tiller force is released, it returns to neutral.
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The computer will close the solenoid valves, releasing the unlock pressure and the centering valve control pressure.
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The spring force of the locking mechanism will move the lock into position.
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The centering valve spring force will move the spool to connect the pressure line to both sides of the actuator, since the actuator has an additional pressure release line in the center position.
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The pressure applied to the head end will be released through this line.
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Therefore, the rod end pressure will be greater than the head end pressure.
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The differential pressure will retract the actuator until it reaches the center position and closes the release line.
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Once closed, the pressure equalizes on both sides, and the actuator is locked hydraulically.
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Simultaneously, the mechanical lock engages when the rod is in its locking position.
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The right steering command works similarly.
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Once the tiller crosses the right steering threshold, the computer operates the solenoid valves to release the mechanical lock and the hydraulic lock.
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The computer will now control the servo valve and apply pressure to the rod end of the actuator.
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The actuator retracts, and when it reaches the desired position, the applied pressure is released.
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Releasing the tiller force will close the solenoid valves.
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Move the mechanical lock into position, operate the centering valve, and apply pressure to both sides of the actuator.
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The release line will reduce the rod end pressure and move the actuator rod to its center position.
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Now the hydraulic and the mechanical lock will be engaged.
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The advantage of using a servo valve is that when the tiller crosses the threshold and the actuator is unlocked, the computer can move the actuator in proportion to the tiller command.
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The computer will control the servo valve with precision to apply the required pressure and set the actuator to the desired position.
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The rod end of the actuator is connected to the right side of the aft axle
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and the head end to the steering crank.
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The steering crank is attached to the truck beam and is free to pivot.
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The aft axle is pivot -mounted at the end of the beam.
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A reaction link connects the steering crank to the left side of the aft axle.
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When the actuator extends, it pushes the axle, causing it to rotate.
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The reaction link will transfer the steering force back to the steering crank.
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The steering crank moves to assist the aft axle rotation and transmit the loads to the truck beam.
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This arrangement reduces the weight and design complexity when compared to the dual steering actuator configuration.
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When the actuator retracts, the aft axle rotates in the opposite direction.
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The reaction link in the steering crank will assist the rotation and help transmit the load to the truck beam.
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The main gear wheel and tire assembly with multiple disc carbon brakes are mounted on the aft axle.
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Therefore, when the actuator extends, the wheels turn right, and when the actuator retracts, they turn left.
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Each main landing gear on the Boeing 777 has a six -wheel truck arrangement, out of which only the aft two wheels are steerable.
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Now let's look at the main gear steering system in action.
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Since the main gear steering is electronically controlled, any failure that leads to inadvertent movement of the aft axle can be dangerous, especially during runway operations.
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Therefore, when there is no steering command, the steering actuators are locked in the center position.
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The tiller is used for steering on the taxiway and the rudder pedals are used on the runway.
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When the tiller command turns the nose gear left, the actuators will remain locked below the threshold value.
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Once the tiller crosses the threshold, which is approximately 10 degrees of the nose wheel turn, the computer unlocks the main gear steering actuators.
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With a further increase in tiller input, the computer will steer the main gears in proportion to the nose gear movement.
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When a full tiller command turns the nose wheel 70 degrees to the left, the main gears turn 7 degrees to the right.
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Releasing the tiller force returns the main gear to the neutral position along with the nose gear.
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A right turn is similar.
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The actuators remain locked below the threshold and are activated when the tiller crosses it.
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The main gears will steer in proportion to the nose gear.
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While steering, if the tiller input is reduced and goes below the threshold, the main gear will be steered back to neutral and the actuators will be locked.
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When a full tiller command turns the nose wheel 70 degrees to the right, the main gears turn 7 degrees to the left.
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When rudder pedals are used for steering, the maximum the nose gear can steer is 7 degrees in either direction, which is below the threshold.
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Therefore, the main gears will not steer and the actuators will remain locked.
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The primary advantage of main gear steering is that it reduces tire scrubbing during sharp turns.
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Consider the transverse axis of the nose gear and the main gear axles.
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When the nose wheel is steered, its transverse axis will intersect the main gear transverse axis at three different points.
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The average point of the intersection will be the instantaneous center of rotation around which the aircraft turns.
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The lines from this point will intersect the forward and the aft axle wheels at an angle.
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The angle becomes worse as the turn becomes sharper.
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Ideally, the wheels should turn on their individual transverse axes, but the six -wheel truck configuration causes the forward and aft axle wheels to drag as they turn.
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This leads to excessive tire scrubbing.
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To counter this, the aft axle wheel is steered in the opposite direction to the nose wheel.
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The shift in the aft axle point of intersection moves the turning center forward and inward.
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This also shifts the average transverse axis of the main gear between the forward and mid axles.
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This reduces the angle of the forward axle and thereby reduces tire scrub.
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Since the aft wheel steers in the opposite direction, its angle is reduced to minimize tire scrub.
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The main gear steering has to be proportional to the nose wheel steering so
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that it intersects the nose wheel transverse axis at a point
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where tire scrubbing is the lowest for all six wheels of the landing gear.
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Tire scrubbing due to dragging will be negligible for lesser angles of nose wheel turn.
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Another advantage of steering the main gear is that by shifting the center of rotation inwards, it reduces the turn radius of the aircraft.
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This helps the wide -body aircraft to maneuver in congested airports.
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The main landing gear, along with the steering system, has another trick up its sleeve that can help steer and further reduce the turning radius of the aircraft.
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And for that, we need to use the differential brakes.
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To allow differential braking, the captain's and the first officer's left pedals are connected to operate the left brake pedal quadrant.
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Likewise, the right pedals are connected and operate the right brake pedal quadrant.
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Let's look at the mechanism in detail.
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When When one of the left pedals is pressed, it operates the left quadrant and, through the transverse rod, moves the other left pedal mechanism,
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but slips over without operating the right quadrant.
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The right pedal force will operate the right quadrant, its connected transverse rod, and the other right pedal mechanism without moving the left quadrant.
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As a result, the left pedals will independently operate the left quadrant and move the left brake cables.
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The right pedals will independently operate the right brake quadrant and move the right brake cables.
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The brake cables are connected to their respective metering valves.
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The right hydraulic system pressurizes the normal brake system.
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The left pedal command operates the left metering valve and applies the left main landing gear brakes.
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The right pedal command operates the right metering valve and applies the right main landing gear brakes.
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Pressing both pedals down will apply all main landing gear brakes.
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The metering valve also controls the amount of brake pressure and directly depends on the applied brake pedal force.
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Less force will result in reduced brake pressure application.
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The hydraulic pressure will operate the main landing gear brakes.
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The brake application on only one side will result in differential braking.
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The applied brakes will convert the kinetic energy of the wheels into heat energy and the wheels will slow down.
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The difference in speed between the two main landing gear wheels will turn the aircraft left.
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The differential braking can be used to steer the aircraft when the primary steering system malfunctions, or to reduce the turning radius of the aircraft.
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While steering the aircraft, the brakes are applied on the inboard landing gear of the turn.
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The use of differential brakes increases the wear on the tires, but is useful when the aircraft has to navigate a tight corner.
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Vocabulaire et conseils d’expression pour cette leçon
Cette vidéo contient 157 phrases et 2452 mots à répéter en shadowing. La partie parlée dure 15:38. Le locuteur parle à un rythme naturel d’environ 157 mots par minute, proche d’une conversation courante. Seuls 69 % des mots font partie des 3 000 mots les plus courants en anglais, le vocabulaire est donc exigeant.
Vocabulaire clé de cette vidéo
15 mots moins courants de la vidéo, avec leur prononciation et leur sens :
- steer /stɪɹ/ (verbe) — conduire, guider. To guide the course of a vessel, vehicle, aircraft etc. (by means of a device such as a rudder, paddle, or steering wheel).
- gear /ɡɪɚ/ (nom) — panoplie, matériel. Equipment or paraphernalia, especially that used for an athletic endeavor.
- valve /ˈvælv/ (nom) — clapet, soupape. A device that controls the flow of a gas or fluid through a space, such as a pipe, manifold, or plenum.
- pedal /ˈpɛdəl/ (nom) — pédale. A lever operated by one's foot that is used to control or power a machine or mechanism, such as a bicycle or piano.
- connect /kəˈnɛkt/ (verbe) — accoupler, connecter. To join (to another object): to attach, or to be intended to attach or capable of attaching, to another object.
- hydraulic /haɪˈdɹɔːlɪk/ (adjectif) — hydraulique. Pertaining to water.
- brake /bɹeɪk/ (nom) — frein. A device used to slow or stop the motion of a wheel, or of a vehicle, usually by friction (although other resistive forces, such as electromagnetic fields or…
- mechanism /ˈmɛk.ə.nɪ.zəm/ (nom) — mécanisme. Any mechanical means for the conversion or control of motion, or the transmission or control of power.
- threshold /ˈθɹɛʃ(h)oʊld/ (nom) — seuil. The lowermost part of a doorway that one crosses to enter; a sill.
- tire /ˈtaɪ̯ɚ/ (verbe) — se fatiguer. To become sleepy or weary.
- voltage /ˈvəʊltɪd͡ʒ/ (nom) — tension, voltage. The difference in electrostatic potential between two points in space, especially between live and neutral conductors or the earth.
- rotate /ˈɹoʊ.teɪt/ (verbe) — tourner, rotationner. To spin, turn, or revolve.
- angle /ˈæŋ.ɡəl/ (nom) — angle. A figure formed by two rays which start from a common point (a plane angle) or by three planes that intersect (a solid angle).
- unlock /ʌnˈlɑk/ (verbe) — déverrouiller. To undo or open a lock or something locked by, for example, turning a key, or selecting a combination.
- scrub /skɹʌb/ (verbe) — récurer, frotter. To rub hard; to wash with rubbing; usually, to rub with a wet brush, or with something coarse or rough, for the purpose of cleaning or brightening
Les verbes à particule que vous entendrez
- look at (verbe) — regarder. To observe or watch.
- slow down (verbe) — ralentir, décélérer. To decelerate.
- turn on (verbe) — ouvrir. To set a flow of fluid or gas running by rotating a tap or valve.
Prononciation à surveiller
- Les sons « th »: threshold /ˈθɹɛʃ(h)oʊld/, thereby /ðɛɹˈbaɪ/
- Les sons « sh » et « zh »: differential [dɪfəˈɹənʃəɫ], threshold /ˈθɹɛʃ(h)oʊld/, rotation /ɹoʊˈteɪʃən/, sharp /ʃɑɹp/, proportion /pɹəˈpɔɹʃən/
- Mots longs — placez bien l’accent: differential [dɪfəˈɹənʃəɫ], mechanism /ˈmɛk.ə.nɪ.zəm/, mechanical /məˈkænəkəl/, simultaneously /ˌsaɪmlˈteɪnɪəsli/, independently /ˌɪndɪˈpɛndəntli/
Comment s’entraîner avec cette vidéo
- Écoutez la vidéo en entier une fois sans parler et notez les mots que vous ne connaissez pas.
- Commencez à la vitesse 0,75×, répétez phrase par phrase, puis revenez à la vitesse normale quand cela devient facile.
- Enregistrez-vous et comparez avec l’original, en faisant attention à des mots comme steer, gear, valve.
La grammaire de cette vidéo
Les structures que le locuteur utilise le plus, avec les mots exacts de la vidéo :
| Structure | Dans la vidéo |
|---|---|
| Voix passive be + participe passé — l’accent est mis sur ce qui arrive, pas sur qui le fait | is assisted · is rotated · is connected |
| Propositions relatives who / which + proposition — une précision sur une personne ou une chose | feedback, which is · threshold, which is · direction, which is |
Qu'est-ce que la technique du Shadowing ?
Le Shadowing est une technique d'apprentissage des langues fondée sur la science, développée à l'origine pour la formation des interprètes professionnels. Le principe est simple mais puissant : vous écoutez de l'anglais natif et le répétez immédiatement à voix haute — comme une ombre suivant le locuteur avec un décalage de 1 à 2 secondes. Les recherches montrent une amélioration significative de la précision de la prononciation, de l'intonation, du rythme, des liaisons, de la compréhension orale et de la fluidité.