Overview
The nose landing gear consists of a nose gear assembly with a built-in shock strut, a folding drag brace incorporating a mechanical downlock, a dual acting hydraulically operated retraction actuator, an uplock assembly and nose steering components.
The nose gear retracts forward into the nose bay area and is held in the retracted position by an uplock mechanism. During normal operation, a downlock actuator within the drag brace ensures downlock when the gear is extended. The nose gear is actuated by hydraulic system No. 3 to operate the uplock, downlock and retraction actuator. Hydraulic system No. 3 pressure to the nose gear is retained while in the extended position.
The nose gear sequencing and position sensing system is controlled and continuously monitored by the LGECU. Any fault or position deviation detected is annunciated on EICAS.
A downlock safety pin is installed within the drag brace assembly to secure the nose landing gear in position while on the ground.
05/19/16
Nose Landing Gear Strut Assembly
The nose gear strut assembly employs a separated gas/oil configuration to provide shock absorbing capability during strut compression, and controlled recoil during extension. The strut assembly consists of an outer cylinder, a telescoping inner cylinder, and a floating piston. Fluid fill ports are located on the outer cylinder and a nitrogen charging point is located on the inner cylinder.
Outer Cylinder
Trunnion pins on either side at the top of the shock strut assembly provide mounting to the airframe at the rear of the nosewheel well. Lubrication points are provided at these points. The trunnion pins are secured with a cross bolt and nut. The outer cylinder includes mounting points for the retraction actuator, nosewheel steering components, landing lights, folding drag brace and aft door control rods.
Inside the base of the outer cylinder is a bearing containing dynamic and static seals and spare seals. Above this bearing is the lower cam, used to center the nosewheel when the strut extends after takeoff. The bearing and lower cam are held in position by dowel pins.
Main Fitting
The main fitting is the upper part of the NLG shock strut. It is attached to the fuselage structure with two trunnion pins which go in fittings installed on the structure of the NLG wheel well.
Trunnion Pins
The trunnions pins are installed in an extension found at the top of the shock strut and they are the pivot point for the NLG retraction. These pins can be removed and are held in the strut by bolts.
Retraction Actuator Fitting
The retraction actuator fitting is found on the left side of the strut and is the attachment point of the NLG retraction actuator.
Drag Brace and Downlock Fitting
The drag brace and downlock fitting is installed at the lower end of the main fitting. It is the point where the drag brace and downlock attaches on the shock strut.
Inner Cylinder
The inner cylinder telescopes into the outer cylinder. Inside, at the top of the inner cylinder, a spring-loaded taxi valve provides primary damping on landing and secondary damping during taxi. The taxi valve allows a high fluid flow as the strut compresses and restricts the flow as the strut extends to prevent proposing. On landing, the sudden compression of the shock strut displaces the spring loaded restrictor in the taxi-recoil valve allowing a large volume of fluid to flow. The impact is absorbed mainly by the nitrogen charge.
Mounted on the upper outer part of the inner cylinder is the upper cam, held in position by dowel pins. This cam, when meshed with the lower cam, ensures the nosewheel will be properly aligned when weight-off-wheels, allowing the nose gear to retract smoothly into the wheel well. The area at the lower part of the outer cylinder, between the inner and outer cylinders is the rebound chamber. It is filled with hydraulic fluid and the size of this chamber varies as the strut compresses and extends. There are two flanges on the outside of the inner cylinder above the upper cam. The two cams are segmented with the lower flange having much larger gaps between the segments.
A rebound ring floats between these two flanges with its outer edge riding along the inner surface of the outer cylinder. This rebound ring controls the flow rate of hydraulic fluid into and out of the rebound chamber. Because of the difference in the gaps along the flanges, fluid flow around the rebound ring is more restricted on extension than on compression. When taxiing, the restrictor remains in place and small amounts of fluid can flow through the opening in the center of the valve. Bumps are mainly absorbed through the action of the rebound ring.
An externally mounted axle is secured to the inner cylinder with dowel pins. The uplock pin is mounted on the front face of the shock strut assembly between the steering actuators. The lower torque link mounts on the rear of the axle.
Piston
The piston is the lower part of the NLG shock strut and is installed in the main fitting. The upper part of the piston is pressurized with hydraulic fluid. A valve with a spring installed in the upper chamber of the piston controls the fluid flow. The movement of the fluid between the upper and lower chambers is a primary damper during landing and ground operation of the aircraft. The lower end of the piston is attached to the wheel axle assembly.
Nose Landing Gear (NLG) Wheel Axle
The NLG wheel axle assembly has lugs to attach the lower torque link assembly. It also has a pad to lift the shock strut to change the wheels.
Floating Piston
The nose strut has two chambers separated by a floating piston. The upper chamber contains MIL-H-5606 hydraulic fluid while the lower chamber contains the nitrogen charge. The chamber containing the nitrogen compresses with the strut assisting in shock absorption.
Torque Links
An upper and lower torque links or scissors connect the nosewheel axle to the steering cuff allowing steering actuator movement to be transmitted to the axle. A dual, pull quick disconnect mechanism, where the two links meet, must be disconnected to allow the nosewheel to turn when the aircraft is being towed. The torque links are spring-loaded apart.
05/19/16
Drag Brace and Downlock
A folding drag brace, with its upper end mounted to the airframe in the nosewheel well and the lower end mounted part way down the front of the outer cylinder, locks the nose gear in the down position. The upper brace assembly is attached to the aircraft with two trunnion pins. The lower brace assembly is attached to the main fitting with a pin. It has an unlock actuator assembly, two downlock springs, and two sensor brackets.
A downlock actuator, allows the drag brace to fold as the nose gear retracts, hydraulically releasing the lock. During emergency extension, two tension springs mechanically pull the drag brace into the lock position. Two proximity sensors provide indication of a down-and-locked condition.
When hydraulic pressure is applied to the retraction actuator, the drag brace assembly extends. This causes the lock link assembly to stay in the center to lock the drag brace in this position. The drag brace can also be locked mechanically in the down position with a ground lock pin which is installed through the lock assembly.
When hydraulic pressure is applied to the unlock port of the retraction actuator, the actuator pulls the forward lock link. This causes the lock link to release from the center to let the drag brace folds when the NLG retracts.
05/19/16
Steering Mechanism
The nosewheel steering mechanism includes two steering actuators and swivel valves which is covered in the nosewheel steering section. These actuators attach to the same collar attached on the body of the shock strut. They give the steering function to the NLG.
Steering Manifold
Retraction Actuator
The retraction actuator is a double-acting actuator used to raise and lower the nose gear. One end is mounted to the roof of the nosewheel well and the other end is connected to the fitting at the top of the gear assembly.
There are two hydraulic ports, one to extend and the other to retract the actuator. The retract port is fitted with a restrictor to control the fluid flow rate in either direction. A grease nipple provides a lubrication point on the bearing at each end.
Uplock
The uplock assembly secures the nose gear in the retracted position. The lock closes mechanically and is opened by a hydraulic actuator for normal extension. The actuator extends hydraulically to release the uplock and allow nose gear extension. On a retract command, the actuator plunger is hydraulically retracted to prevent interference with the lock mechanism. An uplock proximity sensor is installed on the uplock assembly for indication and control purposes. During an alternate gear extension the uplock is mechanically released by a cable input from the manual release handle.
Nose Gear Harness
The NLG has two harnesses. One harness is installed on the left side of the NLG, and the other on the right side of the NLG.
The harness installed on the left side of the shock strut connects, at the top, to the aircraft structure. It is held on the shock strut with clips and brackets. This harness supplies electrical power to:
- The nose-wheel-centered sensors
- The left landing light
- The left feedback position linear-variable differential-transformer (LVDT)
- The left nose weight-off-wheels sensor
- The left nose downlock sensor
The harness installed on the right side of the shock strut also connects, at the top, to the aircraft structure. It is held on the shock strut with clips, junction boxes, and a bracket. This harness supplies electrical power to:
- The right landing light
- The right nose weight-off-wheels sensor
- The right nose downlock sensor
- The right feedback position LVDT
- The steering EHSV
System Operation
Nose Gear Extension
On a gear down selection, once the nose gear doors are open, system No. 3 pressure releases the uplock and operates the retraction actuator to the extend position. As the gear extends, the drag brace unfolds until, assisted by the springs on each side, the downlock engages. System No. 3 pressure remains on the retract side of the actuator while the gear is extended.
Nose Gear Retraction
On a gear up selection, once the nose gear doors are open, system No. 3 pressure is provided to the retraction actuator and the downlock. The restrictor in the retraction actuator ensures the downlock actuator activates first, releasing the downlock. The retraction actuator then moves the gear assembly into the nosewheel well. After the uplock pin has engaged the uplock, hydraulic pressure is removed from the actuator and the gear is secured by the uplock.
09/09/20