Landing Gear and Doors Operation

(1) Takeoff

On takeoff the loads on the MLG decrease. As the aircraft becomes airborne, the shock absorber extends which gives the effect of an increase in the length of the leg. This lets the aircraft move through a large angle during takeoff.

When L/G UP is selected the hydraulically
operated door opens. Hydraulic pressure at the down lock actuator releases the overcenter lock of the lock stay. The MLG actuator retracts the MLG into the bay.

During retraction the BSCU automatically puts the brakes on for a short time. This stops the rotation of the wheels before they go into the bays. The hydraulically operated door closes after the MLG is locked into the bay.

When L/G DOWN is selected the hydraulically operated door opens. The retraction actuator extends to extend the leg. The side stay and the lock stay move to the over center position to lock the gear in

the extended position. The door closes after the gear is extended. With the gear extended the shock absorber absorbs the landing loads.

(2) Nose Landing Gear and Doors

As the NLG wheels leave the ground on takeoff, the shock absorber extends. The cams in the leg make sure that the wheels are in the center. When the shock absorber is fully extended the BSCU prevents steering outputs to the steering mechanism.

When the L/G UP is selected the hydraulically operated doors open. Hydraulic pressure at the NLG downlock actuator releases the overcenter lock of the lock stay. The drag strut assembly folds as the NLG retracts. As the leg retracts a rotating joint on the leg cuts off the hydraulic pressure supply to the steering system. The hydraulically operated doors close after the gear is locked into the bay.

(3) Extension and retraction

(a) Normal Extension and Retraction

When the L/G control lever is moved the LGCIU sends a control signal to the electro-hydraulic safety valve. The LGCIU controls the sequence of operations to extend and retract the gears and open and close the doors. The LGCIU uses the position data from the proximity sensors to control the sequence of operations.

Only one LGCIU controls the sequence of operations. Control changes from one LGCIU to the other after each retraction/extension cycle.

(b) Free Fall Extension

The free fall extension system is controlled by the free fall extension handle in the cockpit. The free fall extension handle is connected to the L/G and door uplocks. When the handle is turned the door uplocks are mechanically opened by the cables. When the doors are open the landing uplocks are also mechanically opened. Gravity then extends the L/G. Springs pull the NLG lock stay and the MLG side stay into the locked position and the L/G doors stay open.

A mechanically operated valve isolates the hydraulic supply. Other mechanically operated valves let fluid move into the hydraulic components of the normal extension and retraction system. This prevents cavitation and hydraulic locks.

The free fall extension system is reset when:

· the free fall extension handle is back in its original configuration

· the L/G control lever (6GA) is put in the down position.

Hydraulic power is then put back into the normal extension/retraction system and the L/G doors close.

(4) Braking and Related Systems

(a) Normal Braking

In the automatic mode the selection of a AUTO/BRK P/BSW (LO MED or MAX) sets a program to give a set deceleration rate. The BSCU automatically starts the program when the aircraft configuration is correct and then controls the pressure sent to the brakes.

In the manual mode the movement of the brake pedals operates the brake-pedal transmitter unit.

The transmitter unit sends a signal to the BSCU which in turn sends the required input signal to the servovalves. The servovalves let a pressure, in proportion to the pedal travel go to the brakes.

The BSCU also controls the anti-skid function. It compares the MLG wheel speeds with the aircraft speed and releases a brake if there are indications of a skid.

(b) Alternate Braking

When the brake pedals are operated the low-pressure hydraulic system operates a dual valve.

This lets hydraulic pressure from the yellow hydraulic system go to the second set of pistons in the brakes. The BSCU controls the anti-skid function. If the BSCU cannot supply this control, braking is in proportion to the amount of pedal travel.

(c) Parking Brakes

When the park brake selector switch is set to ON, pressure from the yellow hydraulic system or the related accumulators is sent to the second set of pistons in the brakes. When this system is in use all other braking systems are hydraulically isolated.

(d) Brake Temperature System

The brake temperature system continuously measures the temperature at each brake. The sensor at each brake sends a signal to a Brake Temperature Monitor Unit (BTMU). Each BTMU receives the signals from a pair of brake sensors and sends the data to the BSCU. The BSCU sends the data to other interface systems.

(e) Brake Cooling (if installed) 

When the P/BSW is set to ON, the fans in the wheel shrouds turn and pull air through the brakes.

This decreases the temperature of the brakes more quickly.

(f) Tyre Pressure Indicator System (if installed) 

The TPIS automatically monitors the tire pressures and shows these values on the WHEEL page of the system display. The tire pressure indicating computer also

supplies other data and warnings when the pressures are unsatisfactory.

(g) Steering 

The BSCU uses the steering input signals from the hand wheel transmitters in the cockpit to calculate:

· the necessary steering angle

· the necessary rate of movement

· the direction of the turn.

It then sends steering output signals to the electro-hydraulic components to operate the steering mechanism.

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