Research on the BAC 1-11 electrical systems

[TheChitterneFlyer]

Landing Gear huh? OK...

DESCRIPTION

General
The retractable tricycle undercarriage is operated by hydraulic power supplied from system No.1. The main and nose undercarriages retract and extend simultaneously and are enclosed by doors when fully retracted. A cable-operated free-fall mechanism is provided for emergency lowering. A nosewheel steering system is provided which incorporates a standby steering system for use when system No.1 has failed or the undercarriage has been lowered by free-fall operation. The main wheel braking system is arranged for foot and hand operation with hydraulic power supplied from system No.2 and incorporates an anti-skid system for the footbrakes only. Gravel deflectors are provided for the nose and each main undercarriage.

Undercarriage controls and indicators are located on the centre instrument panel, the forward cabin intercom area and the left and right-hand consoles on the flight deck. Visual downlock indicators are provided for each main undercarriage and for the nose undercarriage.

Main Undercarriages
The main undercarriages retract inward and are housed jointly in the wings and an unpressurised bay in the fuselage. Three doors enclose each undercarriage when retracted: an outer door, a fixed door and an inner door. The inner doors are hydraulically operated and are retained in the closed position by an uplock mechanism. A panel in each inner door gives access to the bay for maintenance purposes.

Each main undercarriage comprises a shock-absorber strut attached to a pintle supported in the wing torque box structure. The strut is braced by a forestay and a folding sidestay, and is locked down by links held in geometric lock by hydraulic jacks. No uplocks are fitted, the undercarriage being supported in the fully retracted position by the inner doors. Mechanically operated hydraulic sequence valves control the operation of the doors in association with the retraction and extension of the undercarriage. The brakes are automatically applied when the undercarriage selector lever is moved to the UP position. The brakes are released and the undercarriage hydraulic system depressurized, when retraction is completed.

A weight-switch box, fitted to each forestay, is operated by shock-absorber strut extension and compression. Micro-switches in the switch box actuate relays which affect various electrical circuits. An earth wire is attached to the outer wheel brake units, for static discharging.

Nose Undercarriage
The nose undercarriage retracts forward and is housed in an unpressurised bay in the fuselage and when fully retracted is enclosed by doors. It cannot retract until the nosewheels are central.

It comprises a shock-absorber strut assembled in a housing attached to the bay structure and braced by a pair of drag links. Toggle links, operated by a lock jack, lock the undercarriage in the up and down positions. The retraction jack is fitted between the bay roof and an arm on the shock-absorber strut housing.

Spin brake shoes are fitted in the bay roof to arrest wheel spin when the nose gear is retracted.

The mechanically operated doors, two forward and two rear, enclose the under­carriage in the retracted position. The forward doors also close after extension of the undercarriage. The forward doors can be opened to give access to the bay for maintenance purposes.

Steering
The steering system is supplied with hydraulic power from system No.1. Steering handwheels are provided on the left and right-hand consoles adjacent to each pilot's seat position. The handwheels are connected by cables to a steering control valve which directs fluid to and from steering jacks to turn the nose shock-absorber strut. Rotation of the strut actuates a follow-up system of pulleys and cables to restore the control valve toward the neutral position.

Powered steering is limited to 78° either side of the aircraft centre­line, but free castering is possible to 110° on each side.

A cam operated steering and lock change-over valve ensures that the steering system can only be operated when the nose undercarriage is fully extended.
Rudder 'fine' steering is provided to enable the pilot to control the nosewheel steering, in relation to the rudder position, to a maximum of 7° on each side of centre.

A standby steering system is provided for use when system No.1 has failed or when the undercarriage has been lowered by free-fall operation. Hydraulic power is supplied from an accumulator and operated by a guarded switch on the left-hand console, near the steering handwheel. The switch is not effective until the main undercarriage weight-switch relays have operated.

Free-fall Mechanism
A cable-operated free-fall mechanism releases the nose undercarriage uplock and the main undercarriage door uplocks and allows each undercarriage to fall and lock down under its own weight. The free-fall operating lever is located under an access panel in the right-hand console.

Wheels and Brakes
Two wheels are fitted on the nose undercarriage and two on each main under­carriage. Each wheel comprises two half-hubs fitted with tubeless tyres.

Hydraulically operated brakes fitted to each main wheel are arranged for either foot or hand application, hydraulic power being supplied from system No.2. Foot-brake pedal movement is transmitted via master cylinders, at the pedals, to a brake control valve which controls pressure delivery to the brakes and provides for differential braking. The foot-operated system incorporates anti-skid units which permit maximum normal braking application under all conditions without incurring wheel skid. The handbrake system is operated by handles mounted on either side of the centre console. The left handle can be locked in the 'brakes applied' position for parking. Hand operation applies equal pressure to all four brake units, with no anti-skid system incorporated. The brakes are automatically applied when the undercarriage selector lever is moved to the UP position and released when retraction is completed.

In the event of overheating through excessive brake temperatures, fusible plugs incorporated in the main wheel hubs will melt and release the air from the tyre, thus preventing tyre burst. Brake temperature (dual) indicators are provided in the flight deck.

The accumulators are charged with air to 1,000 psi. The auxiliary DC motor driven pump can be used to hydraulically charge the brake system accumulators on the ground for towing purposes and for 'topping-up' the accumulators in flight before an emergency landing, when the No.2 hydraulic system is inoperative. A pressure transmitter connected into the air line to each brake accumulator gives indication of available brake pressure.

Control and Indication
The landing gear UP/DOWN selector lever is located on the right-hand side of the centre instrument panel together with position indicating lights and a warning horn cut-off and test switch which is spring-loaded to the centre position. A spring-loaded knob at the end of the selector lever must be pulled to disengage a latch mechanism before selection. A safety lever, operated by a lock solenoid, prevents normal movement of the selector lever to UP until the aircraft is airborne. The safety lever can be overridden by depressing an override lever fitted under the selector lever.

In the event of failure of the normal indication system, visual indicators show when each undercarriage is locked down. Each main undercarriage indicator is located on the upper surface of the wing trailing edge, and the nose undercarriage indicator is located under a cover in the floor at the rear of the flight deck.

An Undercarriage Doors Warnings & Lights Test Unit (U/C Test Unit) provides further in-flight diagnostic information in the event of a gear or gear door warning. The U/C Test Unit is located on the aft face of the captain’s bulkhead and comprises of a Test Switch (labelled ON - OFF) and six red indicator lights labelled NOSE UP / NOSE DOWN / PORT DOWN / STBD DOWN / PORT DOOR / STBD DOOR. If a ‘gear unsafe’ indication occurs in the cockpit, selection of the Test Switch to ON will illuminate the appropriate warning light associated with the microswitch responsible for the unsafe indication. If none of the lamps illuminate, it is assumed that it is a failure of the microswitch for the gear selector lever.

Gauges are provided to give indication of brake pressures applied, brake accumulator pressures and brake temperatures.
A warning horn will sound continuously during approach and landing when the undercarriage is not locked down and one or both of the following conditions exist: ­

• Flaps are selected to or beyond 26° irrespective of throttle position.
• Either or both throttles are below the approach rpm position and the speed is below 165 knots.

The warning horn can be cancelled by the HORN CUT-OFF/TEST switch. On mod PM 5366 the GEAR UNSAFE light also illuminates when the gear horn is muted by the HORN CUTOFF switch: the gear horn will sound and the light remain on when flaps are selected to or beyond 26°; the light will go out when the landing gear is locked down or throttles advanced above the approach setting.

SYSTEM OPERATION

Undercarriage Retraction
With system No.1 pressurized and the selector lever moved to UP, the brakes are automatically applied, hydraulic pressure releases the door uplocks and the door jacks extend to open the inner doors. When both doors are fully open, a sequence valve directs the hydraulic pressure to the downlock jacks, the downlocks disengage and the downlock jacks fold the toggle links to 'break' the geometric lock and at the same time folds the sidestay upwards. The retraction jacks then rotate the pintles to retract the undercarriages. When both main undercarriages are fully retracted, the door jacks close the inner doors which engage the uplocks and support the main undercarriages.

The nosewheels are centralised, when the aircraft is airborne, by integral cams in the shock-absorber strut. When central, pressure is sequenced to operate the up/down lock jack which 'breaks' the geometric lock formed by the drag and toggle links. At the same time the common pivot point of the drag links is moved upward, the drag links fold and retraction is initiated. The hydraulic pressure is directed to the retraction jack by a steering centring valve when the wheels have centralised, the retraction jack extends and the nose undercarriage is retracted. The undercarriage is held in the retracted position by a geometric lock imposed on the toggle links.

The undercarriage red warning light on the centre instrument panel illuminates when the undercarriage selector lever is moved to UP. As the main and nose under­carriage downlocks 'break' and retraction is initiated, the three green lights extinguish. When all undercarriages are fully retracted and each main undercarriage inner door uplock is engaged, the red undercarriage light extinguishes, the brakes are released and the undercarriage hydraulic system is depressurized.

A cam-operated micro-switch, fitted near the end of the undercarriage selector lever, is operated when the lever knob is pulled to disengage the lever latch from the DOWN or UP gate. The micro-switch is electrically connected to the undercarriage red warning light, so that when the latch is not fully engaged in the latch gate the red warning light illuminates.

Undercarriage Extension
With system No.1 pressurized and the selector lever in the DOWN position, hydraulic pressure releases the inner door uplocks and the door jacks extend to open the inner doors. When both doors are fully open, the hydraulic pressure is sequenced to the down side of the retraction jacks which in turn rotate the pintles and extend the undercarriages. Spring-loading on the down lock jack re-imposes the geometric lock on the toggle link. When each undercarriage is locked down, hydraulic pressure is again sequenced to close the inner doors and engage the door uplocks.

Simultaneously, hydraulic pressure is directed from the 'lower' line to the down side of the nose undercarriage retraction jack via the steering lock and change-over valve to 'break' the uplock. The jack extends the nose undercarriage and when fully extended a geometric lock is imposed on the toggle link.

The U/C unlocked RED warning light illuminates when the undercarriage selector lever is moved to DOWN. When all undercarriages are fully extended and the downlock on each undercarriage engaged, the GREEN light in each of the three indicators on the centre instrument panel illuminate. The main undercarriage inner doors then close and when both the door uplocks are engaged the RED U/C unlocked light extinguishes.



Steering
When the nose undercarriage is fully locked down, the steering and lock change­ over valve is mechanically actuated to allow pressure from the 'lower' line to pass to the steering control valve.

Manual movement of either handwheel is transmitted via cables and a feedback assembly to move the steering control valve from its neutral position, thus connecting one side of each steering jack to return. Pressure present on the other side of each jack causes the jacks to rotate the shock absorber strut in the strut-housing. The subsequent movement of the nosewheels drives the follow-up cables, linkage and the feedback mechanism, to restore the steering control valve towards its neutral position.

With the steering handwheel held stationary, the displacement of the control valve from neutral is maintained. When the handwheel is released, the control valve returns to neutral under influence of a centring spring leaving the nosewheels free to castor.

Rudder 'fine' steering is limited to 7° on each side of the aircraft centre line and is only operative with the undercarriage in the down position and the nose shock-absorber strut compressed. With the undercarriage lowered, a clutch is operated hydraulically, connecting the rudder pedal linkage to the steering cables. Thus steering is achieved through a limited range from the rudder bar, using the normal steering system. Movement of the steering handwheel overrides the signals from the rudder bar.

Standby Steering
The standby steering supply should only be used for landing if the landing gear free-fall system is selected or if pressure is not available in the No.1 hydraulic system and provided that a steering handwheel is held in readiness to prevent nosewheel movement during landing.

The standby steering supply should be selected after the nose landing gear has been locked down, and prior to landing. Following a selection of the standby steering supply, power is not available at the steering cylinders until the main gear oleos are compressed. .

The standby steering accumulator is normally hydraulically charged by system No.1. With the standby steering switch in the ON position and the main undercarriage weight-switch relays actuated, a selector valve is energized to connect the accumulator through the control valve to the steering jacks and steering is effected by normal operation. On completion of landing the standby steering must be switched off. It is important to ensure that it is switched off during all normal operations.

If the accumulator pressure falls below 2,000 psi, irrespective of the standby steering being switched on or off, the warning light illuminates.

Free-fall
When the free-fall lever is pulled upward, the nose undercarriage uplock and the main undercarriage door locks are mechanically released, allowing the undercarriages to lower under their own weight.

The undercarriage control valve is mechanically operated, irrespective of the selector lever position, and the pressure supply is blanked off. The 'raise' and 'lower' lines are connected to the return line and fluid is released from the main retraction jacks, allowing the undercarriages to lower freely. The main wheels push the inner doors open and the doors remain partially open after the undercarriages lock down, leaving sufficient ground clearance.

The position of the undercarriage selector lever does not affect free-fall operation, but it should be left in the DOWN selection and must be in this position before the free-fall lever is reset.

Wheel Brakes
When the footbrake pedals are depressed at either pilot's position, master cylinders transmit the movement by servo pressure to actuate a brake pressure control valve which regulates pressure delivery to the brake units and provides for differential braking.

With No.2 hydraulic system inoperative, the footbrake accumulator provides one full braking stop with the anti-skid units operating.

The anti-skid system is an integrated mechanical-electrical-hydraulic system which controls the aircraft hydraulic brakes to obtain the maximum aircraft retardation under all runway conditions.

Mounted within each wheel axle, and driven by the wheel, is a wheel speed transducer which provides a signal proportional to wheel speed; this signal is fed to the control shield. A dual pressure control valve feeds braking pressure to each main wheel, in conjunction with the control shield, to control excessive brake pressure and to provide wheel protection with optimum braking control.

With the anti-skid system switched on before touchdown, brake application cannot be achieved until the wheels have commenced rotating and the weight switchbox relays have been operated, even if the touchdown is made with the footbrake pedals depressed.

Thermo-couples connected into the brake units give indication of brake temperatures through the brake temperature gauges.

When a handbrake handle is pulled, a cable-operated control valve is actuated and permits an equal pressure to be applied to the four brake units. For parking, the left handbrake is pulled and turned to apply the brakes. On undercarriage retraction, an auto-brake jack is pressurized and actuates the control valve permitting a pressure delivery to the brakes. The handbrake accumulator provides a minimum of six full brake applications with No.2 hydraulic system inoperative.

Weight-switch Operated Circuits
Compression of the shock-absorber struts on landing, and extension on take-off, operates the weight-switch box and the following circuits are affected: ­

• Undercarriage indication and lock control.
• Stall warning system.
• Waste water anti-icing.
• Air conditioning ground cooling.
• Ice detectors.
• Airfoil anti-icing.
• Configuration warning.
• Generator cooling.
• Standby steering and LP warning.
• Elevator isolate.
• Anti-skid system.
• AC hydraulic pumps.
• Flight voice recorder.
• Galley supplies.
• Lift dumper.
• Flight data recorder.
• Engine start and CSDS control.
• Ground proximity warning.