Landing Gear & Brakes
Revised 06-Oct-2001
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The landing gear consists of two dual wheel main gear and one dual nose gear, each main gear is equipped with Disk brakes, anti skid protection and thermal tire deflators (fusible plugs). 
The landing gear is positioned hydraulically as selected by the landing gear lever in the cockpit on the center instrument panel.
Door and gear sequencing is automatic. Except for the nose gear which is mechanically opened and closed by the movement of the gear. There is a door release handle in each main gear well for ground access.
Hydraulic system "A" provides power for the landing gear system and nose wheel brakes if installed. "B" system provides power for the main wheel brakes. System "A" can also be used as an alternate power source by selecting open the brake interconnect switch on the flight engineers panel.

Extension and Retraction
Gear Doors. Each gear is sequenced automatically with its gear door, opening of the door is controlled by the gear lever. The main gear cannot extend or retract unless the gear door is open and cannot close unless the gear is locked in the up or down position all due to sequence valves being installed. The nose gear is controlled mechanically by linkages to the gear. The forward doors are closed in both the gear up and down positions but the aft doors remain open when the gear is down.
Gear Air-Ground Logic. Air ground sensing for various systems is provided by safety switches on the left main gear and nose gear. These are actuated by the extension (air logic) or compression (ground logic) of the left main gear and nose gear. Click here to see some of the system inputs for logic

Nose Gear Steering
The nose wheels can be turned by a steering handle to the left of the captain (some aircraft have two handles one on each side), or by either set of rudder pedals if the nose gear strut is compressed. Internal cams in the strut automatically center the nose wheels if the strut is extended. Power for the nose wheel steering is supplied by hydraulic system "A" through the landing gear down line. Steering wheel movement of 95 deg at the handle will produce 78 deg of nose wheel turning. Full rudder defection will give you about 8 deg of nose wheel turn. If steering hydraulic pressure is lost and the steering control valve is in neutral, restrictions in the hydraulic circuit prevent nose gear castering. Movement of the handle or rudder pedals will displace the control valve and allow castering.

Manual Gear Extension
In the event of a "A" system hydraulic failure the landing gear can be extended and locked mechanically (manual). Three hand crank drums located in the cockpit floor can be operated. The hand crank lever is stowed on the aft left bulkhead of the cockpit. Instructions for operating are placarded on the back of the access panel. Operation of the system for the main gear unlocks the door first and then unlocks the gear which free falls. The gear doors will then remain open. For the nose gear it unlocks and free falls as the doors are mechanically link to the gear. Reversing the hand crank rotation after the gear has extended locks the gear in place.

Mechanical Lock Indicators.
Viewing ports are provided to visually check that the landing gear is down and locked. These are located at the following locations (approx), Nose gear four feet aft of the cockpit door in the center, main gear five feet aft of the over wing exits to the left and right of the centerline. The visual lock indicator is operated  by the down lock linkage and is located near the top end of the side strut. For the nose gear a stripe on the actuating arms aligns with the lock housing. There are lights installed to allow viewing at night and it is controlled by the wheel well light switch on the pilots overhead panel.

NLG View MLG View

If there in case of over rotation on take off and it's this that will first contact the runway.  It is equipped with an energy absorber which consists of a cylinder with a crushable honeycomb core in the upper half. The core is replaceable. An indicator clip is riveted to the strut and attached to a wire. When the clip is sheared off by compression of the tail skid, it will be retained by the wire and a red area beneath the clip will be exposed to indicate that the core has been crushed. Operation is by the electrical system and extends when the landing gear lever is in the down position and the outboard flaps have been lowered 15 deg or more. It retracts when the gear lever is placed in the up position. It has it's own warning light on the flight engineers door annunciator panel and comes on when there is a disagreement with the landing gear lever in the up or down positions.

Normal Operation. Self adjusting, multidisc hydraulic brakes with incorporated brake wear indicators are installed at each main gear wheel. They are operated by the pilots brake pedals or the pneumatic brake handle (main gear only). An anti skid system is installed to maximize normal braking capability and prevent locked wheels. Pressure sources are available from hydraulic system "B", "A", brake accumulator and pneumatic pressure source, nose brake power is from system "A" (if installed). As previously mentioned system "A" may power the brakes via the brake interconnect valve. Check valves retain pressure in the brake system if hydraulic pressure is  lost. A fully charged brake accumulator stores enough fluid under pressure for several brake applications.
There are bake pressure gauges in the cockpit and left hand wheel well this will show hydraulic pressure and accumulator air pressure. Normal operating pressure is 3000 psi. Pressure surges trapped by the check valve may cause indications to rise to 3500 psi. When all the fluid pressure is depleted from the accumulator the indicator will read pecharge pressure, about 1000 psi.
Pilot control of the braking is through the brake pedals to the brake metering valves, one for each main gear. Stepping on the brake pedals actuates the respective metering valve. As the metering valve moves a proportional amount of hydraulic pressure is directed to the anti skid valves and lock out deboosters, then to the wheel brakes. The deboosters reduce hydraulic pressure and isolate the fluid downstream. Thus if a leak occurs between the debooster and the brake, only the isolated fluid is lost. and you won't have a system "B" hydraulic loss. A servicing handle on the debooster replenishes the isolated fluid (part of pre flight).

Pneumatic Braking
The pneumatic braking system is an alternate system and is a way of providing pressure to main brakes in the event of hydraulic system failure. There is no anti skid or differential braking available from the pneumatic source. A pneumatic brake control valve operated by a handle on the captain's instrument panel opens and modulates air bottle pressure to a transfer tube. Pressurized hydraulic fluid from this tube is routed to a shuttle valve on each main wheel brake. The shuttle valve moves to block the hydraulic pressure port of the main brake line and permits fluid from this tube to apply the brakes. Pneumatic braking is only used when hydraulic pressure is lost. ™Most guy's I've know that have used it  have blown a tire or two, (a wheel and tire only costs you about $ 1200 service exchange !).

Gear Up Braking
Light braking is automatically applied to the main wheels from the gear up hydraulic line during retraction. The nose gear tires rub against brake shoes in the forward wheel well (spin brakes) on retraction.

Parking Brake
This provides a means of locking the brake pedal linkage in a brakes applied condition. They maybe set (correct term for brakes on) by depressing either set of brake pedals and pulling up on the parking brake lever, then release pedals. Brakes will be applied if pressure is available the accumulator in the brake system is the backup source and will hold for many hours. The parking brake lever also closes an electric solenoid valve in the anti skid return line on the main gear anti skid valves thus preventing leakage through the anti skid valve as depleting  accumulator pressure. There is light adjacent to the lever to tell you when the brake lever is up (set position). A second light is installed at the external power receptacle panel. Releasing the brakes is by pressing on the pedals.

Anti Skid
A skid is prevented by controlling the deceleration rate of each wheel. Locked wheels due to hydroplaning are prevented by comparing the speed of each wheel to the speed of the other wheels. This is achieved by releasing some or all of the brake pressure applied by the pilot through the modulating anti skid valves. When a brake is released by the anti skid system the corresponding indicator in the cockpit will display REL. touchdown protection prevents landing with the brakes on and keeps all brakes released until landing gear logic is satisfied that the aircraft is firmly on the ground. In flight when a REL is displayed with the gear down, touchdown protection is operative. After landing wheel spinup can override faulty touchdown logic. Do not test the anti skid while applying brakes, brake release may occur.
™A more in depth look at anti skid will come when I get more time.

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