LANDING GEAR 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.
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.
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.
BRAKES 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
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).
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.
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.
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.