There are three hydraulic systems: green, blue and yellow.
All three systems are independent of each other and do not transfer fluid at
any time. Each system has its own accumulator. Priority valves ensure proper
pressure to critical users when system pressure is low.
Green system – 1 pump: engine driven. Two power sources:
engine 1 pump & PTU
Blue system – 2 pumps: 1 electric and the emergency RAT. Two
sources of power: electric pump & RAT pump.
Yellow system – 3 pumps: 1 engine, 1 electric & 1 hand
pump. 4 sources of power: engine 2 pump, electric pump, hand pump and PTU.
Green is the “heavy” system with landing gear, flaps/slats,
nosewheel steering and Normal Brakes.
Blue is basically for redundancy with the only unique items
on it being L & R spoiler 3 and the Emergency Generator which are “backup”
items themselves.
Yellow provides the ground service items of parking brake
and cargo door and also helps power the flaps.
The RAT and Yellow electric pumps do not normally run during
flight. The Yellow electric pump will automatically come on when a cargo door
is operated. Other Yellow system functions are inhibited when automatically
activated by a cargo door. A hand pump is provided on the Yellow system to
provide the ability to open cargo doors with no electric power on the aircraft.
Blue electric operates all the time in-flight and on the ground when at least
one engine is operating.
The RAT hydraulic pump is for emergency use only and will
only deploy manually for hydraulic problems. For electrical problems it will
deploy automatically above 100 kts. with loss of all AC. Note: Min RAT
speed is 140 kts. with A319/321 and modified A320 RATs stalling at less than
125 kts. This speed limit is for electrical power and the RAT will continue to
supply hydraulic power to much slower speeds.
The PTU is able to transfer power but not fluid. It
transfers power between the Green and Yellow systems (the two with the engine
pumps and heavy consumers). The PTU can transfer power in either direction and
is activated when a 500 psi differential is sensed between Green and Yellow.
The PTU can also be powered on the ground by the Yellow electric pump to power
Green hydraulic. Allows Yellow electric pump to power Green on ground (for
example to retract slats on ground).
The PTU is inhibited when (PH 11.1.5):
Ø First
engine is being started. This is identified as when the nosewheel steering
disconnect pin is in and only one ENG MASTER switch is ON. (PTU operation is
tested on second engine start)
Ø Cargo
doors are operated (Yellow electric normally powers cargo doors, this prevents
draining low output of electric pump or accidentally powering Green Hydraulic)
Ø Parking
brake is ON and only one ENG MASTER switch is ON
Ø PTU pb is
off
The engine pumps (Green and Yellow) each have Fire Shut Off
Valves that close when the Engine Fire Pushbuttons are selected open.
Note: If a cargo door is operated and then the 2nd
engine is started within 40 seconds a PTU fault message may be given (I assume
due to inhibition during test period). See PH Chap. 21-209 for detail on reset.
The brakes are carbon, multidiscs actuated by two
independent systems, Normal and Alternate. The normal brakes are powered by the
Green hydraulic system.
Normal brakes are available when:
Ø The A/SKID
& N/W STRG switch is ON
Ø Green
hydraulic pressure is available
Ø The
parking brake is OFF
A BSCU (Brake and Steering Control Unit) controls all normal
braking functions (anti-skid, autobrakes and brake temps.).
Normal brake pressure is 2000 - 2700 psi. w/ full pedal
deflection (PH 3.4.1)
Anti-skid is deactivated below 20 kts. Anti-skid may or may
not be available when on alternate brakes. If antiskid is inop. then alternate
brakes use 1000 psi max to prevent blowing tires.
The alternate brakes are powered by the Yellow hydraulic
system and will automatically become selected if Green hydraulic is
insufficient for normal brakes. Yellow
brakes have the same capabilities as normal brakes except for autobrake
capability.
Alternate brakes can be used with or without anti-skid.
Anti-skid during alternate brakes is inoperative when:
Ø Electrical
power failure
Ø BSCU
failure
Ø A/SKID
& N/W STRG switch turned off
Ø Brake
pressure supplied by Yellow accumulator only
Parking brake disables all other brake modes (319, 320 only). Parking brake is on Yellow system.
A pressure indicator on the instrument panel indicates
Yellow accumulator pressure and Yellow left and right brake (parking brake)
pressure on three needles.
Accumulators maintain good parking brake pressure for at
least 12 hrs. The cargo door operation will restore parking brake (Yellow
system) pressure.
Autobrakes are available on Normal Brakes (Green system)
only. Hold pb for at least one second. LO mode delays for 4 seconds after
touchdown. MED mode delays for 2 seconds. MAX has no delay. Do not use MAX for
landing, MAX is takeoff only (PH 3.12).
The Green DECEL light in the auto brake pb’s indicates
actual deceleration is within 80% of the selected rate (does not indicate that
the autobrake is activated).
Autobrakes activate when ground spoilers are extended. On
takeoff they are not armed until 72 kts. 2 SEC’s are required for Autobrakes.
Brake Fans are installed in the main gear hubs. They will
indicate an amber HOT when the brakes are 300° C or more. Brake temps are shown
on the ECAM WHEELS page. An arc will appear above the hottest brake temp. If
brake temp is above 300° C
then the temp will turn amber. The brakes must be cooled below 300° C before
takeoff. Pilot must manually select brake fans on.
Hot Brakes (PH 3.15) Maintenance action is required if there
is:
Ø 150° C
difference in brake temps on the same strut and one brake 600° or
greater or 60° or less
Ø fuse plug
melted
Ø brake temp
exceeds 900° C (800° C, A321)
Avoid use of the parking brake when brakes are 500° C or
above if able.
Do not set Parking Brake ON in flight.
Landing Gear (PH 11.5.x)
The Airbus
Landing Gear:
Ø Has
enclosed gear bays
Ø Is held by
mechanical uplocks
Ø Uses
manual extension by gravity
Ø Has no
mechanical or visual check for gear position
Ø Uses
autobraking on the mains during retraction
Ø Has a
brake band in the nose gear well
Ø Is
hydraulically locked out from operation above 260 kts.
The LGCIU controls the Airbus landing gear operation. The SD
will show 2 green down triangles on the WHEELS page for each gear down and
locked. There are also gear indicators next to gear handle. Any green triangle
(at least one out of three possible) for a gear confirms the gear down and
locked. One green and two red triangles for a gear still indicates down and
locked. Red shows gear in transit and
no triangle indicates gear uplocked.
The gear doors will remain down after manual gravity
extension (PH 11.5.4).
The gear lights by the gear handle are powered through (hard
wired) LGCIU 1,
if LGCIU 1 is not powered the
lights will not operate (PH 11.6.1).
The gear handle has a red down arrow that will illuminate if
gear is up with flaps 3 or FULL below about 700’ (landing configuration). ECAM
will alert (PH 11.6.2).
Nose Wheel Steering (PH 11.5.5)
Nose Wheel Steering gets inputs from: Capt. & F/O steering hand wheels
(max deflection is 75°, starts reducing above 20 kts to 0° at 70 kts.), Rudder
pedals (max deflection is 6°, starts reducing above 40 knots to 0° at 130
kts.), and Autopilot. A rudder disconnect is on the hand steering wheel
for use during Flight Control Check. A lever on the nose gear deactivates steering
to enable towing. A green NW STRG DISC message will show on ECAM and will turn
amber on second engine start when lever is activated.
Nose wheel steering is enabled with hydraulic pressure when:
Ø Nose gear
doors closed
Ø A/SKID
& N/W STRG switch on
Ø Towing
control lever in normal position
Ø At least
one engine operating
Ø Aircraft
on ground
Nose wheel steering is disabled after manual gear extension.
Note: There have been problems with the Nose Wheel Gear Electrical Box failing to deactivate the nosewheel steering during engine starts when the nosewheel steering lever was in the deactivate position. Airbus has not yet engineered a fix for the problem, therefore US Airways has instituted a (hopefully) temporary solution of turning OFF the A/SKID & N/W STRG switch during the Before Start Checklist flow and turning it back ON during the After Start Checklist flow.