Piper Cherokee Six - Page 216

PIPER
CHEROKEE
SIX SERVICE
MANUAL
The
hydraulic
pump
is a gear
type
unit
driven
by
a 12
to 14
volt
reversible
motor
designed
to operate
in
a pressure
range
as given
in Table
VA-I.A
relief valve
is
incorporated
in
the pump,
which
when
open
allows
fluid
to
flow
into the
reservoir.
Also
incorporated
in
the system
on earlier
models
is a primary
thermal
relief
valve
which
prevents
excessive
buildup
of pressure
in
the
hydraulic
system
due to
expansion.
Other
valves
in the
pump,
channel
fluid to
the
proper
outlet
during
retraction
or extension
of gear.
A
shuttle
valve
located
in
the
base of
the pump
allows
fluid displaced
by
the cylinder
pistons
to return
to
the reservoir
without
back
pressure.
(For
specific
pressures
refer
to
Table
VA-I.)
Also
in the
system
is a
by-pass
or
free-fall
valve
that
allows
the
gear to
drop
should
a
malfunction
in the
pump
system
occur.
To
prevent
the gear
from
extending
too fast,
there
is
a
special
restrictor
nipple
on the
main
gear retraction
line.
The valve
is
controlled
manually
or by
a gear
back-up
extension
device
that
is operated
by
a pressure
sensing
device
which
lowers
the gear
regardless
of gear
selector
handle
position,
depending
upon
airspeed
and
engine
power (propeller
slipstream).
Gear
extension
occurs even
if
the selector
is in
the up
position,
at airspeeds
below
approximately
118
MPH
with engine
power
off. The
device
also
prevents
the gear
from
retracting
at
airspeeds
below
approximately
93 MPH
at
sea level
with
full
power,
though
the selector
switch
may
be in
the up
position.
This
speed increases
with
reduced
power
and/or
increased
altitude.
The sensing
device
operation
is
controlled
by a
differential
air
pressure
across
a flexible
diaphragm
which
is mechanically
linked
to the
hydraulic
valve
and an
electrical
switch
which
actuates
the pump
motor.
A
high pressure
and
static
air source
for
actuating
the
diaphragm
is
provided
in a mast
mounted
on
the left
side
of the
fuselage
above
the
wing.
Manual
override
of the
device
is provided
by an
emergency
gear
lever
located
between
the front
seats
to the
right of
the flap
handle.
The emergency
gear
lever, used
for
emergency
extension
of the
gear, manually
releases
hydraulic
pressure
to
permit
the gear
to
free-fall
with
spring
assistance
on
the nose
gear.
The
lever
must be
held
in the
downward
position
for
emergency
extension.
This
same
lever,
when
held in
the
raised
position,
can
be used
to
override
the
system,
and
gear position
is
controlled
by the
selector
switch
regardless
of
airspeed/power
combinations.
The lever
must
also
be held
in the
raised
position
when
hydraulic
system
operational
checks
are being
conducted.
An override
latch
allows
the emergency
extension
lever
to be
retained
in the
up
override
position.
The latch
is
disengaged
by pulling
up
on the
extension
lever.
The lever
includes
a centering
device
to return
the
handle
to
neutral,
when
not latched
in override.
An
auto extension
off light
is mounted
below
the
gear
selector
switch,
and flashes
to
indicate
whenever
the
latch is
in use.
The
auto
extension
off
light
is controlled
by
a switch
and
flasher mounted
under
the
flap handle
cover.
For a
description
of
the landing
gear and
electrical
switches,
refer
to Section
VIA,
Landing
Gear
and
Brake System.
SEE
CAUTION
NOTE
ON
GRID
IJ3
Revised:
7/29/77
HYDRAULIC
SYSTEM
1124