The
auxiliary power unit, or APU in the Boeing 727 is a small turbine engine
mounted between the main wheel wells. it draws air from the wheel well
area for combustion and cooling and exhausts through louvers in the top
of the right wing root. Most of the accessories are mounted on the
left hand side of the unit. The components of most interest are the APU
starter (electric), hour meter, fuel control unit, 3 speed switch (3 p.s.i
oil, 35%, 95%) Generator, tacho generator (RPM) note that these last two
components are interchangeable with there brothers on the engine. Power
for starting the APU comes directly from the airplane battery. The battery
switch must be on when operating the APU, if the switch is turned off
the APU will shutdown.
The APU uses fuel from the number 2 tank.The APU fuel shutoff valve is located at the tank.It is opened or closed by the APU master switch.Operation of either APU fire switch or activation of the auto fire
shutdown circuitry will also close this valve
The controls for the unit may be found in the flight deck at the flight
engineers auxiliary panel and in the left hand wheel well. It contains
controls for starting and stopping the APU, fire detection and protection,
generator operation, and gauges for monitoring electrical load and exhaust
temperature.
There is a three position control switch marked Off, ON and Start .
It remains in the OFF position when the APU is shut down and in the ON
position when the APU is operating.It must be held in the START position against spring pressure when
starting the APU.ON is the
normal operating position.Selecting
this position prior to starting will open the APU fuel valve. After
the fuel valve is opened, positioning the master switch to START will
initiate the automatic starting sequence.When the APU crank light comes on, the automatic starting sequence
has begun.Once the light
is on, the mster switch may be released to the ON position.During the start sequence, if the EGT does not rise within 15 seconds
or there is no frequency an the AC meter within 30 seconds, the APU fire
shutoff handle should be pulled to interrupt the start sequence.The APU crank light goes out when the starter releases. Click
here to view starting sequence. The self-contained lubrication system requires no crew monitoring.The APU will not operate if the oil pressure fails. An exhaust temperature gauge located in the lower right corner of
the APU control panel shows APU turbine exhaust temperature in degrees
C.The temperature will vary
widely depending on bleed air loads.The green band is the normal operating range, and the red radial
is the maximum operating temperature.
The APU is governor controlled to maintain 100% RPM, thus it can be geared
to drive an AC generator directly.This generator can supply electrical power to all airplane systems
for ground operation.
Bleed air is extracted from the APU compressor to be used for airplane
air conditioning and for engine starting on the ground.An APU bleed air valve is installed on the APU to control flow
of bleed air from the APU to the bleed air distribution system.The APU bleed air valve will open if the APU has reached operating
RPM and either or both Engine No.2/APU BLEED switches are in the OPEN
position. An APU light is located on the door warning annunciator at
the flight engineers panel.This
light will come on any time the fuel valve is open if the number 1 DC
electrical bus is powered.
For maintenance personnel external control of the APU, a second APU control
panel is located in the left wheel well between the fuselage and the gear
strut.It is not used for
normal operation.The start
switch on this panel will start the APU if the battery switch and APU
master switch in the cockpit are on.The stop switch will shut the APU down.Also the panel contains a fire switch, a fire warning light, and
a bottle discharge button.These controls permit fighting an APU fire without going to
the cockpit.
The APU will shut down automatically for the following reasons, loss of
oil pressure or overspeed will cause the fuel to be cut off at the fuel
control in the APU.The fire
detection loop, if it reaches the warning temperature, will close the
fuel shutoff valve in the number 2 tank and at the fuel control and the
APU will stopHeat sensitive
probes in the turbine exhaust provide other protection.First, these probes cause the APU to be unloaded by modulating
the APU bleed air valve toward close; if closing the bleed air valve fails
to solve the high exhaust temperature, the probes will cause the fuel
control to reduce fuel flow until the temperature is lowered sufficiently
or the APU flames out.
On the 200 series aircraft a turbine and compressor assembly called a
flow multiplier is used to improve the airflow from the APU.It draws in additional air from the right wheel well adding it
to the APU compressed air output.This higher volume of air makes it possible for the APU to supply
air to both air conditioning packs.During single pack operation, However, the flow multiplier shut
off valve remains closed and the turbine is bypassed.
A FLOW MULTIPLIER OVERHEAT light on some airplanes and a BLEED AIR light
on others warns of an overheat in the output of the flow multiplier compressor.The overheat will cause the APU bleed air valve to close.Cycling the number two engine bleed switches will reset the APU
bleed air valve.Further
protection is provided by a fusible plug, which should it melt. It will
close the flow multiplier shutoff valve, preventing compressed air from
reaching the flow multiplier turbine.
Pressures in the pneumatic ducts can be read on the duct pressure gauge
under most conditions.The
duct pressures will read zero, when either air conditioning pack is turned
on, if the APU is the only source of air in the pneumatic ducting (200
Series).Turning either pack
switch on prevents APU bleed air from reading on the duct pressure transmitters.
The
APU on the Boeing 727 can be used for ground operation only.Electrical loads and EGT limits must be observed for all operations.The EGT limits are red radial for maximum and the green band for
continuous operation.The electrical
load limit is 165 amps, the higher rating is due to the improved cooling
of the APU installation.The
APU should be operated without pneumatic load for at least one minute after
start or prior to shut down.
A pretty good piece of kit, most of the problems you experience are to do
with starting (3 speed switch or over-speed switch) occasionally the shorting
link on the exhaust disconnect link. Extensive troubleshooting will require
the test set, (not a great deal you can do down route). It's manufactured
by Allied Signal (Garrett).
APU Starter
is limited to 1 min on 4 min off
Max time for APU fire test AC Busses powered 30 sec- 45 sec. Battery
power 60 sec. Max
APU generator load 165 amps
One pack on for cooling (100 Series)
Two packs on for heating
Two packs on for cooling ( If flow multiplier installed)
Normal operating EGT Green Band (Marked @ 700)
Max Operating Red Radial line (Marked 750 - 790)