Auxilary Power Unit (APU)
Revised 24-Mar-2001
 
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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 stop  Heat 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)
 

 

APU EGT Operating  GTCP85-98 and 98C       98CK
Maximum    760 C               710 C
Continuous     710 C            663 C

 

 

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