The APU is a source of bleed air and AC electrics for the
aircraft, this gives independence during turnarounds, electrical backup in the
event of engine failure and provides air conditioning & pressurisation
during an engine bleeds off take-off. It's electrical power source is the battery,
many series -500 aircraft have an extra, dedicated APU battery to preserve main
battery usage.
There are many different APU's available for the 737. The most
common is the
Garrett GTCP (Gas Turbine Compressor [air] Power unit [electrics]) 85-129. This
was standard for the series 1/200 but when the -300 was introduced it was found
that two to three times the energy was needed to start the larger CFM56 engines.
Garrett produced the 85-129[E] which had a stretched compressor, ie the
impellers were lengthened and the tip diameters increased. When the 737-400 was
introduced, even more output was required and Garrett produced the 85-129[H].
This has an Electronic Temperature Control which limits hot section temperatures
depending upon demand and ambient temperatures. By 1989 the 85-129[H] was the
most standard APU in all 737 models, although there are actually 14 different
models of the 85-129 in service with 737's (see table below).
Other APU's available are the Garrett GTCP 36-280(B) and
the
Sundstrand APS 2000 on the 3/4/500; and the Allied Signal GTCP 131-9B
for the NG's.
The main difference between them is that the Garrett is hydro-mechanical
whereas
Sundstrand and Allied Signal are FADEC
controlled. I am told by engineers that whilst the Garrett is more
robust,
the Sundstrand & Allied Signal's are easier to work on. On the
3/4/500's, we pilots prefer the Sundstrand because it
has no EGT limits and faster restart wait times. The easiest way to tell
which
is fitted is to look at the EGT gauge limits; the GTCP 85-129 has an
850C limit
and also runs at 415Hz,
the GTCP 36-280 has an 1100C limit if no EGT limits are marked you have a
Sundstrand. Later aircraft have MAINT instead of LOW OIL QUANTITY and
FAULT instead
of HIGH OIL TEMP warning lights.
The AlliedSignal APU has a 41,000ft start capability and
incorporates a starter/generator, thus eliminating a DC starter and clutch. In
practice this means that it can be started either by battery or AC transfer bus
1 (the classics are battery start only). It
has an educter oil cooling system (see bottom of page) and therefore has no need
for a cooling fan.
It is
rated at
90KVA up
to
31,000ft
and 66KVA
up to
41,000ft. The Garrett and Sundstrand APUs are only rated to 55KVA.
The fuel source is normally from the No 1 main tank and it is
recommended that at least one pump in the supplying tank be on during the start
sequence (and whenever operating) to provide positive fuel pressure and preserve the
service life of the APU fuel control unit.
Boeing responded to this need by installing an extra DC operated APU fuel boost
pump in the No 1 tank on newer series 500 aircraft which automatically operates
during APU start and shuts off when it reaches governed speed. You can quickly
tell if this is installed by looking for the APU BAT position on the metering
panel and the APU BAT OVHT light on the aft
overhead panel.
It is recommended that the APU be operated for one full minute
with no pneumatic load prior to shutdown. This cooling period is to extend the
life of the turbine wheel of the APU.
 |
 |
|
Garrett 85-129 APU panel
EGT limits marked and oil temp & pressure captions.
|
Garrett 36-280 / Sundstrand / AlliedSignal APU panel No EGT limits and MAINT & FAULT captions. |
| Note: NG APU panels do not have an AC ammeter. | |
Components
Sundstrand APS 2000
APU Timer
Some aircraft have APU timers fitted on the aft overhead panel, since
APU running time cannot be measured by aircraft logbook time.
Fire Protection
There is only one APU fire bottle, despite the fact that the
handle can be turned in either direction! It is filled with Freon (the
extinguishant) and Nitrogen (the propellant) at about 800psi. When the fire
handle is turned, the squib is fired which breaks the diaphragm on the bottle,
the pressure of the nitrogen then forces the freon into the APU compartment
which suffocates the fire. Note that after a squib has been fired,
the yellow disc on the fuselage may not blow completely clear, see photos below.
 The APU fire extinguisher bottle indicators comprise of one yellow disc to show if the squib has been fired and one red disc to show if the bottle has over temperatured (130C) or over pressured (1800psi). Some aircraft are fitted with the sight glass to the bottle pressure gauge. Note: Sight glass and bottle indicators are not fitted to NG's. |
 This photo shows the condition of the discs after the APU fire bottle had been discharged. Notice how the yellow disc is displaced slightly but has not been blown away, this could easily be missed on an external inspection. Since the bottle only contains nitrogen and freon, there was no other external evidence of the bottle having been used since the evidence had evaporated away. |
General
The APU will auto-shutdown for the following reasons:
- Fire
- Low oil pressure
- High oil temperature / Fault
- Overspeed
The OVERSPEED light may illuminate for any of the following
reasons:
- An aborted start (overspeed signal given to shutdown). – Further restart may be attempted.
- A real overspeed while running. – Do not restart.
- On shutdown (failed test of the overspeed circuit). – Do not restart.
There is no CSD in the APU because it is a constant speed
engine.
If the APU appears to have started but no APU GEN OFF BUS
light is observed then you may have a hung start.
The current limit is 125A -air and 150A -ground, due to
better airflow cooling on the ground. The galley power will automatically be
load shed if the APU load reaches 165A. Because of these limits, the APU may
only power one bus in the air. However, if you should accidentally take-off with
the APU on the busses then it will continue to power both busses. If the APU EGT
reaches 620-650°C, the bleed
air valve will modulate toward closed. (This can lead to an aborted engine start
if the electrics do not load shed first.)
LOW OIL QTY/MAINT – When illuminated, you may continue to
operate the APU for up to 30 hrs. Note: this light is only armed when APU switch
is ON.
FAULT – Although the malfunction will cause the APU to
auto-shutdown, additional restarts may be attempted.
Max recommended start altitude – 25,000ft Classics; No
limit NG's.
Each start attempt uses approx 7mins of battery life.
Classic: Switching the battery off will shutdown the
APU on the ground only.
NG: Switching the battery off will shutdown the APU
in the air or on the ground.
The APU is enclosed within a fireproof, sound reducing shroud which must be
removed before access can be gained to its components.
There are
two drain masts. The one just aft of the port wheel-well is shared with the
hydraulic reservoir vent and is a shrouded line enclosing the APU fuel supply
line, this collects any leakage of fuel into the shroud which can be drained
when a stop cork is pushed up in the wheel-well. If fuel drains when the stop
cork is pushed, it indicates a leak in the APU fuel line.
|
|
The drain mast
on the APU Cowling (see photo left) mates with the APU shroud and drains oil from the forward
accessory and the compressor bearing.
The opening at the top of the photo is the cooling air
vent.
|
The APU shroud (center), fuel supply line (left),
bleed air duct (right) and cooling air vent (outlined in red). Note this
metal shroud is replaced by thermal fire protection blanket on the NG.
|
The APU cowling showing the lines to the discharge discs and the cooling air overboard exhaust. The small access panel above the cowling is the line of sight oil filler, this is sometime located ventrally in front of the cowling for easy access from the ground. |
|
NG Eductor cooling systemThe 737 NG APU is immediately recognisable by the new "eductor" cooling air inlet above the exhaust. This and the new silencer makes the NG APU 12dB quieter than the classics.The eductor works by using the high speed flow of the APU exhaust which forms a low pressure area. The low pressure pulls outside air through the eductor inlet duct to the APU compartment. The cooling air then goes through the oil cooler and out the APU exhaust duct below, eliminating the need for a separate cooling air vent or fan. The protrusion on the lower right hand side of the photo is the vortex generator on the APU air inlet door. |
 |
Limitations & Operating Techniques:
APU life can be shortened by incorrect operating techniques.
This can be helped by allowing the correct warm-up & cool-down times and bleed
configuration for each type of APU. They all differ slightly due to engine core
and design differences, but the manifestation of the failure is usually a
turbine wheel rotor and/or blade separation. The following table is based on
manufacturers recommendations.
|
|
Garrett 85-129
|
Sundstrand APS 2000
|
Garrett 36-280 | Allied Signal 131-9(B) |
|
|
737-1/200 & some 3/4/500's
|
737-3/4/500
|
737-3/4/500
|
737-NG
|
|
EGT Gauge Markings |
850C Gauge
With colour bands
|
850C Gauge (Pre V14.1 FADEC)
1100C Gauge (V14.1 FADEC onwards)
|
1100C Gauge
|
1100C Gauge
|
|
EGT Limits |
Max start 760C
Max cont 649C
|
No limits
|
Max start 760C
Max cont 710C
|
No limits
|
|
Starter Limits
|
2nd – No wait
3rd – 5 mins
4th – 1 hour
|
1st – 3rd – No wait
4th – 30 mins
|
2nd – No wait
3rd – 5 mins
4th – 1 hour
|
No limits |
| Max alt Bleed & Elec | 10,000ft | 10,000ft | 10,000ft | 10,000ft |
| Max alt Bleeds | 17,000ft | 17,000ft | 17,000ft | 17,000ft |
|
Max alt Elec
|
37,000ft | 37,000ft |
37,000ft
|
41,000ft |
| Warm up period | 3 min | 3 min | 3 min | 3 min |
| Bleed Pack Operation | 1 pack | 1 pack | 2 pack | 2 pack |
| MES to APU shutdown | 1 min (unloaded) | Immediately | Immediately* | Immediately* |
| APU shutdown | 1 min (unloaded) | Immediately | Immediately* | Immediately* |
*Initiates automatic cool down cycle.
Warm up period: The minimum time to run the APU before a pneumatic
load is applied. This allows the turbine wheel temperature to stabilise before a
load is applied. Whilst 3 minutes is the recommended figure, 1 minute should be
the absolute minimum. Note an electrical load may be used with no warm up
period.
Bleed Pack Operation: The number of packs to use on the ground. APU's
which should run both packs have load compressors to supply bleed air. So two
pack operation gives both cooler turbine wheel temperatures and a lower fuel
burn.
Main Engine Start (MES) to APU shutdown: The cool-down time to allow
after main engine start.
Note also that there should be a minimum amount of time between turning off
the pack(s) and starting the first engine. Additionally, minimum delay should
occur between starting the first & second engine. This prevents the turbine
wheel temperature from decreasing and then significantly increasing when the
second engine is started.
APU shutdown: The cool-down time to allow after flight, after the
packs have been switched off. Note, it is important to allow the APU to complete
their shutdown sequence before the battery is switched off.
Ref: Flt Ops tech Bulletin 99-1
Pack Operation and Fuel Flow
Single pack operation is not recommended with the Allied
Signal 131-9 APU. The following 737-700 CDU BITE pages show the reason
why:
A single pack must work harder than two packs to cool the cabin to a
given temperature. Hence the APU must supply higher bleed air
pressures to assure proper environmental control system operation. This
higher pressure requires a greater Inlet Guide Vane (IGV) open position
than that required for 2–pack operation. Since there is less airflow
required to operate 1–pack than is needed, a significant amount of
unused bleed air is exhausted through the Surge Control Valve (SCV).
This higher IGV open position and large quantity of unused air
translates into higher APU fuel burn and higher EGTs during 1–pack
operation. Also, the high airflow levels exhausting through the surge
control valve increases the overall APU generated noise by 2dbA. With 2
packs supplying the cabin cooling requirements the pressure requirement
is lower, resulting in lower turbine inlet temperatures, EGTs and far
less unused air being discharged through the surge valve.
The Future
Boeing has started flight tests of a Solid Oxide Fuel Cell (SOFC)
APU. The SOFC uses jet fuel as the reformer in the proton exchange
membrane to give a 440kW APU that is 75% efficient compared to the
conventional 40-45% efficient APU's. This would give a typical fuel
saving of 1,360t for a 737 over a year. It is actually a hybrid gas
turbine / fuel cell due to the sudden surges in demand eg engine starts
and gear retraction etc. The SOFC will use air from a compressor passed
through a heat exchanger for its gas turbine section. A potential
drawback is that it has a 40min start-up time, so it would have to
remain on for the whole day and depending upon its noise levels this
could be a problem at airports which require the APU to be shutdown
during the turnaround. The technology for the SOFC APU to replace the
current APU is not likely to be available until at least 2015.
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