The auxiliary power unit (APU) has a self-contained lubrication system that includes an oil reservoir integral with the gearbox assembly.

The oil system lubricates the APU gearbox and the main shaft bearings. The low drag coefficient of the system gives better start-up reliability and decreased battery wear. The reservoir is not pressurized, which simplifies its maintainability.

The oil system:

• Supplies the correct quantity of oil to lubricate the applicable APU components
• Keeps the APU oil supply in the correct ranges of pressure and temperature
• Removes contamination from the oil when it flows through the oil filter
• Gives the indications of the oil system conditions to the electronic control unit (ECU)

Oil level may be examined at a liquid level gauge on the right side of the APU gearbox. A filler knob with an attached dipstick is located on the left side of the APU gearbox. Oil may be added and the oil level examined through the filler knob. If there is damage or deterioration of the lubricated components, metal particles usually collect in the sump. The magnetic drain plug can be used to examine the sump oil for indications of such damage or wear. An oil-pressure test port is found in the top area of the gearbox for local pressure tests of the gearbox.

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The oil reservoir is incorporated in the APU gearbox. The lower portion of the gearbox forms the oil sump that stores the oil supply. The oil is supplied to the APU oil system components. The gearbox is vented using an ejector type vacuum pump that maintains a negative gearbox pressure. This negative pressure prevents oil vapors from passing through compressor carbon seal into the air inlet, thus reducing the possibility of oil vapors in the cabin. A liquid level gauge and a oil fill cap allows for servicing the gearbox. A magnetic drain plug provides for oil draining and lubrication system inspection. The oil capacity is 2.0 qt (1.9 liter).

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The oil pump is bolted to the face of the gearbox and has a gear that engages with the gearbox gears. The oil pump draws oil from the oil sump and provides 3.0 gpm of lubricating oil to all moving parts within the engine. A shaft between the fuel control unit and the oil pump transmits mechanical power to the fuel control unit. The fuel control unit is connected to the oil pump with a V-band coupling.

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The oil pressure regulating valve attaches to the APU gearbox. This valve controls the lubrication system pressure. The oil pressure regulating valve sets the system pressure at 40 ± 5 psi (275 ± 34 kPa). Oil pump output pressure above this value is bypassed back to the oil sump. A test port located in the upper area of the gearbox permits direct oil pressure reading.

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The oil filter assembly is bolted to the gearbox above the oil pump. The filter element is held in position by the circular oil filter cover. The filter is a 10-micron disposable filter element constructed of pleated cellulose material encased in a metallic mesh envelope. The oil filter element has no bypass capability.

Pressure is monitored by the LOP switch and when the pressure drops sufficiently, the APU will be shutdown automatically (Auto shutdown).

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The low oil pressure switch is mounted on the top of the gearbox. It is located between the starter motor mounting pad and oil filter. The low oil pressure switch provides protection against low oil pressure conditions.

During the APU start cycle, oil pressure increases, causing the switch to open by 35 psi (241 kPa). If during ground operations the oil pressure drops below 26 psi (179 kPa) for 14 seconds, the ECU initiates a protective shutdown. During inflight operations, the ECU only initiates an EICAS message to alert the flight crew of a low oil pressure condition.

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The oil temperature sensor is a resistive thermal device (RTD) that is installed on the lower front left side of the APU gearbox. If during ground operations the oil temperature becomes higher than 163°C (325°F) for 10 seconds, the ECU initiates a protective shutdown. During inflight operations the ECU only initiates an EICAS message to alert the flight crew of a high oil temperature condition.

Cooling fins on the inlet housing transfer generated heat from the oil to the inlet air.

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The ejector vent is a vacuum pump that prevents oil leaks into the bleed-air supply and oil fumes in the cabin. It is located on the top side of the APU plenum chamber. The ejector vent is operated by compressor discharge air through the plenum drain fitting. The ejector pump discharge is directed into the exhaust pipe. Compressor discharge air flows at a high velocity through the orifice, creating a negative pressure that creates a slightly negative gearbox pressure. The lower pressure within the gearbox helps reduce the possibility of oil leaking past the compressor carbon seal.

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The magnetic drain plug incorporates a drain plug, a chip collector, and a check valve. The magnetic drain plug is installed in the bottom of the gearbox sump and has three openings which let the sump oil flow around the chip collector. The chip collector is a magnet, which collects metal particles from the oil in the sump. The magnetic chip collector is removed to check for metal particles that may indicate internal damage to the engine. A self-closing check valve, part of the drain plug, prevents loss of oil when the magnetic chip collector is removed for inspection. The drain plug serves as a means of draining the oil during programmed intervals.

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The liquid level gauge provides an easy way to check the oil level. It is located on the lower right side of the gearbox. The oil fill plug is used to service the oil system. The fill plug is located on the lower left side of the gearbox. Oil capacity can be checked visually using a floating metal ball and a red circle target on the liquid level gauge lens, or by using the dipstick attached to the fill plug. There is a wire to keep it connected to the APU during removal from the fitting.

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The lubricated APU components and the APU generator use the same oil system. The APU gearbox supplies the sump for this system. The two sensors attached to the gearbox send the oil pressure and temperature indications to the ECU. If the oil becomes too hot or the pressure decreases to the minimum necessary, the ECU sends the applicable signals to the EICAS.

During APU operation, the cooler inlet air from the APU inlet duct flows across the APU inlet housing. This air decreases the temperature of the oil.

Oil may be added and oil level examined through the liquid level gauge on the left side of the APU gearbox. If there is damage or deterioration of the lubricated components, metal particles will usually collect in the sump. The magnetic drain plug can be used to examine the sump oil for indications of such damage or wear.

The oil pump pulls the oil from the sump through a cast passageway that connects to a pickup tube. From the oil pump, the pressurized oil goes to a pressure regulator valve that relieves excess pump pressure in the back of the sump. The pressure regulated oil flows through the oil filter element. Oil is then delivered to the components under pressure through cored passages, using a jet spray or mist method of lubrication. All rotating parts, including the rotating group bearings, FCU shaft, planetary bearings, gear mesh points, and accessory support bearings are spray lubricated. A carbon face seal, aft of the main shaft roller bearing, seals the aft-most lubricated area. All oil returns to the sump by gravity.

Use of an oil cooler is not required since there are no areas of lubrication in the heat soaked turbine areas. Cooling fins on the backside of the planetary gear assembly housing are exposed to compressor inlet flow, which decreases the temperature of the oil. The gearbox vents to the exhaust tailpipe through the ejector vent, which uses bleed air to cause a small negative gearbox pressure. This negative gearbox pressure prevents oil vapors from passing through compressor carbon seal into the air inlet, thus reducing the possibility of oil vapors in the cabin.

If the oil pressure decreases to less than 26 psi (179.4 kPa) for 14 seconds, the low oil pressure switch closes and sends a signal to the ECU. If this occurs, the ECU:

• Causes an APU shutdown for protection (on the ground only)
• Sends a signal to the EICAS display (inflight) and the APU OIL PRESS LOW caution message shows

If the oil temperature becomes higher than 163°C (325°F) for 10 seconds, the ECU:

• Causes an APU shutdown for protection (on the ground only)
• Sends a signal to the EICAS display (inflight) and the APU OIL TEMP HIGH caution message shows

The EICAS messages that follow are related to the oil system:

EICAS MESSAGE(S)	LEVEL (COLOR)
APU OIL PRESS LOW	CAUTION (amber)
APU OIL TEMP HIGH	CAUTION (amber)

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Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
-	MAGNETIC DRAIN PLUG	ZONE(S) 320	49-91-05
-	OIL PUMP	ZONE(S) 320	49-91-09
-	OIL FILTER ELEMENT	ZONE(S) 320	49-91-13
S110	LOW OIL-PRESSURE SWITCH	ZONE(S) 320	49-91-17
-	OIL- PRESSURE REGULATING VALVE	ZONE(S) 320	49-91-21
-	LIQUID LEVEL GAUGE	ZONE(S) 320	49-91-25

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