System Description by Don Pierce: APU

"Don Pierce is a L-1011 Flight Test Engineer (Ship 3), then in Customer Training, then he had his own business doing maintenance training for L-1011 customers. Don is nothing but amazing in his knowledge of the L-1011. I am going to post several conversations on systems that I have had with Don over the past few weeks"





Comments about the APU panel in the photo you sent me:

There are the four red "fault flags" that appear when the APU controller (electronic control unit) receives an auto-shutdown signal. They will "trip" or appear when the appropriate problem is sensed by the APU ECU (electronic control unit). They can be reset back to the normal "blank" position by pressing the momentary RESET button, and the APU Master Power switch is ON. (This switch is spring-loaded Off.) If I remember correctly, all four fault flags will appear simultaneously when the FE's lights TEST switch is used.

N1 = Ng
N2 = Nf ... these terms are used interchangeably.

g = gas generator section of the APU, which operates at a varying speed as necessary to maintain a constant Nf speed.
f = free turbine section of the APU, which runs at a constant speed and therefore there is no need for an IDG, as for the engines.

The four red "fault flags" are:
The OVERSPEED N2 fault flat activates for three reasons 1) if Nf exceeds 110% during operation, or 2) if there is no Nf signal during start when Ng has exceeded 55% Ng, or 3) if there is no Nf signal
from the "alternate" Nf signal when the "primary" Nf reaches 95%.
The LOW OIL PRESS fault flag activates when oil pressure drops below 50 psi for greater than 10 seconds.
The HIGH TEMP OIL fault flag activates when oil temperature exceeds 118°C (245°F)
For TGT, a thermocouple harness is used to measure an average turbine gas temperature. The OVERTEMP TGT fault flag activates when this average is exceeds limits determined by the APU ECU.

I can't read all the lights on the APU panel that you have, but here are the ones I'm aware of:

• The DONT LOAD light illuminates automatically during the startup-up or shutdown sequence and is just an advisory that you can't put any load on the APU yet. The light also comes on if Nf goes above or below limits.
• The next light to the right should be a "blank" (A function, which I have long forgotten, was deleted)
• The INLET FLOW light illuminates to advise of an auto-shutdown that does not trip a "fault flag". It is caused by blockage in the inlet to APU load compressor. This function is inhibited if the APU is operating in Max Mode.
• The LOW OIL QTY light illuminates when oil quantity is less than 0.5 US gallon. This signal is deactivated when the nose gear air-ground sensing circuits are "in the air". Obviously, this is not an auto=shutdown signal.
• The BATTERY CONDITION light illuminates when the ship's battery is overheated. An APU start should not be attempted when this light is illuminated.
• The DOORS IN TRANSIT (or DOOR IN TRANSIT) light will illuminate at the beginning of the start sequence and end of the shutdown sequence to indicate that the ejector doors are in the process of opening or closing.
• The next light to the right might be a "blank". It might read IGV OPEN, indicating that the inlet guide vanes (IGVs) are in the Max Mode position
• The MAX MODE light has varying signals, depending on the airline. It illuminates when Max Mode has been selected or during an engine start cycle. On some L-1011s, the IGV OPEN function is included in the circuitry for the MAX MODE light.

The BLEED AIR S/O switchlight controls a valve that goes by two different names. In ATA 49 (APU) it is referred to as the Bleed Air Shutoff valve. In ATA 36 (Pneumatics) it is referred to as the APU Isolation valve.. The ON light in the switchight is like the "flowbar" on the Pneumatics panel; it indicates valve position.

The rotary MIN MODE/ NORM/ MAX MODE switch controls an input signal to the APU ECU which will cause the ECU to supply more or less fuel to the APU load compressor. MIN MODE is used when the APU is used only for electrical power (however, a single air conditioning pack could be used at the same time). NORM mode is used when both electrical and pneumatic power are being supplied from the APU (now, two air conditioning packs may be used). MAX MODE should be used only when it is necessary to get maximum output from the APU load compressor. Max Mode is automatically selected by the ECU for an engine start (even though the rotary knob is in NORM). When the BLEED AIR S/O valve is closed, the load compressor automatically reverts to Min mode, regardless of rotary knob position.

In flight, the autofire shutdown circuit for the APU is automatically armed when the guarded AUTO FIRE SHUT DOWN switchlight is latched in and the APU MASTER POWER switch is ON. On the ground, the system is automatically armed, with the APU MASTER POWER switch at the ON position. The ARMED legend in the switchlight is "on" for both situations already discussed. This is a part of ATA 26, Fire Protection.

The momentary START switch is pressed in and held for 2-3 seconds to initiate a start of the APU. Pressing the STOP switch will initiate a normal shutdown of the APU. The shutdown can be interrupted during the "cool-down" phase by pressing the START switch. When the DOOR(S) IN TRANSIT and DONT LOAD lights have extinguished, it is safe to move the spring-loaded APU MASTER POWER toggle switch to the OFF position. APU shutdown is now complete.

Not shown here, but there is also an external APU EXTINGUISERS panel, which also includes controls for a normal shutdown of the APU. The panel includes a NORM STOP switch (to shut down the APU) and a momentary switchight that reads PWR ON when the APU MASTER POWER switch is in the ON position. After completing a normal shutdown of the APU, the "PWR ON" switchlight can be pressed to move the spring-loaded APU MASTER POWER toggle switch to the OFF position. Don't forget that the BATTERY switch is still in the ON position, so if the plane is left standing overnight, this will "drain" the battery.

Note: the other controls on the external panel are all related to APU fire.

So, there are six auto-shutdown signals for the APU:

1. Overspeed of the free turbine
2. Low oil pressure
3. high oil temperature
4. excessive exhaust gas temperature (TGT)
5. blockage of the air inlet to the load compressor
6. fire in the APU

Useless information: The APU is built from the PT6 turboprop engine. The load compressor section of the APU is essentially the same as for the PT6. The free turbine replaces the reduction gearbox of the turboprop.

You may need to get an APU ECU, ILCB, GCU and BPP to assist you in all of these light indications - since so many of them are controlled via the various electronic units. I'm only covering "the tip of the iceberg" - telling you what a light indicates. To make the light to illuminate is yet another challenge.