APU B-747
 

Hi there! Is there any sence?-"Before start up APU recommended to set APU start selector to ON position, wait 25-30sec then set to START position"? What will happen if for APU starting I"l set selector to START position without pause?

Is there an APU inlet door on the 747?

How else would it get its air?

The question is- for APU start up should I hold Master control switch in ON position 25-30 sec then set to START position, then to ON OR I may set to START immediately, after that release to ON? Is there any technical reason?

Boeing recommends turning the switch to ON for a few seconds before selecting START to give the APU inlet door time to open completely before the APU actually starts to spin up. The APU will not start properly if it senses that the inlet door is not open.

Is the sequence not automatic like on the 737G ? First it opens the inlet door and then it starts the APU automatically.

On the 744 the APU start is a spring loaded rotary switch.

You momentarily hold it to START and then move to the ON position.

There's some debate that selecting ON to 'open the door' has some supposed advantage, but it makes no difference.

in answer to the question, NO, a momentary pause when going from off to on and then to start is all you need to do.

The APU Controller will not engage the starter relay if the APU door is not sensed open more than 13 degrees. The door goes to 45 degrees on the ground and 15 degrees in the air (if running prior to takeoff).

Unfortunately, there are no warnings that the door is not at 45 degrees for ground operation. The larger opening, I assume, is required for proper cooling and airflow through the engine on the ground.

There will be an EICAS message if the door has been commanded open and the door has not reached 13 degrees after 60 seconds. If you only waited 25~30 seconds, you wouldn't even see this message.

I couldn't find any values for door opening speed, but it wouldn't hurt to wait 25~30 seconds to give the door a little extra time to completely open before going to START on the ground.

@ spannersatcx

Spanners, would you be able to check on the EICAS APU Maintenance Page how long it takes the door to show OPEN (13deg) when going from OFF to ON? From this we could calculate, approximately how long it takes to go to 45 degrees. I've checked this in the past, but have lost my notes.

Not a 747 guy, but one reason I heard (recommended for my current stead) is opening the door allows a cold soaked APU to warm enough to help the start.

GF

Don't know about the B747-400, but I suspect it would be the same as the Classic on which placing the APU Master switch to ON powers the APU Fuel Shutoff to OPEN, initiates the APU Inlet door to OPEN as well as pressurize the APU fuel system. The APU Starter Circuit is deactivated until the APU Inlet Door is FULLY OPEN. That is the way Mr Boeing designed it to work.

Well done Old Fella! That makes sense to me. Nice to see you're still payin' attention!

Thanks Arfur. Just trying to keep dementia at bay.

Looking at my technical training books for the Classic....

"Door Position Switch

There is a door position switch, located adjacent to the door hinge,
which identifies the door position at the 13deg position. The switch
position information is for the ETC and will inhibit the APU start if
the door is not >13deg open.

NOTE: Not all A/C will be fitted with the external 13deg position switch"

The switch is labelled as S1598.

The notes do not specify what happens if the door switch is not fitted. However, if I recall correctly, the start procedure specified that the door transit light be extinguished prior to cranking. The diagrams show 15deg and 45 deg switches in the door transit light circuit. The appropriate position switch is used depending on air/ground status.

P.S. After further reading, I did find a 15deg switch in the Classic starter relay circuit (independent of the 13 deg position/ETC circuit). Again, this is not fully open.

ETC = electronic turbine control unit (an older version of the APUC which is fitted to 747-400s).

The door position switch is located on the air inlet duct, above the door actuator, near the lower hinge position of the inlet door. The purpose of the switch is to detect that the inlet door has opened, working in conjunction with, but independently from the internal switches of the actuator. Should the door fail to open within 60 seconds, or open to a position less than the minimum 15-degree position, the electrical circuitry of the APU control unit is such that the APU will not start and the EICAS fault message APU DOOR will be displayed.

The battery switch on the P5 overhead panel is set to the ON position before an APU start. The APU control switch on the P5 overhead panel is set to the START position, hold at the START position for one second and manually set to the ON position. The air inlet door actuator opens the air inlet door to the fully open position (45°). If the air inlet door does not open in 60 seconds, the EICAS display shows "APU DOOR" on the status page. During an in-flight operation, the ground safety relay, R263, is energized and the air inlet door closes to the partly open position (15°). The APU fuel shutoff valve opens on the rear spar No. 2 main tank. If ac power is available, the ac aft fuel boost pump supplies fuel from the No. 2 main tank of the airplane to the fuel pump. If ac power is not available, the dc fuel boost pump operates from the airplane main battery. After the air inlet door opens, the APU controller does the APU start sequence and supplies 28V dc power to the starter motor. The starter motor starts to turn the low pressure (N1) and high pressure (N2) turbine rotor assemblies. The speed sensor (N1) measures the speed of the input shaft of the load gearbox. The speed sensor (N2) measures the speed of the oil pump drive. When the N2 rotor speed is 8%, the APU controller energizes the ignition exciter and sends a signal to the fuel metering unit to supply fuel to the combustion chamber. The ignition exciter causes the two igniter plugs to make sparks. The APU engine has combustion and starts to increase in speed. When the N2 rotor speed is 40%, the starter motor and ignition exciter are de-energized by the APU controller. When the N2 rotor speed is 55%, the APU controller sets the inlet guide vane (IGV) demand to a minimum. Two seconds after the N1 rotor speed is 95%, the APU is prepared for pneumatic and/or electrical loads

Again this comes back to the definition of OPEN and which switches are used to show open. The wiring schematics don't seem to align with Spanner's notes.

747-400 Wiring Schematic 49-15-01 shows door position switch S1598 sending a signal to the APUC (on pin D11). The switch has the note "Shown closed <13deg". So we can assume that OPEN is 13 deg or greater.

On Wiring Schematic 49-11-02, the APU start system is shown. Inside the APUC, the driver for the APU starter relay has a logic "AND" gate with one of the inputs labelled "APU INLET DR OPEN". I take from this that the start system is looking at the 13 deg switch "S1598".

Then we have the switches on the actuator (Wiring Schematic 49-14-01). Some control actuator power and some are used to generate the "APU DOOR" EICAS message. One switch is labelled 15deg, but I don't see it affecting APU Start (as mentioned in Spanner's notes). None of these switches feed into the start system.

So APU door swx says >13deg - door open signal to APUC to get starter going, actuator then fails, so although the door signal is open from the door swx, the actuator fails to reach full stroke, as it has failed, therefore after 60secs, although the door is 'open' the actuator is not fully open - EICAS - APU DOOR - ? Then get the MEL out! :ok:

My FCOM says :
START, then ON.
This starts the automatic start sequence. If Boeing want anything else doing, they'll put it in the FCOM or recommend it in the FCTM.