Spanners - thank you. We understand. Others don't - say no more. (Big thanks to you, my friend, as always).

Spanners and NSEU,


Disregard the inlet door 15deg position. That's for AIR mode.


My take on this is..... Having observed the 744 APU inlet door open/close from the ground, it takes about 30secs max (if that) either way. The 13deg sw is telling the APUC that the inlet door is indeed open and is on it's way to the full open 45deg position as confirmed by the actuator internal limit sw. Needless to say the APU start sequence is well on it's way inside 60secs, but ONLY after the APUC has seen the actuator internal limit sw signal.


McHale.http://cdn.pprune.org/images/smilies/smile.gif

Seriously, where and how do Boeing recommend that they want you to pause at On? If you can point out the reference, then I'll have learned something. But I've searched for it and can't find it.

Not sure I follow you, McHale. The APUC activates the starter motor (after a start signal from the pilot). There is no input to the APUC from the actuator internal limit switches. The 13 degree switch is external. This does feed the APUC.

I've compared diagrams for 4 different airlines (basically all the same).

I understand Spanner's comment. The EICAS will tell you in a timely manner that the door is not fully open (a half open door is probably not an immediate problem). If the door is not even at 13 degrees, the APU starter won't activate.

NSEU,


Apologies, I should have said EIU's not APUC. Bear with me on this.


The 13deg inlet door sw is really only an opening(not full open) / transit indicator and duly lets the APUC know that the door is moving towards the FULL OPEN position. The APUC will not engage the starter until it sees the inlet door fully open and all other parameters are satisfied. As mentioned previously the door will fully open well under the 60sec timer limit.


With the aircraft on the ground, the air/grd relay R359 is energised, providing a ground for apu door open relay R461 through the inlet door actuator internal OPEN limit sw.


Energising R461 supplies 28V DC power to the inlet door actuator motor open windings, and the 60sec timer in the EIU's.


When the door is opened to full open at 45deg the inlet door actuator open limit sw opens removing the ground from R461 and subsequently power from the timer.


Now it gets a bit hazy with my notes, but I would go so far as to say that the EIU's signal the APUC via the ARINC bus to say that the door is fully open due no timer operation. Or timer has exceeded 60secs and no start and APU DOOR status.


McHale.http://cdn.pprune.org/images/smilies/smile.gif


PS..... We may have similar notes... APU chapter page ref 40/41.

I'm afraid we'll have to agree to disagree on this one.

My notes say:

"Should the door fail to open to position greater than 13 degrees, the electrical circuitry of the APUC prevents the APU from starting...."

13 degrees is enough to start the APU, but it's just not advisable to run the APU on the ground for long periods like this, hence the EICAS message if the door doesn't reach the fully open position. I believe the APU Maintenance Page uses the 13 degree switch to show OPEN, rather than having a circuitous path from the actuator to the EIUs to the APUC.

I wanted to prove this for myself a few years ago, but never got the opportunity to have an observer in near the door and in the cockpit communicating to each other. It's too difficult to see when the door stops moving from the ground... and I'm not going to suggest to a currently serving engineer to pull the EIUs and start the APU just to prove a point :P

My Engineering training notes also say (regarding the APU Maintenance Page):

"Inlet Door Position... The APUC senses door position from the 13 degree inlet door position switch"

also..

"Starter Status
- The APUC commands the starter on and off. The starter engages after the APU selector switch is rotated to start and the APU door opens greater than 13 degrees"

Cheers
NSEU

NSEU
I"m afraide to mistake, but yesterday I try to check on the EICAS APU Maintenance Page (CMC) how long it takes the door to show OPEN (13deg) when going from OFF to ON? Nothing indication, absolutely! No N1, Door open, EGT... It means: start sequence does not start? I wait 30 sec then set to START. After that APU start up has been successfully performed. In this case NO reason to wait some second in ON position before start up APU.
anson harris
You are right, there are not papers where do Boeing recommend that, only "a little bird told".

My mistake. I forgot to mention that (normally) the APUC is not powered until you momentarily go to START. The APUC provides information to EICAS.

However, you might be able to power up the APUC with the Ground Maintenance Test Switch on the overhead panel prior to or during APU start (sorry, my memory is a little vague on this). Unfortunately, whatever happens, the APU has to run though a self test before it can start the APU. This may lead to delays in the parameters appearing. By that time, the door may already have already gone well past 13 degrees. According to my notes, this is the trigger for the OPEN indication on the maintenance page.

Unfortunately, both McHale's and my notes have some serious contradictions in them. It would be a step forward, however, if someone can prove the APU can start without the EIU's being powered before moving the APU selector away from OFF. The only possible signal available to the APUC to initiate the starter motor would then be from the 13 degree switch.

Note that the door opening relays are controlled by both the ON switch and the APUC. This is why you can open the door prior to powering up the APU Controller.

Update:

I've just discovered this duplicate statement in a European airline's maintenance training notes.

"If the door fails to open to a position greater than 13 degress, the electrical circuitry of the APU prevents the APU from starting".

These maintenance training notes do have a common source (Boeing), but each airline adds their own text modifications to suit their aircraft. Right or wrong, these additions are influenced by the editors' understanding of the systems.