APU START SOURCE switch ....TR

[Will]

What's the TR mean?

[Avi]

Transformer Rectifier.
This Unit (TRU) converts 115V AC to 28V DC.

When the switch you refer to is in TR, the APU starter is powered by the APU TRU if the TRU is powered.
If the TRU is un-powered, the APU starter is powered by the APU battery.

What does power the TRU?
Good question. I have different sources that say different things.
One source says Galley bus 2 and other one says Utility bus 4 so ...

[Will]

Ah, thanks!

[Qavion]

What does power the TRU?
Good question. I have different sources that say different things.
One source says Galley bus 2 and other one says Utility bus 4 so ...

Not all 744's have Galley Busses ; )

[Qavion]

Note also that there is a bypass circuit when the Galley Power Energency Cutoff switch has been activated (to allow APU TRU ops with the Galley Power switched off).

Cheers.
Q>

[Avi]

Not all 744's have Galley Busses ; )

Yes, the 2nd source was Atlas Air B744F.

[paojack]

Further question over APU TR function, per AMM as follows, in cold cockpit with no external power, APU START SOURCE switch must be in BAT position otherwise the APU will not be started, unless AC power is available and TR is overheat, in this case the power source will automatically transferred to BAT and start APU, right?

The following conditions must be met in order to start the APU using the TRU:
(a) The APU START SOURCE switch, located on the P5 overhead panel, is in the TR position.
EFFECTIVITY
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(b) The APU battery overheat relay is not energized.
(c) The 115 volt ac power is supplied to galley number 2.

[Avi]

in cold cockpit with no external power, APU START SOURCE switch must be in BAT position otherwise the APU will not be started

No. As far as I can remember (I don't have the time now to re-understand the very complicated APU START electrical chart) if the switch is at TRU but the TRU is not powered, the BAT still will be used. You don't need to set the switch in BAT.

Cheers,

[Hardy Heinlin]

In the real world the TR feature is an airline option; in PSX it's installed on all aircraft (for system training without TR, just deselect TR on the panel) and is powered by AC bus 2 via the left utility bus. Ref: Aerowinx manual page 151.

[John H Watson]

in cold cockpit with no external power, APU START SOURCE switch must be in BAT position otherwise the APU will not be started, unless AC power is available and TR is overheat, in this case the power source will automatically transferred to BAT and start APU, right?

I'm not sure what you mean by "cold cockpit". The battery switch must always be switched to ON to start the APU.

On my old airline, the APU START SOURCE switch was always left in TR. The APU started without external power.

I can't post a wiring diagram for this, but the APU crank relay (R7184) allows battery power to reach the APU starter.

The crank relay is controlled by 3 other relays:

1)The APU Start Relay (R7188), which is energised by the APU Controller (if the APU battery overheat relay is not energised).
2)The APU/TRU Disable Relay (R8303), which needs to be depowered to use battery power for start. This relay is depowered if there is AC power on Galley Feeder Bus #2.
3) The APU/TRU Transfer Relay (R8204), which needs to be energised for either a TRU or battery start.

Basically, the "TR" position is an automatic "TRU/Battery selection" position.

Rgds
JHW