Just noticed that the Fwd Left Tank Fuel Boost Pump "PRESS" light in PIC extinguishes when the APU is being started with the Battery only (no EXT power available/on). This doesn't quite sound right. Shouldn't the APU be supplied with fuel by a different, DC powered fuel pump during this time?Also, I noticed on our 767's, that if EXT power is available on the main busses and you select the APU control switch from OFF to RUN (not all the way to START), the PRESS light extinguishes almost immediately.Incidentally, is this Fwd Boost Pump powered by the Ground Service Bus in PIC? I thought it might be a good indication that power is on the Ground Service Bus and the R Main DC bus when doing electrical checks in PIC (in flight): Looking at our wiring diagrams, the Right Main 28 DC bus appears to be controlling the pump's 3 phase AC power on our aircraft. Just wondering if I've overlooked something here?Thanks,Cheers.Ian.

Hello, just thought I would try to answer your question. The APU in the 767 as well as the 757 gets its fuel from the left main fuel tank and the PRESS light is extinguished as soon an RUN or START is selected. All it is indicating is the fuel sov for the APU is opening. This is the only tank used for APU unless the refuel/defuel panel is opened on the ground...then it feeds from the center tanks. Doing that prevents an imbalance during long turn around times. 600lbs first hour and 500 each additional if I remember correctly.Hope I was able to answer your question.Regards,Weave

"the PRESS light is extinguished as soon an RUN or START is selected. All it is indicating is the fuel sov for the APU is opening." G'day, Weave.Thanks, but I'm still not sure about this...Here is my interpretation of the APU fuel feed taken from the airplane schematics...http://www.ozemail.com.au/~iriddell/767/APUFuel.gifNote the two pressure switches. The one on the main pump, I believe (until I can get my hands on some better diagrams at work), controls the main pump PRESS light in the cockpit. With the main pump feeding the running engine, the light extinguishes because the main pump pressure is being sensed by the switch. However, for the APU DC pump to extinguish the PRESSure light, the fuel would have to flow backwards through 3 check valves to get to the main pump PRESS switch. I doubt that the pressure switch on the APU DC pump would also extinguish the PRESS light, as this may lead to confusion: Say, if the main pump failed... and the APU pump was somehow running on DC in flight due to an Elec Bus failure.... you might think that the main fuel pump was still running).Anyway, thanks for the feedback... and the other interesting info (all new to me). :-)Cheers.Ian.

Ooops... Make that TWO check valves... I drew one too many in my diagram..... Here's a corrected diagram. http://www.ozemail.com.au/~iriddell/767/APUFuel2.gifNote the addition of the CWT pumps.Cheers.Ian.

Here is some reference material from the Maintenance Manual:"Either the ac fuel pumps, or the dc fuel pump may provide APU fuel feed. The ac pumps have priority. Priority and control are automatic, and are a function of ac power availability and the ac fuel pump switches. The APU fuel sov is controlled by the position of the APU switch and the APU fire switch. The APU "FAULT" light provides valve transit indication. Normal APU fuel feed is from the left fuel crossfeed manifold. The same ac pump(s) that are feeding the left engine are also feeding the APU. The left forward boost pump may automatically provide APU fuel feed, even when the pump control switch is "OFF". This occurs when the APU switch is "ON", the airplane is on the ground, and the ac ground service bus has power. When the ac pumps are not operating, the dc pump automatically provides APU fuel feed. The dc pump operates whenever the APU switch is "ON", battery switch is "ON", and the "pump system pressure switch" on the left rear spar senses low fuel pressure on the left crossfeed manifold."Hope this helps.Regards,Weave

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