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APU LOW OIL PRESSure

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The Bulfer manual tells us that the APU "LOW OIL PRESS" light illuminates during start until the APU oil pressure is normal. However, the Boeing Wiring Schematics I have show that the light is inhibited below 95% and then adds a 20 second time delay for the pressure to rise. This seems to indicate that you will never see the LOW OIL PRESS light unless there is actually a problem with the APU (or perhaps the oil is extremely cold???).Are there any variations between NG APU logic? Have any real pilots/engineers seen this light illuminate during a normal start?Thanks.Cheers.Ian.

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Guest wkzzo

Ian ,I can tell you the light illuminates shortly after the switch is moved to start(although it doesn't seem like 20 seconds), it goes out at roughly 30% when monitoring the FMC APU bite page. The light is inhibited with the switch off. The 95% may be for the shutdown mode, because the light does not illuminate on shutdown. I'll try it out monday and let you know.

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Thanks, Wallace.Looking forward to your findings.Cheers.Ian.

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Guest wkzzo

Ian ,the low oil pressure light comes on immediatly when going to start(or just moving the switch to on).It goes out around 22% rpm.the 20 sec time delay is if the speed is 95%+ and oil pressure is low,the ECU performs an auto shutdown after the 20 sec time delay.

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Great stuff, Wallace :-)Engineer "1", Boeing Schematic Diagrams "0" ;-)Cheers.Ian.

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