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EPU activates in flight if EPU switch is positioned to off when throttle positioned to cutoff in flight.


Carbon715
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Was testing some air starts and noticed even when I placed the EPU switch to off, it would still fire off once engine power was cut. Also the air light was on, which with the motor off, no bleed air is being supplied to the EPU to augment hydrazine power.

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Normal and correct. The EPU will fire is if the switch was placed in the ON position any time after takeoff. The triggering condition is main and standby generator offline even if switch is in OFF position.

 

Air light is supposed to be on in this case. It is always on when the EPU is commanded on and pin is pulled, hydrazine augmentation or not.

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Just did some research at work, went through -49FI , -49GS including theory of operation, and asked 3 viper IPs the what if scenario and its all conclusive. If the EPU mode switch is in the off position,  the EPU should not fire regardless of loss of engine , or hydraulic pump output <1000psi or both standby and main generators offline. In norm position, Epu should fire in the air if one of the mentioned issues happens and should fire if commanded on. I work blk 42s, not 50s but I see no reason the logic and system has changed between the blocks.

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  • 2 weeks later...

sdirmitt, you are correct it shouldnt activate if positioned to off, regardless of if the conditions for activation are met, as well the air light should only come on if the engine is running as that if where the EPU gets its bleed air. ive worked block 40s/50s and the EPU works the same on every block

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please include a track replay, and if you have any evidence please PM me. I am happy to take another look 

 

thanks

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To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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Frederf is correct. When the EPU switch is placed to off after takeoff the EPU WILL fire/start automatically. Also the AIR light will always be on when the EPU is commanded on, that is because the HYDRZN and Air light are socalled demand lights. ANd the EPU simply always demand AIR, and when needed the HYDRZN light will come on when it demands that to maintain turbine speed.

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