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BVM UAT not refilling

Old 10-17-2023, 09:16 AM
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ddfwaj
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Default BVM UAT not refilling

Hey guys, hoping for some help from the experts! I am a new turbine pilot and not really sure if this is a problem but thought I would ask.

I am doing a manual pump test to verify my fuel system integrity and I noticed when the MAIN tank becomes empty and the clunk is drawing in some air from the MAIN tank the UAT seems to not refill as fast. Is it normal to have only about 50% of the UAT full when the fuel level is close to empty? I am simulating flight by tipping the plane about 45 deg nose up and down as the fuel got low in the MAIN tank and noticed the UAT was also emptying. The UAT stays 100% full until some air is drawn in from the MAIN tank from low fuel and pretty much stays that way until almost empty at about 50% left in the UAT (see pics). I was under the impression the UAT stays 100% full until literally the last drop of fuel in the MAIN tank?

I attached 4 pics, 2 of the pics show the main and UAT full and the other 2 pics show the low fuel in the main and the half full UAT.

Appreciate any help anyone can offer,

Thanks,
Dennis








Old 10-17-2023, 02:38 PM
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causeitflies
 
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This is normal. The only way to get trapped air out of the UAT is through refilling the tank and pushing the air to the main tank and out the vent. It does it's job by trapping air from the main tank. There is no way that it can refill by itself as all the fuel being drawn in is used by the engine. When the fuel level in the UAT gets below 50%, air will start to be drawn into the engine.
Old 10-17-2023, 03:45 PM
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ddfwaj
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Originally Posted by causeitflies
This is normal. The only way to get trapped air out of the UAT is through refilling the tank and pushing the air to the main tank and out the vent. It does it's job by trapping air from the main tank. There is no way that it can refill by itself as all the fuel being drawn in is used by the engine. When the fuel level in the UAT gets below 50%, air will start to be drawn into the engine.
OK thanks for the explanation, so basically I was close to flaming out the engine as my UAT was at about 50%? I have read here on other threads that the UAT stays pretty much full all the time which seems correct but then it also makes sense the UAT is doing its job if the air coming in from the main tank is being stopped by the UAT until at some point it can't than air bypasses the UAT and introduces air into the system.
Old 10-18-2023, 12:52 PM
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RCFlyerDan
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Originally Posted by ddfwaj
OK thanks for the explanation, so basically I was close to flaming out the engine as my UAT was at about 50%? I have read here on other threads that the UAT stays pretty much full all the time which seems correct but then it also makes sense the UAT is doing its job if the air coming in from the main tank is being stopped by the UAT until at some point it can't than air bypasses the UAT and introduces air into the system.
I would also like to add that after you’re finished flying for the day. Do not pump the fuel out of the UAT. BVM filter bag has some sponge in between the layers. If you pump the fuel out at the end of the day, you are introducing air again. The air becomes bubbles in the sponge and causes flame out. I leave what fuel is in the tank and UAT to also help keep the fuel line limber.

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