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205 Fuel Prob Versus Low Fule Indicator..


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Hello guys..or gal..need to pick the brain of any of you med engineers...I have a fuel problem on a 205 ( the engineering crew I work with is not familiar with the machine enough to figure this one out..another topic please..LOL) heres the issue

 

I was doing a cross country flight..at 1.7 hrs and 400# indicated I got a low fuel light....made it to the airport in good time..the light should come on at 270 # +/- 20...this was a long straight flight...on other occasions doing short hops with low fuel..ie in the 5 to 300 # range ..no light...

 

Upon returning to base and parking for the night we defueled the AC.. at this time we found 2 things ..first the fuel gauge was not calibrated right ,,was reading 100# low...and as they defueled the light came on at 300#..I should add that also when lifting off full fuel..indicating 1300 # once airborn the gauge almost immediately drops to 1200#....

 

My thought is that when I am working doing short moves the fuel is able to transfer fast enough for the system to work in sink but on the long steady flight the fuel transfer is restricted...

should mention that the machine is equipped with dual electric pumps..what in the fuel system will restrict or slow the fuel transfer between the lower cells....????

 

thanks for your help here....

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It's not likely you have a fuel transfer problem as the crossover tubes both front and back are open all the time, unless of course there's something quite large like a rag or something sloshing around in your fwd fuel cells...possible? but not likely. The low fuel level light seems to be coming on close to where it should be (low fuel switch is in left fwd cell). There's two fuel quantity probes in right fwd cell and one in aft center cell, capacitance type. It could be a problem with one or more probes, but more than likely a bad ground or cxonnection in the system. Start by cleaning up your grounds...all of them, checking the connections. Next clean all the electrical connections from front to back, checking for integrity of connections as well as the wiring as you go. Bad wiring, bad connections, bad indications. If no difference, then I'd go after the probe in the center cell, as it's active with full fuel.

Good luck!

Whiskey Cobra

 

Hello guys..or gal..need to pick the brain of any of you med engineers...I have a fuel problem on a 205 ( the engineering crew I work with is not familiar with the machine enough to figure this one out..another topic please..LOL) heres the issue

 

I was doing a cross country flight..at 1.7 hrs and 400# indicated I got a low fuel light....made it to the airport in good time..the light should come on at 270 # +/- 20...this was a long straight flight...on other occasions doing short hops with low fuel..ie in the 5 to 300 # range ..no light...

 

Upon returning to base and parking for the night we defueled the AC.. at this time we found 2 things ..first the fuel gauge was not calibrated right ,,was reading 100# low...and as they defueled the light came on at 300#..I should add that also when lifting off full fuel..indicating 1300 # once airborn the gauge almost immediately drops to 1200#....

 

My thought is that when I am working doing short moves the fuel is able to transfer fast enough for the system to work in sink but on the long steady flight the fuel transfer is restricted...

should mention that the machine is equipped with dual electric pumps..what in the fuel system will restrict or slow the fuel transfer between the lower cells....????

 

thanks for your help here....

 

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With dual electric pumps the low fuel light should come on at 10 minutes of fuel remaining at cruise power, as per the flight manual, and go OUT at 270 lbs +/- 20 lbs during refueling. Also, the LH boost pump pressure should read 0.5 PSI higher than the RH.

 

Unless you have a different system than we do, having the low fuel light come on is pretty serious as you may have as little as 70 lbs remaining.

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Thanks for all your help..seems we had 2 problems...one the fuel gauge was under calibrated by 100# and second upon opening the tanks the baffle in the left tank was found to be faulty...and yes GM you are right very serious problem low fuel light with 70# thinking you have 270 is not a time to be in the air...upon defueling the low fuel prob came on at 300,which because of the fuel guage calibration could have meant 200...all this takes the physiological security in ones aircraft away....this problem has been ongoing for months, the a/c doesnt fly long enough ,or enough in general for the problem to be diagnosed correctly...its going to take a few flights to get a secure feeling on this one....

 

thansk again all..

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check to see if the line that returns fuel back to the aft part of the cell (from the ejector) is dumping fuel into the aft portion of the cell (behind the baffle) and not into the fwd part. This will give you a low fuel indication yet the fuel quanity will read around 300lbs.

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Sorry, this relates to the 212, but it may be similar to the 205 system, which I don't know the exact workings of.

 

If a front ejector pump gets partially plugged with crap in the bottom of the tank, only a tiny amount of fuel will transfer to the back. This means the fuel trapped in the front will still show-up on the fuel gauge (a measure of total quantity), but the low fuel light will activate because there isn't much of that total fuel in the back of the tank where the the low-fuel sensor is located.

NOTE; this can be very serious because the boost pump that sends the fuel to your engine is also in the back and may run out of fuel, even though there is fuel showing on the gauge.

So, have the low-fuel light calibrated regularly and BELIEVE what it tells you!!

 

In the 212 (unsure about 205) you can work around this by opening the interconnect valve. The trapped fuel will connect across to the other front tank, then get ejected back to the rear of that tank. Then it will connect back across to the rear of the tank it originally was in.

However, this is only if the interconnect lines are clear. It is surprising how much crap can accumulate in the bottom of fuel tanks from who-knows-where. If the ejector is plugged, there is a chance there is crap in the interconnect too.

Again, regular tank maintenance can prevent this build-up getting to be excessive.

 

Also, if the front ejector gets completely plugged it will activate the boost pump warning light.

(Descend below 5000', in case it is a real boost pump failure and the good pump also fails).

You can check whether it is a real boost pump failure or a plugged ejector using this method.

On the ground, (NOT in flight !!!), close the cross-feed valve, and watch the fuel pressure gauge on the affected side.

If the pressure stays up, the boost pump is okay, and the ejector is plugged.

If the pressure drops to zero, the boost pump has failed. Re-open the cross-feed valve and the good pump will send fuel across to also supply the affected side.

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ummm i'm going to guess......it was an poorly shimmed flapper valve or.......maybe the hose from the ejector pump not clearing the baffle ...

 

Good info on the 212 OT...the fuel system in the 205 is designed to isolate one side of the fuel system from the other like the 212 is...cross-feed valves and interconnect valves not present...

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ummm i'm going to guess......it was an poorly shimmed flapper valve or.......maybe the hose from the ejector pump not clearing the baffle ...probably the flapper valve. The reason that it happens on long flights is the reason the baffle and ejector pump is there in the first place. A faulty flapper valve allows fuel to leak back into the front of the tank. On long flights there is little attitude change and the fuel will may be leaking forward and burning faster than the fuel coming from the ejector. On short hops...the attitude changes of the aircraft slosh fuel through the flapper into the back of the tank. If you combine this with a ejector hose not postioned properly and not spraying the fuel properly over the baffle wall...beaucoup de problems...

 

 

Good discussion....which reminds me of a little bell medium fuel system trivia question...

 

There are 3 things that can cause a boost pump light to come on in the medium...OT list 2...

 

cheers

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