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Total fuel usable / low fuel warning


NicoN

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I recently tested both low fuel warnings in flight - they are working. But I have no idea how much fuel is left when the low fuel light goes on. On ground the wing-side analog gauge shows roughly 5 gal left.

Another question: I have 72gal tanks (2x36), but it seems that I can fill them more than 36gals. I guess 38 is possible.

What are your experiences?

 

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It may be the year of your plane’s construction...

The Lo Fuel lights come on based on fuel remaining... the POH has the details on... if it is three or six(?) gallons left in the side, when the light comes on...

Somewhere along the way, Mooney realized that the lower level (three gallons?) wasn’t enough of a warning to make a successful change to the flight plan...

Three gallons, when flying at 12 gph, doesn’t give much warning...

Over the years there have been many changes to Mooney fuel tanks, useable fuel, fuel level devices, and lo fuel level warnings...

Compare your POH, to your beliefs, and see if they are both what is set up in your plane... :)

And also remember hitting the test button actually may activate the Lo Fuel warning system.  Without the activation, it probably won’t work at all...

PP thoughts only, fuzzy memories of discussions around MS...

Best regards,

-a-

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The details of how much fuel should be in the tank when the low fuel warning comes on is in the Maintenance Manual.

Remember your fuel tanks and a certain amount of unusable fuel called out in the TCDS - Type Certificate Data Sheet and is also in your POH. The meaning of "Full" is often not clear but is to the bottom of the anti-siphon filler neck. Of course you can add more fuel beyond this but its neither legal to flight plan with the added fuel nor be sure of exactly how much more you're adding beyond full each time unless you measure it from Full each time - because your fighting the trapped air above the filler neck to add more fuel. The tanks also need some some room for expansion due to temperature.

But its very good practice to measure and confirm exactly how much you can fill your tanks (both usable and unusable) so you really know what your tanks hold. We do see some variation on the fleet.

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I have run a tank to engine stumble in cruise and noted the low fuel light time had 26 minutes of fuel between the light and engine stumble.  My guess is that any fuel below that point was unusable on that side. At stumble I immediately switched tanks and landed.  I then, filled that tank noting the fuel I added back. I will have to check my notes listed on my checklist but it seems like 5 gallons left at light and maybe 0.7g unusable per side.

This test also told me that when both lights come on, I have 45 minutes of fuel left and be on the ground with IFR reserves.

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1 hour ago, kortopates said:

The details of how much fuel should be in the tank when the low fuel warning comes on is in the Maintenance Manual.

Remember your fuel tanks and a certain amount of unusable fuel called out in the TCDS - Type Certificate Data Sheet and is also in your POH. The meaning of "Full" is often not clear but is to the bottom of the anti-siphon filler neck. Of course you can add more fuel beyond this but its neither legal to flight plan with the added fuel nor be sure of exactly how much more you're adding beyond full each time unless you measure it from Full each time - because your fighting the trapped air above the filler neck to add more fuel. The tanks also need some some room for expansion due to temperature.

But its very good practice to measure and confirm exactly how much you can fill your tanks (both usable and unusable) so you really know what your tanks hold. We do see some variation on the fleet.

With the M20J, at least, there is no actual filler neck--it's just the bracket that holds the fuel cap.  When I flew the tank empty, and then added 32 gallons of fuel, it was about a half inch below the bracket.  It took about 33 gallons to actually get up to the bracket.  Presumably, 32 gallons leaves a little room for fuel expansion, so even though I could actually get 66 gallons in, I suppose there's no guarantee that the extra 2 gallons wouldn't come sailing out the vent after takeoff.

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5 hours ago, Bryan said:

I have run a tank to engine stumble in cruise and noted the low fuel light time had 26 minutes of fuel between the light and engine stumble.  My guess is that any fuel below that point was unusable on that side. At stumble I immediately switched tanks and landed.  I then, filled that tank noting the fuel I added back. I will have to check my notes listed on my checklist but it seems like 5 gallons left at light and maybe 0.7g unusable per side.

This test also told me that when both lights come on, I have 45 minutes of fuel left and be on the ground with IFR reserves.

Never mind what's in the manual, POH, maintenance Manuel, etc. Do this and you'll KNOW the numbers for your airplane.

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@gsxrpilot: Right, that is also my opinion! Don't trust - proove it!

I also do not want to extend my flight plannings beyound the legal settings!

But it makes me feel safe to have several items to check whether I am running short of fuel or not:

- the fuel gauges. The seem to work  more or less precise.

- the analog fuel gauges on the wing. Perfect!

- the Low fuel warnings and hopefully a good idea of how much fuel is then left usable!

- my EDM830 which tends to count the fuel consumption higher than in real.

So, I will do the procedure to find out for my aircraft.

 

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Use calibrated dip sticks to figure out how much fuel you have remaining when the low fuel light is illuminated. 

Run a tank to low fuel annunciation. Switch tanks. Don’t go back to that low fuel tank. Land. Figure out how many gallons till the fuel just is up to the place where it’s wet at the filler neck.  Make sure plane is on a level surface.  Note gallons added.  Add fuel in 5 gal increments until you reach full.  Mark on dipstick.  

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14 hours ago, gsxrpilot said:

Never mind what's in the manual, POH, maintenance Manuel, etc. Do this and you'll KNOW the numbers for your airplane.

I very much recommend running a tank dry and seeing when the light comes on and how much capacity your tank has.    I did this some time back and one of my tanks was significantly off.  --Maxwell made some adjustments and the tank empty warning is now correct.

In a similar way, I made a calibrated fuel stick by running a tank dry, then adding 5 gallons at a time.  --And yes, I always had a place to land while I was on the near empty tank.

And for those that have not run a tank dry, is your tank full when it reaches the collar or the top of the tank?  You should know!  My manual is rather silent on the topic.

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2 hours ago, chrisk said:

I very much recommend running a tank dry and seeing when the light comes on and how much capacity your tank has.    I did this some time back and one of my tanks was significantly off.  --Maxwell made some adjustments and the tank empty warning is now correct.

In a similar way, I made a calibrated fuel stick by running a tank dry, then adding 5 gallons at a time.  --And yes, I always had a place to land while I was on the near empty tank.

And for those that have not run a tank dry, is your tank full when it reaches the collar or the top of the tank?  You should know!  My manual is rather silent on the topic.

I roll conservative so my calculations are based on top of the filler neck fuel spitting out.  That’s what I use as 64 gal because my POH doesn’t mention it.  

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