Accuracy of fuel low light?

[Wildhorsetrail]

Coming from an older 172, where fuel gages were wildly inaccurate, I'm curious how accurate you've found the fuel low lights to be. I believe they are supposed to come on at 2.5 to 3 gallons of useable fuel remaining. Running a tank dry gives me the heebie jeebies, but if the light is accurate I'll use it.

[pkellercfii]

The low fuel lights in my ‘89 K seem to work pretty well and consistently. One comes on at about three gallons usable remaining, the other closer to five gallons remaining. Both of those results I obtained in carefully controlled tests in which I ran a tank completely out of usable fuel while circling an airport. That’s a test that I highly recommend to other aircraft owners. You don’t know what your usable fuel really is without performing that test, and then topping the emptied tank afterward. I’d also suggest performing the same test yourself on your own A/C before making any assumptions about how much fuel remains when the low fuel lights activate. 
 

I’m in agreement with those opposed to running fuel tanks completely out of usable fuel as a routine operational practice. Knowing that I have 3-5 gallons usable remaining  when the low fuel light(s)activate, I am willing to burn one or two gallons beyond the low fuel light activation before switching fuel tanks, but no more. I’ve done that several times with no ill affects.

—Paul Keller

 

[Schllc]

Given the many accidents over the years due to running out of fuel, intentionally starving an engine of fuel to check the veracity of a gauge seems to be, to me, an extreme method.  
I don’t plan any trip that close, but to each their own…

I haven’t found any gauges dependable enough to give me comfort to within a gallon, but the totalizers have been accurate. All the ones I’ve used, have been well within that range. Most being down to within 2/10ths, which could easily be a rounding error from refueling. 

[larrynimmo]

I bought my plane in 2017…I fly a lot of hours, but I am really uncomfortable with an inflight test of the indicators.

my fuel plan typically is fill to wing indicators 25/25 and refuel 10/10.. and I basically validate my indicators with my JPI cumulative calculation with every fill up…. the lowest I have ever gone is to go down to 12 gallons combined and no lights.

it is my hope that none of my flights will ever be a conversation had by Dan Gryder

Edited June 16, 2022 by larrynimmo
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[carusoam]

8 hours ago, Wildhorsetrail said:

Coming from an older 172, where fuel gages were wildly inaccurate, I'm curious how accurate you've found the fuel low lights to be. I believe they are supposed to come on at 2.5 to 3 gallons of useable fuel remaining. Running a tank dry gives me the heebie jeebies, but if the light is accurate I'll use it.

Coming from reality…

Where all pre-flown machines…. Have unknown qualities somewhere in their lives….

Fuel level sensors and their early warning lights are both accurate, the day they left the factory….

Its been a while since your plane left the factory… so now would be a good time to check…

 

Analog fuel level gauges in Mooneys are incredibly difficult to read.  There just aren’t a lot of lines on the gauge to interpret…

If you have 52gal or less, this accuracy becomes more important…

 

More reality…

most people test their fuel capacities and gauges and fuel selector valves while on the ground…. Before using them in the air…

Our fuel systems are quite digital… they either work, or don’t work…

 

More goofiness… when the low level light comes on… how many minutes do you get to do something about it?

Some Long bodies have the same warning as the mid bodies…. Not enough.

 

 

Soooo…

Measure what you have before relying on it…

PP thoughts only, not a mechanic…

Best regards,

 -a-

[Guest]

From the J maintenance manual. Strangely you don’t have to fly the plane to empty tanks to calibrate the low fuel system

Clarence

[GeeBee]

Fly the airplane until the light comes on, switch tanks and land. Then fill the tank, note how much it took and you have your answer without running it dry.

[Aerodon]

5 hours ago, GeeBee said:

Fly the airplane until the light comes on, switch tanks and land. Then fill the tank, note how much it took and you have your answer without running it dry.

I don't think it is that easy, you need to know exactly where full is on the filler neck.  Its easy to squeeze an extra 1-3G's in, and then you don't know if you low fuel is 1G or 4?

I think it also important to know that the low level is calibrated in the annunciator panel, so there could be a variation from plane to plane, and year to year.  And not work at all?

For my own peace of mind, I would fly one tank to 'light on' and then drain the remaining fuel to know for sure.  And then fill to the filler neck bottom, filler neck top, to know that for sure.  But I'm one of those guys that is quite prepared to fly to 10G remaining, so need to know gauges, warning lights, JPI accuracy, and proper 'full'.

 

Aerodon

 

 

 

 

 

 

 

 

 

[GeeBee]

On a Mooney long body, you have a distinct physical mark (bottom of the neck) that is easily repeated and in fact is in the POH as the "Full Mark". So on those airplanes, yes it is that easy. I know the M20J has a "25 gallon" mark visible you can fuel to that level. Many other airplanes have tabs, slots etc that make easily repeatable volume measurements quite easy. While we could argue small volumetric difference due to fuel density, the maximum variance on a long body standard tank would be 1.8 quarts from the lowest to the highest temperature possible. 

[A64Pilot]

Never trust any light airplane fuel indicating system, ever. They are all crap. I’m not calling a fuel flow meter that tracks fuel used as an indicating system. Even the Cubs with a wire sticking up out of the tank connected a float that floats in the fuel has been known to stick and lie. Always have a way to track fuel used, if we never had gauges I bet there would be fewer engine failures due to fuel starvation. Assuming of course fuel level was actually checked prior to take off (Gimili Glider)

The only reason you should run one dry is if for some reason you have to push fuel, long flight with no airports and strong unforcasted winds or whatever. The reason to run one dry then is that way you know there is nothing left in that side, people have talked themselves into believing there is more fuel in the other tank than there is, if you know it’s empty you will be less likely to talk yourself into believing you have more fuel than you do.

Having said that there is no harm in running one dry, N/A piston airplane anyway.

[jaylw314]

8 hours ago, GeeBee said:

On a Mooney long body, you have a distinct physical mark (bottom of the neck) that is easily repeated and in fact is in the POH as the "Full Mark". So on those airplanes, yes it is that easy. I know the M20J has a "25 gallon" mark visible you can fuel to that level. Many other airplanes have tabs, slots etc that make easily repeatable volume measurements quite easy. While we could argue small volumetric difference due to fuel density, the maximum variance on a long body standard tank would be 1.8 quarts from the lowest to the highest temperature possible. 

I'd also point out that since the filler is on very outboard part of the fuel tank, a half-inch of height is a much smaller volume than a half-inch when half-full.  Hmm, that didn't come out very clear...

[Fly Boomer]

On 6/16/2022 at 8:45 PM, A64Pilot said:

if you know it’s empty you will be less likely to talk yourself into believing you have more fuel than you do.

Also, if I was really bingo due to poor planning or unforeseen circumstances, I would rather have 4 gallons in one tank, and zero in the other, than 2 left and 2 right.

[Will.iam]

Just a data point and in no way is the accuracy repeatable on any other Mooney but,

22 mins from low light to first engine cough at 9.5 gph straight and level.