“Fuel low” warning light accuracy

[toto]

I’ve never once seen this light illuminate before - my fuel gauges said I had 11 gallons in the right tank and 16 in the left. The “Right Fuel Low” light went on and I immediately switched tanks to the left. I was about a half hour from destination, and on landing the gauges showed 11 gallons each. The plane topped off with about 44 gallons of fuel, so it had about 20 gallons left, and it took north of 20 per side - so the gauges agreed pretty closely with reality (i.e., there was about 11 gallons left in each tank on landing and the gauges said 11). 

I didn’t like seeing “fuel low” when the gauge said I had an hour of fuel remaining in that tank, but it’s better than having an empty tank with no warning ..

Are these warning lights just not very accurate, or is there some recalibration to be done?

[toto]

I found this ten year old thread after I posted, and the content there is pretty good:

https://mooneyspace.com/topic/13384-low-fuel-l-r-warning-light/

I think the basic answer is that you *can* calibrate the warning, but it’s not necessarily a bad thing for the warning to come on when you have a bit more than bingo fuel remaining. 

I’ll have a dig around the mx manual to educate myself. 

[anthonydesmet]

Were you in a decent or straight and level flight and was it turbulent? If you were coming out of altitude and had 10/11 ish depending on decent angle it’s see the lights.

In either case I never trust the light or guage…they are advisory only and I still do the mental math for what I have burned and remaining every half hour I fly…..

[toto]

2 minutes ago, anthonydesmet said:

Were you in a decent or straight and level flight and was it turbulent? If you were coming out of altitude and had 10/11 ish depending on decent angle it’s see the lights.

In either case I never trust the light or guage…they are advisory only and I still do the mental math for what I have burned and remaining every half hour I fly…..

Straight and level, negative turbulence. It was all uneventful except for the light. 

I’ve owned the plane for a long time and I’ve never seen the light before, so it’s certainly possible that some very short period of turbulence sloshed the fuel away from the sender and tripped the alarm. Dunno. 

[Hank]

1 hour ago, toto said:

I think the basic answer is that you *can* calibrate the warning, but it’s not necessarily a bad thing for the warning to come on when you have a bit more than bingo fuel remaining. 

The low fuel light in my car comes on usually between 88 -- 102 miles to empty.

Its nice, especially in flight, to.not have to "Stop Right Now!" for fuel.

[rklems]

From other posts, looks like you have a G3X w/EIS. In a factory setup, the sense lines to the annunciator come off terminals that are piggybacked on the stud at the fuel gauge where the fuel sender comes in. The senders in a factory setup are resistive, I don't know for sure it's the same, but going to assume its the same as on a K, where low resistance to high resistance goes low fuel to high fuel. Is your EIS installation new or have you had it a while? There are a lot of guesses/assumptions here, but if you still have the factory senders (as opposed to CiES frequency based senders, in which case I'd say if you are getting a low fuel indication, then the wire for the annunciator was left in and is getting shorted to ground somewhere, as I don't think there is another means to drive that sense line with a CiES sender), there can be a couple different reasons why this might be behaving differently now. Resistive probes are basically a wound wire resistor with a finger that rides up and down that wound wire, varying the resistance. Over time this fine wound wire can wear out, break, develop corrosion, etc. This can cause poor indications, jumpiness, etc. You aren't mentioning any strangeness in the display of quantity, but the G3X uses some smoothing on the data, so that the quantities don't jump around significantly as you bank, encounter turbulence, etc. The annunciator circuit also provides some analog smoothing, so it needs to see a certain low resistance value for a number of seconds before the circuit will actually start to trigger the low fuel annunciation. In theory, over a period of time, both the indication and the annunciation should be indicating a lower fuel quantity if there was a significant departure in the resistance level of the sender. If this is an older G3X install, and thereby used a GEA24 vs GEA24B EIS interface, then there are a couple other reasons this could be occurring. There is an AD for resistive interfaces to the GEA24 EIS interface, and the AD requires reconfiguring the resistive senders through some parallel resistors and resetting the configuration from 0-620 ohm resistive to 0-5V. This AD was effective March 2022, so likely would have been completed previously, but ADs have been missed before, so it's possible this was recently done (and would have likely impacted the low fuel indications, depending on how exactly this was interfaced between the senders, the G3X and the annunciator)? It seems like you recently had an annual (saw the thread about the busted gear switch), so perhaps this AD just got done, or something got touched when the gear switch was replaced, or a sender is starting to vary in resistance a bit, or the annunciator circuit just drifted over time and needs to be recalibrated.

[Mark89114]

My lights seem to be very accurate.  I don't rely on them obviously.

Per the manual they come on around 7-8 gallons, which seems about right to what the fuel gauges say.

My stock gauges also seem very accurate as well.  I have run down to 2 gallons on the digital readout and then got nervous and switched tanks.  

[toto]

5 hours ago, Mark89114 said:

Per the manual they come on around 7-8 gallons, which seems about right to what the fuel gauges say.

I think my manual says 2-3 gallons …. Let me check …

ETA: Yeah, it says 2.5-3 

[Mark89114]

I have an ovation so same but different.

I was pretty close.  Don't have the POH in front of me but found this reference online.  Not even sure how accurate this is as I am serial #243 and it isn't even listed.