Low fuel light - is it independent from tank gauge?

[Adam Frisch]

Flew from Tulsa to Albuquerque today. Lots of buildup and had to dodge the whole way. Anyway, just east of ABQ, my low fuel light comes on again. Although I know it came on early during a steep descent previously, this was in level flight. I was heading to Double Eagle, but when that came on I chose to land at closer KABQ, (despite being fleeced on the fuel price). Was worried there was something going on. But no - when I fill her up, we fit 181gal (inboards fit 237gal). That's 56gal left - which is about an hour's worth. So the low fuel warning light comes on very early in this thing, which, perhaps, is a good thing. Now I know for sure it's not just on steep descent, but in general that it warns early.

But it got me thinking - is the low fuel warning separate fromt he tank gauge, or is it triggered by that? Because the tank gauge is not very precise on this. Anybody know?

[donv]

I don't know about the 680V, but on 690s and subsequent it's independent.

56 gallons is less than 400 pounds, which sounds about right for the low fuel light, at least in a 690. In the Jetprop, it's 150-200 pounds per side, which isn't much. Mine comes on at about 200 pounds/side.

You might consider what sort of reserve you want to have-- and I'd start thinking in pounds, not gallons, since that's what you are seeing in the cockpit.

[Steve binnette]

I think the lights on my 980 come on at 160lbs per side. That is 48 gallons.

As long as you know what it is you are good to go.

It really doesn’t matter if it’s 56 gallons or 48.

Good choice landing at ABQ.  You are still learning how she talks to you. 

I have not had mine go on yet.

[donv]

When they calibrate the fuel system, that is one of the things they check. So they can tell you exactly where in the range they come on (I believe the range is 150-200 pounds).

[Adam Frisch]

Thanks. It was the right decision.

I need to calibrate fuel gauges on next visit. They're too whacky.

[donv]

They may always be wacky. In the 690s, they would typically read full and not budge, and then all of a sudden drop to 1200 pounds and sit there for a while, then be reasonably accurate below 1000 pounds or so. I believe (could be wrong) that in the 690s they only calibrate the gauges at full and something close to empty.

On the Jetprops, they got tired of customers complaining about this and redesigned the system. Jetprop fuel gauges are calibrated every 50 pounds, and they're very accurate.

I have no idea about the 680, but I would guess it's similar (or worse) to the 690.

[Dhavillandpilot]

When I first got the 685, and started to operate it commercially we had a dilemma about exactly how much fuel we had.

To ensure some back up we ensured the low fuel light system worked. And yes it is independent of the gauge.

After 200 hours of guesstimating how much fuel we had for each flight we decided to install a JPI unit.

This proved great. But as a back up every 100hourly we would ensure the tanks were low and then drained the tanks to zero, then compared the actual against the JPI indication. It usually was out by around 40 litres in 100hours of flying.

We are now able to be more accurate with uploads and not have to carry extra "for mother"