Fuel - How Low can you Go

Fuel system maintenance/upgrade prompted me to drain my tanks. I contemplated plugging the vent and doing a quick hose removal and plug of the main line at the tank and keep the fuel in them, but chose to drain the tanks. I don't think it is a bad idea to pull the quick drains and completely drain the tanks once in a while. (I have done it two or three times in 700 hours for various reasons). Follow safety procedures insuring the aircraft and vessel you are draining into are commonly grounded.
As I inspected the fuel tank bung, I noticed that on one side the fuel pick up screen laid very closely to the top of the bung. On the other side, it was quite a bit higher, perhaps 1/4" or so. Really? Had the fuel pick up moved? No, it was tight to the fitting, anti rotation brackets where installed, nope, must have been that way from the beginning. Way too many years ago to remember this.
Now I am a bit concerned. I know I did my unusable fuel tests early on and set my unusable fuel at 1 gallon each tank. I remember that was conservative and purposefully. But, I just had to know, what was the minimal fuel level before the fuel pick up would be uncovered?
I needed to flush my new fuel lines out anyway, so, I lifted the tail to something close to level flight and started with the tank with the fuel pick up that was the lowest. I put in a quart of fuel, hit the boost pump, and in a short few seconds, steady flow was established.
Now onto the tank with the higher pick up. Dump a quart of fuel, hit the boost pump, and much to my surprise, same result.
So, I really don't know how low the tank could get in real flight scenario. However, if the only limiting factor was the fuel pick up, lower than a quart.
I am a fuel chicken. I rarely fly with either tank going lower than 4 gallons. My fuel senders and fuel computer are set at 36 gallons, not 38.
After 9 years and 700 hours since I did any fuel studies on the RV, it feels good to reconfirm.

Minimum fuel

I have run each tank dry on our 6a in flight, obviously not on the same flight. Each time was to empty the tank for service. O-320, carbureted. Might not be wise with fuel injection. There wasn't more than about a tablespoon of gas left inside. Each tank took 19 gallons at the pump to refill.

Every time I fill up, I compare fuel loaded to what the totalizer says I used. It is always a little pessimistic - 3 to 7 tenths, which is fine by me. I don't know about setting the computer 2 gallons low. That's like setting your alarm clock ahead a few minutes. You do the math in your head anyway so you are not really fooled by it. I never land with less than 30 minutes flight time remaining, usually much more, and if I had to stretch it into the reserve, I'd like to know with some degree of certainty just how much I had left. I figure that if I ever had to run it low, I'd run one tank out so I would know where the remaining gas was.

Ed

I check my fuel added against the what the gauges and totalizer tell me every time I fill the tanks. The totalizer is always spot on, but I still use time/burn and watch the gauges.

I've said it here several times and I'll stand by it.

There is NO EXCUSE for running out of gas in this country with airports all over the place. If you do run out, well, what can we say.

I start to get a little uneasy with an indicated 5 gal. per side showing on the gauges, and when the readouts say "0", there's still 1.5 gallons per side available. So my effective reserve is 13 gal., or conservatively 1.5 hours of flight time.

Of course, it'd be folly to rely on just the fuel gauges alone. My EFIS timer is set for a 3-hour max flight duration which, conveniently, is also my max bladder duration. So with topped-off tanks, even in a worst-case-scenario 8.0 gph burn, I've got that 12 or so gallons left. Lots of extra fuel is great peace of mind (and I've always got one eye on the flow rate too).

RV8iator said:

I've said it here several times and I'll stand by it.

There is NO EXCUSE for running out of gas in this country with airports all over the place. If you do run out, well, what can we say.

Click to expand...

Widget,

When I run out of gas, maybe I can get a lawyer to take my case. I mean it can't be MY fault. Would you like to see my bookshelf full of participation trophies?

Greg

I intended this to be more about the effectiveness of the RV's fuel tank pick up and, at some point, it isn't a bad idea to go back to the beginning and reconfirm what you think you already know. If your smart, your doing that each time you fill up, but a fuel system rework is a pretty darn good excuse to revisit. This wasn't intended to be a post about fuel management, but no matter.

It is a personal choice on how you set up your alarms and your useable fuel for flight planning. I accept that not everyone would agree. It is pretty easy to leave your tanks a gallon or so short with some of the high volume pumps out there and the RV's baffle system requires patience when filling up. Of course, I know this, and I know if I set my alarm ten minutes early. I sometimes still do.

I have ran out of fuel and had a forced landing. It was investigated by the FAA and ruled not to be my fault, so there are circumstances that defy "NO EXCUSE" in running out of fuel, but I get the point. Don't need to get into that here, but it is related to the above paragraph, although it wasn't an RV.

Hopefully someone got some value out of the post. If not, it wont ruin my day

I know a guy who ran out of fuel once. He will never forget it.

Bicyclops said:

I have run each tank dry on our 6a in flight, obviously not on the same flight. Each time was to empty the tank for service. O-320, carbureted. Might not be wise with fuel injection. There wasn't more than about a tablespoon of gas left inside. Each tank took 19 gallons at the pump to refill.

Every time I fill up, I compare fuel loaded to what the totalizer says I used. It is always a little pessimistic - 3 to 7 tenths, which is fine by me. I don't know about setting the computer 2 gallons low. That's like setting your alarm clock ahead a few minutes. You do the math in your head anyway so you are not really fooled by it. I never land with less than 30 minutes flight time remaining, usually much more, and if I had to stretch it into the reserve, I'd like to know with some degree of certainty just how much I had left. I figure that if I ever had to run it low, I'd run one tank out so I would know where the remaining gas was.

Ed

Click to expand...

That is exactly the way I look at it. When I am going maximum range, I would much rather have one tank empty and all of the remaining fuel in the other tank. If my rudder isn't perfectly trimmed, then I leave fuel in the tank opposite the ball. I hold the rudder of he tank I am draining and run at tank until it reads "0", then release the rudder and switch to the tank where all my remaining fuel is. In the -10, there is very little unusable fuel. I also switch to the fuller tank several times for a minute or two at a time while draining the lower tank to make sure fuel is flowing well.

I have run a tank dry on my -6A and it took 19 gallons to fill up. The engine didn't miss a beat. I watched my fuel pressure and flow and when they dropped I switched.

I don't like landing with less than 5-6 gallons in my source tank, and if gusty or bumpy I like 7-8, but if I drain one tank dry, then my "unusable" fuel is 5-8 gallons. Otherwise, it is 10-16 gallons.

Know unknowns...

IMHO, determining unusable fuel in the hangar is shortsighted and misleading. What is your unusable fuel during a go-around, steep approach, slip, wing-low crosswind landing, during average pilot technique in [un]coordinated turns? I suspect most of us [me included] are guilty of not knowning the true answers. So to mitigate the unknown, land/maneuver with the selector on a tank that has lots of gas. Fly safe, Jim

fehdxl said:

IMHO, determining unusable fuel in the hangar is shortsighted and misleading. What is your unusable fuel during a go-around, steep approach, slip, wing-low crosswind landing, during average pilot technique in [un]coordinated turns? I suspect most of us [me included] are guilty of not knowning the true answers. So to mitigate the unknown, land/maneuver with the selector on a tank that has lots of gas. Fly safe, Jim

Click to expand...

Agreed, but I am not sure there are practical tests for that and you have to put something down. So, shortsighted, well not sure what else to do. Misleading? You better know the short falls as you describe. Perhaps this is one reason the regs require you to be on the ground with 30 minutes of useable fuel left in the tanks for VFR flight.
I am still comforted to know that in a perfect condition, the fuel pick ups are as effective as they can be getting those last drops. I have owned other airplanes that where not so.

As the OP found out, it depends on how the tanks were built.

My fuel pickups are on the bottom skin of the tanks, so they should be able to pick every last ounce.

As for how low can you go, when I had the small engine in my plane, I landed with three gallons, 1.5 in each tank. That was over a half hour's worth of fuel.

As for the comments about slipping, bumps, etc. I never experienced an issue. However, I kept an eye out for landing spots that last 50 miles.

The Dynon capacitance fuel gauges are right on, in my installation, and as you might guess, I trust my life with them.

Would I run that low with the O-360? Not on your life! Or more appropriately, not on MY life!

Sorry for the thread drift, JonJay, but this is a great discussion with some excellent points made. Just the sort of thought-provoking stuff that makes VAF so useful (and a bargain for $25/year! ).

rightrudder said:

Sorry for the thread drift, JonJay, but this is a great discussion with some excellent points made. Just the sort of thought-provoking stuff that makes VAF so useful (and a bargain for $25/year! ).

Click to expand...

No worries. I think it promoted some good discussion and I don't think it drifted much.