How much fuel is left when sticking the tank shows no fuel?

[salty]

Being semi-anal-retentive, I wasted my morning by draining my tank until there was no fuel directly under then fill cap, and then metering 2 gallons at a time and marking the stick as I filled. I ended up only putting 19.5 gallons in, which was a lot less than I expected. Is there really another 5.5 gallons of usable fuel left when sticking would show 0 fuel?

[Marauder]

6 minutes ago, salty said:

Being semi-anal-retentive, I wasted my morning by draining my tank until there was no fuel directly under then fill cap, and then metering 2 gallons at a time and marking the stick as I filled. I ended up only putting 19.5 gallons in, which was a lot less than I expected. Is there really another 5.5 gallons of usable fuel left when sticking would show 0 fuel?

When I had the wet wings, I remember the unusable was supposed to be around 3 gallons total. Useable being 64. My bladder system is supposed to have 54.8 useable, but we could only get 50 in during JPI calibration.

[salty]

Are you saying I should assume I only have 19.5 gallons per wing rather than 26?  Mooney manual for my plane says 52 gallons *useable* fuel.

Edited August 20, 2017 by salty

[RLCarter]

I'm in the process of replacing the fuel sending units and will be calibrating them along with a "dip stick". My tanks have already been drained so I can't  check until I start filling them, there will be a slight difference as my plane has been leveled to calibrate the fuel gauges and a dip stick is used while sitting on the tires. With 26 gal. tanks (useable) 5.5 gal. would be 1gal. shy of a quarter tank.

[MB65E]

I would just drain them empty from the sumps. Then you have a more accurate number with lesss fluff. Then refill the takes and make your sticks. I've landed not being able to see any fuel in one tank, I'm sure there was about 3-4 gallons in there. Many variables, but you can't have enough gas unless your on fire.

-Matt

[TTaylor]

On the F with bladders our stick starts at 6  which matches your 5.5 very closely.

[AH-1 Cobra Pilot]

When I can no longer see fuel in the tank, I have 1/4 tank left.  Just the way the baffles hide the rest of the tank on my aircraft.

[steingar]

Mine has bladders and has six.

[salty]

Ok, this makes me feel better. I was starting to think I had been taking more chances than I realized having 11 gallons less fuel than I thought I did all this time.

[Bob - S50]

On our J, when the area under the fill cap is just barely getting wet, we have 7 gallons usable.

[Skates97]

2 hours ago, salty said:

Are you saying I should assume I only have 19.5 gallons per wing rather than 26?  Mooney manual for my plane says 52 gallons *useable* fuel.

I am trying to remember what is on my stick but I think I am somewhere at 5 gallons or so when I can just see the fuel barely in the bottom directly below the fill cap. I have run a tank dry and when I went to fill it was only able to put 24.5 in it (26 gal tanks) so I plan for 1.5 gallons as unusable.

[Yetti]

Usable vs. what is in the tank.  Fuel right at the baffle.   Draining 7 out through sump.  The pickup is about an 1" above the bottom so I would say 5 usable.  Don't want to find out what "usable" is when I am up flying around.

[N201MKTurbo]

I have always used the rule of thumb that when the fuel was "on the beach" which means the shoreline was straight down from the fuel cap, there was 1/2 hour left in that tank. 

That corelates well with what you guys are measuring.

[gsxrpilot]

There is only one sure way to know... run the tank dry in level cruise flight.

I do this often on long cross countries when it makes sense to have all my fuel in one tank. It also serves as a good check to know exactly how much USEABLE fuel each tank holds.

[salty]

42 minutes ago, gsxrpilot said:

There is only one sure way to know... run the tank dry in level cruise flight.

I do this often on long cross countries when it makes sense to have all my fuel in one tank. It also serves as a good check to know exactly how much USEABLE fuel each tank holds.

I'll be doing that tomorrow.

[Bob_Belville]

'66E with bladders:

This was done by a previous owner about 15 years ago. (FWIW, when the shop calibrated the JPI they could only get 26 gallons in each side.)

stick 0"             left   7.4          right    7.1

stick 17"           left 27.4          right 27.5

[carolinaflying]

3 hours ago, gsxrpilot said:

There is only one sure way to know... run the tank dry in level cruise flight.

I do this often on long cross countries when it makes sense to have all my fuel in one tank. It also serves as a good check to know exactly how much USEABLE fuel each tank holds.

Having never done that can you advise what it's like when that happens? Obviously you are anticipating that, but how much warning do you get when the tank is finally dry?

[gsxrpilot]

23 minutes ago, carolinaflying said:

Having never done that can you advise what it's like when that happens? Obviously you are anticipating that, but how much warning do you get when the tank is finally dry?

It somewhat depends on carb/fuel injected/turbo. I've done this many times in both a carby M20C and a turbo M20K 252. If in a turbo, it depends on altitude. Are you low between 5K and say 14K or are you up in the flight levels.

Generally what happens is there might be a stumble or you'll notice the RPM stumble or drop. I usually try to anticipate and as soon as I see the RPM drop and feel the engine stumble, then switch tanks. There is sometimes a slight hesitation of maybe 2 to 5 seconds before it comes back strong. It can feel like an eternity the first time ;-) With the carby M20C, it always came back immediately.

I don't touch any of the knobs, black, red or blue. When in the turbo up in the flight levels, I'll be prepared with the low boost or high boost. And it can take as long as 15 seconds to restart when up really high.

The important thing is that it always does restart. I've done this many times and there's never been any issue with it starting as soon as the tanks are switched.

There are three huge benefits to this practice. 

1. I will recognize a fuel issue and instinctively change tanks and reach for boost just incase. If it happens when I'm not anticipating it, the muscle memory is still there. And I'll be quick about it. It's actual live practice for that unlikely, but possible emergency situation.
2. When I land and fuel the dry tank, I know EXACTLY how much fuel it will take and therefore know exactly my unusable fuel. I don't have to fudge +/- 3 to 5 gal just to be sure.
3. I really don't like to be doing anything in the air on my last 5 gal. And 5 gal in each tank doesn't equal 10 gal, it's effectively just 5 gal. Whereas 0 gal in one tank and 10 gal in the other tank is a much better scenario. You can plug in your own numbers for comfort. It might be 15 or even 20 gal for you. But the point is still valid, that the last few gallons in the tank are really not useable at the end of a flight. And so running a tank dry ensures that if my reserve is 10 gallons, it's 10 useable gallons all in the same place and not a much smaller amount sloshing around in different tanks.

[Guest]

If you don't feel comfortable running a tank dry in flight,  you can disconnect the fuel hose from the firewall fitting on the left lower side and pump fuel out until it stops..  This method will leave you with only the un-useable fuel in the tank, based of course on the current attitude of the plane.

Now refuel the plane and mark your fuel stick in the increments you want.

Clarence

[gsxrpilot]

3 minutes ago, M20Doc said:

based of course on the current attitude of the plane.

I'd prefer not to have that or any other * attached to my fuel calculation. When the fuel runs out in cruise flight, I know what the number is.

[Skates97]

1 hour ago, carolinaflying said:

Having never done that can you advise what it's like when that happens? Obviously you are anticipating that, but how much warning do you get when the tank is finally dry?

I'll be honest, despite reading that it is a non-event I was a little nervous the first time. I knew about what my burn was from previous experience (no FF gauge) and so did the math as to about when I expected to run out. I made a note of that time range (a window of about 15 minutes) on my knee-board and when it got to the beginning of the time I gave the fuel pressure gauge a more often glance than just my normal scan. The fuel pressure just slowly started dropping (at a steady rate) and I reached down and switched over before the engine quit. It really was a non-event as others had said.

[carusoam]

Follow Bob...

There is confusion with the OP's description.

when sticking the tank...

The stick isn't anywhere near zero useable fuel.

The fill hole is pretty far away from the lowest point in the tank.  It is uphill from the low point.

When the stick is dry, it doesn't say anything about how much is left in the tank.

Best regards,

-a-

[Hank]

2 hours ago, carolinaflying said:

Having never done that can you advise what it's like when that happens? Obviously you are anticipating that, but how much warning do you get when the tank is finally dry?

As the others say, it's a non-event. I've done it twice in my C; the first time I was distracted near the end of a long XC and missed changing tanks by 20 minutes. It sputtered once and caught my full attention, but everything looked good. Then it sputtered again, I saw the clock well past the red "switch tanks here" hands, leaned over and threw the switch and everything was fine. The second time, I knew fuel would be close, and I was watching and counting on my fingers. I went 15 minutes past where I had originally planned, a few minutes at a time, and had decided to switch tanks when I started my descent almost 50 nm out. Missed it by that much . . .    The engine just quit, no cough, no sputter, no nothing. Also started immediately when I threw the selector over.

Be aware:  when the engine quits in cruise, there's a pretty strong down pitch, and it's immediate.

For fuel capacity, when I had my tanks stripped and resealed, the FBO billed me for 52.2 gallons, so it's pretty accurate. I've had less fuel than "straight down from  the cap," but don't recall how much I put in. I will say that I've flown 4:40 twice, and put in 41-42 gallons each time, leaving me with ~1:15 still in the tanks. The more you fly, the more you play the "how much will it take to fill up" game, the more confidence you will build in your plane and your flying. 

[N201MKTurbo]

It's like "mmmmmmmmmmmmmm............................"

[Hector]

My JPI in my 67C is pretty darn accurate (with 1 gallon after 4.5 hours of flight). I have never run a tank dry but have used 24 gallons on one side and it was still running (level flight).


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