Full Fuel Tanks

[Mooneymite]

As I recall, the M-18 had two gallons of unuseable fuel out of 12 total.  However it was only unuseable in a nose-up attitude...as in a go around.  Someone I know well used that "unuseable fuel" when the flight did not go as planned.  Glad to have it!

[Bob - S50]

14 hours ago, gsxrpilot said:

 

Personal minimums are real and important for any pilot. And my personal minimums on fuel at destination are likely as conservative as anyone. The only difference is I like to KNOW where the fuel is and how much is useable. 10 gallons all together in a single tank is much more useful than the last 5 gallons sloshing around in each tank. So if my personal minimums for a VFR flight was 10 gallons, an additional 2 - 5 gallons in the other tank doesn't help me at all. It just means that my personal minimums are actually 15 gallons, 5 of which are useless.  

I feel the same way.  When I take off, I run one tank until it is 5 gallons below the other.  I then switch tanks every 10 gallons.  Once one tank is at 10 gallons, I run on the other tank until it is either empty or I'm entering the pattern.  I then switch to the 10 gallon tank for the landing.  Fortunately, I have Cies floats and a digital gauge so I'm pretty confident that what my gauge says is what it has.  And since the calibration required me to tell the gauge that the tank was empty when it really has 1.25 gallons in it, I can probably run for a few more minutes even after it says empty.

My point was that if I determine that the engine runs just fine all the way down to .5 gallons in the tank, I'd still rather do my planning under the assumption that the engine will quit with the POH specified 1.25 gallons in the tank.  That gives me another 1.5 gallons of total reserve.  That's another 10 minutes of flying.

Now if I was REALLY interested, I'd run the tank dry, then I'd drain the remaining fuel after I landed and measure it.  If it was less than 1.25 gallons I'd be happy.  If it was more than 1.25 gallons left, I'd have to adjust my planning.  Unfortunately, since I'm usually flying with people who would freak out when the engine sputtered when I ran the tank dry, I have not had a chance to do that.

Edited July 17, 2017 by Bob - S50
typo

[carusoam]

Doc,

Thanks for posting the rib photos.  They really give a hint to what we are seeing and describing...from actual experience.

It looks like the last few gallons going into the tank have to drive an equal number of gallons of air through a hole (or two or three...) at the top most position. And then again a similar exercise for the next rib. The driving force is the weight of fuel pushed by gravity. The resisting force is the size and number of air holes that are open to allow air to pass. Finally the air vent. Make sure it is fully open..

I am looking forward to more reports of CIES accuracy.  I think these are an interesting way to get a real feel of the actual Fuel level in flight when the FF gauge does something goofy...

Best regards,

-a-

[Stephen]

On 7/16/2017 at 7:12 PM, gsxrpilot said:

I've only topped off the tanks one time and it seemed that they would only hold about 73 or 73.5 and that was up past the base of the filler neck. We haven't calibrated the fuel gauges in the JPI yet so we'll get a chance to check it again.

Yeah I was thinking calibrated CIES + JPI EDM 9xx or equivalent seems to mostly mitigate this issue

[FloridaMan]

I've been underfueled by FBOs a number of times, and not physically looking in tanks has caused a number of accidents. Once I watched the guy fuel my M20F. After he left, I looked in the tanks and saw he'd only gone to the tabs and I had a 5 hour trip that I was about to leave for. When the engine monitor is finally installed on my Rocket, my intention is to *tell* the FBO *exactly* how much fuel to add and be very specific that the fuel will get in there, albeit slowly, if there's any reason I can't be there in person during fueling.

[81X]

1 hour ago, Antares said:

I've been underfueled by FBOs a number of times, and not physically looking in tanks has caused a number of accidents. Once I watched the guy fuel my M20F. After he left, I looked in the tanks and saw he'd only gone to the tabs and I had a 5 hour trip that I was about to leave for. When the engine monitor is finally installed on my Rocket, my intention is to *tell* the FBO *exactly* how much fuel to add and be very specific that the fuel will get in there, albeit slowly, if there's any reason I can't be there in person during fueling.

Same thing happened to me just a week ago- I asked the FBO to fill the tanks "really full, and the last few gallons per side take a little bit to get in".  When I went to depart, I pushed the flapper down and it was barely wet.  It took another 10 gallons.  When I fill the tanks myself, I find that if you get the nozzle in the tank good and angle it outboard, it helps get the tanks really full just a little bit quicker.  Sure was nice having that 10 gallons on a 3.75 hour trip!  

 

[Hank]

I always stick the tanks before my first flight of the day, and after fueling if I'm not there. The line boy might forget to gas me up . . .

[Warren]

I have an 85 K model and have run each side dry multiple times (on separate flights) to get an accurate measurement. When I refill to the flapper seat I get 33.8-34 gallons in the tank. I know this doesn’t count legally useable fuel but I know pretty close to when the actual level flight fuel exhaustion will happen. 

I can get more in by being patient but have the extended tanks and have never filled right to the top as I can easily get the fuel I need with the extended tanks. 

I have also checked my low fuel level and I run out in the 2.5-3 gal range after the light comes on. 

[ArtVandelay]

My 78J also holds 67 (usable), I discovered this doing JPI calibration, and that’s without any tricks to squeeze more fuel in.

[Hank]

After having my tanks stripped and resealed, it took 52.6 gal to fill both to the top. That's where FBOs generally fill them, so they overfllw when I open to check quantity. When outting away, i generally leave 1/2" air gap or so, which is 50 gal. Never burned more than 41 gal in a single leg, and don't want to spend 4:45 in the air again  . . .

Kmow your plane, check your fuel. Someone once said "trust but verify"; I prefer the sign my old Quality Manager had in her office:  in God we trust, all othes bring data.

[McMooney]

always top off the tanks and visually check;  even if I fuel the plane myself.

when it comes to fuel, Trust no one.

[gsxrpilot]

4 hours ago, Warren said:

I have an 85 K model and have run each side dry multiple times (on separate flights) to get an accurate measurement. When I refill to the flapper seat I get 33.8-34 gallons in the tank. I know this doesn’t count legally useable fuel but I know pretty close to when the actual level flight fuel exhaustion will happen. 

Good on ya... And this is no question, the MOST accurate way to know how much fuel your tanks will hold. The K is certainly a long distance machine. But it's all predicated on knowing how much fuel you have on board, where it is, and how fast you're burning it.

[FloridaMan]

13 hours ago, teejayevans said:

My 78J also holds 67 (usable), I discovered this doing JPI calibration, and that’s without any tricks to squeeze more fuel in.

My F would take 33-34 after running one side dry. Prior to getting the engine monitor, I would climb out and run for 3 hours before switching tanks. I wouldn’t feel much, if any, imbalance and it gave me confidence that I still had a minimum of 3 hours remaining since you burn more on climbout. 

I remember hearing something about a foreign government accusing someone of having long range tanks for running drugs because they could hold an extra 2 gallons or something ridiculous. 

[jlunseth]

There is some good information in this thread.  

I have a 231 and a JPI.  My fuel flow gauge is very accurate, I ran a set of tests on it a few years ago and it is within .1 gal in 50 gals.  That is what I use.  The JPI readout is extremely accurate, the senders providing the information to the JPI not so much.  

Getting the tanks full is something I work on with the line guys at my FBO.  I sump the tanks and check their filling every single time.  Now, the difficulty of knowing when you really have the tank full is somewhat mitigated by the superior fuel efficiency of the 231.  When I run LOP at cruise I make good speed at around 11 gph.  That is theoretically 7 hours to empty.  I have never flown 7 hours, I have never flown more than 4.5 without a pit stop.  So some inaccuracy is tolerable. I use the numbers from the totalizer though.  I make a little table starting with 37.8 at the top for each tank, and then subtract the fueled used as I switch tanks.  Typically, I will use around 10 to 15 gallons out of the first tank during the climb and level off, then switch and use the second tank for most of the flight.  That might be 2 1/2 - 3 hours.  Then I switch back to the first tank for the descent and landing, I know to a certainty it has 20+ gallons in it when I make that switch.

I was told some years ago that full is to the bottom of the filler neck.  Nevertheless, if I am taking a long trip I make sure it is full to the ring at the top of the neck, that is a few more gallons.  Nine out of ten line guys don’t understand that you have to push the anti-syphon open with the fuel nozzle and keep it open.  I can’t tell you how many times they have “filled” my tanks, there is fuel sitting on the anti-syphon valve, and a big void underneath.  To really fill the tanks well for a long trip the best technique I have found is to gently rock the wing.  This will burp most of the air out.  I find I can usually get about 4 gal more into the tank this way, even if the line guy filled the tank “full” and did it correctly.  

I would not knock that anti-syphon valve by the way.  I did have an O ring fail in the gas cap once.  It sprayed for a little while, but the valve stopped it.  

I ran a tank dry once.  I did it intentionally, but was surprised that it ran dry early.  I agree that they run dry at about 35, not 37.8.  Your mileage may vary.

The sight gauges on my aircraft are not that useful.  The one thing they are good for is knowing when a line guy has mis-fueled a tank.  If the needle moves at all, you can get somewhere between 4 and 10 more gallons into the tank.  The needle will be pegged out if the tank is truly full.

[NicoN]

Thank you for your Information jlunseth!

I am pretty new co-owner of  a 1980 M20k231. The POH says 72 gals usable fuel.

The fuel gauges on my wing have markings for 10,15,20,25,30 gals. And then there is a "6". I am not sure whether it means below 10 or 36 as it is half way between 30 and 10. I  learnt that the wing gauges seem to be accurate for 20-30gals, but seem to be hardly not usabel for really full tanks

I also have this funny vent about 4cm below the filler neck/tank cap. Is there any Information available what is the right fuel capacity when filled up the vent?

Do I risk air trapping when trying to fill tanks above the Level of the "flabber" vent ? ( i do not find a good Translation into german. Google thinks it is netherlands :-)

[NotarPilot]

I have a question regarding heat expansion.  I tend to fill my tanks right up to the fuel cap because I always gas up at other airports with cheaper fuel then immediately fly back to my home airport burning the gas in each tank during the flight back.  This creates very little space in the tanks as the flight is usually just 20 minutes back to my airport after tankering up the plane. I might only burn off 1.5 gallons in each tank from my estimation. The plane is hangared so it’s not subject to extreme heat but any thoughts on whether this is bad practice? Am I allowing enough space for heat expansion in the tanks?

[Danb]

+1 for filling the tank yourself to know how much is in and to mitigate damage from poor linesmen, I left Wisconsin left instructions to fill to bottom of neck, when I checked in the morning I needed to add 7.7 more gallons. It’s our responsibility to ensure what’s inside.

[neilpilot]

2 hours ago, NotarPilot said:

I have a question regarding heat expansion.  I tend to fill my tanks right up to the fuel cap because I always gas up at other airports with cheaper fuel then immediately fly back to my home airport burning the gas in each tank during the flight back.  This creates very little space in the tanks as the flight is usually just 20 minutes back to my airport after tankering up the plane. I might only burn off 1.5 gallons in each tank from my estimation. The plane is hangared so it’s not subject to extreme heat but any thoughts on whether this is bad practice? Am I allowing enough space for heat expansion in the tanks?

That 1.5 gal burn off more than compensates for any potential expansion.  For example a temperature increase of 10C or 18F brings 25 gallons of fuel up by only about a quarter of a gallon.

[NotarPilot]

5 hours ago, neilpilot said:

That 1.5 gal burn off more than compensates for any potential expansion.  For example a temperature increase of 10C or 18F brings 25 gallons of fuel up by only about a quarter of a gallon.

Thank you for the response. That’s good to know.

[Cardinal767]

With my K model, "Full" is all the way to the rivets above the filler neck flap. When filled to the flap, I'm usually short by 2.5 gallons per wing. I installed a EDM830 and came up with these numbers while calibration testing. Running a tank until the low fuel light came on, then filling it verified it all

[carusoam]

A couple of ideas...

Expansion with temperature....

our tank vents are pretty far up hill, the more modern, the more uphil they are.... lots of air between the fuel and the vent....

Our wings have a lot of dihedral Compared to a Cessna that likes to demonstrate fuel density change with air temp....

Where this theory falls down... parking sideways on a hill puts the fuel closer to the vent... on the low side...

 

mechanical wing mounted gauges are slightly different depending on which ones you have...(?)

But, If it says 6... I wouldn’t expect it to mean 36...

 

I consolidated a search on the topic....

https://mooneyspace.com/search/?q=Wing mounted fuel gauge&sortby=relevancy&search_and_or=and

Best regards,

-a-

[kortopates]

10 hours ago, Cardinal767 said:

With my K model, "Full" is all the way to the rivets above the filler neck flap. When filled to the flap, I'm usually short by 2.5 gallons per wing. I installed a EDM830 and came up with these numbers while calibration testing. Running a tank until the low fuel light came on, then filling it verified it all

A lot of K's have reported coming up a bit short on rated capacity. But a couple points, I assume you meant a EDM 930 since the 830 does not report on tank fuel level - only FF. Also, a full accurate calibration empty's the tank till bone dry*, just running it till the low level fuel light leaves a few gallons in it which will reduce how much fuel you can add.

* some owners choose to run it in flight till a tank is empty - to get an idea of their actual experienced unusable fuel. Its a good data point, but not how the factory measures rated capacity. 

[Marauder]

1 hour ago, kortopates said:

A lot of K's have reported coming up a bit short on rated capacity. But a couple points, I assume you meant a EDM 930 since the 830 does not report on tank fuel level - only FF. Also, a full accurate calibration empty's the tank till bone dry*, just running it till the low level fuel light leaves a few gallons in it which will reduce how much fuel you can add.

* some owners choose to run it in flight till a tank is empty - to get an idea of their actual experienced unusable fuel. Its a good data point, but not how the factory measures rated capacity. 

Even the bladder systems can be off a bit. My 54.8 of "useable" is reproducibly 52 gallons. Now, if I fill it the night before and check in the morning, I might be able to squeeze another gallon in. 

[mike_elliott]

On 7/16/2017 at 5:09 PM, M20Doc said:

Sure glad I fly a Piper Comanche, refuelling a Mooney seems as difficult as flying one.  When I fill the tanks and there is no air left the tanks are full and it doesn't settle to some new imaginary "certified full" level.

Clarence

Nor available limit on your credit card

 

[Guest]

9 hours ago, mike_elliott said:

Nor available limit on your credit card

 

I bill it to my Mooney Space account, so far so good.

Clarence