Actual fuel quantity

[Bill Mnich]

I'm at my avionics shop and just had the tanks drained in my Encore to calibrate the fuel quantity indications for a JPI EDM900. The manual says the airplane should have 75.6 gal of useable fuel (3 gal unusable). However, after the gradual refill to establish the 5-point calibration required by JPI, the tanks were brimmed with only 68 gal indicated on the fuel truck meter. 

I know you can dribble fuel into these tanks and sometimes get another gallon or two, but not seven. The tanks were emptied by removing the low point drains under the wings; Could there be 3 1/2 gal of useable fuel remaining in each side inboard below the level of the drain plug? And although it's possible that the fuel truck flow meter was in error and I actually got 7 gallons for free, that seems rather unlikely.

Anyone else have experience measuring actual fuel quantity in a K model? 

Thanks,

Bill

N91618

 

[carusoam]

Welcome aboard, Bill.

Did you follow the procedure in the Maintenance Manual(?) for that?

There may be some details like how to drain the tanks, and how to level the plane.  The other things that may cause a challenge are tiny holes in the ribs between fuel areas and fuel drain hardware.

filling to the top skin, requires air to escape out the vents. Make sure the vents are clear and be patient.  Air flowing through such a small tube takes time.

Also check what the definition of full is.  It may be to the bottom of the the fuel neck. Different planes have different fuel neck designs.  The Eagle got the weirdest one.  Does your Encore have two caps per wing? Do you have a filling procedure to follow? Are there anti-siphoning flaps on your fill necks.

i would bet leveling the plane properly results in finding the few gallons you are looking for...

One last detail to look into is the fuel level floats that you are using.  The accuracy of the original equipment should be checked for how well it is actually working.  There is a company that builds higher quality devices for better level info.  Search for CIES for that...

These are simple PP ideas.  I am not a mechanic.

Best regards,

-a-

[takair]

There may be a requirement to level the aircraft, I don't have the book handy, but the attitude of the aircraft could account for some fuel...

[chrisk]

My suggestion is to run a tank dry.  And yes, my heart does skip a beat when this happens.  When I have done this, I am always within gliding range of an airport and at reasonable altitude (8 to 10,000).  I would be careful at flight level altitudes, as a restart might be interesting. (or at least the POH has left me with that impression, with statements to the effect:  descend be low some altitude and try starting again)   What ever you put back in is the "usable" amount.  Also, filling to the brim is not as exact a definition as you might think.  I would fill until you literally can not put another drop in without an over flow.  It's amazing how much extra you can stuff in there.  --and as others have suggested.  Level ground is important.

[Two Oh One]

2 hours ago, carusoam said:

i would bet leveling the plane properly results in finding the few gallons you are looking for...

I'd second this.  I swear I've got my plane level every time at the pumps, but then my hangar floor says otherwise.

[takair]

2 hours ago, Two Oh One said:

I'd second this.  I swear I've got my plane level every time at the pumps, but then my hangar floor says otherwise.

I think fore and aft level may play into it as well, especially on longer body aircraft sitting nose up.  That said, I am not sure how you would do it as a practical matter, but I wouldn't be surprised if it was done for certification purposes.

[gsxrpilot]

+1 for starting your calibration by running the tank dry at altitude. I've done that with my 252 and really the only way to get an accurate reading on Useable fuel.  I only gave the engine about 3 seconds to restart before using the Low Boost pump. But that got it firing again immediately. I was at 12,500.

With my 252, it's frustrating as hell, but if I just let the fuel dribble in, I can easily get another 5 or 6 gallons in each wing. This is mostly frustrating because there isn't any way to know what the line guy put in the tank unless you watch closely the entire time or fill it yourself. Looking in the tank and seeing the fuel 1/2 inch or so over the flapper indicates a fuel level +/- 5 gallons at best.

[jackn]

When I top my tanks, if I wait a few seconds and am patient, I can get a lot more fuel in. Also, when I'm topping just the mains, I have my Jpi fuel totalizer set to 72 gallons for full. That seems to be about right for me. 

[Guest]

I'm not really sure how running a tank dry in flight would get the tank emptier than draining the tank in a hangar by removing the drain valves.  In fact I would think that unless you run the engine to complete stoppage, there will still be fuel in the system.

Other possibilities are, the tanks were not really empty, or the meter on truck is inaccurate.  Levelling the airplane may result in additional capacity, but no one does this before flying.

Clarence

[kortopates]

The factory defines full rated capacity at the bottom of the filler neck/anti-siphon valve. There is added space for expansion.


Sent from my iPhone using Tapatalk

[Piloto]

With low fuel in the tanks get an articulated mirror through the filler hole and check the upper part of the inboard rib with a flashlight. There should be three vent 1/4" clear holes at the top. If the holes are plugged there would be trap air in the tank upper section. There are also drain holes at the bottom that if not clear it will keep fuel that is not drained, increasing the unuseable fuel. For checking the other inboard ribs you will need to insert a borescope or remove the tanks access panels.

Plugged vent or drain holes will reduce the tank fuel capacity but will not be reflected on the fuel gauges.

José

Edited October 29, 2016 by Piloto

[carusoam]

Running a tank empty in flight seams to be a good way to define where useable fuel ends...

I know that when the M20C noses down towards the runway, the fuel pick up becomes more of an air pick-up.  Useable in level flight may become less useable in a nose down attitude.

It would help to verify the location of your fuel pick-up.

Best regards,

-a-

[gsxrpilot]

11 hours ago, M20Doc said:

I'm not really sure how running a tank dry in flight would get the tank emptier than draining the tank in a hangar by removing the drain valves.  In fact I would think that unless you run the engine to complete stoppage, there will still be fuel in the system.

Other possibilities are, the tanks were not really empty, or the meter on truck is inaccurate.  Levelling the airplane may result in additional capacity, but no one does this before flying.

Clarence

Running a tank dry in flight is the ONLY way to accurately measure useable fuel. And USEABLE fuel is really all we care about. 

Run one tank dry in flight until the engine coughs and quits, then switch tanks and go land. Fill the empty tank to the top and observe how much fuel it takes. That is your actual, verified, useable fuel. Now go do the same on the other tank.

 

 

[Bob - S50]

I'm a conservative kind of guy.  First, once you've drained the tank through the sump drain, do what Piloto says and make sure the vent/drain holes in the ribs aren't plugged.  And while you are in there, look to see if there is any more fuel remaining in the tank.

To me it doesn't matter what the book says I can fit in the tank, what matters is what I can actually get in there using my normal fueling procedures.  If you can only get 68 gallons in using your normal procedures, tell the JPI that full is 68 gallons.  If, some day, you decide you want the extra fuel and spend the extra 15 minutes dribbling in a few extra gallons, the gauge will just show full until you've burned down to 68 actual.

However, when you are calibrating the JPI, be sure to wait long enough for the fuel level to stabilize before telling the JPI you've added your 5 gallon increment.  On our AeroSpace Logic gauge, you fill in 2 gallon increments and watch the resistance readout until it stabilizes before you push the button and continue.  Now if you really want an accurate fuel gauge, you might want to consider putting in Cies floats.  We're waiting for Scott to get the STC approval before we put them in our plane.

[Bartman]

21 hours ago, Bill Mnich said:

Is it possible that the gradual refill itself is the issue ?   I'm not sure exactly how the flow meter works, but I imagine there is some kind of turbine that turns recording flow and it may be dependent on flow rate....I really don't know enough about it, but I can see how this could make a difference.

 

 

Edited October 29, 2016 by Bartman

[Piloto]

2 hours ago, Bartman said:

Is it possible that the gradual refill itself is the issue ?   I'm not sure exactly how the flow meter works, but I imagine there is some kind of turbine that turns recording flow and it may be dependent on flow rate....I really don't know enough about it, but I can see how this could make a difference.

 

That is a possibility specially if the operator resets the meter for every short fill. This may cause the fuel on the hose not to accounted on the meter.

José

[jlunseth]

Whenever I go on a long trip where I want all the fuel I can get, I either fill the tanks myself or help the line guy.  The plane needs to be rocked to burp all the air out.  It gathers in spots on the underside of the tank tops.  The person filling the tank also needs to make sure to depress the anti-siphon valve with the fill nozzle.  If these two things are not done, you can go out and put between 5 and 7 gallons in the plane just be doing them.  I believe that "full" in my aircraft is the bottom of the ring, but I always fill to the top and that is the only way I can get the full, 75.6 useable into the tank.

Yes, you can run a tank dry to help figure out what you have for useable, but you have to start with a tank that is actually full.  If you have not done that, then you will have the wrong measure of useable.

However, I don't find the gauges very helpful.  They depend on the senders on the tank, which are old, inaccurate technology.  When doing a long trip I rely on the fuel flow meter and the USD number on my JPI, not on the tank gauges.  Depending on which tank you are looking at, the tanks go to zero with somewhere between 6 and 10 gallons left, and that is well over an hour of fuel in my plane.  My fuel flow meter, on the other hand, is accurate to within .1 gallons out of 50, so I go with that.

[LANCECASPER]

15 hours ago, Piloto said:

With low fuel in the tanks get an articulated mirror through the filler hole and check the upper part of the inboard rib with a flashlight. . . .

José

Remember to use a flashlight and not a match 

Although that might prove whether you really ran it dry or not . .

[Bill Mnich]

Thanks for all the inputs guys. 

To address some of the comments:

Leveling the aircraft... I find no requirement in the Mooney MM to level the aircraft when draining the tanks, and in this case the airplane was on a hard, flat (as near as I could tell) concrete floor of an avionics shop. I concede that there could be a minor slope to the floor that was not apparent and the airplane could have been tilted very slightly laterally and/or longitudinally. But given the dihedral of this wing and normal tire pressures, I can't believe that a total of 7 gallons got trapped. And when the drain valve in the bottom of each wing is removed and the tanks drained through those holes, each fuel tank should be well and truly empty. Good comments from "Piloto" on the drain holes and vent holes in the inboard rib and I'll check that, but again, I doubt that accounts for so much fuel.  

Now, leveling the aircraft when filling the tanks is another issue. In fact there is a procedure in the Mooney Maintenance Manual for Fuel Tank Transmitter Adjustment which states that "AIRCRAFT MUST BE LEVEL TO CALIBRATE FUEL GAUGES". In my airplane the factory analog gauges are gone and fuel quantity is now on the JPI, but the refill procedure from empty tanks would be the same to get a 5-point calibration for the EDM-900 (for empty, 1/4, 1/2, 3/4, full). I wasn't there to observe this process but no doubt the airplane was rolled onto the ramp to get fuel from the truck, and that ramp is not nearly as level as the hangar floor. From the MM, the final step in the procedure to establish the FULL reading on the gauge is to "Finish filling tank until fuel just starts to spill over outer wing surface". Well, when I got to the airplane after the fuel calibration, my tanks were extremely full, but not that full! 

So, my first target is to do a slow, methodical top-off of each tank to where the fuel starts to spill over the wing. I know from experience in this and previous Mooney's that you can squeeze a lot of fuel into those wings if you're patient. Assuming that in fact 68 gallons was loaded from the truck into assumed empty tanks, because my fuel flow meter is pretty accurate, I know how much I burned. Sure, there's the possibility of an error stack from the fuel truck meter that loaded that 68 gallons, variations in fuel temp and therefore fuel density, aircraft attitude on the ramp at the self-serve pump, etc. But it's a start. 

If I still don't discover those 7 "missing" gallons, I'll dig deeper. And if this fueling technique from the MM is what it takes to actually load 75 gal into my tanks, that's not something I'm willing to do at every fuel stop so at least I'll know how much fuel I really have using more conventional techniques that don't involve spilling fuel over the wing!

Thanks again for the inputs.

Bill 

 

[gsxrpilot]

1 hour ago, jlunseth said:

Yes, you can run a tank dry to help figure out what you have for useable, but you have to start with a tank that is actually full.  If you have not done that, then you will have the wrong measure of useable.

I'll have to disagree. I actually like to do this test when I can take off with only 5 or 6 gallons in one of the tanks. Just run it dry and return on the other tank. Now fill the tank and take as much time as needed to get as much fuel as possible in the tank. I keep filling until the fuel level stays above the lip on the retaining ring for the cap. The amount I can add to an empty tank is the useable fuel.

[Guest]

9 hours ago, gsxrpilot said:

Running a tank dry in flight is the ONLY way to accurately measure useable fuel. And USEABLE fuel is really all we care about. 

Run one tank dry in flight until the engine coughs and quits, then switch tanks and go land. Fill the empty tank to the top and observe how much fuel it takes. That is your actual, verified, useable fuel. Now go do the same on the other tank.

 

 

I'll have to respectfully disagree, flying the plane till fuel exhaustion on one tank is not the only way to get to unusable fuel.  Imagine for a moment that one wishes to do a re weigh for a weight and balance, would you fly over the airport running each tank dry, then glide back to the airport for the weighing?  Not very likely.

Pumping the fuel out with the boost pump would also get you to unusable just as easily without extraordinary measures.

Clarence

[carusoam]

Known inaccuracies....

1) the sight gauge fuel level gauges are calibrated to be accurate on the ground.

2) The instrument panel gauges are calibrated to be accurate in flight.

3) The FF gauge can be calibrated to +/- 1 gallon out of 100 gallons in the Tanks.  What I rely on for accuracy in flight.

4) The CIES floats with a digital display can bring the readout accuracy to a level similar to the FF gauge.

5) Fuel float accuracy is a bit wacky at the extreme ends of their travel.  The fuel continues to fill the tank, but the floats cannot continue to float up.  Note: see how many floats you have per tank, see what their limitations are.

6) know how your FL system accounts for the missing volumes caused by discreet volume changes like the speed brake volume.

7) I can only use my panel's FL gauges to generally know the amount used from each tank. Really rough numbers by comparison to FF.

8) I can find room for about five gallons by allowing the tank to be filled slowly and waiting for distribution.

9) the fuel delivery hose's back splash safety valve is a challenge when doing this.  Know the challenge, get the extra five gallons...

10) Fuel neck designs have been altered since they were first installed.  The K's anti-siphon flap is one of the most interesting.  The Eagle's is the most annoying (for it's resistance to filling).  The C's is the most disappointing (for it's materials of construction.)  The B's hid their fill neck and tank cap under a door.  The O's is simple (and it works).

11) Use Patience...

12) know what O-rings you need and how many are in each cap...

Fuel tank discussions go back to the beginning of MS.  We are fortunate to have Dr. Monroy on board.  Our Cies engineer stops in every now and then.  There are some really great pictures by people that have stripped and resealed their tanks.  Check out the photos to see where the fuel pick-up is compared to the lowest corner of the tank.  See the size of the water stain that can be left behind...  low corner, drain, and pick-up, are not all one ideal single point.

Things I have learned here.  I am a PP, not a mechanic.

Best regards,

-a-

[Bob - S50]

22 hours ago, jlunseth said:

However, I don't find the gauges very helpful.  They depend on the senders on the tank, which are old, inaccurate technology.  When doing a long trip I rely on the fuel flow meter and the USD number on my JPI, not on the tank gauges.  Depending on which tank you are looking at, the tanks go to zero with somewhere between 6 and 10 gallons left, and that is well over an hour of fuel in my plane.  My fuel flow meter, on the other hand, is accurate to within .1 gallons out of 50, so I go with that.

However, fuel flow will not catch fuel leaks.  You would think I work for Cies (but I don't), but if you get their floats and a digital fuel gauge you can have an accurate indication that WILL account for leaks.  Hopefully the STC will be out for my J model soon.

[fuellevel]

The FAA indicated that Tuesday Nov. 1 would be the day for the STC.  

We held off after Oshkosh to capture any nuance Textron wanted to add to their aircraft.  We have included a lot of new features.

With all the variations in capability of filling aircraft - I find pilot reports of FF accuracy to be curious based on a relatively non precision procedure.

So yes ramp or hanger floor angle makes a large difference in fuel level capability.  

The other element is gear set, when you fill one side of the aircraft, the gear on that side compresses and the opposite side extends.

Venting s the other unknown as the ability to fill depends on the ability to remove air from that final pocket - there is always an expansion space 

Venting does not confuse any fuel quantity system 

The idea that floats impead fuel measurement in aircraft is a bit over reported, location is key.   As there is unusable fuel at the bottom and vent space at the top, the float is usually not an issue.    CiES uses this, as we maintain a neutral buoyancy.  We use a denser float to  give a measure of stability to output. 

Other measurement systems like capacitive have similar issues, you can't extend a capacitive probe to the top and bottom of the wing surface and you don't want to on the inbd end as water messes with the output.   Equally due to lightning strike consideration it isn't a good idea to bring an isolated capacitive probe close to the top surface of the wing surface 

The more you know 

Good luck getting your full 76 gallon capacity - you will be on the right path - Leveled and jack supported aircraft, clear vent lines - +++++

 

Note - Typically for retrofit aircraft pilots report that the CiES fuel senders are inaccurate as the reported digital value conflicts with their fuel totalizer value.  I have a standing offer of $1000 dollars that in an aircraft I have done or supervised the calibration, that displayed amount in a leveled aircraft will be the amount drained from the aircraft within a gallon, that $1000 is still firmly attached to my wallet.   The reason being is that the assumptions pilots make about their starting and entered fuel value are not nearly as accurate as they believe.   Those pilots are now our most fervent supporters. 

[Marcopolo]

Just interjecting my .02, I think the only way to find useable fuel is to run one tank empty in flight (with plenty of fuel in the other) as this gives you the useable fuel in the flight, airframe attitude that you need the information from.  I think (opinion) it will also give you a chance to measure the fuel level low light to exhaustion timing.  Draing the fuel through the sump drains should empty the tank almost completely but should not be taken as any sort of measurement in useable fuel as the fuel pickup for the engine and the sump drain are not co-located.  Running the pump on the ground would definately be the safer route but I believe it would be slightly inaccurate compared to the inflight conditions experienced most of the time.  

 

Again, just my .02, or less!

 

Ron