Fuel gauges vs. fuel burn

[wombat]

On the new-to-me Mooney Rocket, I'm noticing that the fuel gauges show gallons, and read about 36 Gallons per side when completely full.

I've got the Monroy long range tanks, so I'd expect full to be 52.5.     When I was flying, they seemed to decrease at about the rate of fuel burn (20 GPH).  But they are starting at  a total of 72 Gallons, so when the fuel burn shows I should have 41 Gallons left, the gauges are showing I've only got about 25.    I don't know if the fuel is leaking out that fast or if the gauges are miscalibrated.    Guess I'll know more the next time I fuel it up, since I now have a record of what the gauges and the 'fuel remaining' from the flow meter say after having it topped off to start.

[Hank]

Hopefully some Rocket owners will be along soon. @aviatoreb?

[A64Pilot]

I don’t think it has anything to do with being a Rocket, I think it’s an MVP-50 thing.

Fortunately it’s very easy to re-calibrate, hardest part will be getting the tanks empty, and you may have to call EI and get a one time use code. I’m not sure there, but they will know, give them a call they are very helpful. At least a Mooney is at or close to flight attitude, tailwheel aircraft have to have the tail lifted high to calibrate.

It’s been a few years, but I’ve set up dozens of MVP-50T’s, the T just means turbine, but it’s nearly identical to a “P”

The Fuel burned calibration is also very easy to do and very accurate, I’d recommending burning several loads of fuel, doesn’t matter how far down you burn the tanks just keep up with the total, then go into the add fuel page and it will read something along the lines of you burned 100 gls and a spot for you to put actual fuel burned, then it will tell  you the difference and ask if you want it to recompute, answer yes and your good. You may want to write down the K factor, because one day if you accidentally really screw up, simply re-enter the K factor and your back right where you were.

The larger the amount of fuel you’re working with the more accurate the adjustment will be. I could get fuel burn within a gallon in a crop duster that held 228 gls.

The MVP-50 is an astonishingly accurate and powerful device and thankfully the senders are readily available and reasonably priced.

Depending on if you have EI fuel sending units or not you may have a little black box that your factory Mooney sending units attach to that converts something so they will work with the MVP.

If you don’t have the manual I’d download them and read about it.

I believe you can find all the manuals here

https://iflyei.com/product/mvp-50p-engine-monitor/

[carusoam]

Every new 2U aircraft gets to go through the same challenge….

Check the Calibration of the fuel instruments….

Check the actual useful volume of the tanks…

Know their limitations…

Some tanks can get filled beyond what their instruments can measure….

Some Monroy installations add extra volume, but may not have a method of measuring that extra volume… and the tanks don’t empty in series… (this could be why the EVP is only measuring the original fuel volume, and not including the extended tanks…)

Some digital displays are really good…

Some digital fuel floats are really good…

Some fuel flow meters and fuel totalizers are really nice…

If you have fuel gauges on the wing… know they are calibrated while the plane is on the ground…

The fuel gauges on the instrument panel are calibrated while the plane is level in flight…  (on the ground requires using a level on the sheet metal seam on tail cone….)

If you have to rely on 1 gal accuracy… it can be done.

 

Start with an empty tank… with the last drops emptied by the engine or through the fuel separator… fill it one or two gallons at a time… (park next to the fuel pump)

If you have Ceis fuel gauges… there is a procedure for that…

If you are building your own fuel stick… this works as well…

Know how many fuel floats you actually have… some big tanks get two each…

It is important to have the most accuracy as the tanks reach zero… but it is also really important to have them be accurate at half full too…

How you fill the tanks can also affect the details… start on a nice level pad…

 

PP thoughts only, not a mechanic…

Best regards,

-a-

[Pinecone]

Its a Monroy tank thing.  The gauges don't know about the Monroy tanks, and they way they work is, the first few gallons are out of the Monroy extension, then the fuel comes from both the Monroy and the factory tanks.

So, what you need to do is calibrate the wing gauges (and panel fuel gauges) so you know what you have at various points.

On my 252, when the fuel gauges (panel) read 27 gallons, I have about 34 gallons on that side.

[midlifeflyer]

I don't know the MVP-50, but I took a quick peek at the manual. Fuel management shows two values. Its similar to other similar systems.

One is the fuel in the tanks. That's based on and only as good as the fuel sensors (which may be old school) and the MVP-to-tank calibration. So, as already suggested, that calibration might be the issue.. 

The other is the Fuel Used/Remaining/ etc information. That's based on the number you give it on the Add Fuel? screen. The MVP reads the actual fuel flow and deducts from the number you gave it. That should be dead on so long as you gave it a good starting number and do not have a fuel leak. 

So the question to me is, is there a discrepancy between the fuel gauge and the gallons remaining per the totalizer. So long as you input good numbers for Add Fuel, if there is a discrepancy, I'd suspect the sensors or the calibration.

[wombat]

12 minutes ago, midlifeflyer said:

I don't know the MVP-50, but I took a quick peek at the manual. Fuel management shows two values. Its similar to other similar systems.

One is the fuel in the tanks. That's based on and only as good as the fuel sensors (which may be old school) and the MVP-to-tank calibration. So, as already suggested, that calibration might be the issue.. 

The other is the Fuel Used/Remaining/ etc information. That's based on the number you give it on the Add Fuel? screen. The MVP reads the actual fuel flow and deducts from the number you gave it. That should be dead on so long as you gave it a good starting number and do not have a fuel leak. 

So the question to me is, is there a discrepancy between the fuel gauge and the gallons remaining per the totalizer. So long as you input good numbers for Add Fuel, if there is a discrepancy, I'd suspect the sensors or the calibration.

Yes, those are my questions too.   Additionally, since the fuel senders are in the main tanks, I think there will be significant fuel added after they are full but before the monroy long range tanks are full. 

After the last fill-up, I set the MVP-50 to 104 gallons.  The official 'book' capacity is 105.    At this point after one flight since then, the 'fuel remaining' and the fuel gauges are significantly different.  41 Gallons for fuel remaining (from the fuel totalizer) vs 25 for the gauges.

In a perfect world, the gauges would read correct on empty, and for all fuel added until the fuel senders reach their 'full' limit, even though that is not actually full  fuel.  However much they read when they hit their limit of travel is what they would read for any actual quantity of fuel above that.  Or if there is a way to make it say "At least XX gallons"   

If I fill the tanks to completely full, the gauges would start by reading a value less than 105 total and would stay there until I consume enough fuel to let the senders start moving.  Then they would be accurate down to 0 gallons.

So right now I don't know how much fuel I have.   If I lost a lot of fuel that didn't go into the engine, the fuel gauges might be right.  But I won't know until I fill the plane all the way back up.  And I don't want to do that because I'll be dripping fuel.

[midlifeflyer]

3 minutes ago, wombat said:

In a perfect world, the gauges would read correct on empty, and for all fuel added until the fuel senders reach their 'full' limit, even though that is not actually full  fuel.  However much they read when they hit their limit of travel is what they would read for any actual quantity of fuel above that.  Or if there is a way to make it say "At least XX gallons"   

Hmmm... This may be way off, but that part sounds similar to an oddity in the Diamond DA40NG. The tanks hold 39 gallons usable but the fuel gauges only go up to 28 (14 each). That's weird but OK. The odd part (which I don't like) is that the totalizer won't allow us to input anything above 28 gallons even if we fill the tanks.  So, if we have 39 gallons and use 8 GPH, after two hours (keeping the tanks fairly even), the fuel gauges will read 23 gallons. But the fuel remaining per the totalizer will read only 12 gallons. In that case, it's not the sensors nor the calibration, just a design "feature". Do you think that might be what you are seeing?

[Hank]

2 minutes ago, midlifeflyer said:

Hmmm... This may be way off, but that part sounds similar to an oddity in the Diamond DA40NG. The tanks hold 39 gallons usable but the fuel gauges only go up to 28 (14 each). That's weird but OK. The odd part (which I don't like) is that the totalizer won't allow us to input anything above 28 gallons even if we fill the tanks.  So, if we have 39 gallons and use 8 GPH, after two hours (keeping the tanks fairly even), the fuel gauges will read 23 gallons. But the fuel remaining per the totalizer will read only 12 gallons. In that case, it's not the sensors nor the calibration, just a design "feature". Do you think that might be what you are seeing?

I would find that annoying! More like a bug than a "feature" . . . .

[midlifeflyer]

54 minutes ago, Hank said:

I would find that annoying! More like a bug than a "feature" . . . .

Yeah, that's why I put "feature in" quotes. I think it's beyond annoying. Downright stupid. I can even figure out why someone would think it's particularly safe.

This is an airplane that uses 8 GPH at 92% power (normal cruise - it's a diesel). So when we have about 1.9 hours of fuel with a 1-hour reserve, the totalizer is telling us we only have a half hour before hitting the hour reserve. I have no idea what would make that "safe" in terms of calculating things like enroute alternates. My way of handling it is to wait until I have 13 showing on both gauges and updating the totalizer quantity to get a more realistic read. 

[wombat]

I'm going to call EI today and ask about this.

[wombat]

Just got off the phone with EI.

For fuel gauge calibration the MVP-50 has 4 or 5 calibration points with what I'm assuming is linear interpolation between them.   They are user-settable, so I can set them to whatever I want.   I will probably set them to read however much is in the tanks when the travel is at the full upper limit, and of course empty, and pick two spots in the middle where the it will have good linear interpolation between them.

 

The engine percentage power is configurable too, so I can fix that too.

 

@midlifeflyer I can set my fuel totalizer to be 105 gallons, even though the gauges will only read a maximum of 72 gallons total, so it's not quite the same problem you have on the Diamond.

 

 

EI sells the EI P-300M magnetic float sensor.   Much cheaper than the CIES ones (< $500 each instead of $1,100 each).   So I wonder if I should do this now since the fuel tanks will be getting resealed next month anyway....  But the current ones seem to work just fine.   Hmm....   If it was just the $1,000 I'd probably do it, but it's also going to be a bunch of labor to drain, remove, then re-install the sensors.    Hmmm....

 

 

[A64Pilot]

4 hours ago, wombat said:

I'm going to call EI today and ask about this.

Best thing to do

[Hank]

2 hours ago, wombat said:

EI sells the EI P-300M magnetic float sensor.   Much cheaper than the CIES ones (< $500 each instead of $1,100 each).   So I wonder if I should do this now since the fuel tanks will be getting resealed next month anyway....  But the current ones seem to work just fine.   Hmm....   If it was just the $1,000 I'd probably do it, but it's also going to be a bunch of labor to drain, remove, then re-install the sensors.    Hmmm....

Your tanks must be drained to reseal, may as well swap out the sensors while the sealant is curing. 

[Fly Boomer]

9 hours ago, wombat said:

If I fill the tanks to completely full, the gauges would start by reading a value less than 105 total and would stay there until I consume enough fuel to let the senders start moving.  Then they would be accurate down to 0 gallons.

Are you saying this is the way you wish it worked?  Because I don't it currently works that way.  As far as I know, when the panel fuel gauges first come off "full", the mains are still being fed by the aux tanks.  At some point when the aux tanks are empty, the panel gauges are correct and will remain so until they read "empty".

[wombat]

@Fly Boomer Yes, that was intended to be part of my "In a perfect world" statement about how I wish it worked.

 

 

[wombat]

I'm considering replacing the fuel senders in the plane.   Does anyone know if this should be done before having the fuel tanks resealed?   i.e. Is there any fuel tank sealant over the fuel senders?      If I'm spending the money to have the 'best' tank sealers do their work, I don't want to break through that seal shortly after.

[ArtVandelay]

I'm considering replacing the fuel senders in the plane.   Does anyone know if this should be done before having the fuel tanks resealed?   i.e. Is there any fuel tank sealant over the fuel senders?      If I'm spending the money to have the 'best' tank sealers do their work, I don't want to break through that seal shortly after.

No, they are sealed using gaskets, not sealer.

[carusoam]

43 minutes ago, wombat said:

I'm considering replacing the fuel senders in the plane.   Does anyone know if this should be done before having the fuel tanks resealed?   i.e. Is there any fuel tank sealant over the fuel senders?      If I'm spending the money to have the 'best' tank sealers do their work, I don't want to break through that seal shortly after.

 

On 5/5/2023 at 9:07 PM, Hank said:

Your tanks must be drained to reseal, may as well swap out the sensors while the sealant is curing. 

For selecting the specific Fuel eleven sensor…

Use Ceis as the base line… where their accuracy is about 1 gallon over a 100 gal load…

With hundreds of installs around here…

 

See if you can find a pirep on the other brand… it may be as good, but you have more effort required to know for yourself if …  there should be a few around here…

do you have any speed brakes or any other oddities inside the tanks?

This is where the linearity causes issues…

Ceis has the ability to calibrate for a bunch of non-linear challenges….

You mentioned the EVP doing the linear interpolation…

 

Sounds like you may want to discuss this with your fuel tank expert to get the results that you want…

I bet your fuel tank guy knows the details you want to have…

 

You are paying a lot for that expertise… make sure you get what you are asking for…

If you are an engineer, or enjoy electronics… this is a relatively simple challenge to go through…

If this is a forever-plane… having two fuel level sensors in each tank is really good…

• The inner one provides accuracy as the tank(s) runs down…
• the outer one provides accuracy as the tank is filled near the top…
• the tanks go up hill a lot as they go towards the wing tips…

overall challenge…

1) Old plane

2) Updated fuel tanks (not a simple single rectangular tank, but two… that empty simultaneously from uphill to downhill)

3) Fuel level sensors in the orginal location(s) (?)  one per tank or two?

4) Upgrading tanks with new sealant

5) Upgrading fuel level sensors to digital accuracy

 

6) You have a fuel totalizer that probably gets used as your best source of info of fuel used…

Between the two systems… you really have a good method of knowing how much fuel is really in there… at all times.

7) Unless you forget to

• reset the fuel used in the totalizer before flight…
• fill the tanks to the top…
• Or…. don’t have a method of partially filling the tanks… calibrated stick, or mechanical gauge in the wing.

Compare these notes to what you have in mind…  see if I missed anything…   
 

8) I have a simple FT101 fuel totalizer… precisely calibrated.

mechanical fuel level sensors… accurate to about 1/8 of a tank (not very precise at all)

mechanical fuel level sensors in the wing for partial fueling.  Really good accuracy to about 25 gal.

 

There is nothing more stressful than running low on fuel, and being unsure how much you have used, or how much is really left…

Running out of fuel is in the top 4 issues we are constantly battling…    IMC, ice, and thunderstorms are the others…

Update your electronics check list items… resetting the fuel used / totalizer details is really nice to remember…

PP thoughts only, not a mechanic…

Best regards,

-a-

 

[Pinecone]

On 5/5/2023 at 9:51 PM, Fly Boomer said:

Are you saying this is the way you wish it worked?  Because I don't it currently works that way.  As far as I know, when the panel fuel gauges first come off "full", the mains are still being fed by the aux tanks.  At some point when the aux tanks are empty, the panel gauges are correct and will remain so until they read "empty".

No, the main start feeding before the auxs are empty.   I don't have the numbers handy here (not at home), but will post later what amount of fuel there starts to be visible fuel in the Monroy section.  On my 252, when the gauges read 27 gallons, I have about 34 gallons on that side.

You can't really think of the Monroy tanks as Aux tanks, they are extended main tanks.   But the outboard senders are at the end of the main tanks, so once the hit the top, they can't measure the extra fuel.

[kris_adams]

Not in a million years would I put EI float sensors in my plane.  I went with CIES.  I can't find many reviews on EI senders and most here probably know my experience with EI (not senders btw) was terrible.  Here's a note on CIES vs EI senders from Pilots of America.  Not vouching for the validity but I think there's a reason you see CIES everywhere including many (maybe most) new planes.  EI commented on this thread as well.

 The method is similar but it is different technology. EI uses a Hall effect sensor, and CiES uses an AMR sensor. AMR is used extensively in automotive for drive by wire functions. Throttle pedal and throttle plate, brakes, crankshaft position sensing i has supplanted hall devices as it is temperature tolerant and stable. Patent still in force

[1980Mooney]

On 5/5/2023 at 10:22 AM, Pinecone said:

Its a Monroy tank thing.  The gauges don't know about the Monroy tanks, and they way they work is, the first few gallons are out of the Monroy extension, then the fuel comes from both the Monroy and the factory tanks.

So, what you need to do is calibrate the wing gauges (and panel fuel gauges) so you know what you have at various points.

On my 252, when the fuel gauges (panel) read 27 gallons, I have about 34 gallons on that side.

Exactly right.

On 5/5/2023 at 11:33 AM, wombat said:

In a perfect world, the gauges would read correct on empty, and for all fuel added until the fuel senders reach their 'full' limit, even though that is not actually full  fuel.  However much they read when they hit their limit of travel is what they would read for any actual quantity of fuel above that.  Or if there is a way to make it say "At least XX gallons"   

If I fill the tanks to completely full, the gauges would start by reading a value less than 105 total and would stay there until I consume enough fuel to let the senders start moving.  Then they would be accurate down to 0 gallons.

So right now I don't know how much fuel I have.   If I lost a lot of fuel that didn't go into the engine, the fuel gauges might be right.  But I won't know until I fill the plane all the way back up.  And I don't want to do that because I'll be dripping fuel.

You are right that the current factory senders hit their upper "Full" limit before you completely fill the Monroy auxiliary tanks. 

The Monroy tanks sit outboard on the dihedral wing and are slightly above the main tank.  The Monroy auxiliary tank is connected to the Main tank with a single flow line with 6D fittings and a single vent line.  I think that 6D line is about 3/8 inch ID. It flows between Main and Monroy Auxillary tanks by gravity. The point is that it flows slowly.  If you fill only the Main tanks to the lip with 75.6 gallons (and none in the Monroys), the fuel does not rush into the Monroy tank.  Your panel gauges will say "Full" at the moment after you fill the Mains.   But if you wait 15 minutes or more the levels even out.  The Main tank level drops and the Monroy will now have fuel in it rising by gravity to the same vertical level as the Main.  Your panel gauges will then say about 3/4 or something like that (maybe 55-60 gallons).

If you fill the Main to the lip and then the Monroy to the lip you will have your full 105 gallons.  When you are flying and get down to about 1/2 tank as shown on your panel, then your panel fuel gauge will read accurately.  The Monroys will have drained out completely into the Main.  Only the Mains will have fuel and the factory sender will read correctly.

So in the "current world", your current gauges read accurately between 1/2 and Zero.  Your bottom half of your tank in each wign is 19 gallons.  But the top half of your tank in each wing is 33 gallons.  Your Monroy Supplement should have a conversion for you to stick measure the Monroy so you can determine how much is in each Monroy (while sitting level on the ground) whch you can add to what your panel is saying (reading only the Main).  It is not perfect but you can rely upon your panle gauges when it matters - when fuel reads below 1/2 tank.

On 5/5/2023 at 5:22 PM, wombat said:

Just got off the phone with EI.  For fuel gauge calibration the MVP-50 has 4 or 5 calibration points with what I'm assuming is linear interpolation between them.   They are user-settable, so I can set them to whatever I want.   I will probably set them to read however much is in the tanks when the travel is at the full upper limit, and of course empty, and pick two spots in the middle where the it will have good linear interpolation between them.

As explained above it is not linear.

This is similar with the Monroys in a J.  There is a copy of the J Supplement.

 

Edited May 11, 2023 by 1980Mooney

[Z W]

I like to think of it as a "zone of confusion" like flying over a VOR.  With the Monroy tanks, your gauges are accurate at full, then they get confused for a while and stop moving, then they come back to their senses and start working again after the Monroy tanks are empty, which is when your main tank is down to about 20 gallons. 

It works fine, but makes it very difficult to know to the gallon how much fuel is on board at any given time unless you topped up to 105 gallons.  It's rare I have the useful load available to do that, so more often, I top the mains which is 75 gallons (although I feel like a gallon or two probably flows up into the aux tanks while fueling) and use the fuel totalizer from there.  Topping the mains, which is perfect for 1-3 people and most missions (maybe different for a Rocket), puts you right into the zone of confusion.

You could use a dip stick on all four tanks and a spreadsheet and get pretty accurate numbers if it really mattered.  Requires draining to empty, filling a few gallons at a time, and noting your data.

[rbp]

Right now the G3X EIS records, fuel flow rate in gph, but not fuel  flow quantity (gal). Yesterday I sent off a feature request to Garmin asking them to record fuel flow quantity in each sample so that I can do my own consumption rate calculations over a longer period of time (e.g., top of climb to beginning of descent), and compare it to the fuel quantity levels 

[Pinecone]

On 5/10/2023 at 10:51 PM, 1980Mooney said:

Exactly right.

You are right that the current factory senders hit their upper "Full" limit before you completely fill the Monroy auxiliary tanks. 

The Monroy tanks sit outboard on the dihedral wing and are slightly above the main tank.  The Monroy auxiliary tank is connected to the Main tank with a single flow line with 6D fittings and a single vent line.  I think that 6D line is about 3/8 inch ID. It flows between Main and Monroy Auxillary tanks by gravity. The point is that it flows slowly.  If you fill only the Main tanks to the lip with 75.6 gallons (and none in the Monroys), the fuel does not rush into the Monroy tank.  Your panel gauges will say "Full" at the moment after you fill the Mains.   But if you wait 15 minutes or more the levels even out.  The Main tank level drops and the Monroy will now have fuel in it rising by gravity to the same vertical level as the Main.  Your panel gauges will then say about 3/4 or something like that (maybe 55-60 gallons).

If you fill the Main to the lip and then the Monroy to the lip you will have your full 105 gallons.  When you are flying and get down to about 1/2 tank as shown on your panel, then your panel fuel gauge will read accurately.  The Monroys will have drained out completely into the Main.  Only the Mains will have fuel and the factory sender will read correctly.

So in the "current world", your current gauges read accurately between 1/2 and Zero.  Your bottom half of your tank in each wign is 19 gallons.  But the top half of your tank in each wing is 33 gallons.  Your Monroy Supplement should have a conversion for you to stick measure the Monroy so you can determine how much is in each Monroy (while sitting level on the ground) whch you can add to what your panel is saying (reading only the Main).  It is not perfect but you can rely upon your panle gauges when it matters - when fuel reads below 1/2 tank.

Close.

The hard part is fully fueling the tanks.  If you fill the "mains" and then fill the "Aux" you will NOT have full tanks.  Last time I filled all the way up, I filled each main, then put on the cap and filled the aux.  Then went back and topped up the first aux, and then the second, and repeated.  Then went to lunch.  After lunch, I was able to get about 9 more gallons in the plane.

I recently calibrated some dip sticks.  252 with Monroy.  So 37.5 each main, 14.5 each aux, for  a total of 104.

When I got to a 25 gallons, I could see fuel in the Monroy, but not measure it.  I was adding in 2.5 gallon increments.  At 27.5, there was about 1/2 in the Monroy.  The wing gauges read 30 gallons when there was 37.5 gallons added.

At the point the mains were pretty much full, I had put in 42.5 gallons.  I managed to put in 55.0 gallons and adding that last bit, it was pour, wait a bit for the level to drop and pour some more.  I could still hear bubbling as the level went down.

For each addition of 2.5 gallons, I waited at least 5 minutes before measuring.