MooneyMark

Calculated fuel on board versus indicated fuel on board

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With the gauges on the wings and in the cockpit near zero, my M20K never takes more fuel then 50 gallons at the pump. The capacity is 76 gallons so I always land with way more fuel on board then I think. Yesterday I pushed my flight a little longer with the gauges uncomfortably reading 1 gallon and both the low fuel annunciators burning, yet the calculated landing fuel on board was 18 gallons. 

She took 60 gallons this time, so I guess the calculated fuel on board wins. Some uneasy flying tho.

I refueled with the nose a little higher then on a completely level surface. Does that impact the amount of fuel I can add to the tanks? When refueling, the fuel gets up to those ‘anti-siphon’ doors easily. But then the topping up starts. This seems it could go on endlessly, if I just rock the wings a little back and forth. When do you guys say, this is full?

I always fly with a big fuel margin, but to never use about one third of the fuel on board is silly, so I thought I’d dig in and see if it was really there. And it was. I stayed close to the airport by the way. Good to get to know the aircraft a little better again.

It was a sad day for flying, last weekend we lost three young dutch glider pilots in two separate accidents. I started out flying gliders and I heard the ELT blaring on the 121.5 from the last crash, gave me the shivers. Stay safe! 

 

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The calculated NEVER wins. It is one merely a confirmation of a known set of facts, IF you have mapped your gauges and lights.

The problem is........you never know if you have a leak and if so which side of the flow transducer. If you have a leak before the flow transducer, you'll think your calculated is just fine, but in fact you've lost a bunch of fuel and the low quantity gauges or lights are in fact telling you tragedy is around the corner. Ask Air Transat 236 when they deadsticked an A330 into the Azores  how that works.

There is always gauge hysteresis on the tank levels and you should have a map of it for each tank. I have mapped mine (it is in the form of an Excel spread sheet) and I know for instance what the gauge should read based upon the calculated flow. If the gauge is significantly less than that of the mapped quantity compared to the flow, you know you have a leak before the transducer. You should know, to the gallon when your low fuel lights come on and you should be able to predict that almost to the minute with the flow meter. Again, if the lights come on early, something is amiss.

 

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Remove all doubt.  Install Cies and a digital gauge.  My gauge/fuel counter total always stay within a gallon of each other.  I believe whichever says I have the least gas.

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Or just run one side dry. Figure out how far off the gauge actually is.

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I have never seen low fuel light up on the annunciator panel! How low does fuel have to be for it to come on? 

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I have never seen low fuel light up on the annunciator panel! How low does fuel have to be for it to come on? 

POH says 2.5-3 gallons.
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4 hours ago, MooneyMark said:

With the gauges on the wings and in the cockpit near zero, my M20K never takes more fuel then 50 gallons at the pump. The capacity is 76 gallons so I always land with way more fuel on board then I think. Yesterday I pushed my flight a little longer with the gauges uncomfortably reading 1 gallon and both the low fuel annunciators burning, yet the calculated landing fuel on board was 18 gallons. 

She took 60 gallons this time, so I guess the calculated fuel on board wins. Some uneasy flying tho.

I refueled with the nose a little higher then on a completely level surface. Does that impact the amount of fuel I can add to the tanks? When refueling, the fuel gets up to those ‘anti-siphon’ doors easily. But then the topping up starts. This seems it could go on endlessly, if I just rock the wings a little back and forth. When do you guys say, this is full?

I always fly with a big fuel margin, but to never use about one third of the fuel on board is silly, so I thought I’d dig in and see if it was really there. And it was. I stayed close to the airport by the way. Good to get to know the aircraft a little better again.

This is one of the reasons for running a tank dry. Run one of the tanks dry while in level flight. Then land and go fill it up. Make notes of the gauge indications at regular intervals, for example every 5 gal. I make notes of both the wing gauge indications as well as the panel fuel gauge indications. The tanks in the M20K are a real pain in the ass to fill properly. So make a note of how many gallons to get to the anti-siphon door. Then now many gallons you can go beyond that. Take your time and make lots of notes. Then go do the same to the other wing.

There is nothing quite a comforting as accurately knowing your fuel situation at all times. I recommend the CiES digital senders along with a good engine monitor such as the EDM900 as the fuel gauges. 

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53 minutes ago, Bob - S50 said:

Remove all doubt.  Install Cies and a digital gauge.  My gauge/fuel counter total always stay within a gallon of each other.  I believe whichever says I have the least gas.

Capacitance senders are in general more accurate, but any electrical circuit is subject to hysteresis. 

Running one tank dry will tell you how far the gauge is off when empty. But what about at say 25 gallons? You need to know you have a "fuel disagreement" before the tank leak has bled dry.

 

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Some Mooneys get the light sooner than others...

3gal left isn’t enough warning to do anything but land now.... @10gph...

The long bodies went to the next level... probably 6gal per side... (I’ll have to look that up for sure...) @15gph...


Things we can have now that really helps with knowing what you have...

1) Know how much fuel fits in the tank... from empty to full... watch for the lights while you fill it.... (More detail to check on, because there is intentional hysteresis here too)  the hysteresis keeps the light from going on and off In the narrow range of activation...

2) Kfactor for your FF gauge... should net accuracy to near 1 gallon...

3) Fuel float gauges get improved by going digital, using frequency... Cies does both better than the old original Mooney ones... Again 1 gallon accuracy...

4) On that one odd situation where you have a leak develop mid flight... compare your Cies fuel used to your FF totalizer... they should be matching... if they don’t... it may be time to plan the landing sooner, rather than later....

5) Ground attitude definitely affects the gauges... we know this because the wing sight gauges Read differently than the panel gauges...

6) We discussed using Cies to better represent fill levels while on the ground... technically it could be done, but not very easily...

7) Until you run your tanks dry, one at a time... it is really hard to know what the real useable fuel level actually is...

8) Every Mooney has a different fuel neck... some needed to be changed because they would rust... others got the anti-siphon gate... some tanks got two fuel caps on each side... the original Eagles got the sinful long fuel neck of shame...(modifiable with a simple hole maker)... the best were the necks that got a calibrated vent hole...

9) Once you have all that.... develop a fuel use strategy... climb on one tank, then hourly switches after that... knowing your fuel burn rate for at least phases of flight climb and cruise... you should know pretty well what your fuel use should be...

10) Level on the ground is the best way to fill the tanks... fuel never comes out the vents, because they are really far up hill... this ain’t no Cessna... :)

11) Sitting in the sun probably won’t make the fuel expand enough to reach the vent either...still not a Cessna.

12) Stainless steel fuel necks are better than mild steel...mild steel rusts and drops rust particles in the fuel... that show up on and below the fuel strainer screen...

13) +1 for blue fluorosilicone pairs of Orings... helps by not adding water to the fuel tanks when it rains....

14) when running a tank dry, be familiar with the procedure...  not much to it... turbo restarts at altitude can be messy if the turbo spins down... don’t be slow...

15) topping off can take extra time... there is tiny groups of holes in the stringers... at the top for air... at the bottom for fuel.... If they get filled with sealant, that can add a few challenges....

16) it is really nice to have the wing top sight gauges... available to all Mooneys, they get installed in the access panel...

Did I miss anything...? Or... how many things did I miss?

Sorry to hear about the glider tragedies... :( prayers...

Best regards,

-a-

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My light was on on one side and I would estimate I had 6 or 8 gal when it went on based on my fill up. I found my cockpit gauges and tank gauges reasonably accurate.  I did however install the cies senders hooked to my TXi since then and I do feel more confident with them. Anyone want to buy my old senders? Like I said, they were pretty good! 

 

:D

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I haven't checked the K models for a limitation like this one for the R models, but it's something to consider before running both tanks down to the warning lights.  Think about what might happen if you need to go around...

Screen Shot 2020-07-13 at 8.23.55 PM.png

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Take three trips where you burn about 35 gallons or more each time. You should personally fill the tank every time, including before the first trip (but you don’t include that fuel in the following total), or supervise the filling, and make sure to fill the tank in the same way to the same place each time. Get the reading off the pump each time and write it down. Also get the calculated total gallons used from the engine monitor or fuel flow monitor. When you have the three trips done, total the fuel readings from the pumps and the fuel readings from the monitor and compare the two. You will then know if your calculated fuel is off. The fuel pump readings you are comparing them to are required by law to be tested and accurate, at least here in the US.  There is usually a way to adjust the fuel flow calculation. I have a JPI 930 and it is called the K factor, you need to go into the programming function and change that. Adjust the fuel flow reading until you know it is accurate.  Then, during each trip, record the fuel used out of a given tank each time you change tanks and total it up so you know how much fuel is in each tank.  This will give you a very accurate reading, and much more accurate than the gauge readings unless you have the new CiES sensors.

If you take a long trip and need all the fuel you can load, then supervise the loading yourself and waggle the wing to burp air out of the tanks.

It is worth checking the fuel flow accuracy every couple of years.

When I did this I found my fuel flow was off by 0.1 gallon for every fifty gallons indicated used, and the slight deviation was in the conservative direction. Fuel flow is a way better way to know what fuel is left and in what tanks, than the gauges.

I eventually got the CiES sensors installed and they are quite a bit more accurate than the factory sensors, but indicated fuel flow is still the way to go.

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19 minutes ago, Greg_D said:

I haven't checked the K models for a limitation like this one for the R models, but it's something to consider before running both tanks down to the warning lights.  Think about what might happen if you need to go around...

Screen Shot 2020-07-13 at 8.23.55 PM.png


Expect the same for all Mooneys...  not so much the exact number of gallons, but the ability to unport the fuel pick-up is pretty similar across the breed...

Under ideal conditions nose up attitude puts the fuel near the pick-up... nose down, the fuel runs away from the pick-up... and things do get quiet...

under less ideal conditions, like a cross wind... selecting the tank that is in the higher wing can help...

PP thoughts only, not a CFI...

Best regards,

-a-

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Expect the same for all Mooneys...  not so much the exact number of gallons, but the ability to unport the fuel pick-up is pretty similar across the breed...
Under ideal conditions nose up attitude puts the fuel near the pick-up... nose down, the fuel runs away from the pick-up... and things do get quiet...
under less ideal conditions, like a cross wind... selecting the tank that is in the higher wing can help...
PP thoughts only, not a CFI...
Best regards,
-a-

If I ever get to a point I have to consider which wing is the high wing for fuel pickup....I have done something horribly wrong.
My record is 14 gallons left in the tanks.
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5 minutes ago, ArtVandelay said:


If I ever get to a point I have to consider which wing is the high wing for fuel pickup....I have done something horribly wrong.
My record is 14 gallons left in the tanks.

I goofed Up a maintenance flight...

Plan A... don’t run out of fuel...

Plan B... use the tank with the most in it...

Plan C... switch tanks when the fuel runs out...

In reality... fuel ran out when the nose was lowered on short final...

Hmmm... is that exhaust noise or just wind noise...?

Jockey the throttle just to find out....

Switch tanks and turning on the fuel pump while on short final... and count....   
 

Failure to plan 101...

putting gas in would have taken a whole other five minutes...
 

Not my best showing... :)

Best regards,

-a-

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4 hours ago, GeeBee said:

Capacitance senders are in general more accurate, but any electrical circuit is subject to hysteresis. 

Running one tank dry will tell you how far the gauge is off when empty. But what about at say 25 gallons? You need to know you have a "fuel disagreement" before the tank leak has bled dry.

 

Cies floats are not capacitance.  I think they work on something called the Hall effect.  Our gauges are consistently accurate to within .3 gallons/tank.

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===14) when running a tank dry, be familiar with the procedure...  not much to it... turbo restarts at altitude can be messy if the turbo spins down... don’t be slow...

Never having flown a turbo I am interested in this.   I have done restarts in a twin, with a feathered prop that wouldn't un-feather because of a low accumulator - that was a challenge.

The engine is still pumping some air and driving the turbo some,  no?  At altitude, with no boost, seems like the mixture would rich.  Where are the problems?

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8 hours ago, Bob - S50 said:

Cies floats are not capacitance.  I think they work on something called the Hall effect.  Our gauges are consistently accurate to within .3 gallons/tank.

Point taken, and with Hall effect which is magnetic field disruption, you still have hysteresis both in the field and the circuit. I certainly would agree they are an improvement over resistance sensors.

 

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1 hour ago, skykrawler said:

===14) when running a tank dry, be familiar with the procedure...  not much to it... turbo restarts at altitude can be messy if the turbo spins down... don’t be slow...

Never having flown a turbo I am interested in this.   I have done restarts in a twin, with a feathered prop that wouldn't un-feather because of a low accumulator - that was a challenge.

The engine is still pumping some air and driving the turbo some,  no?  At altitude, with no boost, seems like the mixture would rich.  Where are the problems?

I've run each of my tanks dry, half a dozen times or so. I've not run one dry above FL180 though. There is a procedure set out in the POH for restarts at high altitude.

But each time I've run a tank dry, it's been a very deliberate action. I'm ready with my hand on the fuel selector, I know within about +/- 30 sec when the engine will start to stumble. And at the first indication, I switch tanks. This doesn't require the boost pump, or any other action other than just switching the fuel tank selector and the engine immediately restarts.

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6 minutes ago, gsxrpilot said:

I've run each of my tanks dry, half a dozen times or so. I've not run one dry above FL180 though. There is a procedure set out in the POH for restarts at high altitude.

But each time I've run a tank dry, it's been a very deliberate action. I'm ready with my hand on the fuel selector, I know within about +/- 30 sec when the engine will start to stumble. And at the first indication, I switch tanks. This doesn't require the boost pump, or any other action other than just switching the fuel tank selector and the engine immediately restarts.

Hi Paul,  do you see the FP flicker or drop prior? Had an aux tank in a Pitts, you could see FP fluctuate prior to sucking air. 
-Matt

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5 minutes ago, MB65E said:

Hi Paul,  do you see the FP flicker or drop prior? Had an aux tank in a Pitts, you could see FP fluctuate prior to sucking air. 
-Matt

I've only run one tank dry.  I did something similar to Paul, except I was watching the fuel pressure indicator.  Yes, it began to vary before the engine quit.  I too, switched tanks as soon as I saw that.  The engine never quit.  And as I've stated here before, our Aerospace Logic FL202 gauge, provided input by our Cies floats, showed the tank had 0.0 gallons left about 5 minutes before that happened.  That's probably because some of the 'unusable' fuel in the tank is in fact usable in level flight.

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7 minutes ago, MB65E said:

Hi Paul,  do you see the FP flicker or drop prior? Had an aux tank in a Pitts, you could see FP fluctuate prior to sucking air. 
-Matt

Neither one of my Mooneys had a fuel pressure gauge. The C was carbureted and the K doesn't have one either. I see the MP needle moving along with the sound and feel of the engine.

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M20C should have an fuel pressure gauge... something showing the range of 0 - 6psi... or so...(old fuzzy memory)

M20R definitely doesn’t come with a fuelP gauge,  just a FF gauge that will get fuzzy logic as air gets drawn into the system...

Just a couple of data points tossed on the pile...

one of the tests for the fuel system prior to start...

  • watch the FP rise with the electric fuel pump on...
  • See that it stays up for a while...
  • watch it fall with each pump of the accelerator pump...

PP thoughts only, not a mechanic...

Best regards,

-a-

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14 minutes ago, carusoam said:

M20C should have an fuel pressure gauge... something showing the range of 0 - 6psi... or so...(old fuzzy memory)

Maybe it did... I've got the same old fuzzy memory problem

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Fuel pressure seems like a valuable gauge to have to diagnose problems with our engines, especially since we only have one.

Personally I would add one if my plane didn’t already have it.

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