Full Fuel Tanks

[CoffeeCan]

This is a timely thread for me. I just completed my first long-distance trip in my 1985 M20K (2260 NM) and was very interested in the fuel gauges and fuel burn en route, as well as before and after fill-ups. The discrepancies I noted during my trip are at least partly explicated by the posts I'm reading on here. 

FWIW, I met with the avionics man at my home airport this morning and we arranged for him to install a fuel flow transducer on my JPI monitor (as well as an engine oil temp transducer). After that is in, I'm going to work on nailing down the numbers for my useful fuel capacity, fuel flow, and range. 

jlunseth, thanks for your comment in particular... I like your approach to fuel management. 

[FJC]

Curious to know how brave one must be to run a tank dry. I have the Monroys and some say blessing, other people say a curse. At some point, I will start using 4 seats for travel and will HAVE TO HAVE a really good handle on the fuel situation. BTW, my daughter is 45lbs. Up until now, it is just me and one other occupant. Pretty easy from a fuel perspective.

I think for me, I'd like to tackle the problem from a flow/usage perspective. In other words, fuel up to a prescribed place such as the bottom of the neck, then fly using that tank for an hour or so and then filling up to that same level. At that point I'll have the gallons used as measured by fuel pump and also what Shadin says i've used and HOPEFULLY the correlation is close. If the accuracy is off, I have a flow meter calibration issue or if gross, a stuck vane.

When I fly 4 seats for my intended 2 hour trips, I simply add the fuel that is needed to complete the trip. The only down side is that I will really never know what fuel was already in tanks so there is a risk that I could be 10-20 gallons over max take-off weight and maybe even landing weight at destination. Also, I just cannot resolve the fact that when one cannot see fuel immediately below the neck does not mean there isnt fuel in the lower wing. Anyone want to comment on how much there could be?

I really dont think I have the nerve to empty a tank in flight (someone please describe in detail what you've done...maybe I just do not understand the risk enough to consider it low in my mind). To get this info, my idea would be to sump a tank dry by pumping the fuel to the other side or a container, then repeating and doing the same for the other side which would tell me fuel onboard (assuming the sumps are pretty low and would have to account for unusable?). Even if I find equipment to do this and it does not take forever, I still would have to test Shadin totalizer for accuracy. Thoughts?

 

Freddy

[A64Pilot]

If your pushing fuel, run one dry. I was told that by a very experienced ferry pilot. his reasoning was then you know there is nothing left in that wing, many have convinced themselves there is enough when there isn’t. But when you know all there is is what’s in the one wing, your more likely to find a place to land.

Runing one dry at altitude isn’t big deal really, first it’s unlikely it will suddenly quit. most likely it will droop in power and maybe surge a bit, usually you have the selector on the other tank by that time and it picks up and is fine.

Personally I won’t go below 5 gls in a tank if I’m below cruise altitude, afraid I’ll get busy and forget and run out at low altitude.

 

The calibrated stick is a pretty good way to know about how much fuel there is on a partial tank. you can also buy ones that look like a tube and you stick your finger over the end to hold the fuel into read the level.

Best plan is to start with an empty tank and add some amount, say 5 gls at a time and mark the stick, once done transfer the marks to another stick so when you lose it, you have a master to mark another.

Edited June 1, 2021 by A64Pilot

[jlunseth]

There was a quite a bit of discussion about running a tank dry on mooneyspace a few years back, several people said they do it frequently. It is a little different issue in a turbo (like mine) than an NA because the turbocharger will spool down even if the prop keeps windmilling. The engine becomes an NA but with turbo compression ratio. Special procedures in the manual about airstarts above 12,500. Less of a deal with an NA I would think, because first of all you are not likely to be at an altitude where the engine can't run without compressed induction air. The classic method is to start at full MP full rich and lean out until the engine starts.

I did it once after all that previous discussion. What I can tell you is that the time from the first stumble to the hand reaching the switch and switching tanks can be measured in microseconds. Restart was a non issue. As long as the prop is windmilling and you have a reasonable amount of altitude it works out ok. I fly so much under IFR flight plans that I am really not willing to do it again. Don't want to discover that the restart takes longer than expected and have to report to ATC that the engine stopped and you are restarting so will lose altitude.  Maybe do it VFR.

CoffeeCan mentioned the fuel management method I have written about in the past. To add to that method, if I am going to fly a mission where I need to carry, let's say, 15 gallons less because of weight and balance, I will contrive to fly a prior flight where I can use the fuel flow meter to draw down at least 15 gallons from one tank. Then when the mission comes up, I fill the other tank completely full. Now I have one full tank and another tank that I know from fuel meter readings is down 15 gallons, and I have made weight and balance.  Then I would use the partially full tank for takeoff and climb, leaving some fuel in that tank. At cruise, I switch to the completely full tank, and now have about 2 1/2 hours in that tank to get where I want to go (or three hours if I am LOP), plus some extra fuel in the tank that started partially full just in case I need it for descent and landing. My Mooney flies just fine with the tanks complete "unbalanced," there is no good reason to switch back and forth every hour or whatever. It just makes the fuel calculation confusing, and you don't want confusing.

Obviously, if during the prior flight you drew one tank down, say 25 gallons, and you want it at 15, then the easy thing to do is to put 10 gallons in that tank (so you are now short just 15 gallons), and fill the opposite tank completely full. This let's you use the fuel flow meter, which I have found in my plane to be very accurate.

If you don't have the antisiphon valve you can also make or buy a measuring stick and actually measure what is in the tanks. 

[A64Pilot]

I knew that restarts at altitude are a big deal on a Diesel as they are a compression ignition engine, but never knew it about  a spark ignition turbo.

[dogbocks]

14 minutes ago, A64Pilot said:

The calibrated stick is a pretty good way to know about how much fuel there is on a partial tank. you can also buy ones that look like a tube and you stick your finger over the end to hold the fuel into read the level.

Best plan is to start with an empty tank and add some amount, say 5 gls at a time and mark the stick, once done transfer the marks to another stick so when you lose it, you have a master to mark another.

On an M20R I think the stick would be dry around 20 gallons or so, not sure about a J....

[carusoam]

Fuel Fill holes in Mooneys seem to vary all over the place...

In My M20C if you didn’t see fuel at the bottom... the tank was nearly empty...

In the M20R if you don’t see fuel at the bottom...  you have to look from more of an angle to see it...

Either way... there isn’t much if you are seeing a dry surface at the bottom...

Note for Freddy...

It takes practice.

1) know the procedure...   (recognition of the signs, fuel pump on/off)

2) know the selector switch is working smoothly...

3) try it without running the tank out...

4) ultimately... do it when the tank is allowing air bubbles in...

5) It helps if you have a fuelP instrument...   many Os and Eagles don’t...

6) Know the system is designed to work this way...  so there shouldn’t be any surprises if you follow the procedure...

7) Done at the first sign of air bubbles entering the line....  the engine is still running very strongly.... and the fuel pumps don’t know the difference...

8) Advise any and all passengers what you are doing... they surely won’t like the experience... 

9) If you really fall asleep at the switch.... and the fuel line runs dry, and air goes all the way to the fuel injectors...  expect 10 seconds for fuel to make its way all the way back up there...

10) You can see how well things are working by watching the FF meter...  it may be full of bubbly fuel, but it is moving, and the bubbles will get vented overboard...

See if that helps any..?

 

Note for TC’d engine drivers...  as JL mentioned above...  there is a critical altitude to be below... if your TC has slowed too much...  this is the altitude where the overall compression needs the TC to be spinning... or have the mixture pulled back to match...  be prepared to follow the restart procedures for your engine...

 

Got any other fears?

PP thoughts only, not a mechanic...

Best regards,

-a-

[kortopates]

Highly recommend calibrating the stick to get a good estimate on your remaining fuel. But it won't work well if the wings are terribly out of balance - one wing near full and one wing near empty. If you really want to be precise, either fill both tanks evenly together or make sure you level the wings with jacks that will hold leve and measure level with a level gauge across the seat rails. Also after any addition of fuel, be sure to wait for several minutes for the fuel to distribute evenly between mains and long range tanks - this actually takes some time.

Of course when your sticking the tanks later down the road, its unlikley you'll ever have perfectly even tanks but any out a balance is going to skew your results so best to start with an accurate stick just as if you were calibratuing new fuel senders.

[Will.iam]

I have found my wing gauges to be more accurate than a stick measure but i have the flapper valve, so hard to get that stick vertical. I flew one tank empty and put in 5 gals at a time and thought my wing gauge was off by a gallon at each mark. But when i towed the plane to the hanger and got it on the hanger floor i realized our gas pump pad is not level and the gauge was dead on with the 20 gals i put in. So i use the gauges as a starting point and usually just fill up the moroy outer tanks to 15 gals each. That gives me 30 gals plus what ever i had before i started refueling as a bonus. The outer wing tanks drain into the main tanks in under 10 mins so usually I’m reading the 15 gal increase on the wing gauge before takeoff. 

[RJBrown]

Wing gauges are all I ever trusted. And my fuel totalizer, accurate to .2 gallons over the use of 50.

As shown above if you need another opinion.

If needed I would drain one completely. Once it stumbles switch the valve. Then you know there is no fuel there and all remaining is in the other. This allows you to use most of what Mooney calls “unusable” Did it in a 231 and same plane later as a Rocket.

once as a 231 I tried to get the prop to stop and could not. The wind kept it turning even at stall speeds. I did it before there was an internet telling me I couldn’t.

once in the Rocket I went Brownsville to Centennial 900.5 kts direct non stop just to find  Centennial closed. Used all I had in one tank just to know for sure I could land with over 10 in the other. 

[A64Pilot]

Even if you stop the prop, depending on the health of the engine a quick decent will get it spinning again.

‘This is my C-140, it has strong compression and it took 120 MPH to get it spinning again. The flying club in Americus Ga has or had a C-172, motor ran fine but it was so low in compression that you couldn’t get it to stop, even in a stall it was windmilling.

If you choose to stop a prop inflight, just make sure the engine is nice a cool, low power descent for a while, you don’t want to shock cool a hot motor, be sure to have a forced landing area made too, just in case I guess. Glide ratio does improve and it’s nice and quiet.

[A64Pilot]

Wing gauges would be nice, but I have bladders and I don’t think they are an option with bladders

[jlunseth]

That's another issue with engine stoppage in the turbo. If it takes time to restart, then the engine, and particularly the OT, can cool below temps where full power can be safely applied even if you can restart. Read your POH. It would take a little time to get to cool enough temps, but you would be surprised at how little, especially in the cold temps up high. The bottom line is it is probably not smart to run a tank dry above 12k. If it restarts instantly, great, if it doesn't, not so great. On the other hand, you would generally have plenty of altitude and time to fuss with it, unless, of course, you are crossing the Rockies....

[A64Pilot]

I’ve gotten complacent a time or two and had one go dry unintentionally, I’ve never had one quit,  as was said as soon as that engine stumbles, you realize why and have the fuel switched before the Wife has time to say anything. I’ve never even had to turn on the boost, switching tanks it picks up immediately. C-210 didn’t have a totalizer, so you used the gauges and the clock so it wasn’t all that accurate.

‘I’ve not run my Mooney dry, but think it would be the same?

‘I’m really, really surprised a turbo won’t restart up high, I thought one would, but be really rich until boost built back up, but wouldn’t that happen very quickly? I mean like a couple of seconds?

[jaylw314]

I safe way of measuring unusable fuel would be to simply empty most of the fuel, have your mechanic disconnect the fuel line north of the electric fuel pump into a bucket, and then run the electric pump until it stops picking up fuel.  The only gotcha is in planes where the attitude on landing gear is different than the attitude in level flight (taildraggers!), but I think we're okay there.

[Will.iam]

6 hours ago, jaylw314 said:

I safe way of measuring unusable fuel would be to simply empty most of the fuel, have your mechanic disconnect the fuel line north of the electric fuel pump into a bucket, and then run the electric pump until it stops picking up fuel.  The only gotcha is in planes where the attitude on landing gear is different than the attitude in level flight (taildraggers!), but I think we're okay there.

Why disconnect the fuel line? Wouldn't just pulling on gascolator ring until it stops flowing fuel do the same thing?

[carusoam]

6 hours ago, jaylw314 said:

The only gotcha is in planes where the attitude on landing gear is different than the attitude in level flight (taildraggers!), but I think we're okay there.

This applies to Mooneys as well...

While they are sitting on their legs... they are pointed a few degrees up...

Which does benefit picking up fuel...

It will be different in level flight...

This is why the wing sight gauges are calibrated using the ground attitude...

And for that last gallon... know where you are in the Balance part of the envelope...

 

So... follow the procedure... level the plane... to measure useable fuel...  then know of the differences in actual flight...

PP thoughts only...

Best regards,

-a-

[carusoam]

31 minutes ago, Will.iam said:

Why disconnect the fuel line? Wouldn't just pulling on gascolator ring until it stops flowing fuel do the same thing?

No...

Due to A tiny technicality...

When using the fuel pump to drain the system...  it is a test of the fuel delivery system to pull fuel, and some air from the tank...

 

it will start pulling air while leaving the gascolator half full of fuel...

The gascolator’s drain is at the bottom... the fuel line is half way up...

 

What you want to learn from this test... is how much fuel is left in the system when that first bubble enters the line... on its way to the engine...

After that first bubble... things are going to get dodgier from there...

PP thoughts only, not a CFI...

Just things to consider...

Best regards,

-a-

[Will.iam]

7 hours ago, carusoam said:

No...

Due to A tiny technicality...

When using the fuel pump to drain the system...  it is a test of the fuel delivery system to pull fuel, and some air from the tank...

 

it will start pulling air while leaving the gascolator half full of fuel...

The gascolator’s drain is at the bottom... the fuel line is half way up...

 

What you want to learn from this test... is how much fuel is left in the system when that first bubble enters the line... on its way to the engine...

After that first bubble... things are going to get dodgier from there...

PP thoughts only, not a CFI...

Just things to consider...

Best regards,

-a-

But if you are just draining the system why have the fuel pump on at all? Just let it drain by gravity until nothing comes out.

[carusoam]

4 hours ago, Will.iam said:

But if you are just draining the system why have the fuel pump on at all? Just let it drain by gravity until nothing comes out.

hmmmm

i may be on a different page.... 

one part is emptying tanks...

The other part is calibrating fuel sticks and gauges...

 

the reason for the extra effort in finding where empty is...

empty is different depending on what method you use to get there...

Draining one way, you are taking out more than useable fuel...

This will upset your data collecting by some amount...

 

As far as the physics of draining the tank goes...

Syphons, and pumps, and good grounded containers...works.

removing a sump drain can work too...

Safety is everything when working with fuel...

 

Draining through the fuel sep valve can work, it has some added logistics challenges...

 

So... it’s more of an accuracy thing with the last few gallons...

Then the bulk emptying the tank phase...

Best regards,

-a-

[Will.iam]

Ah so draining will get more out than just when the pump starts grabbing air causing the engine to quit. Got it.
 

On your previous remark the plane sits a few degrees higher than when flying what about the 2-3 degrees nose up to keep level? And the slower you go the more nose up to keep level, so at some point you will reach that same pitch attitude you had on the ground if you were not there already and the engine would pickup that remaining fuel if the pitch was different when you first ran out of gas. 

[carusoam]

Ground attitude is selected for T/O purposes...

Which occurs near 65ias... with T/O flaps deployed.... during acceleration...

The fuel pick-up locations are at the back... to ensure that fuel is there for climb out...

LBs have a couple of extra degrees of ground attitude compared to shorter, lighter, Mooneys...

Lots of small decisions have been made along the way... for every functional piece of a Mooney...

 

Soooo.... when you find yourself in that terrible position of being on your last cup of fuel...

Nose up, wing high...  (wing with the last cup of fuel in it...)

When that stops working... go level, and best glide speed...

PP amazement only, not a mechanic, or CFI... 

Best regards,

-a-

[Will.iam]

16 hours ago, carusoam said:

Ground attitude is selected for T/O purposes...

Which occurs near 65ias... with T/O flaps deployed.... during acceleration...

The fuel pick-up locations are at the back... to ensure that fuel is there for climb out...

LBs have a couple of extra degrees of ground attitude compared to shorter, lighter, Mooneys...

Lots of small decisions have been made along the way... for every functional piece of a Mooney...

 

Soooo.... when you find yourself in that terrible position of being on your last cup of fuel...

Nose up, wing high...  (wing with the last cup of fuel in it...)

When that stops working... go level, and best glide speed...

PP amazement only, not a mechanic, or CFI... 

Best regards,

-a-

Yea i hope to never have to test or experience that. 

[M20F]

My F holds more then 64 gallons, my fuel totalizer works great and is set for 64.  When it reads 0 I have fuel left.  I really don’t see the value in knowing if it holds 66, 70, etc.  My gauges read 0 when there are @ 4 gallons of useable left. 
 
When you get to the point of worrying over 2-3 gallons, you have already probably gone too far. 
 
 

[squeaky.stow]

On 6/1/2021 at 6:45 PM, FJC said:

Curious to know how brave one must be to run a tank dry

If running a tank dry makes you uncomfortable, (me too) there is another option. Before I could have full confidence in my fuel system I had to know three things: the accuracy of my low fuel light, the accuracy of my external wing gauges, the accuracy of my FF indicator. It was a multi-step process.

1) Calibrate the low fuel light.

During annual, show up with one tank fairly low, drain the rest of it manually and run the Maintenance Manual calibration procedure for your LOW FUEL annunciator light. It’s pretty simple and it gives you a benchmark that doesn’t require worrying about whether the engine will restart when you run it dry. Now you know pretty accurately when you are down to the last 2.5 to 3 useable gallons per tank. (at least for my K)  

2) Calibrate your external wing gauges (or dipstick)

Add fuel to the almost empty tank in increments until you reach your preferred maximum fill point, recording what the wing gauges say while you do this. Doesn’t matter what that maximum point is as long as you always use the same point every time. I use the bottom of the anti-siphon flapper. Now you know the accuracy of your wing gauge or your dipstick. 

3) Calibrate your FF indicator.

Fly a long cross-country with the power setting you always flight plan for. During that X/C run one tank until the LOW FUEL light illuminates, refill the tank to the same “full” reference point used in step 2 and do the math to determine the accuracy of your FF indicator. Mine was bang on. (I recorded the FF and time to TOC to allow for the climb portion.)

My POH has a warning in the limitations section that “Takeoff maneuvers when the selected tank contains less than 12 gallons...have not been demonstrated”. While not technically a hard limitation, it is a “Warning” and I think it constitutes useful guidance for a minimum landing fuel as I never know for sure when a go-around might be required. 

Therefore if I were to plan a trip that required running one tank all the way to the LOW FUEL light, I would plan to have at least 12 gallons in the other one for landing. That may strike some as overly conservative. It’s simply my personal technique, nothing more. It means I will always land with a comfortable hour+ of extra fuel and don’t have to worry about whether the engine might sputter if I have to go around on landing.

Once I had determined with some accuracy what my fuel burn was during cruise, the internal fuel gauges became nothing more than a reminder to switch tanks every now and then and/or alert me to a major fuel leak. I don’t find them useful for much more than that. That being said, I do crosscheck them against my calculated fuel on board every 30 minutes or so. If my FF guage happens to die or lose accuracy, my GPH will not change over time for a given power setting so I will always be able to calculate my fuel over destination even if all of my avionics and gauges quit.

Because I am confident in the accuracy of my wing gauges, I can also fuel to lower quantities when I need the extra payload. I generally plan higher reserves for these scenarios just to be on the safe side.