Excessive fuel usage

[flyingchump]

I'm still learning this plane so yesterday I tried to verify fuel usage while my and my wife went for a flight.

1966 M20E, w/ bladders and powerflow exhaust.  No other major mods. Sorry, No engine monitor data to upload.  I know, I know.  I need to install one soon.  Mags first, then monitor.

I filled up my tanks at an airport with cheap gas 52 miles away from my base.  I sticked the tanks 5 minutes after I finished filling the tanks.  Left 23.5 gallons, right tank 24 gallons.  Neither tank was 100% filled but the bottoms of each fuel cap was just touching the fuel.  We flew home for 32 minutes at an alt of 4500-5000ft.  (Flight aware link below) WOT throttle, MP 24.  I started the flight using 2400 RPM for the first half of the flight but then lowered the RPM to 2050 while leaving the MP at 24.  She ran with a lot less vibration.  I leaned to roughness then turned back to just smooth.  Worth noting that my mags are at 500 hrs each and probably aren't sparking quite as hot as they should be so I can't lean as far a some others..  Plugs were removed last week, cleaned and gaped by my A&P.  The entire flight was using the right fuel tank.

After we landed I sticked the tanks again as was a bit surprised...  The left tank (which I didn't use for the flight) was down 1.5 gallons.  Where did it go?  No noticeable fuel staining above or below the wing.  The right tank sticked at 14 gallons meaning I burned 10 gallons for a 32 minute flight???  Could that be right?

One of the reasons I bought into the Mooney club was the all around efficiency.  I suspect I'm doing something wrong.  Too much throttle or mixture.  COuld my fuel stick be misleading me?  Could I be leaking fuel out of the caps or breathers?  I've been trying to find the right o-ring kits for the caps anyway as I can't find any information on the last time they were changed.  Of course I have to have the "oddball" fuel caps that no one caries a kit for.  I found the outer o-ring but haven't yet found the inner.

 The fuel stick I used was based off of another Mooneyspace post.  https://mooneyspace.com/topic/1037-calibrated-fuel-sticks/?tab=comments#comment-582710

https://flightaware.com/live/flight/N3460X

 

[ShuRugal]

I don't have an E POH handy, what's it say about fuel burn rate at the power settings you were using?

[Oldguy]

For the left tank, it could be a few things.

• Could there be some air trapped in the bladders that settles out after the tank sits or vibrates? You might try filling the tank to the level you did, close it up and rock the wing up and down and back and forward. Wait a few minutes and check to see if it changes the level. 
• Is there any fuel smell in the cabin? You can check behind the sidewalls for any blue staining indicating leaks at the sender or fuel line.
• Do the drain plugs seal fully?
• Was the fuel warmer when it was pumped than when you arrived at your destination (probably not enough to make a difference)?
• Checked for a leak in the fuel lines (boost on until pressure rises with mixture off/boost off - pressure drop?)?

Right tank could be some of the same and/or a higher them expected fuel burn.

You put 47+ gals. in your tanks. How empty were they and do you have the 64 gallon extensions in your bladders?

Sorry to have so many questions with no real answers, but it may help you chase down an answer.

[drapo]

I'm not sure about using the M20E normal tanks dipstick with the bladders. I would have leaned and usage for your flight would have been around 7 or 8 gallons total as take-off and climb use more than cruise for an average of 10.5GPH. Finally, about the 1.5 gallons usage on the other tank, could be that there are wrinkles in the bladders and fuel dispersed itself during the flight or simply that you used your checklist and switched to the fullest tank during your pre-landing checklist.

Hope it's one of those things and not the dreaded fuel leak in a bladder!

[takair]

Sticking the tanks level is critical.  Very few ramps are level and variations will cause big differences out at the caps.  You will also likely find that the gauges are not linear.  Your apparent burn rate will be quicker below half tanks, so be aware of that and use caution the first few trips.  This is due to tank geometry, especially since the senders are not well matched to bladders.

[flyingchump]

  On 6/27/2020 at 4:17 PM, Oldguy said:

For the left tank, it could be a few things.

• Could there be some air trapped in the bladders that settles out after the tank sits or vibrates? You might try filling the tank to the level you did, close it up and rock the wing up and down and back and forward. Wait a few minutes and check to see if it changes the level. 
  • I'll try that next time I fill up.
• Is there any fuel smell in the cabin? You can check behind the sidewalls for any blue staining indicating leaks at the sender or fuel line.
  • No smell.  No staining that I have noticed.
• Do the drain plugs seal fully?
  • Yup.  I check often.  I have learned my lesson about this.
• Was the fuel warmer when it was pumped than when you arrived at your destination (probably not enough to make a difference)?
  • I didn't check the temp of the fuel.  The fuel could have cooled in flight...  I doubt this is my problem
• Checked for a leak in the fuel lines (boost on until pressure rises with mixture off/boost off - pressure drop?)?
  • I'll check next time i'm at the plane.

Right tank could be some of the same and/or a higher them expected fuel burn.

You put 47+ gals. in your tanks. How empty were they and do you have the 64 gallon extensions in your bladders?

I do not have the 64 gallon tanks.  I was at around half.  I didn't fill up with 47 gallons.  I had a total of 47 gallons when I was done.

Sorry to have so many questions with no real answers, but it may help you chase down an answer.

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[flyingchump]

  On 6/27/2020 at 4:18 PM, drapo said:

I'm not sure about using the M20E normal tanks dipstick with the bladders. I would have leaned and usage for your flight would have been around 7 or 8 gallons total as take-off and climb use more than cruise for an average of 10.5GPH. Finally, about the 1.5 gallons usage on the other tank, could be that there are wrinkles in the bladders and fuel dispersed itself during the flight or simply that you used your checklist and switched to the fullest tank during your pre-landing checklist.

Hope it's one of those things and not the dreaded fuel leak in a bladder!

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God I hope not!  There is no staining above or below the wing.  No fuel smell in the plane.  

Is there a stick that is made for use with the bladders?

[flyingchump]

  On 6/27/2020 at 4:22 PM, takair said:

Sticking the tanks level is critical.  Very few ramps are level and variations will cause big differences out at the caps.  You will also likely find that the gauges are not linear.  Your apparent burn rate will be quicker below half tanks, so be aware of that and use caution the first few trips.  This is due to tank geometry, especially since the senders are not well matched to bladders.

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I'm going to keep records for the next few flights so I can really get to know this plane.  I understand the geometry of the tank which is why I was soo surprised to see such a drop after 32 minutes at the top end of the tank.

[Bob - S50]

I agree that the 1.5 gallons out of the left wing was most likely due to trapped air.  If the same thing happened on the right then you only burned 8.5 gallons, not 10.

I don't dawdle on the ground.  For me, it's usually about 7 minutes from start to takeoff for the first flight of the day and more like 3 or 4 minutes on subsequent flights.  I plan on burning .5 gallons for start, taxi, and takeoff.  If you spend more time than me then you may burn closer to a gallon.  At any rate, you are down to about 8 gallons burned.

For climb, I figure it costs me a quart of gas to climb 1000'.  That means you probably spent about 1 to 1.25 gallons for the climb.  That leaves about 7 gallons burned in cruise.  That's still a lot for a 32 minute flight, but not unreasonable if you were well ROP.

If you were running 2050 RPM and 24" MP, that is well over square and I'm guessing you didn't even get close to peak EGT before the engine appeared to run rough (because I'm guessing it was more like lugging the engine).  So I'm guessing you were in fact well ROP.

Do you have an EGT gauge?  Did you reference it while leaning?

Next time, instead of 2050 RPM and 24", try it the other way around: pull the MP back to 20" and then reduce the RPM to 2400 (about the same power).  After you reduce RPM you'll probably notice the MP has risen so pull it back to 20" again, then lean to roughness and enrichen until smooth.  See how that affects your burn.

[Skates97]

  On 6/27/2020 at 4:22 PM, takair said:

Sticking the tanks level is critical.  Very few ramps are level and variations will cause big differences out at the caps.  You will also likely find that the gauges are not linear.  Your apparent burn rate will be quicker below half tanks, so be aware of that and use caution the first few trips.  This is due to tank geometry, especially since the senders are not well matched to bladders.

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I'm going to lean towards a ramp that wasn't level. I will often stop at Riverside and fill up, it's close to my home base at Fullerton and fuel is often at least 0.50 cheaper. The ramp there is "almost" level. If I fill all the way up and fly back using just one tank (20 minute flight) when I put the plane in the hangar the tank I didn't use is always showing a gallon less in my dipstick. It doesn't take much of a slope in the ramp to account for 1+ gallons that can't get in the tank. 

[chriscalandro]

Where in the world did you get those power setting from?

 

A 32 minute flight is going to have a high percentage of rich high power settings and  I would think your fuel usage wouldn’t be abnormal. 

Edited June 27, 2020 by chriscalandro

[jaylw314]

There's no way your fuel stick is giving you useful information--that post was for a fuel tank without fuel bladders.  Heck, I'm not even sure dipsticks would be accurate in fuel bladders, since I imagine the flexible bladders have enough play that the height of the fuel in the tank will vary wildly.  Think about the difference between your coffee in a thermos, and your coffee in one of those starbucks to-go plastic bladder boxes.  If you open the starbucks box and drink some coffee, you'll lower the coffee level in the box.  But if you squeeze the sides, the coffee comes shooting out the top because the fluid level changes as the shape of the box changes.  With the rigid thermos, the fluid level will always be the same for any given volume.

That being said, I imagine the bladder shape should be pretty consistent whenever it is full, but I don't know how common the air bubble thing is in fuel bladders.  Either way, refueling is the best way to confirm how much fuel you've used

  On 6/27/2020 at 5:10 PM, chriscalandro said:

A 32 minute flight is going to have a high percentage of rich high power settings and  I would think your fuel usage wouldn’t be abnormal. 

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He had to land, though   You figure 15 gph for 5 minutes and 5 gph for 10 minutes is about the same or a little less than cruise.  I know doing pattern work I'll only use about 8 or so gallons in an hour

[Ross Taylor]

I’ve got a ‘66 E with bladders, also...and I’ve found that I can’t rely on a stick at all.

[flyingchump]

  On 6/27/2020 at 5:42 PM, Ross Taylor said:

I’ve got a ‘66 E with bladders, also...and I’ve found that I can’t rely on a stick at all.

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Thats good to know.  What methods do you recommend for tracking fuel usage?

I'm sure as I get more familiar with my plane this will become less of a concern.

[takair]

One more thought on bladders. When I had my bladders done by the OEM, they gave me hand made sticks for the bladders. I can try to get the dimensions next time at the airport, PM me a reminder.   Another thing with my bladders. They take a loooong time to settle. The pass throughs don’t pass fuel quite as fast as the original tanks, so if you don’t wait at least 20 seconds after you think you are topped off, then next time you stick them they will seem low. The stick I have is reasonably accurate when level. The best way to know for sure is to top off again from a known top off. 

[takair]

....also, with my bladders, when level....if you JUST barely see fuel in the tank, you have under 10 gallons. If you can’t see anything assume you have less than 7 gallons. Not a perfect art, but line guys may call you out on landing empty when you may have 2 hours to empty.  Don’t take off like that, but you may land like that and catch some rubbing from the line guys. 

[jaylw314]

  On 6/27/2020 at 6:00 PM, flyingchump said:

Thats good to know.  What methods do you recommend for tracking fuel usage?

I'm sure as I get more familiar with my plane this will become less of a concern.

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Like I said, refueling is best.  If you have a self-service fuel pump at your home base, go out and top off the tanks.  That way you'll have let the fuel settle as long as is reasonable.  The next time you refuel, use the fuel pump at your home base and record the amount, then let the plane sit overnight to make sure everything's settled.  The next day, if the fuel level has dropped any,  refuel it again and add that amount to the previous day's.  Now, you know as close to exactly how much fuel you used between those to refuelings, and can compare it to your flight time (and fuel totalizer, if you have one).  It'll also tell you if your fuel level actually drops after waiting or not.

[skydvrboy]

There is a really accurate way to calibrate your fuel stick and getting the numbers off the internet isn't it.  The best method I have found is to stick the tank before you add any fuel and mark that line on your stick.  Then top it off and subtract that from your total usable gallons.  Now label the line you already marked.  Over the course of a few months you will get a REALLY accurate fuel gauge (to within one or two tenth's of a gallon accurate).

If you don't know your (actual) usable fuel, intentionally run a tank dry.  When you land, fill it up and now you'll know exactly how much usable fuel that tank holds.  Now repeat for the other side as they may be different.

[Ross Taylor]

I do as @jaylw314 describes above.  I have run each tank dry (on different flights, of course!) and filled them to verify capacity.  With the bladders, as mentioned above, make sure you wait for fuel to work through the baffles.  And, if you're sticking, make sure you put the stick on the same side of the filler opening every time...I think that wide mouth has enough lateral room to cause some small measurement differences.

I tend to work off topped off tanks most of the time, since my wife and I can still bring a couple hundred pounds of stuff (if needed) with full tanks.  So, topping off again gives me a pretty accurate usage after each leg.  Even at our DA, this works just fine for us.  I do have to plan ahead, though, for dirt strip trips or extra load.   I may adjust this work-off-the-top plan, though, after I fly more with the Shadin MiniFlo I recently got from a helpful MSer. 

[M20F-1968]

I did not read this entire thread, but it may be helpful to know that if the fuel selector is no working properly (internal O rings leaking) fuel can transfer from one tank to the tank on the other side.  A hint is if the engine does not stop when the fuel selector  is turned to off.

John Breda