NicoN Posted June 5, 2019 Report Share Posted June 5, 2019 According to POH and the filler caps my M20K (25-0378) has 72gals usable fuel. I also know , that Mooney defines "full" with the tanks filled to the trap door valve. Typically, we fill our tanks to 25gals each indicating from the wing side gauges. Doing this, I get in flight an indication of something between "18" and "27". Yesterday , I had 2 findings, that raise several questions: 1. I ran the left tank down to indicating "9". Back to the fuel station, the wing side gauge showed exact "15". Hmm. That my be explained by the form of the tank - correct ? I also learned from earlier experiments, that it takes a long time of flying where the fuel gauge needle nearly does not move before the low fuel triggers. AGain, I know be have a more or less linear indicating with irregular shaped tanks. 2. Filling the tanks to the max is not easy, as the filling system tends to switch off when the fuel level reaches the gas pump nozzle. You have to pull it back and continue slowly and manually control the nozzle. But then the trap door valve closes and it gets even more difficult. So, I help myself by keeping the trap valve open with a short piece of wood. Nevertheless, even filling the tanks up to the absolute maximum, the wing side gauges never reach the "6". Is this also the experience from the communitiy? If it is true, that fuel level to the valve means 36gals, thus indicating about 32 on the gauge, then I can add at least 4gals each side. Without correct indication and of course ignoring the expansion space. But I have never run a tank dry from some known point. Also the wing side gauge nature is linear, while the tanks are not. 3. Yesterday the left tanl was filled to the max indicating the something about 34. The RH tank started at 25 indication, I added about 5 gallons, the indication was shortly above 30 and the fuel level already far above the trap door valve! Is this an effect of air trapping? I did not wait for a longer time to check whether the fuel level disappears. Thank you and excuse my bad nglish Quote Link to comment Share on other sites More sharing options...
carusoam Posted June 5, 2019 Report Share Posted June 5, 2019 Nico, This is a favorite topic of the Mooney community. It was so difficult for Mooney to standardize how this was done... every model has some variation. What the MS community has done... recommend not trusting the fuel system until the useable amount has been proven for each machine. For each new owner... The Ks with the flapper valves and second fuel caps on the extended tanks are the most complex... You will see the phrase used often... “Trust, but verify....” Some things have changed... 1) location of the fuel pick-up, shape / size of parts, tank sealant... 2) air trapped, small holes in ribs get covered with sealant... 3) procedure for filling... 4) Then there is annuciator differences for the amount of fuel left when the lo fuel light comes on... 5) Then there is the procedure to enable the lo fuel light... press to test. 6) The fuel neck... the metal insert that the cap fits in... it changed a lot over the years and models... some trap a lot of air... some have vent holes in them some aren’t very tall... The M20S had the worst... it trapped 10s of gallons of air to artificially limit fuel capacity.... 7) MS has threads specific to how to drain a tank, and refill them properly to measure the useable fuel.... including leveling the plane to fill the tanks evenly... 8) calibrate your fuel level sensors at the same time... 9) The flapper valve was pretty limited... some Ks got them. More modern Mooneys skipped them... I have no idea what the flap is supposed to seal, that the primary rubber cap seal does on its own.... PP thoughts only, stuff I read around here... and Ronald Reagan who said trust but verify.... Best regards, -a- Quote Link to comment Share on other sites More sharing options...
NicoN Posted June 5, 2019 Author Report Share Posted June 5, 2019 Thank you for your answer. @ #2: Yes, of course we had tanke re-sealing about 3 years ago. And yes, after this procedure, the left fuel gauge indicated max. 18 gals. This was corrected last year and works now flawlessly. @ #6: I suppose, those holes should not be necessary as there must be a separate vent line. Right? Otherwise the effect of the flabber vent should be void. BTW: WHat's the purpose of that flabber valve? I am not sure, but I think we don't have any holes in the filler neck Quote Link to comment Share on other sites More sharing options...
aviatoreb Posted June 5, 2019 Report Share Posted June 5, 2019 It is one of the great mysteries of the M20K how much fuel they actually hold. I have personally note that I could fill 39 gallons into the 36 gallon wings. This is done by filling then giggling the wing (with a few trusty hip bumps) which allows some air to burp out of the system. Then fill again. You can squeeze in 3 or 4 more gallons per side after it first seems full. Also note if you don't do this and you fill the wings to the full tab then come back in a few hours and peek in you will see there is now space for more fuel. Then I got the LR tank extenders, which is even more difficult to decide how much you can actually get in there. I think it is 102 total if you do no burping. I have heard it is as high as 115 if you diligently burp your baby. I have my EDM marked so that when I hit the fill-up button it is 99 gallons. Better to err on the side of conservative. Quote Link to comment Share on other sites More sharing options...
gsxrpilot Posted June 5, 2019 Report Share Posted June 5, 2019 59 minutes ago, aviatoreb said: It is one of the great mysteries of the M20K how much fuel they actually hold. So true. My "calibration" procedure was to go fly and run a tank dry. Then on landing, add fuel in known amounts, noting the indication on both the wing gauge and the EDM900 gauge at 5 gal, 10 gal, 15 gal, etc... until full. During this procedure I took the time to get as much fuel as I could in the wing. Just to work out where the MAX was. I did this for each wing. 4 Quote Link to comment Share on other sites More sharing options...
aviatoreb Posted June 5, 2019 Report Share Posted June 5, 2019 1 hour ago, gsxrpilot said: So true. My "calibration" procedure was to go fly and run a tank dry. Then on landing, add fuel in known amounts, noting the indication on both the wing gauge and the EDM900 gauge at 5 gal, 10 gal, 15 gal, etc... until full. During this procedure I took the time to get as much fuel as I could in the wing. Just to work out where the MAX was. I did this for each wing. What totals did you get? did you get the same on each wing? im am certain it’s greater than 72 and 102 so I feel safe just using those numbers as my soft filled numbers. Ie fill to the top and not worry about jiggling the wings to add more. Which I then do if I’m trying to stretch range but I don’t count it in fuel planning. Quote Link to comment Share on other sites More sharing options...
gsxrpilot Posted June 5, 2019 Report Share Posted June 5, 2019 2 minutes ago, aviatoreb said: What totals did you get? did you get the same on each wing? im am certain it’s greater than 72 and 102 so I feel safe just using those numbers as my soft filled numbers. Ie fill to the top and not worry about jiggling the wings to add more. Which I then do if I’m trying to stretch range but I don’t count it in fuel planning. I don't remember the exact totals, but I'm confident I can get to 35 gal in each wing. So my default is 70 gal total when full, no wing shaking, etc. I can check my cheat sheet in the plane next time I'm there. Quote Link to comment Share on other sites More sharing options...
Greg Ellis Posted June 5, 2019 Report Share Posted June 5, 2019 I also burp my tanks when I am filling and set the EDM 900 to 48 gals (1963 M20C). When a lineman fills from a fuel truck I just add the amount they fill on the EDM 900 because they never burp the tanks nor do they even make it to the top sometimes. This seems to be quite accurate for my airplane. Quote Link to comment Share on other sites More sharing options...
DanM20C Posted June 5, 2019 Report Share Posted June 5, 2019 1 hour ago, aviatoreb said: What totals did you get? did you get the same on each wing? My 79 claims 80gal capacity with 72 usable. The later 231's 83-84?? are listed at 78.6gal capacity with 75.6 usable. I flew my right tank dry on my 79 and was able to squeeze 39 gal in it. If I'm going on a long trip I fill as full as I can and plan on 72 usable to be conservative. I haven't tried the left yet, hopefully it is the same. If it is I have an extra 30-40min at cruise as an extra cushion beyond my normal reserves. Cheers, Dan 1 Quote Link to comment Share on other sites More sharing options...
gsxrpilot Posted June 5, 2019 Report Share Posted June 5, 2019 The nice thing about measuring by running a tank dry in flight, is that you KNOW your useable fuel number. And that's really the only number that matters. 2 Quote Link to comment Share on other sites More sharing options...
aviatoreb Posted June 5, 2019 Report Share Posted June 5, 2019 2 hours ago, DanM20C said: My 79 claims 80gal capacity with 72 usable. The later 231's 83-84?? are listed at 78.6gal capacity with 75.6 usable. I flew my right tank dry on my 79 and was able to squeeze 39 gal in it. If I'm going on a long trip I fill as full as I can and plan on 72 usable to be conservative. I haven't tried the left yet, hopefully it is the same. If it is I have an extra 30-40min at cruise as an extra cushion beyond my normal reserves. Cheers, Dan That almost matches what I was able to do when I flew one wing dry and I mentioned above. I was able to squeeze 39 gallons into one wing from dry, with lots of jiggling. And the soft fill amounted to about 36 gallons. So that makes 72 and 78 gallons total resp. And since then (3 years ago?) I got LR tanks installed and I have not done the fly til dry test but I have easily filled them to 51 on a side with only flying it to very low (low on the gauge but still the light did not come on) after having filled it to the extreme. 1 Quote Link to comment Share on other sites More sharing options...
carusoam Posted June 5, 2019 Report Share Posted June 5, 2019 Now for the flapper valve purpose.... 1) It is good for hiding the actual fuel level in the tank... 2) It was supposed to keep fuel in the tank, in the event the cap’s seal failed... (so I believe) 3) high altitude, high speed, leaking cap seal, or missing cap... the flapper is supposed to keep the fuel in place. I believe the K is the only Mooney that has this. The other TC’d Mooneys rely on good cap seals. Including Bravo and Acclaim... Blue Teflon (?) based seals seem to be the way to improve on faulty seals... ask @OSUAV8TER for insight. Improper Orings And other rubber seals have been known to expand and elongate making the seal not work... got a loose Oring? Signs of seals not working... rain water getting in the tank... a few drops or gallons.... (while on the ground...) In flight, in rain, leaky seals become visible as air bubbles actually come out of the cap through the rain soaked wing top.... Fuel cap location... how far out on the wing it is... leads to how much fuel can be put in the tank... the hole defines the top of the fuel level. Also note some Mooney extended fuel tanks do have a section separating the extended part from the standard part... plumbing is used to bridge the two sections... Some Mooneys may have speed brakes that occupy some volume of the fuel tank. Rain is drained out of the bottom some how.... (not sure how correct this statement can be...) PP thoughts only, not a mechanic... PP thoughts only, not even complete thoughts.... Best regards, -a- 1 Quote Link to comment Share on other sites More sharing options...
NicoN Posted June 6, 2019 Author Report Share Posted June 6, 2019 Okay, I learnt a lot. I was not in rain for a longer time to see bubbles - or was concentrating on other things. New O-rings are on the list already My first suspect to check is whether holes in the ribs are blocked with sealant. On my M20K the wing side gauge keeps moving up after closing the gas nozzle. May be normal or indicating a lower than normal fuel dissipation in the tank compartments. 1 Quote Link to comment Share on other sites More sharing options...
carusoam Posted June 6, 2019 Report Share Posted June 6, 2019 Interesting observation for the delayed response of the sight gauges (mechanical floats magnetically connected to the display needle) could be fuel taking time to get where it wants to go... air distribution through the small holes... Could be a float sticking, wanting to be cleaned...? my floats respond quickly to sloshing fuel (pushing the wing, causing waves). That’s a hint they are not sticking... PP thoughts only, not a mechanic. Best regards, -a- Quote Link to comment Share on other sites More sharing options...
Austintatious Posted October 14, 2020 Report Share Posted October 14, 2020 I just found this thread... Did a pretty long flight in the rocket yesterday. M20k. https://flightaware.com/live/flight/N305RK/history/20201013/1830Z/2R4/KFTW I Topped it off to the point that fuel was about an inch OVER the plate. I then took off, burned 20 gallons from the right tank and then ran the left tank until the engine started to falter. Total fuel burn at that point was 53 gallons. This means I only got 33 usable gallons from the left tank. Next time I top off I will do this but start on the left tank. Then I will know how much total usable I have. I am a bit miffed I only got 33 out of the left tank. If the right is the same, That is pretty pathetic. 66 usable gallons with 15 remaining for reserve leaves 51 gallons for T/O cruise and descent. I typically burn 20-25 gallons the first hour depending on how high I climb. Then about 17 per hour after that doing from 180-210 ktas depending on altitude. I made it from Pensicola to Fort worth non stop with a headwind, but I was really working the numbers to do so. Right Fuel guage was showing a tad over the 9 gallon mark when I landed, totalizator said I burned 58 gallons.. During descent and arrival I had it pulled back to 10gph doing about 140 kias. By my calculation I had at least 30 min of reserve with the 6-10 gallons I had left. What sucks is that if I had the full 37 gallons per side, I would have still had 15 gallons on board. Maybe I am still just not used to such small fuel numbers. In the aircraft I fly for a living, if I have less than 600 gallons on board after I land I have messed up! Quote Link to comment Share on other sites More sharing options...
jlunseth Posted October 14, 2020 Report Share Posted October 14, 2020 I haven’t ever found the gauges to be accurate to the nut. I put in CiES senders and they are better, but remember their accuracy depends on how the installer did the calibration and the installer and the guy who wrote the instructions wants to make sure you don’t get to no fuel before the gauge reads low or zero, so with both my factory gauges and the CiES senders, the calibration is conservative. If I still have 5 gallons of fuel in each tank when it says zero, that is almost an hour of fuel in my aircraft. Fuel flow is the more accurate measure. So in the early days when I got my 231 I followed a procedure I read, probably in the JPI manual. Fill the tanks your self so you know they are being filled the same way. Take three flights of an hour or more and refill the tanks yourself each time. Note the amount that goes in and note the fuel flow out. Total each for the three flights and compare the two. My fuel flow meter was off (conservative) by one tenth of a gallon out of 50+ gallons. My aircraft is supposed to hold 37.8 usable (total of 75.6 gallons), but it was never clear to me at what fill level. The tanks do not hold 37.8 when filled to the anti-siphon valve (bottom of the neck), it is more in the range of 34-35. If I am going on a long trip and want all the fuel in the tanks I can get I help the fuel guy fill the tank. I fold my arms over the wing tip and rock it while he fills. This burps the air out. Then I tell him I want the fuel at the top ring. That’s as close to 37.8 as I can get. Someone asked what the flappers are for. A gas cap O ring failed in my aircraft on takeoff at a little airport in Arkansas. There was an impressive stream of fuel spraying out of the cap and up into the air. I landed, refastened the cap and was trying to figure where I could go to get this fixed. Took off again and still got the stream but it only lasted for the cup or so of fuel that sits on top of the anti-siphon valve. I was able to fly home, several hours, where I got the cap fixed. That’s what that flap is for. Quote Link to comment Share on other sites More sharing options...
ArtVandelay Posted October 14, 2020 Report Share Posted October 14, 2020 Someone asked what the flappers are for. A gas cap O ring failed in my aircraft on takeoff at a little airport in Arkansas. There was an impressive stream of fuel spraying out of the cap and up into the air. I landed, refastened the cap and was trying to figure where I could go to get this fixed. Took off again and still got the stream but it only lasted for the cup or so of fuel that sits on top of the anti-siphon valve. I was able to fly home, several hours, where I got the cap fixed. That’s what that flap is for.How did you let your gas cap O ring get so bad that it failed? You literally can check it’s condition every time you refuel. Quote Link to comment Share on other sites More sharing options...
jlunseth Posted October 14, 2020 Report Share Posted October 14, 2020 (edited) I do, and I have them checked every year at annual. It was a newly installed gas cap, must have been some issue with the way the cap was assembled. The leak was around the stem, not the big O ring. Edited October 14, 2020 by jlunseth Quote Link to comment Share on other sites More sharing options...
larrynimmo Posted October 14, 2020 Report Share Posted October 14, 2020 I find the leaks I have had with the caps has been on the shaft o ring on the Inside of the cap...it only takes like 3 minutes to replace and reassemble. you can test the cap my propping on a table making it sit flat edge level and filling insert area with water...it has to hold water to be safe 1 Quote Link to comment Share on other sites More sharing options...
flyer338 Posted October 15, 2020 Report Share Posted October 15, 2020 On 6/5/2019 at 10:35 AM, gsxrpilot said: The nice thing about measuring by running a tank dry in flight, is that you KNOW your useable fuel number. And that's really the only number that matters. I used to routinely run a tank dry when I flew a carbureted Mooney. In cruise flight, I knew the normal fuel pressure reading. When the fuel gauge got to the place, where I knew from experience that the tank was about empty, I watched the fuel pressure. When it started to fall it was boost pump “on” and switch tanks. I never had the engine even stumble. Now that I fly a fuel-injected airplane, I wonder what happens when a tank runs dry? I suppose I shall have to try it. (Of course the engine quits, but after what warning, and after switching tanks how long before there is power again.) 1 Quote Link to comment Share on other sites More sharing options...
gsxrpilot Posted October 15, 2020 Report Share Posted October 15, 2020 51 minutes ago, flyer338 said: Now that I fly a fuel-injected airplane, I wonder what happens when a tank runs dry? I suppose I shall have to try it. (Of course the engine quits, but after what warning, and after switching tanks how long before there is power again.) Watch the Manifold Pressure. It will bobble and drop. *Switch tanks and you'll be running strong again before you can take your hand off the tank selector. No need for a boost pump. Just switch tanks and keep going. *Note: it's a little different above 18,000 ft. 2 Quote Link to comment Share on other sites More sharing options...
carusoam Posted October 16, 2020 Report Share Posted October 16, 2020 Essentially, Both engines are seeing air bubbles start to enter the fuel distribution system... Carburetor can easily separate air out within the carb’s bowl... Fuel injection system also has a method of separating out air bubbles... The FuelP gauge starts to show for a while before Things go silent... Having things go silent at FL180 and above... may lead to having to descend to a more fuel ignition friendly altitude... before the turbo can get back up to speed. Expect to need the mixture to be pulled pretty far back to match the NA airflow... Don’t fear the fuel tank change... until the valve doesn’t work right... Practice the first couple of times over a safe alternative... in case some other surprise shows up at the same time... practice a few times without running the fuel all the way out... This way you are practiced at the entire procedure, before you need to execute it in a timely manner... practice with a delay to changing the tank valve... watch the FP drop... don’t leave it here, this would be tougher on the fuel pump than necessary... the fuel pump can draw a vacuum pretty well... enough to oil can the wing’s top surface, when the vent gets plugged... wait... why fumble trying to get the fuel pump on...? Let’s get the fuel flowing first... if the fuel isn’t flowing, the pump isn’t going to be very helpful... Check the POH for proper order of steps for this... PP thoughts only, not a mechanic... Best regards, -a- Quote Link to comment Share on other sites More sharing options...
tmo Posted October 16, 2020 Report Share Posted October 16, 2020 9 hours ago, gsxrpilot said: it's a little different above 18,000 ft. So what is the proper way to run a tank dry and switch to the one with fuel above 18,000 ft? Quote Link to comment Share on other sites More sharing options...
gsxrpilot Posted October 16, 2020 Report Share Posted October 16, 2020 9 hours ago, tmo said: So what is the proper way to run a tank dry and switch to the one with fuel above 18,000 ft? The procedure is in the POH. Basically it just takes longer to restart. I think the POH says it could take 13 seconds or longer... which seems like 13 minutes. You also might have to reduce the mixture until it fires. I've done it a few times just to know what it's like. It really only took a couple of seconds to come back to life. 1 Quote Link to comment Share on other sites More sharing options...
tmo Posted October 16, 2020 Report Share Posted October 16, 2020 Thanks. I wanted to be sure there isn't some "better than book" procedure, known by the more experienced, which sometimes is the case. Quote Link to comment Share on other sites More sharing options...
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