Bigdaddie Posted January 24 Report Posted January 24 We have a 1980 M20K. I was out at the hanger updating my Garmin databases and was met with fuel imbalance message. The G500txi showed “0” fuel in the left tank and 26gal in the right tank. I should have had about 50gal remaining in the aircraft after my last flight (25 each tank). The left wing gauge also showed close to zero. Since there was no fuel on the hanger floor, my first thought was “who took my fuel?” Are the wing gauge and the inner fuel transmitter tied together (I have CEIS senders)? I can’t believe someone broke into my hanger and just took fuel and left everything else alone. It seems like there’s fuel in the tank when I shake the airplane.
Fritz1 Posted January 25 Report Posted January 25 Fuel gauge on top of wing is independent from fuel senders in wing, top of wing gauge has float which may have gotten stuck 1
ArtVandelay Posted January 25 Report Posted January 25 Since both gauges show close to zero, did you open the fuel cap and use a flashlight to look inside? Shaking the wings won’t help you much, fuel noises from the other wing and last few gallons can still make noise.Check floor for blue stains! Avgas would evaporate quickly so don’t expect any wetness.Usually leaks are so slow they never reach the floor. Are you sure someone didn’t take your plane for a flight? Check FlightAware for last flight. 1
ArtVandelay Posted January 25 Report Posted January 25 Fuel gauge on top of wing is independent from fuel senders in wing, top of wing gauge has float which may have gotten stuckHe said BOTH gauges show low levels.
GeeBee Posted January 25 Report Posted January 25 I find the wing gauges highly accurate and very simple. That said, easy way to find out. Fill the tank!
Fly Boomer Posted January 25 Report Posted January 25 19 minutes ago, GeeBee said: I find the wing gauges highly accurate and very simple. That said, easy way to find out. Fill the tank! Yep. Or stick the tank.
skykrawler Posted January 25 Report Posted January 25 (edited) Once saw a M20M TLS with a bad seal on the mechanical fuel pump. It was pissing fuel out the drain hose at a substantial rate. Since that precedes the fuel flow sensor the owner had no clue to the actual fuel 'consumption' (those pumps are crazy expensive to overhaul, but the replacement seal was cheap) Edited January 25 by skykrawler 1
GeeBee Posted January 25 Report Posted January 25 1 hour ago, skykrawler said: Once saw a M20M TLS with a bad seal on the mechanical fuel pump. It was pissing fuel out the drain hose at a substantial rate. Since that precedes the fuel flow sensor the owner had no clue to the actual fuel 'consumption' (those pumps are crazy expensive to overhaul, but the replacement seal was cheap) Had one of those last month. $2000 for a new one.
Bigdaddie Posted January 25 Author Report Posted January 25 3 hours ago, GeeBee said: I find the wing gauges highly accurate and very simple. That said, easy way to find out. Fill the tank! Yea, thought of filling the tank IF we had fuel on the field.
Bigdaddie Posted January 25 Author Report Posted January 25 9 hours ago, ArtVandelay said: Since both gauges show close to zero, did you open the fuel cap and use a flashlight to look inside? Shaking the wings won’t help you much, fuel noises from the other wing and last few gallons can still make noise. Check floor for blue stains! Avgas would evaporate quickly so don’t expect any wetness. Usually leaks are so slow they never reach the floor. Are you sure someone didn’t take your plane for a flight? Check FlightAware for last flight. No stains on the floor. Well, maybe one about the size of a coffee cup under the center fuselage. I would think 25gal of AVGAS would make a bigger mark. There should have been about 25gal remaining in that tank. I did a quick look inside the tank but most of the fuel would be inboard of the filler neck at half tanks. I'm going to take a camera out there and have a look inside the tank. It had major avionics update 1 1/2 years ago including CiES fuel senders. I just had back surgery a couple of weeks ago so my ability to do too much work is still limited. I can't believe someone broke into the hanger and just took fuel, even though that was my first thought. There’re 2 Bose headsets, a GPU, BestTug and other goodies out there, all undisturbed. Thanks for the ideas guys.
Bigdaddie Posted January 25 Author Report Posted January 25 Took a run out to the hanger. I DO, in fact, have a fuel leak coming from the belly pan area. When it warms up a bit and I heal up some more, I'll pull the pan off and see what’s leaking. My luck dictates that it will be the most expensive component there. But hey, for a 45 year old airplane and what it does, it’s worth every penny. 1
ArtVandelay Posted January 25 Report Posted January 25 Took a run out to the hanger. I DO, in fact, have a fuel leak coming from the belly pan area. When it warms up a bit and I heal up some more, I'll pull the pan off and see what’s leaking. My luck dictates that it will be the most expensive component there. But hey, for a 45 year old airplane and what it does, it’s worth every penny.Could be the senders, have you noticed any avgas smell when you open the door? See if you pull the carpeting up to look at the flooring.
Bigdaddie Posted January 26 Author Report Posted January 26 5 hours ago, ArtVandelay said: Could be the senders, have you noticed any avgas smell when you open the door? See if you pull the carpeting up to look at the flooring. I'll take a look. No AVGAS smell in the cabin.
Yetti Posted January 26 Report Posted January 26 Cold weather causes more fuel leaks to show up. Belly Pan suggests pull side panel in the cabin and inspect the rubber fuel line. I would stick a inspection camera down the side panel before I would stick in the tank. Replace fuel line and tighten the clamps. Or the fuel sensor that is there. Tighten screws. New fuel wing sensors gaskets can be found at Brown Aircraft https://www.brownaircraft.com/ 1
Pinecone Posted January 26 Report Posted January 26 FYI, you can see the fuel by looking in the filler hole at about 5 gallons. I can get a dip stick reading at 7.5 gallons.
Ragsf15e Posted January 26 Report Posted January 26 Losing 25 gallons in a sudden leak would be almost unheard of. Losing that much without resulting evidence is even more intriguing. Look around carefully.
Bigdaddie Posted January 26 Author Report Posted January 26 4 hours ago, Pinecone said: FYI, you can see the fuel by looking in the filler hole at about 5 gallons. I can get a dip stick reading at 7.5 gallons. Yea, I was just too lazy to move the flapper way out of the way.
Bigdaddie Posted January 26 Author Report Posted January 26 1 hour ago, Ragsf15e said: Losing 25 gallons in a sudden leak would be almost unheard of. Losing that much without resulting evidence is even more intriguing. Look around carefully. It would not have had to be sudden. I had my third blood clot in my leg in October (that’s what you get when your “other airplane” is an Airbus A350 with 16-hour legs). So, I haven’t flown the airplane since then but have been out at the hanger dabbling. I also had back surgery since I figured I would do a heavy maintenance visit on my body while I was out of work. That precluded any serious bending for inspection. My guess is that it is the rubber fuel line since the tank is pretty much dry (reads “0” on the Garmin G500 with the CIES senders). If it was the sender, I would think there would be usable fuel remaining due to their location. However, the senders were replaced with the avionics upgrade 1 ½ ago. Does anyone know if the rubber fuel line is ¼”?
Yetti Posted January 26 Report Posted January 26 I will bet a dollar it's not 1/4". I think it needs to go over 3/8" tubing. But don't quote me.
PT20J Posted January 26 Report Posted January 26 Unless it all leaked out in flight, a leak at the wing root (sender gasket or fuel line) should produce a very noticeable odor in the cabin. I'd pull the belly panel and look underneath first because that's easier than opening the interior. 2
Bigdaddie Posted January 26 Author Report Posted January 26 1 hour ago, Yetti said: I will bet a dollar it's not 1/4". I think it needs to go over 3/8" tubing. But don't quote me. You're right. 3/8" is a more common fuel line size.
Bigdaddie Posted January 26 Author Report Posted January 26 (edited) 1 hour ago, PT20J said: Unless it all leaked out in flight, a leak at the wing root (sender gasket or fuel line) should produce a very noticeable odor in the cabin. I'd pull the belly panel and look underneath first because that's easier than opening the interior. I'm going to start at the belly pan. The residual fuel needs to be cleaned up anyway. This is going to turn into a project I can see coming. If I pull the interior, I might as well do the insulation kit that has been sitting in a corner. With the seats out, might as well change the rollers. And so on, and so on. Good time to buy a new Milwaukee screw gun. The Chinese one I've been using is okay for light work but ANY excuse to buy a new tool. Oh yea, start with a NO SMOKING sign in the hanger. Edited January 26 by Bigdaddie 1
gabez Posted January 26 Report Posted January 26 On 1/25/2026 at 6:00 AM, GeeBee said: I find the wing gauges highly accurate and very simple. That said, easy way to find out. Fill the tank! I second this. on my m20K the wings are very accurate when on the ground, the one inside are kinda okay more like full, middle and low.
amillet Posted January 28 Report Posted January 28 I had a leaking tank a few years back that got so bad I saw fuel dripping out of the belly pan drain hole when I came to the hangar one day. Greg at Advanced Aircraft in Troutdale fixed it. He said there was an area at the top of the tank that looked like it never had sealant applied at the factory. Didn’t leak for more than 20 years after leaving the factory.
Rick Junkin Posted February 24 Report Posted February 24 @Bigdaddie any resolution on the cause of the missing fuel?
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