Would it siphon the entire tank?

[AggieMike88]

As I checked fuel levels during my preflight this morning, I thought how fuel will be siphoned out should the fuel cap become dislodged or disappear in flight.

Question of the morning, would the siphoning action remove all of the fuel from that tank?

 

[NorthEast Ohio]

Once, I may, or may not have forgotten to replace my fuel cap after refilling. I didn’t lose one drop. But I have a fuselage tank, so YMMV

 

[unsafervguy]

yes, no, maybe. ive had it happen twice. first time on a twin comanche, outboard thermos cap came loose, it was still held in by the cover door but it siphoned out all 15 gallons. second time was in my RV-4, I left the cap levers up and lost both caps on takeoff. didn't loose a drop on that one. so the answer is it depends.

 

[Fearless Tower]

It may depend a little on the design and airflow around the cap, but generally I don’t think you’d have to worry about siphoning.

I’ve had fuel caps that didn’t seal well and lost a little fuel during climb when the tank was topped off but nothing significant.

I also once lost the oil cap on a Stearman without realizing it and flew an hour and a half without losing any oil.

 

[flyingron]

I looked over at the wing of a borrowed Navion one time and noticed the cap dangling. Didn't seem to have lost any fuel.

On the other hand, I did once, years ago, forget to replace the oil filler cap on a 170. Probably lost less than a quart of oil but I spent an hour washing the plane. My instructor pointed out that oil is a lot like blood. A little looks like a whole lot.

 

[porttack]

Lost a wing fuel cap on take off on my Glasair, lost 16 of 18 gallons in the time it took to loop around the pattern.

 

[Matthew Rogers]

I looked over at the wing of a borrowed Navion one time and noticed the cap dangling. Didn't seem to have lost any fuel.

On the other hand, I did once, years ago, forget to replace the oil filler cap on a 170. Probably lost less than a quart of oil but I spent an hour washing the plane. My instructor pointed out that oil is a lot like blood. A little looks like a whole lot.

I may (or may not) have left the oil cap off my wife's Subaru after adding a quart of oil and we drove for 2 hours to the beach. Oil level light comes back on and sure enough, the engine is nicely lubricated - all over the outside. Luckily I bought an extra two quarts at the same time and the cap stayed inside the engine compartment, albeit deep down a small hot, oily passage.

But I have also left the fuel cap off once on the C150 and flew for a while (30-60 minutes) and don't seem to have lost any fuel. Looking at these reports, it seems that if the cap is loose or slightly open, the fuel is more likely to come out, while if the cap is completely off, it seems to stay put. Perhaps someone with better fluid dynamics can explain that.

 

[Salty]

Suction not siphon. Don’t blame gravity, he’s doing his best to keep the fuel in the tank.

I think it depends where your caps are relative to the center of pressure, the angle of attack, and the amount of fuel in the tank. The first 10 gals will likely go a lot faster than the last 10 gallons.

 

[Stewartb]

If you see a Cessna take off without a fuel cap you’ll see a trail of fuel from the open filler. I’ve heard tower advise pilots to land because it it.

 

[SoonerAviator]

I may (or may not) have left the oil cap off my wife's Subaru after adding a quart of oil and we drove for 2 hours to the beach. Oil level light comes back on and sure enough, the engine is nicely lubricated - all over the outside. Luckily I bought an extra two quarts at the same time and the cap stayed inside the engine compartment, albeit deep down a small hot, oily passage.

But I have also left the fuel cap off once on the C150 and flew for a while (30-60 minutes) and don't seem to have lost any fuel. Looking at these reports, it seems that if the cap is loose or slightly open, the fuel is more likely to come out, while if the cap is completely off, it seems to stay put. Perhaps someone with better fluid dynamics can explain that.

At least you won't have to worry about the Subaru engine/engine compartment rusting.

 

[Dan Thomas]

A leaking or missing cap on a bladder-type wing tank can clean that tank out. The suction pulls the bladder restraints loose and it collapses like a toothpaste tube and squeezes out all the fuel.

 

[GaryV]

I had the gasket between the top of the fuel tank and the wing on my first 172 replaced. The mechanic didn’t do the job properly and the gasket leaked.

In the 10 minutes it took me to notice the leak after takeoff and successfully land back at the airport I took off from I’d lost 44 of the 50 gallons of fuel I had between the tanks when I started the plane. In hindsight I probably should have switched from both to the tank that wasn’t leaking but I didn’t think of that at the time.

I could actually see fuel leaking. I just small the very rapid drop in the fuel level. When I landed the entire side of the plane had a bright blue streak down the side with the bad gasket. I’m just glad I was in the habit of checking my fuel gauges frequently.

Gary

 

[TheFB]

I’ve flown both a Tiger and Bonanza V tail with the cap off. Lost very minimal fuel on both.

 

[kyleb]

Launched once upon a time in the RV-6 with a cap unseated on a full tank. The cap departed on takeoff and I made one quick trip around the pattern. Fuel siphoned out of the tank at high AOA, but the fuel loss diminished significantly as speed increased.

 

[Wagondriver]

I had the gasket between the top of the fuel tank and the wing on my first 172 replaced. The mechanic didn’t do the job properly and the gasket leaked.

In the 10 minutes it took me to notice the leak after takeoff and successfully land back at the airport I took off from I’d lost 44 of the 50 gallons of fuel I had between the tanks when I started the plane. In hindsight I probably should have switched from both to the tank that wasn’t leaking but I didn’t think of that at the time.

I could actually see fuel leaking. I just small the very rapid drop in the fuel level. When I landed the entire side of the plane had a bright blue streak down the side with the bad gasket. I’m just glad I was in the habit of checking my fuel gauges frequently.

Gary

I think if I had a tank leaking or losing fuel, I'd switch TO that tank. That way I could use at least some of it before it was all gone. Of course if you are just in the pattern or are going to land soon, it probably doesn't matter. If the engine sputters, switch to the other one. But, definitely don't run on both!

 

[Matthew Rogers]

I think if I had a tank leaking or losing fuel, I'd switch TO that tank. That way I could use at least some of it before it was all gone. Of course if you are just in the pattern or are going to land soon, it probably doesn't matter. If the engine sputters, switch to the other one. But, definitely don't run on both!

My C150 only runs on both, no left or right. Luckily, I am in the “didn't have a problem flying with the cap off” group for some reason. Maybe Cessna actually tested that at some point, who knows.

 

[Dan Thomas]

My C150 only runs on both, no left or right. Luckily, I am in the “didn't have a problem flying with the cap off” group for some reason. Maybe Cessna actually tested that at some point, who knows.

We had a 150 that a student flew with a cap off. Once around the circuit lost about four gallons, IIRC.

 

[CharlieD3]

I remember panicking over a wing sump that wouldn't quit dripping when I preflighted the plane 250 miles from home... First trip as PPL... Called home base FBO owner.... "Fly it home, use that tank first, go to both about 50 miles out if you need to equilibrate the tanks...

And don't sump on frosty mornings!"

I watched it drip allaway home. Lost maybe a gallon. Thought I was dooooomed!

 

[Capt. Geoffrey Thorpe]

I remember panicking over a wing sump that wouldn't quit dripping when I preflighted the plane

Find a bit of rubber tube, push it up against the drain and blow into it as hard as you can while opening the drain. Chances are you will dislodge the debris and the drip will stop.