Small fuel loads, risks and limits

[AndyMac]

I took a big friend in a small plane today. I did a good W&B and was happy with the reserve (2hr fuel for a 1hr flight) but it made me wonder; how low do the tanks get before they don’t get the fuel they need, in, say, a bank or climb?

Legally I could have departed for a 30min flight with less than 3 gallons in each tank (1972 C150L, 22.5 usable) but is that just rainbows and cupcakes?

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[sarangan]

I took a big friend in a small plane today. I did a good W&B and was happy with the reserve (2hr fuel for a 1hr flight) but it made me wonder; how low do the tanks get before they don’t get the fuel they need, in, say, a bank or climb?

Legally I could have departed for a 30min flight with less than 3 gallons in each tank (1972 C150L, 22.5 usable) but is that just rainbows and cupcakes?

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Bank should not make a difference if you are co-ordinated. Pitch attitude does make a difference, but that is already accounted for in the usable fuel of the aircraft. As long as you are not flying at unusual pitch attitudes, you should be fine.

 

[OKDonn]

I took a big friend in a small plane today. I did a good W&B and was happy with the reserve (2hr fuel for a 1hr flight) but it made me wonder; how low do the tanks get before they don’t get the fuel they need, in, say, a bank or climb?

Legally I could have departed for a 30min flight with less than 3 gallons in each tank (1972 C150L, 22.5 usable) but is that just rainbows and cupcakes?

All that is covered in the type certificate - it's FAA approved.

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[Craig]

Bank angle can make a difference.....Inboard pickup, under a couple of gallons in the tank and 20 degrees of bank, that tank down, and you have the possibility of slugging air in the pickup. Tank size, configuration, pickup location, fuel level and bank angle all come into play and are aircraft specific.

 

[Matthew Rogers]

Here's a question - in an airplane with two high wing tanks, connected together with a "BOTH" fuel selector arrangement (like the C150), does it matter if a single tank pickup gets uncovered? Since there is no way of equally using the tanks in this arrangement, one tank will always have less fuel than the other. Has anyone actually flown a C150 and seen one tank at E while the other has usable fuel showing on the other gauge?

One guy at my club once landed his C150 in below minimum IFR after getting caught in a marine layer in CA. He was in contact with ATC and had as they vectored him back for another approach, he had to tell them "you need to keep this short, I am down to a couple of minutes of fuel." I can't remember if the engine died on the runway or not, but it was close.

 

[William Pete Hodges]

>>>>I did a good W&B and was happy with the reserve (2hr fuel for a 1hr flight) but it made me wonder; how low do the tanks get before they don’t get the fuel they need, in, say, a bank or climb?>>>>>

Do you really want to find out? And what will happen next when you do? It has been a long time since I flew a C150, but I recall my instructor saying that the last 2 gallons of fuel will not make it to the engine. That alone means the fuel reserve must have 2 gallons added.

>>>>> Legally I could have departed for a 30min flight with less than 3 gallons in each tank (1972 C150L, 22.5 usable) but is that just rainbows and cupcakes? >>>>

This is where personal minimums come into play. If it is legal to leave with 3 gallons in each tank, 6 gallons total, is that smart? When I get close to the bottom of my tanks and I want a short fly, I feel comfortable if I have at least minimum fuel in each tank. In your above scenario (for me) that would mean 6 gallons in each tank. At the very least you should add 2 gallons for unusable fuel, that would be 8 gallons total instead of 6.

Don't get me wrong. I am not suggesting that you press your fuel to the last drop, I never do, but there is greater understanding of your options if you know where the limits are for the specific aircraft you are flying. The only way to absolutly know is to run a fuel tank dry in the air, then switch to the tank with fuel before the engine quits. This is a little tricky but it can be done. But to answer the question properly, the aircraft would need to be cross coordinated in a slip to verify the correct point of fuel starvation. Do you really want to go that far? Isn't it far easier just to assume the last 2 gallons won't make it to the engine? That's what I do.

 

[AndyMac]

>>>>I did a good W&B and was happy with the reserve (2hr fuel for a 1hr flight) but it made me wonder; how low do the tanks get before they don’t get the fuel they need, in, say, a bank or climb?>>>>>

Do you really want to find out? And what will happen next when you do? It has been a long time since I flew a C150, but I recall my instructor saying that the last 2 gallons of fuel will not make it to the engine. That alone means the fuel reserve must have 2 gallons added.

>>>>> Legally I could have departed for a 30min flight with less than 3 gallons in each tank (1972 C150L, 22.5 usable) but is that just rainbows and cupcakes? >>>>

This is where personal minimums come into play. If it is legal to leave with 3 gallons in each tank, 6 gallons total, is that smart? When I get close to the bottom of my tanks and I want a short fly, I feel comfortable if I have at least minimum fuel in each tank. In your above scenario (for me) that would mean 6 gallons in each tank. At the very least you should add 2 gallons for unusable fuel, that would be 8 gallons total instead of 6.

Don't get me wrong. I am not suggesting that you press your fuel to the last drop, I never do, but there is greater understanding of your options if you know where the limits are for the specific aircraft you are flying. The only way to absolutly know is to run a fuel tank dry in the air, then switch to the tank with fuel before the engine quits. This is a little tricky but it can be done. But to answer the question properly, the aircraft would need to be cross coordinated in a slip to verify the correct point of fuel starvation. Do you really want to go that far? Isn't it far easier just to assume the last 2 gallons won't make it to the engine? That's what I do.

Nothing you say or imply is wrong - this is why I asked on PoA instead of trying to figure it out!

They’re only 11.5 gal capacity tanks on each wing, so at 3gal, they’re still at >25% capacity.

Lastly, the valve is “both” only, unlike on a C172. This always bugged me, as the tanks don’t drain evenly - but they do both drain.


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[AndyMac]

Run a tanks dry and find out. Just be at least semi-smart about it - don't do a John Denver and run it dry at a few hundred AGL.

I don’t know how many more hours I’ll get in this plane, or if I’d need to take a big passenger again, but I will certainly be careful not to forget the lessons John (re)taught us that day.


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[donjohnston]

Bank angle can make a difference.....Inboard pickup, under a couple of gallons in the tank and 20 degrees of bank, that tank down, and you have the possibility of slugging air in the pickup. Tank size, configuration, pickup location, fuel level and bank angle all come into play and are aircraft specific.

If you're in a coordinated turn, how would bank angle make a difference?

 

[GeorgeC]

Legally I could have departed for a 30min flight with less than 3 gallons in each tank (1972 C150L, 22.5 usable) but is that just rainbows and cupcakes?

tl;dr: yes. The pucker factor isn't worth it.

Story time!

50 minute flight home. Half tank (1 hour) in the right, quarter tank (half hour) in the left. Gauges known to be reasonably accurate. We're legal, let's go. Take off on the right tank, fly for half an hour, then switch to both. Cut off a friend in the pattern, fly straight to the numbers and land. Plane takes 20 gallons. That means 5 were left, almost an hour! Wait, out of 22 usable. Now, look sad and say d'oh...

 

[Bell206]

how low do the tanks get before they don’t get the fuel they need, in, say, a bank or climb?

FYI: from strictly a system standpoint, a properly functioning fuel system should provide all usable fuel quantity to the engine. If there is a "normal/intended" maneuver that could cause a greater than 20 second (?) power or fuel flow interruption of that usable fuel there will be a limitation stated in the POH/AFM/TCDS depending on the age of the aircraft.

 

[Capt. Geoffrey Thorpe]

Both only makes it harder to run one tank dry to find out what is really going on. All you can really do is circle an airport until you run out of nerve and then land and see what is left.

Of course, per the FAA usable fuel is usable. And we all know that certificates trump physics. But I can tell you from experience that in a Cessna 120 (no "both"), a hard slip into a tank that is less than 1/4 full will result in silence - usually when you decide to go around and shove the throttle in.

 

[Matthew Rogers]

Both only makes it harder to run one tank dry to find out what is really going on. All you can really do is circle an airport until you run out of nerve and then land and see what is left.

Of course, per the FAA usable fuel is usable. And we all know that certificates trump physics. But I can tell you from experience that in a Cessna 120 (no "both"), a hard slip into a tank that is less than 1/4 full will result in silence - usually when you decide to go around and shove the throttle in.

Exactly why I asked if anyone knows if the same thing happens in the 150 when there is only a “BOTH” setting and one tank is dry.

 

[Dan Thomas]

Exactly why I asked if anyone knows if the same thing happens in the 150 when there is only a “BOTH” setting and one tank is dry.

No. In a gravity-feed system, which the 150 has, there is always fuel going to the engine unless there's no fuel left. If you slip hard one way, the lower tank might unport, but the higher tank still has fuel against the outlet and it goes to the engine. In fact, there will even be fuel flowing though the system to the lower tank from the higher one.

A low-wing airplane doesn't have a Both position. It's either left or right. That's because there's a pump on the engine that's pulling the fuel upward, and if there was a Both position and one tank unported, the pump will pull that air instead of the fuel. The air destroys the suction of the pump, see?

There should be fuel capacity placards on the wing next to the fuel fillers and on the fuel selector. They will list the capacities as per the TCDS. The ones next to the filler will be the total fuel in the tank. The one on the selector will be the useable fuel, the fuel that will flow from the tank under conditions like a Vx climb or power-off, full-flap descent. But here's a problem: a lot of owners make their own dipsticks and start with a dry tank, so that the dipstick ends up reading total fuel instead of useable. I know of two incidents due to that---engine failures on final. Fortunately, no damage or injuries. Both were in 150s. The 150 has a flat-bottomed tank that has nearly two gallons unusable per tank. 26 total between the two tanks, 22.5 useable.

 

[sarangan]

If you're in a coordinated turn, how would bank angle make a difference?

Exactly.

But if you in a slip, that could make a difference. Crosswind landings are always a slip anyway.

In any case, if one has to be concerned if the last couple of gallons are usable, then I would recommend carrying more fuel. Its not worth the stress.

 

[Huckster79]

Make sure you know the plane well...

I had a similar story to George’s. Helped a friend haul a new to him bird home to MI from Texas and used POH figures. Lucky I’m an “hour of reserve always guy”. I landed first leg and we filled up- she had a half gallon of usable fuel! Something was wrong with the carb snd it drank fuel fat in excess of what it was supposed to. If idda had a charcoal suppository in idda had a 5karat diamond when I seen the gallons pumped.

had I done a legal 30 minute reserve idda been a glider pilot that we all shake our heads at...

 

[sarangan]

No. In a gravity-feed system, which the 150 has, there is always fuel going to the engine unless there's no fuel left. If you slip hard one way, the lower tank might unport, but the higher tank still has fuel against the outlet and it goes to the engine. In fact, there will even be fuel flowing though the system to the lower tank from the higher one.

A low-wing airplane doesn't have a Both position. It's either left or right. That's because there's a pump on the engine that's pulling the fuel upward, and if there was a Both position and one tank unported, the pump will pull that air instead of the fuel. The air destroys the suction of the pump, see?

There should be fuel capacity placards on the wing next to the fuel fillers and on the fuel selector. They will list the capacities as per the TCDS. The ones next to the filler will be the total fuel in the tank. The one on the selector will be the useable fuel, the fuel that will flow from the tank under conditions like a Vx climb or power-off, full-flap descent. But here's a problem: a lot of owners make their own dipsticks and start with a dry tank, so that the dipstick ends up reading total fuel instead of useable. I know of two incidents due to that---engine failures on final. Fortunately, no damage or injuries. Both were in 150s. The 150 has a flat-bottomed tank that has nearly two gallons unusable per tank. 26 total between the two tanks, 22.5 useable.

Thanks for that explanation. I didn't realize that the fuel filler placard was total and the selector placard was usable!

 

[Huckster79]

Another point- not 150 applicable- but if you have a L/R/both- I prefer to run one tank darn near dry in cruise then switch as if I’m safe but right on fuel I have one tank much more full at landing even going to Both... that’s how I fly my c140 as it has a “no take off” zone on the gauge that’s just under a quarter tank- to me I won’t land in that zone either as a go around is a take off- So I prefer to actively manage my fuel so one tank is plenty above that at landing

 

[Jeff Oslick]

I remember as a student pilot doing a preflight on a Cessna 150 I was flying, to find that slightly less than the unusable fuel was remaining in the tanks.

A student had returned the evening before from a cross-country flight. Apparently, they were supposed to have gassed up, but did not.

My personal minimums is to not go below an hour useable. Ever. This is one link in an accident chain we all have complete control over.

 

[farmrjohn]

No. In a gravity-feed system, which the 150 has, there is always fuel going to the engine unless there's no fuel left. If you slip hard one way, the lower tank might unport, but the higher tank still has fuel against the outlet and it goes to the engine. In fact, there will even be fuel flowing though the system to the lower tank from the higher one.

A low-wing airplane doesn't have a Both position. It's either left or right. That's because there's a pump on the engine that's pulling the fuel upward, and if there was a Both position and one tank unported, the pump will pull that air instead of the fuel. The air destroys the suction of the pump, see?

Ercoupes/Aircoupes have both wing tanks teed together with no selector valve. The fuel from them is pumped to a header tank which then gravity feeds the engine (unless the system has been modified).

 

[AndyMac]

Ercoupes/Aircoupes have both wing tanks teed together with no selector valve. The fuel from them is pumped to a header tank which then gravity feeds the engine (unless the system has been modified).

Now THAT is interesting. That is one innovative plane. Peculiar, but innovative!


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[AndyMac]

What this conversation does is confirm that I was right to A. insist on a 1hr reserve and B. Be concerned about low values of available fuel, even if they’re “legal”.

Glad for the reminder about “normal” maneuvers, as well as the specifics of maneuvers that can cause premature exhaustion, AND a refresher on the way the BOTH selector works.

Great discussion all, thank you!


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[Dan Thomas]

The Ercoupe had lots of dihedral to allow that. Simple airplane. Less dihedral would lead to trouble. The Cessna 210 has very little dihedral and savvy owners don't run the fuel too low.

Edit: The Ercoupe also had no rudder pedals, and had the rudders connected to the ailerons so you couldn't fly uncoordinated.

 

[Capt. Geoffrey Thorpe]

The 150 has a flat-bottomed tank that has nearly two gallons unusable per tank.

Outlet in the back? Push come to shove, throttle back, nose up, and get another 15 minutes???

 

[William Pete Hodges]

I will add a short story here. We used to own a 1968 Cherokee 140 that we would routinely fly 400 nautical miles to visit family on holidays. This flight would typically burn 34 gallons in 4 hours, so I would fill to 44 gallons for the flight, leaving 6 gallons short for an added 36 lbs of payload. I also typically switch tanks every 20 to 30 minutes as required to keep the airplane in proper trim with no aileron input on my part. This keeps the two tanks relatively even during the flight.

One Christmas we flew down there and as we neared our destination I noticed one of the tanks was lower than I expected. I completed the flight without incident but when we got out on the ground there was an oil slick on the underside of the airplane. It turns out one of the jugs had lost compression in flight, but I hadn't noticed it because it must have happened after we got to cruise and the loss of power was not readily apparent in that aircraft without a manifold pressure gauge.

When I checked the tanks in my post flight inspection I had six gallons remaining instead of the 10 that I had planned for. There are lots of good reasons to keep a bit of reserve on hand even if you don't think you'll need it. Have fun, fly safe!

 

[Dan Thomas]

Outlet in the back? Push come to shove, throttle back, nose up, and get another 15 minutes???

A 150's left fuel tank:

The outlet is at the bottom on the side facing the camera, about 60% of the way back from the front. That point is a compromise, and allows more fuel to reach the system in a steep climb (does any 150 have a steep climb?) than it would in a steep descent. If the outlet was at the rear, the unusable fuel would go way up, since the engine could starve in a power-off, full-flap descent even with considerable fuel still in the tank. If it was more toward the front, the engine could die on climbout. Compromises are everywhere in airplanes.

Now, the 152's tank:



Outlets at front and rear. The system looks like this:

Lines run from the rear outlets down the rear doorposts, and from the front outlets down the front doorposts, and teed together under the floor.. That reduces the unusable fuel somewhat.

The 150's TCDS on fuel:
26 gal. (22.5 gal. usable, two 13 gal. tanks in wings

So 3.5 gallons unusable, or 1.75 gallons per tank.

The 152's:
Fuel Capacity 26 gal. (24.5 gal. usable, two 13 gal. tanks in wings

1.5 gallons unusable, or .75 gallons per tank. Plumbing makes a difference.

The lines that tee off the top of the rear lines and go forward to the vent crossover line are to let bubbles in those lines bleed back up and into the tank's airspace. Later 172s also had this system, and some earlier ones were modified to have this bubble-bleed line. There was a condition where the downflowing fuel would hold a bubble in one spot in the vertical section, forming a dam around which the fuel had to flow, and there were some engine hiccups due to it. As the fuel sloshed around in the tank some air could get in the outlet. That bleed line took it back to the top of the tank.

 

[MauleSkinner]

Edit: The Ercoupe also had no rudder pedals, and had the rudders connected to the ailerons so you couldn't fly uncoordinated.

Either that, or you couldn’t fly coordinated. Both are true, depending on speed/phase of flight.

and they are spinnable.

 

[farmrjohn]

The Ercoupe had lots of dihedral to allow that. Simple airplane. Less dihedral would lead to trouble. The Cessna 210 has very little dihedral and savvy owners don't run the fuel too low.

Edit: The Ercoupe also had no rudder pedals, and had the rudders connected to the ailerons so you couldn't fly uncoordinated.

FYI later ones did have rudder pedals standard, and the same fuel system.

 

[Cervieres]

My personal minimums is to not go below an hour useable. Ever. This is one link in an accident chain we all have complete control over.

Same. With anything less, all it would take is a disabled aircraft on the runway at your destination or any other numerous scenarios to ruin your day.

 

[Dan Thomas]

FYI later ones did have rudder pedals standard, and the same fuel system.

Yes, I know that, and have a few hours in an Alon Aircoupe. Just try slipping that thing. Those tiny rudders are a joke. The pedals mostly made steering on the ground feel normal.

 

[David Megginson]

I took a big friend in a small plane today. I did a good W&B and was happy with the reserve (2hr fuel for a 1hr flight) but it made me wonder; how low do the tanks get before they don’t get the fuel they need, in, say, a bank or climb?

Legally I could have departed for a 30min flight with less than 3 gallons in each tank (1972 C150L, 22.5 usable) but is that just rainbows and cupcakes?

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It depends on the plane. For later Pipers like my 1979 PA-28-161, 1 gallon is flagged unusable in each tank, though I expect I could unport the fuel with more than that in the tank by carrying out extreme accelerated or uncoordinated maneuvers.

In 19 years of flying, I don't think I've ever landed with less than 5 gallons in each tank (not counting the "unusable" gallon), so over a 1 hour reserve. That's my comfort level. When a tank's down to 5 gal, it looks and sounds almost empty after lifting a fuel cap.

 

[Snowmass]

If you have two tanks and REALLY want know fly the plane in all realistic flight positions with very little fuel in one tank, at altitude, until engine cuts out. Switch to other tank and measure remaining fuel in "empty" tank. One test is worth a thousand opinions. Or just wonder.

 

[Juliet Hotel]

If you have two tanks and REALLY want know fly the plane in all realistic flight positions with very little fuel in one tank, at altitude, until engine cuts out. Switch to other tank and measure remaining fuel in "empty" tank. One test is worth a thousand opinions.

Purposely run a tank until the engine quits? Anyone who do such a thing must surely have a death wish.

Says the guy who used to routinely run tanks until the engine quit while flying 350 agl over water.

 

[Lawson Laslo]

Yes, I know that, and have a few hours in an Alon Aircoupe. Just try slipping that thing. Those tiny rudders are a joke. The pedals mostly made steering on the ground feel normal.

My Ercoupe has rudder pedals
The Coupe's has ample amount of rudder authority to slip it in as needed, although typically pulling the power is all you need to start dropping...

 

[Dan Thomas]

My Ercoupe has rudder pedals
The Coupe's has ample amount of rudder authority to slip it in as needed, although typically pulling the power is all you need to start dropping...

Yes, those short wings drop pretty fast.

The Alon I flew had the rudder pedals but its slip was pretty tame. I was used to my Jodel, which had enormous slipping capability, and the Citabrias I instructed in, which also slip well.

 

[bluerooster]

Bank angle can make a difference.....Inboard pickup, under a couple of gallons in the tank and 20 degrees of bank, that tank down, and you have the possibility of slugging air in the pickup. Tank size, configuration, pickup location, fuel level and bank angle all come into play and are aircraft specific.

In coordinated flight, bank angle will not cause the gas to "run down hill". For the same reason that the ball remains centered regardless of bank angle. Now, un-coordinated flight such as slips, and skids, all bets are off.