Fuel Flow Discussion, Moved from Float Mounting

[Battson]

short story; Running on "Both" without having interconnecting tanks. Pressure differential between the tanks caused fuel flow stoppage.

I have gone full circle on this and I'm not convinced that interconnecting the tanks will actually prevent this; having the benefit of clarity via Bob.
I am really sorry to "flip-flop" on this.
Having had the event at our home airfield I got very interested and I cannot understand some of the logic.

The facts are:

• This issue seems to only affect engines with a fuel pump.
• Any fuel pump will rather suck air than fuel, if air is available.
• Low fuel in one tank is universal in the incidents discussed.
• If the engine sucks for air long enough it will stop, whether the selector is on both or L/R

Occam's razor: It seems much more likely that sucking air bubbles due to prolonged unporting or running a tank dry caused these incidents. A cross vent will not prevent that.

The whole mysterious scenario of one tank robbing fuel from the other just seems unlikely. Sorry to the supporters of this idea.... but nobody has offered an adequate explanation, the necessary pressure difference is too great, and the experts don't support it.

Also the hypothesis of any plane with a "Both" selector needing a cross vent, we haven't provided evidence. I THINK the reason for the cross vent is probably as a backup in case one of the external vent blocks. That way, you can leave the selector on both and if there is fuel you're likely to get it, even if a vent blocks. That was the whole point of the both selector, avoiding accidents due to fuel mismanagement.

I understand Bob is considering providing some additional guidance about fuel system design.

[Bcone1381]

The Patrol Book specifies a four way L-Both-R-Off fuel valve.

[Battson]

I think it's worth remembering that Bob designed and recommends a gravity feed system to a carb engine without any fuel pumps.
In Bob's fuel system, operating with both tanks selected in any fuel configuration will be safe at all times.

Introducing fuel pumps and injection brings some benefits, as well as many new design considerations and operational considerations. As a community, I think we are still learning how to manage those risks, there is no one size fits all.

[zkelley2]

I think you can make a good arguement he designed it only fot L/R even if he didnt know it, considering there isn't a cross vent. And I think that argument has been made.

[Bcone1381]

We are thinking now! That is good.

Bob designed the fuel system for a R-Both-L selector. If I fear moving it we have a problem that needs to be vetted out.

[Battson]

I would like to elaborate my post #114. It seems to me that fuel cross-flowing through the fuel selector from one tank to another reduces the amount of fuel flow available for the engine. So therefore, I am wondering if this safety notice or limitation might not be prudent....

"When a significant fuel imbalance exists do not place the fuel valve to BOTH, rather place the selector to either the L or R tank as appropriate."

The trouble is, you need to know why the imbalance occurred, if you select the fullest tank - you are selecting the tank most likely to have a venting issue or negative pressure in flight, assuming the imbalance occurred unintentionally. Technically you should select the emptier tank and refuel as soon as possible.

****

Statistically speaking, playing with the fuel selector gets more pilots killed than tanks without a cross-vent. I feel it's important to remember that fact, we are only human.

[Battson]

I confess to having done exactly the same thing, and I agree the system *MADE THE WAY BOB DESIGNED IT* is very tolerant to issues. I must disagree with zkelley2 above, as I always fly on Both and this was no exception.
We flew for a long time without one fuel cap and I hardly lost a drop of fuel overboard. I was able to tape over the open tank and fly back and get the missing cap (it was a short flight and the closest option).

[svyolo]

There is more head pressure to feed to carb or FI pump, then to cross-feed to the other tank. The head between the two tanks is only the difference between the fuel levels, 1-8 inches. The head to the carb or pump is 4 feet.

To my mind, I think the flow goes to the engine before it goes all the way up the other fuel line to the opposite tank.

[robcaldwell]

I've thought long and hard about cross venting, but have decided to simply be disciplined about fueling and maintaining both sides as equal as possible without ever going below 1/4 indicated.

[Bcone1381]

I would like to elaborate my post #114. It seems to me that fuel cross-flowing through the fuel selector from one tank to another reduces the amount of fuel flow available for the engine. So therefore, I am wondering if this safety notice or limitation might not be prudent....

"When a significant fuel imbalance exists do not place the fuel valve to BOTH, rather place the selector to either the L or R tank as appropriate."

[zkelley2]

Thank you for sharing. The fuel system as designed is tolerant of this error, provided you don't run it on both.

I've had a leaking fuel tank before, on a previous airplane, on a long leg with no fuel until destination. It's a really good idea, IMO, that if you suspect you're losing fuel from a tank, to immediately switch to that tank to try to burn as much as you can before you lose it.

[jaredyates]

I need to add a story here from the "not particularly proud to tell it" category. But I'm going to fess up because it may save someone else for doing the same.

A couple of months ago I took the Bearhawk on a solo trip the lake to tow paragliders. It's a 3-hour drive or a 1-hour flight. It's a great airplane mission, because the weather demands of paragliders are much more stringent than GA planes, so weather isn't usually much of a concern. I secured permission to tie down at a tiny airport with no attendance or services at all. It's very much like the airport where we keep our airplane, and best of all, it is really close to the boat ramp we use for the towing. The morning flight over was easy. I knew I was going to have to fuel before getting home that evening, but I didn't fuel in the morning, and parked the plane with around 12 gallons on board. That's not much, but it is certainly enough to fly a few miles to the big self-serve airport on the way home.

Having snacked/worked through lunch, we had a great day and were pounding away some much needed calories at the Mexican place. I happened to check the radar image on my phone, and there was a giant line of rain that was headed right for us. The line was parallel to my course home, and probably 40 miles upwind. I didn't have to get home, and knew that. But I also had a hunch that if I hustled over to the airport and got on my way, I'd have a good chance to make it home. It was safe in the sense that the line was barely moving, so I could always make a right turn and be in clear blue sky. If the rain moved over the home airport, I might have an adventure, which is no big deal.

The hop over to the big airport was uneventful, but I knew that time was not on my side. I figured, why not just fill up the right tank? That would be 17 more gallons than I'd need to get home, and over the course of the flight, it would be able to drain itself to the left tank. I'd save the time of having to move the ladder over to the other side, and be on my way 5 minutes sooner.

Skip ahead 15 minutes or so. I was on the radio with Greer approach, flying on course and parallel to an ominous looking line to the left, with a perfect day on the right. I could see the rain falling hard to the left, right at the Blue Ridge Escarpment. I was feeling pretty good about surfing the updraft curling around from that upflow, set at my usual 8gph cruise power and getting a bonus 20-25 knots of IAS over the usual cruise. (note! This technique is not recommended for beginners. Or really for anyone else. It's not the first time I've done it, but it's not without risk.)

I did a cursory and typical check of the fuel sight gauges and got quite a surprise. The left tank was as empty as I've ever seen it, and the right tank was still completely full. This is not what I was expecting, since I'd had some time for the tanks to balance. The left tank level was actually going down, not up!

At this point, my brain was showing the little hourglass symbol. Thankfully and perhaps luckily, I came up with a hypothesis. What if, in my slightly rushed refueling, I forgot to replace the cap? The left tank would have positive pressure, the right tank would have at best ambient pressure, and maybe low pressure. The corrective action that I needed, perhaps very quickly, was to select the right tank only. The pressure in the right tank was lower than the pressure in the left tank, but not so low that the engine wouldn't run. To confirm this, I selected the right tank with a close watch on the fuel flow, and thankfully it never changed. Everything was still good, and I pressed on towards home.

As I got closer, I could see that the rain had not reached home yet. I landed in strong gusty winds, which is also not recommended for beginners or anyone else. But I put the plane away and checked the right tank. Sure enough, the cap was gone.

So while I don't expect that I'll be adding a cross vent in the near term, I do want to point out that I encountered a scenario where the designed system was not tolerant of a particular series of errors- that is, fueling only one tank, and failing to recap the tank. It is totally fair that the system does not need to be tolerant of these errors. It may very well be that the engine would have kept on running. I wasn't in a position to test it.

As I said in the beginning, this isn't a story of great piloting. But I thought it was worth sharing.

[svyolo]

I am using a vent from the inboard sides of the tank because I already have the fittings welded there. I am pretty sure I will use a 12v shutoff valve to automatically close with the loss of 12v power. I will put a switch and light on the valve to ensure it functions.

Outboard is probably a better location, but inboard is better than nothing. Still using vented fuel caps.

[rodsmith]

After reading through this whole discussion again a few weeks ago, I decided to add a cross vent tube between the two main tanks, similar to what Paul documented. Will post pictures when complete. Jon in Idaho was quite persuasive advocating that in his posts.

[Battson]

I am just becoming aware of a "near miss" involving a Bearhawk here, everyone is OK and the plane landed safely at an airfield.

Early reports are saying an engine failure after takeoff occurred during a short flight (only a few miles long) caused by one tank being half full and the other tank being almost empty. It sounds like the symptoms of the issues presented in this thread.

He was able to land and pump in more fuel to re-balance the fuel load and continue the trip without further incident.

I understand the pilot lost power just after takeoff, and immediately took the correct action of changing tanks which allowed him to restart the engine and continue to climb out. However he left the throttle wide open which prevented the fuel system from fully recovering, based on what we know here, which resulted in intermittent power surges throughout the remainder of the flight. Based on what we know, I believe that had he retarded the throttle the engine would have continued to run smoothly.

Note that this particular fuel system was not built exactly as Bob intended it, and it tends to draw from one tank faster than the other.