Fuel Flow Discussion, Moved from Float Mounting

[Bcone1381]

Something like this is pretty non-invasive. This suggests fabricating and installing a cross vent between the top sight tubes, adding a vent like Whee's fuel cap nozzle on the cross tube and placing it at the wing root faring, plus adding short vent lines between the rear fuel supply lines and the cross tube.

Alternatively the atmospheric vent for the tank interconnect vent could be run outboard of a tank and down through the bottom wing skin as long as the the atmospheric vent line rises above the highest point in the tanks.
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[Battson]

Until you park on a slope and then you have an automatic fuel drain, but I can't see any way around this.
I like the cap vents and the cross vent between fuel gauge lines

[Battson]

I can't think of any way to use check valves on the cross vent line. If you prevent cross feeding in both directions, then you prevent the pressure difference from equalising too.

Funny how the issues with a fuel system design are not obvious until you think about it critically, next minute they stare you in the face.

[JJohnston]

Wouldn't the Cessna 177 system prevent spillage from parking on a slope? The vent from each tank crosses over to the other side of the opposite tank before turning down; plus I think that little short branch connecting the two crossvents must be a "siphon break". On a slope, the fuel from the low side would only go into the vent until it was at the same height as the high side. Or am I way off? I'm a civil engineer, but structural; my hydraulics are weak.

[Mark Goldberg]

One data point to mention is the size of the vent hole on Bob's fuel caps. He designed the hole quite small so that insects are unlikely to block them. I say unlikely because there are a lot of planes out there using Bob's fuel caps, and I have never heard of a case of insect blockage. But it is a possibility. Icing up the vent holes would seem like a bigger problem. Mark

[Bcone1381]

The image below is more invasive than the image in post #94. It shows a vent line running from the right outboard tank across and passed the left tank. Its pilot proof. If parked on a slope with full tanks fuel would only drain out of the lowest fuel cap.

A tank can be isolated with shutoff valves (Like installing one in the fuel tank interconnect vent). Although valves like that are far from pilot proof they should be installed so the are accessible in flight.
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[JimParker256]

Since this discussion was on my mind, I asked my A&P/IA if all high-wing certified planes have a vent line between the left and right tanks. All three of the mechanics in the room immediately nodded "Yes" and the IA said that there's a mandatory test in the annual and 100-hour checklists to ensure the vent is properly working. Just another data point for the discussion.

[Ed.Meyer]

I see this thread got new life after being quiet for a few days. I read that two people have had engine failures. They state that the failures were due to venting issues. I wonder how they KNOW this. Is it possible that the problems might have been something else that was never discovered?

When I had an engine fail in a Cherokee, I thought it must be the pump so I changed it. I now think it was a vapor lock problem. Looking back on it, the conditions were right for vapor lock: hot day, low power setting so low fuel flow prior to failure, and burning auto gas. The reality is that I don’t actually KNOW positively what caused it. I quit burning auto fuel and never had the problem again but I might not have ever been in quite the same circumstances again.

My concern is that we are doing a lot of chasing theory with the possibility of adding risk with added system complication that might introduce other unintended consequences.

I still don’t see how this venting imbalance can possibly occur on a stock Bearhawk short of a blocked or improperly installed fuel cap vent. The only scenario I have been able to theorize is, as mentioned the other day, the possibility that with differing amounts of fuel in the tanks resulting in differing amounts of air above the fuel and with outside air pressure change, like altitude change, differing amounts air would be needed to equalize the pressure. I suspect this difference would be relatively small unless the altitude changes were pretty extreme.

[Archer39J]

I asked my A&P/IA if all high-wing certified planes have a vent line between the left and right tanks. All three of the mechanics in the room immediately nodded "Yes".

Makes sense, it used to be required by regulation but it isn't any more.

I still don’t see how this venting imbalance can possibly occur on a stock Bearhawk short of a blocked or improperly installed fuel cap vent.

You're right it can't, when everything is working correctly. If you run BOTH the fuel level will equalize between the tanks, and with proper venting altitude changes won't affect anything.

[whee]

I see this thread got new life after being quiet for a few days. I read that two people have had engine failures. They state that the failures were due to venting issues. I wonder how they KNOW this. Is it possible that the problems might have been something else that was never discovered?

When I had an engine fail in a Cherokee, I thought it must be the pump so I changed it. I now think it was a vapor lock problem. Looking back on it, the conditions were right for vapor lock: hot day, low power setting so low fuel flow prior to failure, and burning auto gas. The reality is that I don’t actually KNOW positively what caused it. I quit burning auto fuel and never had the problem again but I might not have ever been in quite the same circumstances again.

My concern is that we are doing a lot of chasing theory with the possibility of adding risk with added system complication that might introduce other unintended consequences.

I still don’t see how this venting imbalance can possibly occur on a stock Bearhawk short of a blocked or improperly installed fuel cap vent. The only scenario I have been able to theorize is, as mentioned the other day, the possibility that with differing amounts of fuel in the tanks resulting in differing amounts of air above the fuel and with outside air pressure change, like altitude change, differing amounts air would be needed to equalize the pressure. I suspect this difference would be relatively small unless the altitude changes were pretty extreme.

Perhaps I should have used the same wording as the NTSB? "Probable Cause." First hand knowledge of two failures, detailed inspection of the aircraft after each failure, theory researched and developed, mathematical prediction of the conditions required for another failure, predicted failure occurred. Seems like a pretty solid "probable cause" to me.

You can't assume that because both tanks are venting to atmosphere that their vent pressures are the same. If the atmosphere was static then yes, that assumption is acceptable. The conditions at the inlet of each cap vent are dynamic on a flying airplane. Prop-wash, wing contamination, VG placement, uncoordinated flight are things off the top of my head that could affect individual vent pressure. Sure, most of the time the conditions will be close enough it won't matter but that may not always be the case.

As I said before, I do believe Bob's caps reduce to likelihood of this failure occurring because they seal well and the size of the vent hole is small.

I do wonder: If Matt or me and my family had died as a result of either of these failures would you all be so willing to pass it off as an anomaly that occurred due to not using Bob's caps or installing a fuel injected Continental?

[Archer39J]

"Sure, most of the time the conditions will be close enough it won't matter but that may not always be the case."

I think most of us discussing this understand that, if our saying that it's not really possible to have an imbalance under normal conditions is being taken in the most strict sense.

[Battson]

Jon - you have to admit, it's a big assumption everyone here is making. We have no firsthand sight or knowledge of the investigation and determination of the cause. It's only natural for people to question it. Nothing personal.

[whee]

Since this discussion was on my mind, I asked my A&P/IA if all high-wing certified planes have a vent line between the left and right tanks. All three of the mechanics in the room immediately nodded "Yes" and the IA said that there's a mandatory test in the annual and 100-hour checklists to ensure the vent is properly working. Just another data point for the discussion.

Luscombe 8, C206, PA18 are the ones I know of off-hand. But I do appreciate you thinking about this discussion and talking about it with folks you trust.

[alaskabearhawk]

I spoke with my mechanic friend about this. After a lot of noodling and discussion we decided it wouldn’t hurt to add a vent between the tanks. Then the debate centered on where to put the vents. We decided on the outboard side of the mains, at the top (tallest part). After this and Mark’s comment about his conversation with Bob and his thoughts I called Bob to run our idea by him. He agreed that it wouldn’t hurt to put in the vent and he said the outboard location would be his choice too, but at the top forward corner. He said the main thing is to be an aware pilot. He’s flown many hours and is always aware of what flight profile may cause a tank to unport in a low-fuel situation. Airmanship and awareness I guess. About the vents, I did read somewhere that venting between the sight gauges is a bad idea. I also commented on post #89 about the pressure caps.

[robcaldwell]

How will the vent line be routed through the cabin? I guess you could place a bulk head fitting on each inboard wing rib? Then join the line on both sides after the wings are installed? With the vent line in the outboard forward location should prevent cross feeding in a wing high parking position unless the tanks were totally slap full, even then not much should transfer.

[svyolo]

It might be tough to put a fitting on the outboard edge, at least on a QB kit. To give myself space for fittings on the inboard side, I put the tank fairly far outboard in the bay. It might fit better to put it on the forward side of the tank. I think I had about 1.7 inches or so from the front of the tank, to the spar. That also makes installing the whole tank a bit of fun. I think it would have to be a flexible line. The flexible line would have to be secured so that it doesn't chafe on the tank straps.

[MattS]

On Murphy Rebels, they used to have the interconnecting vent connected through the sight gauges. After some problems, they issued a bulletin to move it to a separate port.

[Battson]

On Murphy Rebels, they used to have the interconnecting vent connected through the sight gauges. After some problems, they issued a bulletin to move it to a separate port.

It would be good to know what the issue was.

[MattS]

Below is the bulletin. There is a previous one that talks about the tanks needing to be interconnected. I can't find that one.

[Battson]

That is interesting, thanks for sharing!
Again, they have been unable to come up with hard test results to prove what was happening, and were forced to assume it was the sight gauge causing the issue. I have not thought about the problem hard enough to understand how interconnected sight gauges could cause the problem they were having - air bubbles in the main fuel lines. Seems very counter-intuitive, and I am getting a sore head already!

[Nev]

This Safety Bulletin shows surprisingly similar symptoms to the Cessna AD from 1969, although they prescribe different fixes.
Murphy term it an “air locked” fuel line and suspect air somehow entering the fuel lines. Cessna suspected vapor lock from fuel essentially boiling in the lines at bends under specific atmospheric conditions.

[JJohnston]

That's not the same problem as we've been talking about, either. They think their problem was from having the fuel feed at the bottom of the sight gauges, and their solution is to move the fuel feed to its own fitting. Bearhawk tanks are already that way.

[Battson]

In which case, there is no known problem from Murphy Aircraft with using fuel sight gauges to connect the cross vent system.

[alaskabearhawk]

Spoke with my mechanic friend again about the vent. Tentative plan is to weld bungs on the top/fwd/outside of each fuel tank. Put in a barbed fitting and connect the two tanks with clear tubing and watch how that goes. There is plenty of room for the fitting. The only potential negative is the tubing would have to go down a bit to fit through the front spar lightening hole. After that it's a straight shot to the other side. If it doesn't work just plug both tank ports and call it good.

[svyolo]

There is only a degree of dihedral. I doubt it would be possible to have the line run slightly uphill to the center of the fuselage. There will probably be a slug of fuel most of the time in the middle, but I think it still works. I wonder what Cessna's implementation looks like.