Fuel Flow Discussion, Moved from Float Mounting

[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.

[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.

[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.

[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.

[AKKen07]

Thanks, I haven’t uncrated the wings yet, so lots of time to make whatever changes seem prudent.

[yateselden]

AKKen07 definitely better tapping into outboard tank ends if your still assembling the wings. I would need to remove the tanks, work in the closed up wing. But I don't like the idea of connecting the sight gages or having a valve either. Welding in bungs in tanks that have had fuel in them doesn't excite me at all. All things considered I think I'll let it be.

[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.

[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.

[alaskabearhawk]

FWIW,
Spoke with a mechanic friend about these caps. I see them often on Super Cubs. He said the STC for 180hp conversions require the pressure vent caps to meet fuel flow requirements.

[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.

[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.

[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]

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.

[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.

[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.