Fuel Flow Discussion, Moved from Float Mounting

[Nev]

Brooks mentioned earlier in this thread that he wondered if fuel could be flowing laterally through the fuel selector when in the BOTH position. I also got to wondering further about the whole "can a fuel pump suck air" issue. Previously I'd assumed it could. But I now have an issue with this - the fuel pump is flooded from the gascolator. The gascolator would have to empty first, before the pump could "suck" air.

In the BOTH position, at least one tank should be keeping the gascolator topped up.

Could a lateral fuel flow through the gascolator in the BOTH position create a Venturi like condition ? With a fuel pump "pulling" from the front, and a lower pressure at the back, could fuel vapor form? Could it even empty the gascolator back into the high tank ?

I realize that I'm going right off the reservation, but I'm not certain that the issue was ever resolved conclusively.

Some of you have a fluid dynamics or fuel system design background and can probably shed light on this for me.

[Nev]

I've just re-read post 37 by MattS and I believe this is what he was saying in that post that caused his engine stoppage.

[jaredyates]

The gascolator doesn't deliver from the bottom, so it wouldn't need to be empty in order to deliver air. It would only need to lose enough fuel to unport the fittings.

[Nev]

That's very interesting. I had also thought that it might act as a mini header tank, but in this case it wouldn't.

[svyolo]

I believe that is correct. But if the fuel system is feeding correctly by gravity, the gascolator should be full, and providing about 1 psi boost from the gravity feed if I remember right. But feeding from the top, not a header tank or reservous of any kind.

[Redneckmech]

There is a lot of great information in this thread and I have learned a lot about fuel systems in the process of reading through it. I have a slight background in fluid dynamics as that was one of my main focuses when I was in engineering school, having said that, I am far from an expert.

Looking through the history of this conversation, there has been a lot of discussion on whether a cross vent at the top of the tank was absolutely necessary when using the selector in the both position.

I may have missed someone else pointing this out, but much of the discussion was related to whether there could be a pressure difference between tanks when both were vented to the open atmosphere. The assumption that goes along with that, is that both tank vents are seeing the same pressure (atmospheric) which may not always be the case when the aircraft is in flight.
As a wing produces lift, it creates a pressure differential between the upper and lower wing surface with a corresponding pressure gradient that changes based on the angle of attack (see attached photo). If there are rigging differences, the wings are not perfectly aligned, or the aircraft is slipping/skidding, there will be a variation in the pressure that is seen by the vent since the pressure will change relative to the velocity of air over the surface. When that happens, the pressure will want to equalize between tanks and will do so through the easiest path.

If there is an upper vent hose, air will flow through it to the lower pressure tank (in through the higher pressure vent, and out through the lower pressure vent). If there is no upper vent, and the pressure difference is great enough, the only path that exists is from the higher pressure tank, down through the lines to the fuel selector (when on both) and back up the line to the lower pressure tank.

This sets up the scenario that may have happened to the OP where the higher pressure (from the vent) from the empty tank was great enough to push air down through the fuel selector back up the line to the lower pressure tank (that was full of fuel) preventing the fuel in the full tank from being able to reach the engine.

Hopefully this helps,

Bill angleofattackpressure.png

[Nev]

That's a very interesting point Bill. Any idea which side the aircraft would need to out of balance to increase the tank pressure on a particular side ?

[Redneckmech]

Hi Nev,
Sorry for the delayed response but I tried to take some time to puzzle over this to come up with a solid answer. Unfortunately, the more I dug into it, the more variables I thought of. I am not sure if there is any one definitive answer since there are so many variables that can affect the pressure seen by the fuel vent, take for example if it were a twist in the wing it could affect the pressure distribution over the fuel vent differently than if the rigging were off, which would be different than if the fuselage were misaligned with the wings, which in turn would be different than if one wing were a a slightly different angle of attack, etc..
I guess the short answer is that I really do not know.

[Bcone1381]

Troy,

Do you think a faulty seal in one of the fuel caps might cause a pressure differential to exist between the two tanks? Then I wonder if it is combined with an airspeed change how it might effect things.

[Bcone1381]

Ops....Sorry....Bill, not Troy. (Bill from Troy, ID)

[Redneckmech]

All good! After you commented I realized that my sig was confusing so I updated it. I figured out who you were talking to! haha

[schu]

After re-reading this thread I have removed my previous post because I was wrong. This entire thread was about someone that had fuel starvation with a pump, I forgot that the float plane had a Continental in it.

Here is an updated summary:

There seems to be two modes of failure for the fuel system:

• Unporting a tank and sucking in air instead of fuel.
• Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.

The first failure can happen with any pump but is greatly exaggerated by a return to tank fuel system such as in the case of EFI because it's pumping a lot more fuel because not all of it is being used.

The second failure has appeared on one gravity fed o-360 bearhawk without a cross vent, and also the IO-360 Continental in this thread, which if memory serves uses a return unused fuel to tank fuel system.

I am unaware of any failures with returnless/bendix/afp fuel injection systems, however if the imbalance was the issue in the case of the float plane, and enough to stop the continental pump, there is no reason to believe that it wouldn't also stop the lycoming IO fuel pump.

For me the take-away is:

• If you have a significant imbalance between your tanks, then there is obviously some pressure difference between them, and probably best to avoid the both setting.
• The vent between the tanks almost certainly makes it nearly impossible to have the above mentioned imbalance, even if your venting system has an issue.
• If you are returning a significant amount of fuel to the tank, then a header tank is probably best. Those are much harder to unport, even if you are cycling 100GPH.
• Pilot awareness is super important. Monitor the fuel system.

[Redneckmech]

Hi Brooks,

That is a great point! It absolutely could, since the pressure is relative to the velocity distribution of the air over the wing and the velocity of the air will approach zero as you reach the skin due to the boundary layer, there will be a pressure change relative to the height above the wing skin. The flow of air around the fuel cap will also be disrupted and that could create an even greater pressure differential due to the turbulence of that disrupted flow.

The below photo shows the velocity profile of the boundary layer. Velocity and pressure have an inverse relationship, as the velocity decreases, the pressure will increase.

boundary layer.png

As I said in my earlier post, I am by no means an expert and I haven't really revisited fluid dynamics in about five years. I know there are others here with far greater knowledge and experience in aerodynamics that could provide much greater detail and insight but hopefully this helps to clarify the general fluid dynamic mechanisms that are in effect.

Bill

[Redneckmech]

After re-reading this thread I have removed my previous post because I was wrong. This entire thread was about someone that had fuel starvation with a pump, I forgot that the float plane had a Continental in it.

Here is an updated summary:

There seems to be two modes of failure for the fuel system:

• Unporting a tank and sucking in air instead of fuel.
• Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.

The first failure can happen with any pump but is greatly exaggerated by a return to tank fuel system such as in the case of EFI because it's pumping a lot more fuel because not all of it is being used.

The second failure has appeared on one gravity fed o-360 bearhawk without a cross vent, and also the IO-360 Continental in this thread, which if memory serves uses a return unused fuel to tank fuel system.

I am unaware of any failures with returnless/bendix/afp fuel injection systems, however if the imbalance was the issue in the case of the float plane, and enough to stop the continental pump, there is no reason to believe that it wouldn't also stop the lycoming IO fuel pump.

For me the take-away is:

• If you have a significant imbalance between your tanks, then there is obviously some pressure difference between them, and probably best to avoid the both setting.
• The vent between the tanks almost certainly makes it nearly impossible to have the above mentioned imbalance, even if your venting system has an issue.
• If you are returning a significant amount of fuel to the tank, then a header tank is probably best. Those are much harder to unport, even if you are cycling 100GPH.
• Pilot awareness is super important. Monitor the fuel system.

Great summary,

There are an incredible number of variables that are in play and that is likely why one of the leading causes of experimental accidents is related fuel system issues.
Since each our systems are in effect, one-off creations tailored to our own needs, fuel line routing, line size, selector location, engine type, etc., will all be slightly different and will produce different results in fuel flow.

It is still a very long time until I start on my fuel system, but this thread has been incredibly thought provoking and brought up many different considerations that I had not contemplated until they were mentioned.

Many thanks to everyone who has contributed.

Bill

[Bcone1381]

After re-reading this thread I have removed my previous post because I was wrong. This entire thread was about someone that had fuel starvation with a pump, I forgot that the float plane had a Continental in it.

Here is an updated summary:

There seems to be two modes of failure for the fuel system:

• Unporting a tank and sucking in air instead of fuel.
• Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.

The first failure can happen with any pump but is greatly exaggerated by a return to tank fuel system such as in the case of EFI because it's pumping a lot more fuel because not all of it is being used.

The second failure has appeared on one gravity fed o-360 bearhawk without a cross vent, and also the IO-360 Continental in this thread, which if memory serves uses a return unused fuel to tank fuel system.

I am unaware of any failures with returnless/bendix/afp fuel injection systems, however if the imbalance was the issue in the case of the float plane, and enough to stop the continental pump, there is no reason to believe that it wouldn't also stop the lycoming IO fuel pump.

For me the take-away is:

• If you have a significant imbalance between your tanks, then there is obviously some pressure difference between them, and probably best to avoid the both setting.
• The vent between the tanks almost certainly makes it nearly impossible to have the above mentioned imbalance, even if your venting system has an issue.
• If you are returning a significant amount of fuel to the tank, then a header tank is probably best. Those are much harder to unport, even if you are cycling 100GPH.
• Pilot awareness is super important. Monitor the fuel system.

I use to believe and shared something I now believe was incorrect…. It had to do with sucking air if the front tanks port is uncovered.

I can’t recall who shared this…..maybe it’s in this thread…

I Now I believe if the front port is uncovered (like when slowed way down with a tank that is 20% full) then gravity will keep the the front fuel line essentially full by way of that tanks TEE (prior to the fuel selector). I can’t see an EFI with very significant fuel return changing this. The tank and the associated fuel lines would need to be empty before the pump would have the chance to suck air.

Here is a diagram of the fuel system from Bob's Patrol Book to help visualize this.
Screen Shot 2022-04-23 at 3.24.10 PM.png

[schu]

I Now I believe if the front port is uncovered (like when slowed way down with a tank that is 20% full) then gravity will keep the the front fuel line essentially full by way of that tanks TEE (prior to the fuel selector).

Sure, but what if your EFI system is cycling 100GPH of fuel? Do you think that gravity will keep up and keep the inlet flooded? Or do you think that the fuel will be cycling fast enough for the pump to pull in some air when either side is unported?

I suspect the second one, and heard about an EFI install on a cub in Alaska that had an unport which caused fuel starvation from someone I trust. It's anecdotal, but it makes sense to me that in the case of EFI the second that there is air exposed to a port that is drawing fuel, you will get a bubble, while if you have a port exposed that is draining fuel, it might not matter that much. Given the bearhawk gravity system is only good for around 40-50GPH, I think that makes both of your ports drawing fuel not simply draining it.

One more point:

There was a bearhawk that had an engine stoppage with EFI. I'd take a look at that NTSB if you are planning on EFI so see if there is something to be learned. Also, I don't think EFI is any more fuel efficient than AFP given how our systems tend to remain at static RPMS. I think the only reason for EFI is the ability to run garbage ethanol mogas fuel, which is for sure cheaper than avgas.

schu

[Bcone1381]

Schu, I had not thought about 100 gph. More to consider.

[svyolo]

I spent a bunch of time trying to figure all this out a couple of years ago. The best info, was Cessna's several decades of evolution of their fuel systems on MFI Continental engines, which also flow extra fuel and return fuel. I mostly used the kit stuff, but added a cross vent, and a small header tank (the local DAR, retired Boeing fuel guy) referred to it as a "collector tank". I return fuel to the small tank(2L) under the front seats. The fluid logic is what Cessna went to after 30-40 years with injected Continentals. I have never flown one of those planes, and don't know if my implementation is the same, but the system logic is as close as I could get it.

If one front or aft line is unported, I am pretty sure gravity will try to refill the other line, one way or another. Will it refill the other line, to the feed detriment of the fuel pump? Not sure.

I will say the Newton valve is not that impressive other than low weight and cost. Lots of sharp edges internally, and I think it is the biggest constriction in the fuel lines.

[RatherBFlying]

This was also my impression, after comparing the Newton valve and an Andair valve side by side I decided to use the Andair due to the concerns you've noted.

[Nev]

There seems to be two modes of failure for the fuel system:

• Unporting a tank and sucking in air instead of fuel.
• Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.

A couple of thoughts:

Unporting a tank and sucking in air instead of fuel.
My feeling is that this is more likely to occur when a selector is in either the L or R position, and the aircraft is being flown out of balance. My understanding is that the aircraft is designed to be normally flown in the BOTH position to prevent this.

Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.
My current thinking is that it's not having a severe imbalance that's causing the issue, rather the other way around. Flying out of balance when in the BOTH position is leading to a severe fuel imbalance, and the fuel flowing through the selector may be causing an issue at the gascolator/selector. Many years ago I flew Metroliners and if we ever had a fuel imbalance we would simply open the cross flow valve (similar to our BOTH position) and fly out of balance for a few minutes. The fuel would transfer from one side to the other really quick. Sometimes we forgot to close the valve for a few minutes and would quickly develop an imbalance to the opposite side.

My own experience is that I found the Bearhawk particularly difficult to fly in balance initially. More recently I am getting much better at keeping it in balance, and as a consequence I seldom get more than a couple of liters of fuel imbalance between refueling. I mention this not to knock the Bearhawk, but to reiterate that it did get flown out of balance alot when I was new to it. Other pilots found the same difficulty when flying my aircraft. When I refueled I would always find that I pumped far more gas into one tank than the other - though not always the same tank. MattS mentions in post 27 that his Bearhawk with floats was more demanding to fly in balance, and that after the mishap they found 55 liters in the Right tank and only 10 liters in the Left tank. This was after feeding off the Right tank for 30 mins in an attempt to balance the fuel.

I think it's probably very important to maintain balanced flight (side slipping excluded ), and (for my own bearhawk), to use the BOTH position of the fuel selector whenever possible.

[AZBearhawk272]

Very interesting thread and I have been following and re reading for a considerable time.
Just about to perform my fuel flow tests and was wondering what tests I can do while I am in the testing mode to gather additional information about modes and scenarios.

Configuration is: Two main tanks, gravity feed down to 2 gascolators, one for each tank located at the lowest point on the fuselage bottom just inboard of each wing strut.
Gascolators feed fuel to the Newton Fuel Valve. R, L, B, OFF. OFF is off with no cross feed. L,R isolate. Fuel valve feeds a pass thru electric boost pump to the Continental engine driven mechanical pump. Mechanical pump has a Vapor / Excess fuel return to the left tank via a check valve.

That said, I am making a test matrix that isolates various system elements, vents, gascolators, Tank levels, Mechanical fuel level sensors, Redcube fuel flow transducer.
With all the data collection through the Dynon I should be able to have a pretty good picture of baselines and influences of changes.


Fringe technical comments: With respect to aircraft and most fuel systems in general. Very few fuel pumps are designed to " Suck or have Suction" on the inlet to the pump. The desired baseline is to have a flooded inlet with positive head pressure on the inlet of the pump. " Suction" at the inlet will cause fuel to vapor lock and create bubbles and possibly associated cavitation. The pumps are designed to move a volume of liquid from inlet to outlet. Any air, entrained or otherwise will cause a substantial decrease in the pumps ability to move fuel from inlet to outlet.
Pressure seen at the outlet of the pump is the result of trying to force the liquid fuel through a downstream restriction. Pumps do not make pressure, pressure is from a restriction to flow. If there is not flow, there is no pressure.

With respect to slipping with low fuel causing the fuel to move away from the tank outlets and pickups becoming un-ported.
When slipping to landing, fuel burn is ounces per minute. Very little flow consumed by the engine. At altitude performed numerous full cross controlled slips, engine idle with less that 1\4 tank
and the engine never faltered while in BOTH. With the un-ported tank selected it generally took 3 minutes of sustained slip before engine roughness and it smoothed out within 7 seconds of wings level coordinated flight.

Now it gets interesting, this is a ground based test you can safely perform yourself. Have the left tank less than 1/4 full. Select left tank, and taxi at a nice fast walking pace in a fairly tight right turn... take a Dramamine first and bring a lunch. When the engine finally quits, stop the plane and immediately try to start the engine. If it does not start wait a minute and try again. If it still does not start, switch to BOTH, If it still does not start, walk back to the hangar and get the truck and a tow strap......... Been there. Done that. Now the question is why would it not start with no airfow over the wings and exactly the same tank air vent pressures and conditions??????

More will be revealed..........

Kevin D

KCHD

[Nev]

One fuel flow test I'd be interested to see that had me stumped. Start with an empty L tank. Pour a small quantity of fuel (say 3 gallons) into the left tank. Then disconnect the line where it exits the firewall and select L on the selector. Does any fuel come out ? If not, select both, the L again. Any change ? On mine it only started flowing after making another selection then back to the original tank.

[jaredyates]

Nev, in this case, imagine if an air bubble was trapped in the left line near the fuel selector. It was trying to rise up the line and there was enough head pressure from the 3 gallons to keep it trapped from rising, but not enough pressure to purge the air out through the firewall end. Selecting both allowed the head pressure from the right tank to overpower the small pressure on the left, thus burping the bubble back up into the left tank. I haven't experienced this type of thing with the fuel system but have with gravity-fed water systems as we collect lots of rainwater here on the farm.

[Nev]

Jared, that's exactly what we think was happening. Eventually after a lot of head scratching, we were able to repeat it consistently on both sides. Specifically we think a bubble was being trapped between the electric pump and the Gold Cube transducer right behind the firewall (we were able to release it by untorquing the AN fittings). Changing the fuel selector to another position cleared it every time and it then flowed from the original position as well. We tested with the opposite tank empty which then probably acted as a vent. When it later contained fuel it probably sufficiently increased the head pressure.

[schu]

Unporting a tank and sucking in air instead of fuel.
My feeling is that this is more likely to occur when a selector is in either the L or R position, and the aircraft is being flown out of balance. My understanding is that the aircraft is designed to be normally flown in the BOTH position to prevent this.

I think that unporting a tank and sucking air are two different issues. With gravity and almost certainly bendix/returnless, the system is gravity to the inlet of the pump, and as long as the engine isn't using more fuel than gravity provides, I imagine they are the same, and that unporting a tank isn't a huge deal, if it's brief. In the case of a returning fuel system, it's possible to have the pump pulling more fuel than gravity can supply and I suspect we are now firmly in the realm of sucking air.

Either way, do you think that unporting is less or more likely when using one tank vs two in a slip assuming you have the correct tank selected? What if you have 10Gallons of fuel in the airplane, which is more likely to unport? 6/4 split with both or 9/1 split with the 9 gallon tank selected?

Having a severe imbalance between the tanks which combined with a switch to "Both" appears cause the system to prefer balancing rather than feeding the engine.
My current thinking is that it's not having a severe imbalance that's causing the issue, rather the other way around. Flying out of balance when in the BOTH position is leading to a severe fuel imbalance, and the fuel flowing through the selector may be causing an issue at the gascolator/selector. Many years ago I flew Metroliners and if we ever had a fuel imbalance we would simply open the cross flow valve (similar to our BOTH position) and fly out of balance for a few minutes. The fuel would transfer from one side to the other really quick. Sometimes we forgot to close the valve for a few minutes and would quickly develop an imbalance to the opposite side.

My own experience is that I found the Bearhawk particularly difficult to fly in balance initially. More recently I am getting much better at keeping it in balance, and as a consequence I seldom get more than a couple of liters of fuel imbalance between refueling. I mention this not to knock the Bearhawk, but to reiterate that it did get flown out of balance alot when I was new to it. Other pilots found the same difficulty when flying my aircraft. When I refueled I would always find that I pumped far more gas into one tank than the other - though not always the same tank. MattS mentions in post 27 that his Bearhawk with floats was more demanding to fly in balance, and that after the mishap they found 55 liters in the Right tank and only 10 liters in the Left tank. This was after feeding off the Right tank for 30 mins in an attempt to balance the fuel.

I think it's probably very important to maintain balanced flight (side slipping excluded ), and (for my own bearhawk), to use the BOTH position of the fuel selector whenever possible.

Given you evidence that imbalanced fuel load is due to flying in a slip gives a lot of credibility to Bill's theory above. If imbalance is due to one wing flying more or less than the other, then it seems that putting the vent in the cap and not having a vent is for sure a factor.

I stand by my original thought that having a cross vent probably goes a long way to dealing with this, and that single tank operation also gets around this because balancing is explicit and the tanks are discrete systems.

In regard to the metroliner, who knows how that worked. I suspect the vents aren't in the caps, which changes everything.

Thanks for the dialog!
schu

[Nev]

Either way, do you think that unporting is less or more likely when using one tank vs two in a slip assuming you have the correct tank selected? What if you have 10Gallons of fuel in the airplane, which is more likely to unport? 6/4 split with both or 9/1 split with the 9 gallon tank selected?

More likely when only feeding from one tank when lower on fuel. I would think the 6/4 is more likely to unport if feeding from the L tank and the ball well out to the Left. Problem is that eventually the individual tank selected will contain LESS fuel because it's been burn off, and unporting is even more likely if flying uncoordinated.

Using the 9/1 as a more extreme scenario to illustrate, with only the R tank selected and the ball out to the right indicating a skid/slip situation, the fuel in the right tank will be at the right hand (outboard) end of the tank away from the ports so unporting is very likely. Fuel from the left tank in this situation is not available because only the right tank is selected. If the BOTH position was selected, the fuel in the left tank would be at the right hand (inboard) end of the left tank covering the ports, and available to feed the engine.

This effect can be observed inflight by putting the aircraft out of balance. If you look at the sight gauges, one gauge will read very full (it has fuel at the inboard end of the tank), and the other gauge will read very low (fuel is at the outboard end of the tank). Both tanks might actually contain the same amount of fuel. What is happening is that the fuel in both tanks is moving in the same direction as the ball and occupying that end of the respective tanks. Hence the recommendation to normally run with BOTH tanks selected.

In my mind, whether fuel can transfer from one tank to the other is not a question, I've seen it happen often. It results easily from flying in an uncoordinated situation and the fuel will always flow in the direction of the ball. The Bearhawk can be challenging to fly in balance initially so it makes sense to me that this could be happening and would result in more fuel being consumed from one tank, and more fuel remaining in the other tank. But importantly it also causes the APPEARANCE of more fuel in one tank due to the sight gauges indicating a fuel imbalance situation that may or may not exist.

Try it next time you go flying, With about half fuel in each tank, push the left rudder pedal and observe the ball move to the right. Then look at the fuel sight gauges. The left sight gauge will suddenly be indicating alot more fuel. The right sight gauge will suddenly be indicating less fuel. Then put the aircraft back into balanced flight and the fuel sight gauges will indicate the same fuel in both tanks. All that has happened is that the fuel has moved to a different position in each tank, either towards the sight gauges (and ports) or away from the sight gauges (and ports).

Another way to observe this is to park on a slope with say 5 gallons each side. The downhill sight gauge will show empty while the uphill gauge shows full. The engine will start in the BOTH position, and the Uphill tank position. But select the downhill tank, and it'll likely be completely unported. Leave the fuel selector in BOTH while you go and have lunch and see where the fuel is when you come back. It flowed to the downhill tank through the fuel selector and while it did this there was fuel flowing back up the lines and into that tank. Block either tank vent before you go to lunch and the fuel will likely stay exactly where you left it.

When inflight in these out of balance situations, we tend to then select what appears to be the fullest tank in an effort to rebalance the fuel. The tanks may actually contain the same amount of fuel, but by selecting the tank the appears fullest we are lowering the amount of fuel in that tank more than the other side.

Eventually, we will select the opposite tank when it shows a large enough real fuel imbalance. At this point (if we're still flying uncoordinated to the same side) a very large fuel imbalance may have developed that has overcome the tendency for the sight gauges to show the "uphill" tank as being the fullest. Incidentally, if the fuel imbalance had been caused by a blocked/restricted vent issue for example (instead of flying uncoordinated) we would now be selecting the tank that is most resistant to feeding fuel to the engine (if that was how it ended up with more fuel in it). In a more extreme out of balance situation (with BOTH selected) the fullest tank may even have a fuel flow into that tank (up the lines instead of down the lines) as the fuel flows across from the other tank. The question in my mind is what happens if we now select that (fullest) tank - is it able to adequately feed the engine ?

Just to clarify as Kevin mentioned above, we're not talking about a conventional side slip to land here. In that situation the aircraft is normally high on approach, and engine at idle, so fuel demand is very low.

Incidentally, in the case of the Metroliner, it was a twin engine aircraft and the cross feed was a separate line to the engine supply. So having fuel flowing into one tank didn't impede the fuel supply out of that tank to the engine.

[Bcone1381]

"4-8-1-3 green - flight idle - low - beacons on - parallel- starting engine check - clear capped flat on the right - clear capped flat on the left."

[Nev]

Haha better memory than me!

[jaredyates]

I have a question about cross venting. If the plane is parked on a slope and the fuel selector is set to isolate the tanks, what is to keep the high side from draining to the low side through the cross vent? A ball valve?

[zkelley2]

Generally nothing.

[schu]

I have a 170 with a cross vent, if the fuel selector is set to isolate the tanks nothing happens because the vent is on top of the tank where there is just air.