Brilliant post, WN.
I reckon you could un-port the tank outlet in almost any type of plane if you side-slip it with low enough fuel level. It’s hard to imagine the contrary because it would leave a lot of unusable fuel.
I would reiterate the caution about knowing your type. I can think of one particular type even without especially low fuel that you definitely didnt want to slosh the fuel to one side of the tank!
IIRC, from a brief but highly enjoyable time with a Socata, the TB9 POH states select the high wing fuel tank for pronounced or prolonged side slipping. So perhaps that’s one type where the fuel sloshes too far the wrong way!
Slip away from the tank in use and all will be well. This is not rocket science and should be common knowledge.
Slip away from the tank in use and all will be well.
In many types, the fuel selector will be on “both” and I really don’t know what will happen if either tank will supply air instead of fuel. On a C421 I once co-owned we had to install a very expensive modification to the fuel tanks which was supposed to ensure that only fuel and not air could be drawn from any tank. For this modification to become mandatory a sufficient number of people had lo lose their lives. Luckily I was not among that group.
This is not rocket science and should be common knowledge.
As a matter of fact I am a rocket scientist (or as close as it gets) and I have never heard of that before today.
There’s only one direction you can slip to witha given crosswind …. I would say
Peter wrote:
I reckon you could un-port the tank outlet in almost any type of plane if you side-slip it with low enough fuel level. It’s hard to imagine the contrary because it would leave a lot of unusable fuel.
Planes that have issue with unporting are generally those with long, thin wing tanks, where the only fuel ports are at the inboard end of the wing tanks – where dihedral and simplicity means they are typically located. Fuselage tanks (as opposed to wing tanks) aren’t often long and thin enough to be affected by typical slip angles, and planes with wing tanks having fuel ports both inboard and outboard would also not be affected.
Planes with wing tanks supplied through a valve with a ‘both’ setting may transfer fuel to the wing-down tank in a slip, even to the point where that tank may overflow through the vent if it was quite full to begin with, but there will not typically be any issue in delivering fuel to the engine regardless of the wing-down tank (maybe) unporting.
Alexis wrote:
here’s only one direction you can slip to with a given crosswind …. I would say
And if you plan to slip in either direction for an extended period, for any reason but let’s say to lose a lot of height, you would want to select tanks appropriately before doing so. This thread is actually a good reminder for me in that regard because neither of my planes have a ‘both’ setting.
Silvaire wrote:
…but there will not typically be any issue in delivering fuel to the engine regardless of the wing-down tank unporting.
How can you know that? If sufficient fuel flows from the wing-up to the wing-down tank (which takes ages in real life because the pipes are very thin… ( * ) ) then at one point the wing-up fuel outlet will become uncovered. And from that point on, air will enter the fuel line, no matter how much fuel remains in the wing-down tank. Unless there is some kind of float valve to prevent this from happening. Are there float valves in every plane that has a fuel selector with “both” as default setting? Who knows… Finding that out at 50ft is certainly good for one’s adrenaline balance though. As I wrote above, in the C421 they found about this issue by counting casualties.
( * ) The Metroliner I once flew for a while used this “technique” to balance the fuel in the tanks. A crossfeed-switch in the middle of the glare shield opened a valve in a line connecting the tanks and one was supposed to fly with crossed controls holding one wing down until the fuel was balanced. This could take up to 15 minutes in case of a major imbalance, e.g. after prolonged simulated single engine flight during a checkride. There was certainly no danger of overfilling a tank due to fuel flowing by gravity from one side to the other…
Silvaire wrote:
Planes with wing tanks supplied through a valve with a ‘both’ setting may transfer fuel to the wing-down tank in a slip, even to the point where that tank may overflow through the vent if it was quite full to begin with, but there will not typically be any issue in delivering fuel to the engine regardless of the wing-down tank (maybe) unporting.
what_next replied:
If sufficient fuel flows from the wing-up to the wing-down tank (which takes ages in real life because the pipes are very thin… ( * ) ) then at one point the wing-up fuel outlet will become uncovered.
I said “typically” on purpose, knowing that it is not typical to slip so long that the up-wing fuel tank will completely drain into the down-wing fuel tank through a small diameter interconnecting fuel line. Even with 1/4 fuel in the up-wing tank this would take a long time, and is obviously unlikely. Equally obviously, I would never want to be landing with ‘both’ selected and fuel level so low that this was anywhere near being an issue.
There is one somewhat more likely scenario along those lines: if the plane were flown on one tank until that tank were empty, then the pilot selected both tanks, then before long he slipped for an extended period away from the tank that he’d earlier emptied.
Silvaire wrote:
Even with 1/4 fuel in the up-wing tank …
All this is a non-issue with sufficient fuel in the tanks. 1/4 is a lot. I rarely land with so much fuel left. It only becomes critical and/or dangerous in low-fuel situations. The problem with those (been there, done it) is that they occur rarely and therefore are not usually trained or briefed. A low-fuel situation alone is quite unnerving (been there, done it…) and doesn’t leave the average pilot – to which group I count myself – with much mental capacity to figure out the optimum tank selection. One selects “both” or the fuller tank and what happens after that will happen.
I’d recommend not slipping for extended periods with dangerously low fuel in wing tanks, with ‘both’ fuel tanks selected (on planes where that is possible)
