I was out flying the other day and decided to descend for some sightseeing, wanted to get down quick so I put in a slip left wing low, had the fuel selector on the left tank...caught me by surprise when the engine started to sputter and lost power. My first thought was possible carb ice so I went through that drill, didn't fix the problem. It didn't occur to me until after the fact that I likely unported the fuel tank. I had approx. 6 gallons in the tank and I was established in the slip for maybe 20-30 seconds, I always assumed I'd be safe from unporting with more than about 1/4 tank. I'm still scratching my head about why the header tank couldn't keep up for that amount of time. I must have had a lot of air in the line at that point because it wouldn't make good power even after a coordinated power-off descent for what seemed like another 15-20 seconds. I selected the right tank and power was reestablished immediately, albeit closer to the trees than was comfortable. After I landed I ran the left tank at 1800rpm for a few minutes and everything was normal. My assumptions were challenged and proven wrong, be wary of your assumptions and be quicker than I was to go for the other tank. Another reason why I'm glad to have two tanks in a 18-95...
Unported my left tank in a slip
- Thread starter MZ18
- Start date
BC12D-4-85
SPONSOR
Fairbanks, AK.
Can a slip like you describe force fuel from the full header back into the main tank? Just a question no answer here. You might try it again at altitude and see if it reoccurs.
Gary
Gary
aktango58
FRIEND
18AA
I was thinking that the header tank will also have the fuel slide 'down' and away from the out port, and with no fuel in the line pressurizing it.
This is the exact scenario that Jason G. talked about where the header tank is worth having, at least the fuel is super close to the tank, so when you do unport you have a chance of getting fuel back to the carb quickly.
My guess is that the slip was mighty steep?
This is the exact scenario that Jason G. talked about where the header tank is worth having, at least the fuel is super close to the tank, so when you do unport you have a chance of getting fuel back to the carb quickly.
My guess is that the slip was mighty steep?
BC12D-4-85
SPONSOR
Fairbanks, AK.
Header tank support for fuel in level flight versus turns...coordinated turns or slips and skids. Discussion?
GAry
GAry
bob turner
Registered User
I am not an expert on wing tank vent systems, but have had an 18 quit at reduced power in level coordinated approach. Plenty of fuel in both tanks, and a switch to the opposite tank cured it almost immediately.
Do you have those little “pffft” thingies on your caps?
Do you have those little “pffft” thingies on your caps?
cubdriver2
FRIEND
upstate NY
I've done a million slips on left tank only and never any problems. Even with only 3 or 4gls
Glenn
Glenn
Dan Gervae
FRIEND
Upper Peninsula of Michigan
What configuration do you have Glenn? Dual tanks with header? Headerless? Just curious since I am finishing fabric on my 11EX and will be assembling this summer. I did headerless with dual 18 gal tanks.
skywagon8a
MEMBER
SE Mass MA6
A both position on the fuel selector valve eliminates this issue.
cubdriver2
FRIEND
upstate NY
Yes Bob I have vented fuel caps, it wasn't blocked. Glenn I haven't done a million slips but I've done enough and it has never happened to me before which is probably why I didn't consider unporting as the problem in real time. I'm guessing it was just the right combination of left wing down, deck angle, uncoordination, amount of time in the slip, etc. for it to unport. The header tanks failure to keep the engine running is what I'm curious about, and the other thing about this that is striking is that I couldn't get the engine running again from the left tank for a considerable amount of time after the slip was let out. If I had a 1 tank system I would have ended up in the trees. I'm curious if anyone is running a boost pump on a cub type fuel system?
Steve Pierce
BENEFACTOR
Graham, TX
I run very low fuel levels a lot with header tanks and have never had an issue. I can't see the header tank unporting. I would put some more fuel in the right tank and go up and try to recreate that scenario. I take it all the lines and tanks are installed correctly?
skywagon8a
MEMBER
SE Mass MA6
Steve has a point. Once the lines get full of air, if there is a high point anywhere the air can form a bubble which is strong enough to oppose letting fuel gravity feed. Sometimes this would require a strong slip in the opposite direction to give the fuel in the tank a high enough head pressure to start flowing.
BC12D-4-85
SPONSOR
Fairbanks, AK.
Needs to be tested and fixed no doubt. Confirm all the plumbing and vents are correct and unrestricted. Stuff like this does bad and the aftermath is fodder for the scene's investigators.
Gary
Gary
Thanks for all the input with troubleshooting tips, I think the next step will be pulling panels and checking vent line from left tank to header. I agree Steve, after I've looked over everything and the weather allows I'm going to see if I can duplicate the scenario overhead the field. I'll report back with findings...
BC12D-4-85
SPONSOR
Fairbanks, AK.
How about draining the affected portion of the fuel system then inspecting and carefully blowing air through the components to clear obstructions? I've had finger screens on my Taylorcraft wing tanks restricted by insect parts. They get in the large forward cap vents apparently. Spiders and their webs won't dissolve. Removing the quick drain, blocking the vents, and blowing through the tank outlet sent them out flying again. Maybe do same for your header and fuel valve then replace any hose splices that look compromised?
Gary
Gary
courierguy
MEMBER
Inkom, Idaho
My header tank is unvented, didn't see the need when I built the S-7S 13 years ago, zero issues. Didn't know it was a thing to vent them, ignorance is bliss!
SuperCub MD
Registered User
Collins, Wisconsin
The left(forward) header is vented to the forward corner of the left fuel tank. The vent line is small,gets little if any flow through it, so it is very prone to corrosion and clogging in older aircraft. That is the first thing I would look at.
Gordon Misch
MEMBER
Toledo, Wa (KTDO)
How does it fill if there's no place for air to go?
BC12D-4-85
SPONSOR
Fairbanks, AK.
Plus the header's air fitting should be upper or at least higher than the fuel fitting(s).
Gary
Gary
skywagon8a
MEMBER
SE Mass MA6
If there is no place for air to be trapped in the header, then a vent is likely not needed. It all depends on the design of your system.
BC12D-4-85
SPONSOR
Fairbanks, AK.
Question on front Cub header: It's been some years since but aren't the vent and fuel nipples on the same end of the tank? If so would a left slip force fuel into both and maybe slow drainage by filling the air inlet? That assumes the main tank's outlet is unported when partially filled and in a left slip.
Gary
Gary
cubdriver2
FRIEND
upstate NY
I don't think it has a dedicated vent? Fuel in fuel out, vents up to tank? This is a Pa11 tank
Glenn
Glenn
courierguy
MEMBER
Inkom, Idaho
What skywagon said....when I run my floorboard mounted fuel transfer pump when refueling out of a 5 gallon jug or bush bag, when the pump starts sucking air I can see the air bubble gurgle up the header tank's sight gauge, up into the wing tanks where it vents out their vents. 2650 hrs TT and no in flight anomalies of any kind.
BC12D-4-85
SPONSOR
Fairbanks, AK.
Subject plane is a PA18-95 with dual tanks and headers. The PA-11 with single left tank originally had none? (http://www.univair.com/content/partcatalog/11PM-flipbook/mobile/index.html#p=30) but I guess one could be added later. My PA-11 had dual tanks and two headers.
Either way the fittings I believe should be one above the other not horizontal so any air when empty can be replaced by fuel and the header fully fueled to capacity. PA-18 fuel system (https://www.univair.com/content/partcatalog/18PM-flipbook/mobile/index.html#p=120) The UNIVAIR front header might be drawn wrong for the outlet location (see 61-63). See Atlee's PA-18 front header tank picture below.
Gary
Either way the fittings I believe should be one above the other not horizontal so any air when empty can be replaced by fuel and the header fully fueled to capacity. PA-18 fuel system (https://www.univair.com/content/partcatalog/18PM-flipbook/mobile/index.html#p=120) The UNIVAIR front header might be drawn wrong for the outlet location (see 61-63). See Atlee's PA-18 front header tank picture below.
Gary
Attachments
jjack
FRIEND
coastal n.c.
I did a left 180 , non coordinated descending dive towards my brothers dock and 1/2 way thru the turn the engine quit. Switched tanks , engine came to life and averted a crash landing in his yard.
Rebuilt early cub, 24 gallons in each wing. No other issues in 300 hours.
Never forget that incidence.
Rebuilt early cub, 24 gallons in each wing. No other issues in 300 hours.
Never forget that incidence.
SuperCub MD
Registered User
Collins, Wisconsin
That is the stock 18 front header. It was added to the 11 by a SB. Fuel line on the bottom goes down towards the selector,vent line on the top goes up to the main tank. Atlee front header has pipe fittings as shown in the other picture.I don't think it has a dedicated vent? Fuel in fuel out, vents up to tank? This is a Pa11 tank
Glenn
