3465 Check Valve question
10-29-2008, 11:14 AM
seawally | 3465 Check Valve question
My recent question (#3458) to Fred Lohr's post #2400 was attached to his and, of course, went way back and was effectively not seen. Can anyone tell me of their experience or knowledge of the problem of fuel being forced out the tail static vent after having installed a pair of header tank feed check valves? -- not in flight, but in the hangar. My valves are very effective, and stop return to the wings, so any fuel expansion from heat forces it up the static vent. How about adding a return line from high in the static vent down to either wing feed line UPSTREAM of it's check valve so fuel rising in the vent could migrate back to a tank? It sounds good to me, but I'm seeking first hand knowledge. Thanks, Wally
10-29-2008, 11:16 AM
Fred Lohr | 3466
Thanks Wally, that post solved the riddle of why i have seen some fuel stain around the static vent on the tail. Like you said, I quess my new check valves are actually working like they are supposed to. I have only noticed this recently so it happened after the valves were upgraded this year. I don't think you want to run a line back to a wing tank though, cause the concept is to have positive pressure in the wing tanks and negative or zero in the header tank and you sure don't want to mix those two. Although both vents are zero on the ground, the difference between the two increases the faster you go. If memory serves, the seawind crash in south america was due to mixing the two up. The engine ran fine on the ground and did't quit till after takeoff when the header tank emptied due to the pressure differential. (By now you can tell that I am solidly in the "if its a tank, VENT it" camp) One additional thought is that with the left tank flowing to the header tank(no shut off valve on that side)and with repeated heat/cool cyles you will tend to pump a little fuel from an always full header tank every day up the vent, because it cannot go back thru the check valves. In a parked aircraft, first morning, the cool fuel starts from the height in the vent line which is about the level in the tank, creeps up the line with heat expansion of fuel. When it cools back down, it recedes back down to the level in the header vent line which has filled in from the left fuel tank. With zero leak check valves,repeated cycles, this process could gradually fill the vent line. Too bad the standard set up does't have a valve to close the left tank fuel line at the header tank.....Hmmmmmm? Maybe we could come off the header vent line and put a saf t drain in the wheel well like there is for the other vent line. Any other ideas out there?
10-30-2008, 11:18 AM
Ed Lynch | 3468
ast time in Duluth, Phil de Ridder mentioned the same problem. The quantity of fuel coming from the vent was not great, but consistent after every shutdown. Whether it is caused by heat soak of the fuel in the engine compartment, or drainbacck from a not perfectly sealed system up there, I think that just adding some volume to that vent line would contain the fuel. A simple fuel filter installed in the line would add a couple of ounces to the capacity, and not affect anything else. That's what I plan to do if I notice any run down on my plane. Ed Lynch
11-04-2008, 12:21 PM
keithw | 3473
I have had simalar events where I noticed the tell tail blue stain on the tail at the low pressure side vent. It has been so rare (THREE OR FOUR TIMES IN OVER 360 HRS.) of flying that I just wipe it off and go on. No big deal for me but yes it does occur. Now the one time that did cause a concern was cross country to Manteo N. C. My stop for fuel at Dayton Ohio caused alarm as I looked at that tail and it was realy stained BLUE. I figured my fuel consumption on the first leg and could no figure that I had a significant fuel loss. I cleaned it off' filled the mains and aux tanks and headed across the Mountains to Manteo. At Manteo, no BLUE. I have never changed anything on fuel system since I built it. I have never had a problem and Ive been at 11500 and chirping along happy as can be. oooOPPP'sforgot to put on the altitude boost pump. So I reached down and clicked it on and I could not see any diference at all. In fact this happened on the 2nd leg Dayton/Manteo N.C. I love my fuel system and would not change it for Love or Money. Keith Walljasper N80CC
11-10-2008, 12:19 PM
Fred Lohr | 3477
I have been trying to get this to happen again, without success. I have tried flying with tanks full, parking with tanks full, and it has not recurred. My stains were straight down, not windblown toward the back like Keith's. It must be a fairly rare event so I think i'm going to leave this one alone for a while. I wonder if transfering auxillary fuel into the mains when the mains are full would force fuel up and out the vent?
11-10-2008, 12:22 PM
Mike | 3479
When I brought the plane from Fla to NY I noticed the same. Full tanks, hot day, up the vent....... I ripped out and rebuilt the entire fuel system and haven't seen that problem since.
05-12-2009, 09:46 AM
Originally Posted by Ed Lynch View Post
Last time in Duluth, Phil de Ridder mentioned the same problem. The quantity of fuel coming from the vent was not great, but consistent after every shutdown. Whether it is caused by heat soak of the fuel in the engine compartment, or drainbacck from a not perfectly sealed system up there, I think that just adding some volume to that vent line would contain the fuel. A simple fuel filter installed in the line would add a couple of ounces to the capacity, and not affect anything else. That's what I plan to do if I notice any run down on my plane. Ed Lynch