Back a couple of weeks ago we were having a discussion about the IVPT fuel system and it suddenly faded away. Well, not really... it was starting to get a bit contentious and a little emotional, so I headed off some posts at the pass in order to allow things to cool off a bit. Since it has never been the policy of the LML to censor anything submitted to it as long as it was directly Lancair related, I thought I would paraphrase a couple of the posts that were sent in and present their hypotheses solely as bases for discussion and to keep within the LML's mission of providing operational and safety related data dispassionately and without bias.
Just to be sure that we all understand what we're talking about, we need to understand the basic layout of the IVPT fuel system as regards tankage, fuel feed and venting. As for tankage, the system includes both wing tanks (typically 46-50 gallons each) and a belly-mounted sump tank (typically 33 gallons) from which fuel is pumped to the engine. (I am reluctant to call the belly tank a header tank, simply because it is not at the "head" of the system, ie, physically located in an elevated position as is the header tank in the LNC2 and capable of gravity feed.) The fuel pickup is located at the low point in the belly tank (not sure how much distance exists between the end of the pickup tube and the floor of the tank) with the fuselage in the level condition. The fuel is retained in the immediate vicinity of the fuel pickup by baffles with slosh doors. I do not know if the baffled areas are inter-vented. Fuel is delivered to the belly tank from the wings through the fuel selector which can select off-left-right. (Some systems use off-left-right-both selectors.) The belly tank is vented from the high point at the front of the tank through 2 vent lines that go forward to the firewall, up the firewall and are topped off with a pair of electrically operated solenoid valves .The valves open to the atmosphere when they are unpowered and close off the vents when the master switch is on and the aircraft is under power. The wing tanks are vented normally, through aluminum tubes (were 1/4", now are 3/8" if I understand correctly) that start at the high points at the top of the tanks and rise higher still in the winglets where they terminate at a small NACA duct. There is a vertical segment of hose that goes upward from the NACA and then back down to connect to the vent line from the wing to minimize the possibility of moisture being trapped in the vent line.
If I've screwed up any of the details I would appreciate having anyone more knowledgable then myself jumping in here and setting me straight. This is, after all what discussions are all about <g>.
Now for the meat and potatoes....
Apparently this system closely mimics the fuel system that was in the early turboprop Caravans. Back in those early days there were apparently a number of accidents caused by pilots forgetting to turn the fuel on. The sump tank system held just enough fuel that one could start, taxi, takeoff, then flame out soon after. With the vent closing and fuel selector in the off position, the engine starts ok, but the belly tank soon comes under a vacuum which just keeps getting worse. The engine keeps pulling, until first indication to the pilot, which is either a rupture of the tank (causing flameout) or the the engine can no longer overcome the ever increasing vacuum and then again flames out. Either way, the first indication is a flameout at the worst possible time. The question here is whether or not this is a possibility with the Lancair system, and if so, what can be done to alleviate it?
Hypothesis #2.... it appears that if air gets into the sump tank, (in flight) there is no way for it to get out other than go through the engine. What if someone takes off without having the wings and belly full? Fuel is being fed into the belly from a wing and that wing empties. Now there is air getting into the belly. Once the pilot realizes this is going on he selects to the other tank. But now there is air in the belly. What happens if the pilot reaches his destination, but has to do a go around? . Power is added, the nose is raised and a climb is established. The air goes to the front of the belly tank and the main pickup is now sucking air. The engine flames out because the FCU is full of air.. Unfortunately this is a go around and so the aircraft is too close to the ground to have enough time to wait for the turbine to wind down so he can relight it. Once again, the question is whether or not this is an unrealistic scenario, and again, what can we do to alleviate it if it's not?
Hypothesis #3... there is air already in the belly because last leg of a flight was a long one and one wing emptied. The pilot pulls up to the pumps, shuts down and fills the wing tanks. What if the solenoid valves stick or were sluggish and didn't open when the master was off? Alternatively, assume that the solenoids have opened but the pilot rushed through the fill up in order to get back in the air ASAP and didn't allow enough time for all the air to leave the belly tank and the fuel to percolate into the belly to replace the air. The air in the sump tank remains trapped. What if the pilot doesn't realize that the sump tank has this air bubble in it, takes off and unports the fuel pickup in his rapid climb? Again, this is a hypothetical situation... is there a possibility of it becoming reality? I don't know, that's why the questions. Let's talk about it.
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