Andair check valve

[PapuaPilot]

You definitely want to send the return fuel to the header tank. The reason is to get rid of any vapor. That is the reason it goes in the top of the header tank.

It would not be wise to put it back in the gascolator.

[jrevens]

I could be mistaken, but I believe that RANS says to run it back to the gascolator. At least I've seen it done that way on several S-19s. I wonder if there is any value at all in doing that? If the gascolator is down low on the firewall, as they usually are, the fuel in it is probably cooler than that in the fuel line running on top of a hot engine. Also then, you are mixing the warmer returning fuel with the cooler fuel entering the gascolator, possibly giving close to the same effect, with shorter hose runs & one less line running through the cockpit.

[Dutch]

Correct me if I'm wrong. I allways thought the reason for the return line was to relieve the pressure of the expanding fuel between the fuel pump and the carbs after shut down. Thats why I routed it back to a metered port on top of the gascollater. I reasoned that the 8 or 9 lb would easily push the fuel back up line.

Dutch

[Dave S]

Ideally, I believe the instructions from Rotax say to route the relief line back to the fuel tank.

The idea behind that is to get any vapors (if they exist) out of the incoming fuel stream.

The Kitfox tactic favors the return/relief line going back to the header tank which has a vent to the fuel tank; and, has sufficient thermal mass in the amount of fuel stored in the fuel tank to recondense any gasolive vapors anyway.

Don't know anything about the Rans & gascolator deal; but, I would estimate that someone has evaluated, to some degree, the option of going to the gascolator and found that it works in that design (I hope?)......however, the line to the gascolator entirely depends on sufficient thermal mass at the gascolator (gascolator + fuel) to recondense gasoline vapors (which has far less thermal mass than a kitfox header tank)....since it does not have a vent daisy chained back to the fuel tank through a header or other venting system there is no way to exclude vapors from the fuel line to the engine if the stuff does not recondense in the gascolator...theoretically anyway.

So, why do people plumb a system short of the fuel tank?.....probably for simplictity and avoiding the extra work/plumbing that would be needed to go there......on a kitfox, there are no addittional ports for a return line at the wing tanks; but there is at the header which is also vented so it serves the same purpose as going to the wing tank. On the Rans, I don't know; but most gascolators have a primer outlet - if it isn't being used...it may be a target of opportunity for a return line - but it is not vented; and, the proximity of the primer port to the fuel pickup port to the engine in a gascolator is pretty darn close.

Dutch is correct on this point " I allways thought the reason for the return line was to relieve the pressure of the expanding fuel between the fuel pump and the carbs after shut down."

PaPua Pilot is correct on this "You definitely want to send the return fuel to the header tank. The reason is to get rid of any vapor. That is the reason it goes in the top of the header tank."

Jott is correct on this " one of the main purposes of the return line is to recirculate cool fuel from the tank back to the engine to help prevent vapor lock. Recirculating it only to the gascolator (usually mounted in the hot engine compartment) seems to partly defeat this purpose."

The trick here, as the Kitfox factory installation tactic has determined and suggests in its installation guide, is to: 1) Cure the pressure build-up between the fuel pump and carbs with a bleed/return line; while 2) Not taking a chance of sucking vapors back into the fuel line to the engine through a short circuit.

Mission accomplished by running the line back to the header tank where the bleed fluid can be recondensed and if not, vented to the fuel tank.

Sincerely,

Dave S
KF7 Trigear
912ULS Warp Drive

[jiott]

IMHO there is too much emphasis here on recondensing vapors in the fuel line. What you really want to do is to prevent the formation of any vapors in the first place, thus the reason for the recirculation of as cool fuel as possible from the header tank. If vapors form in the fuel line during flight I think the horse is already out of the barn and there is a real danger of vapor lock in spite of any recirc line (the recirculated fuel is a very small percentage of the total fuel flowing to the carbs).

I do agree that there is some value to recondensing vapors that may have formed during a hot soak after shutdown on the ground. This is one of the most likely times to develop vapor lock and it can make the engine very hard to restart. The cool recirculated fuel can really help here by flushing out and recondensing any vapor pockets.

[Dutch]

I have never experienced vapors forming in the fuel line during flight. If that were to happen it seems to me that would indicate a serious problem with the routing of the lines. Especialy with the use of fire sleeves. The only problem is the hot starts, with the pump putting out 4 lbs and the pressure in line up to 9 lbs you need to relieve the pressure. Anyway thats my 2 cents worth.

Dutch

[HighWing]

I guess I'm with Dutch on this one to some degree. The return line is a late addition to the Rotax system. I wonder if the apparent problems attendant to it's being a recommended practice is Kitfox related or if it developed in another environment entirely. I doubt you will find this discussion in any of the old List or forum archives and with almost ten years in my old IV and lots of hours alongside other Kitfoxes and Rans with hundreds of flight of six or eight engine starts and shut downs on extended cross countrys without it ever being a discussion item during the hangar talk sessions, it makes me wonder. I don't have one on my current IV and until I hear of the exact reason I should relative to the Kitfox installation, I probably won't have one on my new IV either. It would sure help me understand this issue if someone could give me a real life Kitfox experience pointing to the risks of not having one or even the potential benefits of having one.

[Russell320]

I'm with Dutch and Lowell. When I purchased my model IV, it had a nagging hot start problem. The fuel lines were routed against the case, no fire sleeves, and no return line. I rerouted the fuel lines with fire sleeves and added a fuel return line to the wing tank (with the proper orifice). That solved all the hot start problems.

These engines, IMO, do not have vapor issues. When they are shut down, the heated fuel in the lines increase in pressure to overwhelm the float needle allowing fuel to be dumped into the carburetor, and the fun begins. I'll bet one of the biggest problems with starter and battery wear is hot starts.

Here in AZ, we try the hardest to keep our fuel cool.

Russ

[jrevens]

I tend to agree with the last 3 posts also. When I first heard of this return line deal, I believed it was to help with hot start problems, then read about the vapor issue. What Russ wrote is exactly what I had discussed with friends before that. When you shut down, & the fuel downstream of the fuel pump cannot flow backwards through the pump, that solid column of fuel can produce tremendous hydrostatic pressure with just a relatively small temperature rise. That section of line, sitting as it does on the top of the engine, certainly gets heat-soaked after shutdown & might easily overpower the float needle & possibly overfill the carbs. Result- hard start. Routing the line into a gascolator or teeing in anywhere upstream of the pump will definitely take care of that issue.
The few long-time Kitfox owners I know around here have never said anything about vapor lock problems, & don't have the line. Again - good installation practice & fire sleeve are probably most important. I'd like to know what exactly caused this to become an "issue" with Rotax. Is it just further a** cover to guard against some poor installations? It would be good to know.

[HighWing]

I guess what prompted my comment was a 1999 Service Letter SB-912-026UL which mandated the inspection and possible replacement of the Stator Assembly.

The only difference between old and new stator assemblies was the size of the Adel style clamps holding the wiring to the assembly before exiting to the ignition modules or the voltage regulator. It finally came out that the certified version in certain airplanes used for training were, as expected, only inspected and maintained during the periodic mandatory inspections. Since the oil was checked and replenished through a small door on the cowl by student pilots and instructors, the engine compartment was an oily mess at the the time of the inspections. Some A&Ps were hosing down the engine with some sort of solvent that softened the insulation on the wiring within the "undersized" Adel clamps and it resulted in some incidents of shorts there. Definitely not good, but on the other hand not a real hazard with a properly maintained engine.

I did the deed. Not a big project and parts were complimentary from Rotax with free loan of needed tools, but it did require removing the engine. Would I have done it if I had really known the reason behind the failures? Not sure I would have. Another thought on that, I would definitely have been convinced to do the swap if they had really done the right thing and replaced the wire in the stator assembly with an aircraft grade wire using Tefzel or similar insulation. Correct me if I am wrong, but I believe to this day they still use wire straight out of Pep Boys.

In searching for the above service letter number, I found a new one specifically addressed, I guess, to my friend Hal (and others) regarding his large bore kit. For sure, this one is a CYA letter and the customer should definitely beware, but...