Jump to content

parallel fuel lines

Guest micgrace

Recommended Posts

Guest micgrace

Hi everyone


Something that could possibly be a potential very


nasty problem. Whether this has been the cause of any accidents I can't


seem to find a reference to. Maybe someone has come across it.




ultralights are fitted with an engine fuel pump as well as a parallel


electric fuel pump. With both connected just foward of the outlet of


the engine pump.


WhereI tend to voice a bit of concern, is the separation or breakage of either fuel line while one remains operative. (i.e. fuel still getting to engine)




believe it would be quite possible for the engine to be running


normally with the pilot unaware the fuel is rapidly disappearing from


the tank (into engine bay, cockpit or whatever). Especially in those aircraft where fuel level cannot be easily seen.


The consequences are possibly an inflight fire or no fuel.


In doing an inspection I would STRONGLY reccomend to check the fuel lines (hopefully aircraft grade) , cracks, swelling, (especially around clamps)age, aircraft clamps and barbed fittings. correct routing of lines. Correct use of grommets.


To check for cracks bend hose back on itself, if in doubt change.




while on the subject I have noticed some use of Automotive paper fuel


filters on 2 stroke engines. The oil and fuel mix reacts with the resin


in the filter, swelling the fibres plugging it. Nylon / brass type


screens only


Micgrace :)


what you don't know will get you



Link to comment
Share on other sites

Guest TOSGcentral

The Thrusters have a semi dual system standard.


Single outlet from tank. Splits into main fuel line and the other to


the electric boost pump. Then both lines have clear view filters. Then


they conjoin again and a single line leads up to the engine.


Simple and effective - except owners get excited about the plumbing and


some really weird results have been seen to happen. Drawing a mud map


on a piece of paper and then putting it into practical use effect does


no end of good!



Link to comment
Share on other sites

Guest micgrace

Right on Tony


Do not take chances in this area. If in doubt ask.


I do not know about the Thruster layout, however Tony's the expert on


them>but this set upwas commonly used on early drifters and such


which have (more than likely) suffered incorrect mods.


I'm sure I seen a dual pickup used on a chinook 95.10 (fuel tank mounted in ex passenger seat place)as well. even a worse idea.


By the way with rotax 2 stroke you DO NOT run engine/electric fuel


pump in series. It will create excess fuel pressure and flood the




Micgrace :)



Link to comment
Share on other sites

Guest micgrace

A few more notes


The use of in series fuel pumps is a trick


used in high performance engines to 1. increase the overall fuel


pressure 2. deliver an increased volume of fuel.


Think about this


statement for a moment. Fuel pump A supplies fuel at a certain


pressure. which is supplied to fuel pump B which no longer has to do


the work of pulling fuel from tankas well asraising the fuel pressure


to an acceptable level. The end result is increased pressure and flow.


When this system is used on high performance carby equipped engines a


fuel pressure regulator is used to prevent flooding due to excess


pressure. On injected engines a pressure regulator is used to lower the


load on the efi pumps at idle.


OK so someone will say but that


doesn't relate to aircraft does it? But think about it before jumping


to conclusions. This is where confusion arises.


The reason a parallell system is used in some aircraft is somewhat different to this.


This is to assist with fuel flow with a high angle of attack generally associated with takeoff climb.


Think for a moment what happens when an aircraft with a gravity supplied fuel tank climbs. The tank is now lower than before, (compared to the engine)hence the fuel pressure and flow reduces.


By using a electric fuel pump in parrallel sufficient fuel will be


supplied. This effect would certainly apply to a thruster. Other


arrangements would certainly benefit from it to ensure no shortage of


fuel flow (not increased pressure, although the two are interconnected) for takeoff. Hence the parallel design.


Since most aircraft engines are fitted with a float type carburettor it is


quite possible to exceed the fuel inlet pressure which takes only a


small increase to actually push the float in the fuel and flood the




At wide open throttle with electric fuel pump on a pilot


would probably not notice any flooding. BUT at around idle it is quite


possible to flood an engine. A pilot who turns the electric pump on for


climb out then turns it off for cruise would likely be none the wiser.


I have witnessed this effect and it takes a couple of minutes to develop,


the effect is not instant bit like leaving the choke on too long. All


the pilot seems to think he's left the choke on, need new needle/jet.


Fuel consumption too high, or whatever pet theory they have without


identifying the real cause.


The pilot then "clears" the engine and everything seems to be fine.


Needless to say, operating in this condition will not help the engine. Fuel wash


will remove oil from cylinders/pistons etc, create excess carbon, jam


rings foul plugs.


Like all good things this design MUST be correctly maintained, installed and operatedhence the earlier post.




Micgrace :)



Link to comment
Share on other sites

Guest micgrace

The problem with an unauthorised change on the Gazelle arose from


removal of engine pump and substitution with an aftermarket fuel pump.


No doubt this came from a misguided attempt to save money.




person who did not know any better removed the plumbing from the


parallel fuel circuit effectively boosting the inline pressure as such


this jury rigged idea would have to be run full time instead of the


usual practice.


i.e direct from wing tanks to header tank then


through the aftermarket electric fuel pump straight to the carby. If


the pump was not on this provides a restriction otherwise so pump would


have to run full time.


There is (from memory) a fuel return on the engine pump (I'll check the parts catalogue to be sure)


This acts to reduce pressure if the flow dimensions are carefully


matched to pump output.if anything is any different, I'll make a new


post with any corrections. This is the simplest way for a designer to


attend to this design problem. No moving parts to control fuel


pressure. (= safer)


The Gazelle of course has a boost pump for climb out, which is then turned off. It forms part of the takeoff/landing procedure.


4.5 NORMAL PROCEDURES & CHECKLIST (extract CA-25N Flight Manual)


(F) Check before Takeoff:


4. Fuel Pump Switched on.


(even marked red)


Electric fuel pump plus engine pump fitted from factory on R912


These comments equally well apply to 4 stroke (carby)


engines. Although it is safer for a spark ignition engine to run too


rich, compared to too lean, though not ideal over the long term. The


opposite condition (rich = death to engine) applies to diesils (yes they are made for aircraft)




idea of the post was to bring to everyones attention the necessity of


fuel hose inspection, however, this has uncovered another problem that


people have created themselves by deviating from manufacturers plans


without the prerequisite knowledge to do so.


There are other


ways to increase flow to engines. The simplest is to increase the size


of the fittings. I remember seeing something from TOSG about the exact


same problem in Thrusters (i.e. fuel line diameter too small) Tony can comment on this I'm sure. Of course to be legal (95.55 factory) even this change must have Reg 35 approval.




point is, a backyard "fix" can seriously endager safe operation if you


do not have the prerequisite knowledge. I'm not going into a full


theoretical debate on this issue. It took me a full engineering degree


to learn about such things.


Micgrace :)



Link to comment
Share on other sites

Guest micgrace

A slight correction to the above. The fuel return line (not on pump)uses


a separate fuel pressure regulator. This style of arrangement is very


German and gives very precise control, however adds to cost. It is


possible to overpower these devices with too large a pump (or simple fuel return). I have done very little work on R912 so someone who does could clarify for me .


Thanks Micgrace :)



Link to comment
Share on other sites

Guest carlsnilsson

Surely the Skyfox Gazelle installation is a series system, i.e. the


electric fuel pump is in series with the mechanical, which is why you


can/should switch it off out of the circuit? This is fundamentally


different to the parallel system with which this thread started.





Link to comment
Share on other sites

Guest micgrace

I'll have a proper look at the plumbing on a Gazelle and confirm it one way or the other. And I'll post the results.


There is no reason it couldn't be in series as the r912 does use a proper fuel pressure regulator (although on the return line). Although to be fair I never paid much attention. All I do is follow the exact layout by the manufacturer when doing repairs.


Micgrace :)



Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...