parallel fuel lines

[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.

 

Some

 

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)

 

I

 

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.

 

Also,

 

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

 

 

[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!

 

 

[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

 

engine.

 

Micgrace :)

 

 

[Thruster87]

I've not exprienced any problems with my Thruster which is setup in series. Cheers Alan

 

 

[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

 

engine.

 

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.

 

Beware.

 

Micgrace :)

 

 

[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.

 

The

 

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)

 

The

 

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.

 

The

 

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 :)

 

 

[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 :)

 

 

[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.

 

Carl

 

 

[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 :)