To bypass or not to bypass, that's the question!

Hi RW,

I respect your extensive experince & knowledge of Rotax engines, however you are human (I hope) and to to be human is to err from time to time.

I believe you erred in your description of the fuel supply to the carburettors - the  fact that the float bowl is open to atmosphere, suggest that a Rotax 9 (carby) does run on unpressurised fuel .

I agree with your statement & reasons why Rotax have designed the fuel delivery to the float bowl, to be pressurised (wider application).

Your point is valid in that a Rotax, depending on gravity fed fuel, through a failed mechanical pump, may not deliver sufficient fuel to the float bowl for sustained higher power - surely someone has tested this hypothesis???

I would expect the engine to keep running - sufficient to sustain flight? I don't know.

Every Rotax (all low wings) I have flown had a back up pump - I am comfortable with their function & management, so would likly prefer to have one, even in a high wing (fuel tank) - this is habit not science.

You will note that I suggested that a gravity fuel delivery should be tested at reserve fuel capacity (not a full tank), with the aircraft in the climb attitude. The gravity fuel flow at the carburettor will determine fuel availability, to the float bowl, from this an idea of sustained engine power. 

😈

And since I don't want to be a test pilot (at least when I'm not getting paid to be one ;-) ), I will for sure install the elec aux pump.

But that wasn't up for debate anyway. It was about the bypass fuel line and most of you have been in favor of that design, so I will install it accordingly.

Thanks everyone for your contributions to this debate.

Mark

Hi Mark,

I don't think you would need to be a "test pilot"

What I am suggesting is a ground based assessment - no risk to life or limb..

It should not be difficult to determine the gravity fuel flow (supply) to the carburettors;

Engine off- reserve fuel quantity - aircraft tilted in climb attitude - remove fuel hose from carburettors - timed fuel drain into calibrated container = available fuel (without pump running).

Compare result with Rotax fuel consumption rates.

Reserve fuel & aircraft in climb attitude is the worst case scenario. Full fuel, level aircraft,the best.

I would be intersted in the result , as I am sure RW would.😈

A bypass to an electric auxiliary pump does do no harm. If there are any doubts that the e-pump does not guarantee fuel flow in case of a failure you are well prepared. Personally I cannot see a reason to also fit a fuel filter for the bypass assuming that you have fuel filter(s) fitted anyway.

Peter K

"A bypass to an electric auxiliary pump does do no harm."

I beg to differ.

First it should be established that an auxiliary /backup /boost pump is required.

If its not required - extra joins, potential pump failure (risk) & unnecessary cost.

Fuel Filters

My last aircraft had an in line fuel filter on both tanks and the auxiliary fuel line. If a filter blocked, I would still have clean fuel supply back up.

Filters are a point of potential failure, in whole or part - if  fuel system/design allows, its good to have redundancy.

😈