ROTAX 912 low fuel pressure at take off

Jeff Blakeslee wrote:

Sean,

In your earlier posts, I thought you stated that the fuel pressure was in the normal range whenever using your (existing) Boost Pump, and that the low pressure only occurred once the Boost Pump was turned off, then slowly recovered in level flight.  Did I get that wrong?  In other words, I thought you were trying to achieve normal pressure under all flight conditions without the use of your boost pump.  

Hi Jeff,

Sorry for any confusion.

"I thought you stated that the fuel pressure was in the normal range whenever using your (existing) Boost Pump, and that the low pressure only occurred once the Boost Pump was turned off, then slowly recovered in level flight. "

We (my Son & I are the listed test pilots for our new Sonex home build), have as a Prestart check - boost/auxiliary pump ON, check pressure - boost OFF.  Always use boost ON for TO/Landing, OFF when safe altitude attained.

On the ground (engine running) -Fuel pressures with & without boost pump, is in the 4.5-5psi range.

Take off/Climb- Fuel pressure drops to below 2psi (Dynan gives warning). Due to low pressure warning, boost pump has been left on, until cruise level achieved ie well above safe altitude.

Cruise - Boost off. Fuel pressure rises back up to 4.5 psi.

I thought you were trying to achieve normal pressure under all flight conditions without the use of your boost pump

Yes - (normal being a range). I see the boost pump as a back up, in the event of the mechanical pump failing, rather than essential for continued flight. (I am amazed to hear of some aircraft where the boost remains on for the duration of the flight)

So Roger L's solution, (installing a more powerful boost pump) in this conversation, might be seen as covering a problem, rather than correcting it.

Sean, 

I’m curious, do you have a bypass circuit around the electric boost pump or are you counting on the pump flow-through when the pump is off (or fails).  If the latter, have you done a flow test through the boost pump (with it off).     

The pump is in line, there is no bypass circuit.

Not sure how I would go about testing the mechanical pumps flow rate - I suppose cranking the engine with ignition off and a hose from the pump delivery side into a graduated cylinder, might give some indication but that's all it would be - not an accurate assessment.

Drawing from the wing tanks, I have tested the boost pump flow rate - not wonderful but I think its more of a pressure pump than a volume supplier. The boost pump is not far below the bottom of the wing tanks so has little head to assist it - when boost off, the wing tank fuel still percolates  through.

My Sonex uses the standard (in fuselage, above crew legs) tank position to house a 40L header tank which gravity feeds to the main/mechanical pump - it delivers fuel faster than the boost pump.

So there is no boost pump between the header tank and mechanical fuel pump?

Jeff Blakeslee wrote:

So there is no boost pump between the header tank and mechanical fuel pump?

Correct.

For this reason the header tank is not used for TO/Landing.