In my last Fuel Injected plane I only used the boost pump for starting the engine . Does any one use it taking off ? If so why ?
Tom
Tom, the reason is a technical one. Fuel will vaporize if the absolute pressure is low enough and the temperature of the fuel is high enough.
So - if the boost pump is not on, the pressure drop from the fuel tank to the mechanical pump is proportional roughly to the square of the fuel flow. At takeoff, the fuel flow, full throttle, full rich, is at the maximum, so -therefore - is the pressure drop at the inlet of the mechanical pump. Just after rotation there is an uphill flow that adds to this pressure drop. This is the worst case for flow and pressure. Temperature and actual vapor pressure of the fuel is another matter.
If the fuel is at that high end of the vapor pressure, and combined with hot fuel on a hot day, maybe even a hot start, then this would be the most likely condition to create vapor due to "suction" (low absolute pressure) at the suction of the mechanical pump.
So, the condition of the fuel filter, size of the lines, altitude and temperatures all remain variables to address.
Keeping the boost pump on during TO is a precaution to avoid all the thinking and measurements and unknown variables.
I hope this makes sense. Other variables of the various ways people build the fuel system from tank to M. pump affect that pressure drop. Lycoming says -2psi is the max drop to the pump inlet. I have not quantified what this means terms of temperature tolerance (yet). My building takes priority.